Merging upstream version 1.69.0+dfsg1.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

21 files changed, 3288 insertions, 1567 deletions

diff --git a/compiler/rustc_parse/Cargo.toml b/compiler/rustc_parse/Cargo.toml
index dbcfb3903..3eb158c81 100644
--- a/compiler/rustc_parse/Cargo.toml
+++ b/compiler/rustc_parse/Cargo.toml
@@ -16,7 +16,7 @@ rustc_lexer = { path = "../rustc_lexer" }
 rustc_macros = { path = "../rustc_macros" }
 rustc_session = { path = "../rustc_session" }
 rustc_span = { path = "../rustc_span" }
-thin-vec = "0.2.8"
+thin-vec = "0.2.12"
 tracing = "0.1"
 unicode-normalization = "0.1.11"
 unicode-width = "0.1.4"
diff --git a/compiler/rustc_parse/locales/en-US.ftl b/compiler/rustc_parse/locales/en-US.ftl
new file mode 100644
index 000000000..e76e91fc1
--- /dev/null
+++ b/compiler/rustc_parse/locales/en-US.ftl
@@ -0,0 +1,734 @@
+parse_struct_literal_body_without_path =
+    struct literal body without path
+    .suggestion = you might have forgotten to add the struct literal inside the block
+
+parse_struct_literal_needing_parens =
+    invalid struct literal
+    .suggestion = you might need to surround the struct literal in parentheses
+
+parse_maybe_report_ambiguous_plus =
+    ambiguous `+` in a type
+    .suggestion = use parentheses to disambiguate
+
+parse_maybe_recover_from_bad_type_plus =
+    expected a path on the left-hand side of `+`, not `{$ty}`
+
+parse_add_paren = try adding parentheses
+
+parse_forgot_paren = perhaps you forgot parentheses?
+
+parse_expect_path = expected a path
+
+parse_maybe_recover_from_bad_qpath_stage_2 =
+    missing angle brackets in associated item path
+    .suggestion = try: `{$ty}`
+
+parse_incorrect_semicolon =
+    expected item, found `;`
+    .suggestion = remove this semicolon
+    .help = {$name} declarations are not followed by a semicolon
+
+parse_incorrect_use_of_await =
+    incorrect use of `await`
+    .parentheses_suggestion = `await` is not a method call, remove the parentheses
+    .postfix_suggestion = `await` is a postfix operation
+
+parse_in_in_typo =
+    expected iterable, found keyword `in`
+    .suggestion = remove the duplicated `in`
+
+parse_invalid_variable_declaration =
+    invalid variable declaration
+
+parse_switch_mut_let_order =
+    switch the order of `mut` and `let`
+parse_missing_let_before_mut = missing keyword
+parse_use_let_not_auto = write `let` instead of `auto` to introduce a new variable
+parse_use_let_not_var = write `let` instead of `var` to introduce a new variable
+
+parse_invalid_comparison_operator = invalid comparison operator `{$invalid}`
+    .use_instead = `{$invalid}` is not a valid comparison operator, use `{$correct}`
+    .spaceship_operator_invalid = `<=>` is not a valid comparison operator, use `std::cmp::Ordering`
+
+parse_invalid_logical_operator = `{$incorrect}` is not a logical operator
+    .note = unlike in e.g., Python and PHP, `&&` and `||` are used for logical operators
+    .use_amp_amp_for_conjunction = use `&&` to perform logical conjunction
+    .use_pipe_pipe_for_disjunction = use `||` to perform logical disjunction
+
+parse_tilde_is_not_unary_operator = `~` cannot be used as a unary operator
+    .suggestion = use `!` to perform bitwise not
+
+parse_unexpected_if_with_if = unexpected `if` in the condition expression
+    .suggestion = remove the `if`
+
+parse_unexpected_token_after_not = unexpected {$negated_desc} after identifier
+parse_unexpected_token_after_not_bitwise = use `!` to perform bitwise not
+parse_unexpected_token_after_not_logical = use `!` to perform logical negation
+parse_unexpected_token_after_not_default = use `!` to perform logical negation or bitwise not
+
+parse_malformed_loop_label = malformed loop label
+    .suggestion = use the correct loop label format
+
+parse_lifetime_in_borrow_expression = borrow expressions cannot be annotated with lifetimes
+    .suggestion = remove the lifetime annotation
+    .label = annotated with lifetime here
+
+parse_field_expression_with_generic = field expressions cannot have generic arguments
+
+parse_macro_invocation_with_qualified_path = macros cannot use qualified paths
+
+parse_unexpected_token_after_label = expected `while`, `for`, `loop` or `{"{"}` after a label
+    .suggestion_remove_label = consider removing the label
+    .suggestion_enclose_in_block = consider enclosing expression in a block
+
+parse_require_colon_after_labeled_expression = labeled expression must be followed by `:`
+    .note = labels are used before loops and blocks, allowing e.g., `break 'label` to them
+    .label = the label
+    .suggestion = add `:` after the label
+
+parse_do_catch_syntax_removed = found removed `do catch` syntax
+    .note = following RFC #2388, the new non-placeholder syntax is `try`
+    .suggestion = replace with the new syntax
+
+parse_float_literal_requires_integer_part = float literals must have an integer part
+    .suggestion = must have an integer part
+
+parse_invalid_int_literal_width = invalid width `{$width}` for integer literal
+    .help = valid widths are 8, 16, 32, 64 and 128
+
+parse_invalid_num_literal_base_prefix = invalid base prefix for number literal
+    .note = base prefixes (`0xff`, `0b1010`, `0o755`) are lowercase
+    .suggestion = try making the prefix lowercase
+
+parse_invalid_num_literal_suffix = invalid suffix `{$suffix}` for number literal
+    .label = invalid suffix `{$suffix}`
+    .help = the suffix must be one of the numeric types (`u32`, `isize`, `f32`, etc.)
+
+parse_invalid_float_literal_width = invalid width `{$width}` for float literal
+    .help = valid widths are 32 and 64
+
+parse_invalid_float_literal_suffix = invalid suffix `{$suffix}` for float literal
+    .label = invalid suffix `{$suffix}`
+    .help = valid suffixes are `f32` and `f64`
+
+parse_int_literal_too_large = integer literal is too large
+
+parse_missing_semicolon_before_array = expected `;`, found `[`
+    .suggestion = consider adding `;` here
+
+parse_invalid_block_macro_segment = cannot use a `block` macro fragment here
+    .label = the `block` fragment is within this context
+
+parse_expect_dotdot_not_dotdotdot = expected `..`, found `...`
+    .suggestion = use `..` to fill in the rest of the fields
+
+parse_if_expression_missing_then_block = this `if` expression is missing a block after the condition
+    .add_then_block = add a block here
+    .condition_possibly_unfinished = this binary operation is possibly unfinished
+
+parse_if_expression_missing_condition = missing condition for `if` expression
+    .condition_label = expected condition here
+    .block_label = if this block is the condition of the `if` expression, then it must be followed by another block
+
+parse_expected_expression_found_let = expected expression, found `let` statement
+
+parse_expect_eq_instead_of_eqeq = expected `=`, found `==`
+    .suggestion = consider using `=` here
+
+parse_expected_else_block = expected `{"{"}`, found {$first_tok}
+    .label = expected an `if` or a block after this `else`
+    .suggestion = add an `if` if this is the condition of a chained `else if` statement
+
+parse_outer_attribute_not_allowed_on_if_else = outer attributes are not allowed on `if` and `else` branches
+    .branch_label = the attributes are attached to this branch
+    .ctx_label = the branch belongs to this `{$ctx}`
+    .suggestion = remove the attributes
+
+parse_missing_in_in_for_loop = missing `in` in `for` loop
+    .use_in_not_of = try using `in` here instead
+    .add_in = try adding `in` here
+
+parse_missing_expression_in_for_loop = missing expression to iterate on in `for` loop
+    .suggestion = try adding an expression to the `for` loop
+
+parse_loop_else = `{$loop_kind}...else` loops are not supported
+    .note = consider moving this `else` clause to a separate `if` statement and use a `bool` variable to control if it should run
+    .loop_keyword = `else` is attached to this loop
+
+parse_missing_comma_after_match_arm = expected `,` following `match` arm
+    .suggestion = missing a comma here to end this `match` arm
+
+parse_catch_after_try = keyword `catch` cannot follow a `try` block
+    .help = try using `match` on the result of the `try` block instead
+
+parse_comma_after_base_struct = cannot use a comma after the base struct
+    .note = the base struct must always be the last field
+    .suggestion = remove this comma
+
+parse_eq_field_init = expected `:`, found `=`
+    .suggestion = replace equals symbol with a colon
+
+parse_dotdotdot = unexpected token: `...`
+    .suggest_exclusive_range = use `..` for an exclusive range
+    .suggest_inclusive_range = or `..=` for an inclusive range
+
+parse_left_arrow_operator = unexpected token: `<-`
+    .suggestion = if you meant to write a comparison against a negative value, add a space in between `<` and `-`
+
+parse_remove_let = expected pattern, found `let`
+    .suggestion = remove the unnecessary `let` keyword
+
+parse_use_eq_instead = unexpected `==`
+    .suggestion = try using `=` instead
+
+parse_use_empty_block_not_semi = expected { "`{}`" }, found `;`
+    .suggestion = try using { "`{}`" } instead
+
+parse_comparison_interpreted_as_generic =
+    `<` is interpreted as a start of generic arguments for `{$type}`, not a comparison
+    .label_args = interpreted as generic arguments
+    .label_comparison = not interpreted as comparison
+    .suggestion = try comparing the cast value
+
+parse_shift_interpreted_as_generic =
+    `<<` is interpreted as a start of generic arguments for `{$type}`, not a shift
+    .label_args = interpreted as generic arguments
+    .label_comparison = not interpreted as shift
+    .suggestion = try shifting the cast value
+
+parse_found_expr_would_be_stmt = expected expression, found `{$token}`
+    .label = expected expression
+
+parse_leading_plus_not_supported = leading `+` is not supported
+    .label = unexpected `+`
+    .suggestion_remove_plus = try removing the `+`
+
+parse_parentheses_with_struct_fields = invalid `struct` delimiters or `fn` call arguments
+    .suggestion_braces_for_struct = if `{$type}` is a struct, use braces as delimiters
+    .suggestion_no_fields_for_fn = if `{$type}` is a function, use the arguments directly
+
+parse_labeled_loop_in_break = parentheses are required around this expression to avoid confusion with a labeled break expression
+
+parse_sugg_wrap_expression_in_parentheses = wrap the expression in parentheses
+
+parse_array_brackets_instead_of_braces = this is a block expression, not an array
+    .suggestion = to make an array, use square brackets instead of curly braces
+
+parse_match_arm_body_without_braces = `match` arm body without braces
+    .label_statements = {$num_statements ->
+            [one] this statement is not surrounded by a body
+           *[other] these statements are not surrounded by a body
+        }
+    .label_arrow = while parsing the `match` arm starting here
+    .suggestion_add_braces = surround the {$num_statements ->
+            [one] statement
+           *[other] statements
+        } with a body
+    .suggestion_use_comma_not_semicolon = replace `;` with `,` to end a `match` arm expression
+
+parse_inclusive_range_extra_equals = unexpected `=` after inclusive range
+    .suggestion_remove_eq = use `..=` instead
+    .note = inclusive ranges end with a single equals sign (`..=`)
+
+parse_inclusive_range_match_arrow = unexpected `>` after inclusive range
+    .label = this is parsed as an inclusive range `..=`
+    .suggestion = add a space between the pattern and `=>`
+
+parse_inclusive_range_no_end = inclusive range with no end
+    .suggestion_open_range = use `..` instead
+    .note = inclusive ranges must be bounded at the end (`..=b` or `a..=b`)
+
+parse_struct_literal_not_allowed_here = struct literals are not allowed here
+    .suggestion = surround the struct literal with parentheses
+
+parse_invalid_interpolated_expression = invalid interpolated expression
+
+parse_hexadecimal_float_literal_not_supported = hexadecimal float literal is not supported
+parse_octal_float_literal_not_supported = octal float literal is not supported
+parse_binary_float_literal_not_supported = binary float literal is not supported
+parse_not_supported = not supported
+
+parse_invalid_literal_suffix = suffixes on {$kind} literals are invalid
+    .label = invalid suffix `{$suffix}`
+
+parse_invalid_literal_suffix_on_tuple_index = suffixes on a tuple index are invalid
+    .label = invalid suffix `{$suffix}`
+    .tuple_exception_line_1 = `{$suffix}` is *temporarily* accepted on tuple index fields as it was incorrectly accepted on stable for a few releases
+    .tuple_exception_line_2 = on proc macros, you'll want to use `syn::Index::from` or `proc_macro::Literal::*_unsuffixed` for code that will desugar to tuple field access
+    .tuple_exception_line_3 = see issue #60210 <https://github.com/rust-lang/rust/issues/60210> for more information
+
+parse_non_string_abi_literal = non-string ABI literal
+    .suggestion = specify the ABI with a string literal
+
+parse_mismatched_closing_delimiter = mismatched closing delimiter: `{$delimiter}`
+    .label_unmatched = mismatched closing delimiter
+    .label_opening_candidate = closing delimiter possibly meant for this
+    .label_unclosed = unclosed delimiter
+
+parse_incorrect_visibility_restriction = incorrect visibility restriction
+    .help = some possible visibility restrictions are:
+            `pub(crate)`: visible only on the current crate
+            `pub(super)`: visible only in the current module's parent
+            `pub(in path::to::module)`: visible only on the specified path
+    .suggestion = make this visible only to module `{$inner_str}` with `in`
+
+parse_assignment_else_not_allowed = <assignment> ... else {"{"} ... {"}"} is not allowed
+
+parse_expected_statement_after_outer_attr = expected statement after outer attribute
+
+parse_doc_comment_does_not_document_anything = found a documentation comment that doesn't document anything
+    .help = doc comments must come before what they document, if a comment was intended use `//`
+    .suggestion = missing comma here
+
+parse_const_let_mutually_exclusive = `const` and `let` are mutually exclusive
+    .suggestion = remove `let`
+
+parse_invalid_expression_in_let_else = a `{$operator}` expression cannot be directly assigned in `let...else`
+parse_invalid_curly_in_let_else = right curly brace `{"}"}` before `else` in a `let...else` statement not allowed
+parse_extra_if_in_let_else = remove the `if` if you meant to write a `let...else` statement
+
+parse_compound_assignment_expression_in_let = can't reassign to an uninitialized variable
+    .suggestion = initialize the variable
+    .help = if you meant to overwrite, remove the `let` binding
+
+parse_suffixed_literal_in_attribute = suffixed literals are not allowed in attributes
+    .help = instead of using a suffixed literal (`1u8`, `1.0f32`, etc.), use an unsuffixed version (`1`, `1.0`, etc.)
+
+parse_invalid_meta_item = expected unsuffixed literal or identifier, found `{$token}`
+
+parse_label_inner_attr_does_not_annotate_this = the inner attribute doesn't annotate this {$item}
+parse_sugg_change_inner_attr_to_outer = to annotate the {$item}, change the attribute from inner to outer style
+
+parse_inner_attr_not_permitted_after_outer_doc_comment = an inner attribute is not permitted following an outer doc comment
+    .label_attr = not permitted following an outer doc comment
+    .label_prev_doc_comment = previous doc comment
+    .label_does_not_annotate_this = {parse_label_inner_attr_does_not_annotate_this}
+    .sugg_change_inner_to_outer = {parse_sugg_change_inner_attr_to_outer}
+
+parse_inner_attr_not_permitted_after_outer_attr = an inner attribute is not permitted following an outer attribute
+    .label_attr = not permitted following an outer attribute
+    .label_prev_attr = previous outer attribute
+    .label_does_not_annotate_this = {parse_label_inner_attr_does_not_annotate_this}
+    .sugg_change_inner_to_outer = {parse_sugg_change_inner_attr_to_outer}
+
+parse_inner_attr_not_permitted = an inner attribute is not permitted in this context
+    .label_does_not_annotate_this = {parse_label_inner_attr_does_not_annotate_this}
+    .sugg_change_inner_to_outer = {parse_sugg_change_inner_attr_to_outer}
+
+parse_inner_attr_explanation = inner attributes, like `#![no_std]`, annotate the item enclosing them, and are usually found at the beginning of source files
+parse_outer_attr_explanation = outer attributes, like `#[test]`, annotate the item following them
+
+parse_inner_doc_comment_not_permitted = expected outer doc comment
+    .note = inner doc comments like this (starting with `//!` or `/*!`) can only appear before items
+    .suggestion = you might have meant to write a regular comment
+    .label_does_not_annotate_this = the inner doc comment doesn't annotate this {$item}
+    .sugg_change_inner_to_outer = to annotate the {$item}, change the doc comment from inner to outer style
+
+parse_expected_identifier_found_reserved_identifier_str = expected identifier, found reserved identifier `{$token}`
+parse_expected_identifier_found_keyword_str = expected identifier, found keyword `{$token}`
+parse_expected_identifier_found_reserved_keyword_str = expected identifier, found reserved keyword `{$token}`
+parse_expected_identifier_found_doc_comment_str = expected identifier, found doc comment `{$token}`
+parse_expected_identifier_found_str = expected identifier, found `{$token}`
+
+parse_expected_identifier_found_reserved_identifier = expected identifier, found reserved identifier
+parse_expected_identifier_found_keyword = expected identifier, found keyword
+parse_expected_identifier_found_reserved_keyword = expected identifier, found reserved keyword
+parse_expected_identifier_found_doc_comment = expected identifier, found doc comment
+parse_expected_identifier = expected identifier
+
+parse_sugg_escape_to_use_as_identifier = escape `{$ident_name}` to use it as an identifier
+
+parse_sugg_remove_comma = remove this comma
+
+parse_expected_semi_found_reserved_identifier_str = expected `;`, found reserved identifier `{$token}`
+parse_expected_semi_found_keyword_str = expected `;`, found keyword `{$token}`
+parse_expected_semi_found_reserved_keyword_str = expected `;`, found reserved keyword `{$token}`
+parse_expected_semi_found_doc_comment_str = expected `;`, found doc comment `{$token}`
+parse_expected_semi_found_str = expected `;`, found `{$token}`
+
+parse_sugg_change_this_to_semi = change this to `;`
+parse_sugg_add_semi = add `;` here
+parse_label_unexpected_token = unexpected token
+
+parse_unmatched_angle_brackets = {$num_extra_brackets ->
+        [one] unmatched angle bracket
+       *[other] unmatched angle brackets
+    }
+    .suggestion = {$num_extra_brackets ->
+            [one] remove extra angle bracket
+           *[other] remove extra angle brackets
+        }
+
+parse_generic_parameters_without_angle_brackets = generic parameters without surrounding angle brackets
+    .suggestion = surround the type parameters with angle brackets
+
+parse_comparison_operators_cannot_be_chained = comparison operators cannot be chained
+    .sugg_parentheses_for_function_args = or use `(...)` if you meant to specify fn arguments
+    .sugg_split_comparison = split the comparison into two
+    .sugg_parenthesize = parenthesize the comparison
+parse_sugg_turbofish_syntax = use `::<...>` instead of `<...>` to specify lifetime, type, or const arguments
+
+parse_question_mark_in_type = invalid `?` in type
+    .label = `?` is only allowed on expressions, not types
+    .suggestion = if you meant to express that the type might not contain a value, use the `Option` wrapper type
+
+parse_unexpected_parentheses_in_for_head = unexpected parentheses surrounding `for` loop head
+    .suggestion = remove parentheses in `for` loop
+
+parse_doc_comment_on_param_type = documentation comments cannot be applied to a function parameter's type
+    .label = doc comments are not allowed here
+
+parse_attribute_on_param_type = attributes cannot be applied to a function parameter's type
+    .label = attributes are not allowed here
+
+parse_pattern_method_param_without_body = patterns aren't allowed in methods without bodies
+    .suggestion = give this argument a name or use an underscore to ignore it
+
+parse_self_param_not_first = unexpected `self` parameter in function
+    .label = must be the first parameter of an associated function
+
+parse_const_generic_without_braces = expressions must be enclosed in braces to be used as const generic arguments
+    .suggestion = enclose the `const` expression in braces
+
+parse_unexpected_const_param_declaration = unexpected `const` parameter declaration
+    .label = expected a `const` expression, not a parameter declaration
+    .suggestion = `const` parameters must be declared for the `impl`
+
+parse_unexpected_const_in_generic_param = expected lifetime, type, or constant, found keyword `const`
+    .suggestion = the `const` keyword is only needed in the definition of the type
+
+parse_async_move_order_incorrect = the order of `move` and `async` is incorrect
+    .suggestion = try switching the order
+
+parse_double_colon_in_bound = expected `:` followed by trait or lifetime
+    .suggestion = use single colon
+
+parse_fn_ptr_with_generics = function pointer types may not have generic parameters
+    .suggestion = consider moving the lifetime {$arity ->
+        [one] parameter
+        *[other] parameters
+    } to {$for_param_list_exists ->
+        [true] the
+        *[false] a
+    } `for` parameter list
+
+parse_invalid_identifier_with_leading_number = expected identifier, found number literal
+    .label = identifiers cannot start with a number
+
+parse_maybe_fn_typo_with_impl = you might have meant to write `impl` instead of `fn`
+    .suggestion = replace `fn` with `impl` here
+
+parse_expected_fn_path_found_fn_keyword = expected identifier, found keyword `fn`
+    .suggestion = use `Fn` to refer to the trait
+
+parse_where_clause_before_tuple_struct_body = where clauses are not allowed before tuple struct bodies
+    .label = unexpected where clause
+    .name_label = while parsing this tuple struct
+    .body_label = the struct body
+    .suggestion = move the body before the where clause
+
+parse_async_fn_in_2015 = `async fn` is not permitted in Rust 2015
+    .label = to use `async fn`, switch to Rust 2018 or later
+
+parse_async_block_in_2015 = `async` blocks are only allowed in Rust 2018 or later
+
+parse_self_argument_pointer = cannot pass `self` by raw pointer
+    .label = cannot pass `self` by raw pointer
+
+parse_visibility_not_followed_by_item = visibility `{$vis}` is not followed by an item
+    .label = the visibility
+    .help = you likely meant to define an item, e.g., `{$vis} fn foo() {"{}"}`
+
+parse_default_not_followed_by_item = `default` is not followed by an item
+    .label = the `default` qualifier
+    .note = only `fn`, `const`, `type`, or `impl` items may be prefixed by `default`
+
+parse_missing_struct_for_struct_definition = missing `struct` for struct definition
+    .suggestion = add `struct` here to parse `{$ident}` as a public struct
+
+parse_missing_fn_for_function_definition = missing `fn` for function definition
+    .suggestion = add `fn` here to parse `{$ident}` as a public function
+
+parse_missing_fn_for_method_definition = missing `fn` for method definition
+    .suggestion = add `fn` here to parse `{$ident}` as a public method
+
+parse_ambiguous_missing_keyword_for_item_definition = missing `fn` or `struct` for function or struct definition
+    .suggestion = if you meant to call a macro, try
+    .help = if you meant to call a macro, remove the `pub` and add a trailing `!` after the identifier
+
+parse_missing_trait_in_trait_impl = missing trait in a trait impl
+    .suggestion_add_trait = add a trait here
+    .suggestion_remove_for = for an inherent impl, drop this `for`
+
+parse_missing_for_in_trait_impl = missing `for` in a trait impl
+    .suggestion = add `for` here
+
+parse_expected_trait_in_trait_impl_found_type = expected a trait, found type
+
+parse_non_item_in_item_list = non-item in item list
+    .suggestion_use_const_not_let = consider using `const` instead of `let` for associated const
+    .label_list_start = item list starts here
+    .label_non_item = non-item starts here
+    .label_list_end = item list ends here
+    .suggestion_remove_semicolon = consider removing this semicolon
+
+parse_bounds_not_allowed_on_trait_aliases = bounds are not allowed on trait aliases
+
+parse_trait_alias_cannot_be_auto = trait aliases cannot be `auto`
+parse_trait_alias_cannot_be_unsafe = trait aliases cannot be `unsafe`
+
+parse_associated_static_item_not_allowed = associated `static` items are not allowed
+
+parse_extern_crate_name_with_dashes = crate name using dashes are not valid in `extern crate` statements
+    .label = dash-separated idents are not valid
+    .suggestion = if the original crate name uses dashes you need to use underscores in the code
+
+parse_extern_item_cannot_be_const = extern items cannot be `const`
+    .suggestion = try using a static value
+    .note = for more information, visit https://doc.rust-lang.org/std/keyword.extern.html
+
+parse_const_global_cannot_be_mutable = const globals cannot be mutable
+    .label = cannot be mutable
+    .suggestion = you might want to declare a static instead
+
+parse_missing_const_type = missing type for `{$kind}` item
+    .suggestion = provide a type for the item
+
+parse_enum_struct_mutually_exclusive = `enum` and `struct` are mutually exclusive
+    .suggestion = replace `enum struct` with
+
+parse_unexpected_token_after_struct_name = expected `where`, `{"{"}`, `(`, or `;` after struct name
+parse_unexpected_token_after_struct_name_found_reserved_identifier = expected `where`, `{"{"}`, `(`, or `;` after struct name, found reserved identifier `{$token}`
+parse_unexpected_token_after_struct_name_found_keyword = expected `where`, `{"{"}`, `(`, or `;` after struct name, found keyword `{$token}`
+parse_unexpected_token_after_struct_name_found_reserved_keyword = expected `where`, `{"{"}`, `(`, or `;` after struct name, found reserved keyword `{$token}`
+parse_unexpected_token_after_struct_name_found_doc_comment = expected `where`, `{"{"}`, `(`, or `;` after struct name, found doc comment `{$token}`
+parse_unexpected_token_after_struct_name_found_other = expected `where`, `{"{"}`, `(`, or `;` after struct name, found `{$token}`
+
+parse_unexpected_self_in_generic_parameters = unexpected keyword `Self` in generic parameters
+    .note = you cannot use `Self` as a generic parameter because it is reserved for associated items
+
+parse_unexpected_default_value_for_lifetime_in_generic_parameters = unexpected default lifetime parameter
+    .label = lifetime parameters cannot have default values
+
+parse_multiple_where_clauses = cannot define duplicate `where` clauses on an item
+    .label = previous `where` clause starts here
+    .suggestion = consider joining the two `where` clauses into one
+
+parse_nonterminal_expected_item_keyword = expected an item keyword
+parse_nonterminal_expected_statement = expected a statement
+parse_nonterminal_expected_ident = expected ident, found `{$token}`
+parse_nonterminal_expected_lifetime = expected a lifetime, found `{$token}`
+
+parse_or_pattern_not_allowed_in_let_binding = top-level or-patterns are not allowed in `let` bindings
+parse_or_pattern_not_allowed_in_fn_parameters = top-level or-patterns are not allowed in function parameters
+parse_sugg_remove_leading_vert_in_pattern = remove the `|`
+parse_sugg_wrap_pattern_in_parens = wrap the pattern in parentheses
+
+parse_note_pattern_alternatives_use_single_vert = alternatives in or-patterns are separated with `|`, not `||`
+
+parse_unexpected_vert_vert_before_function_parameter = unexpected `||` before function parameter
+    .suggestion = remove the `||`
+
+parse_label_while_parsing_or_pattern_here = while parsing this or-pattern starting here
+
+parse_unexpected_vert_vert_in_pattern = unexpected token `||` in pattern
+    .suggestion = use a single `|` to separate multiple alternative patterns
+
+parse_trailing_vert_not_allowed = a trailing `|` is not allowed in an or-pattern
+    .suggestion = remove the `{$token}`
+
+parse_dotdotdot_rest_pattern = unexpected `...`
+    .label = not a valid pattern
+    .suggestion = for a rest pattern, use `..` instead of `...`
+
+parse_pattern_on_wrong_side_of_at = pattern on wrong side of `@`
+    .label_pattern = pattern on the left, should be on the right
+    .label_binding = binding on the right, should be on the left
+    .suggestion = switch the order
+
+parse_expected_binding_left_of_at = left-hand side of `@` must be a binding
+    .label_lhs = interpreted as a pattern, not a binding
+    .label_rhs = also a pattern
+    .note = bindings are `x`, `mut x`, `ref x`, and `ref mut x`
+
+parse_ambiguous_range_pattern = the range pattern here has ambiguous interpretation
+    .suggestion = add parentheses to clarify the precedence
+
+parse_unexpected_lifetime_in_pattern = unexpected lifetime `{$symbol}` in pattern
+    .suggestion = remove the lifetime
+
+parse_ref_mut_order_incorrect = the order of `mut` and `ref` is incorrect
+    .suggestion = try switching the order
+
+parse_mut_on_nested_ident_pattern = `mut` must be attached to each individual binding
+    .suggestion = add `mut` to each binding
+parse_mut_on_non_ident_pattern = `mut` must be followed by a named binding
+    .suggestion = remove the `mut` prefix
+parse_note_mut_pattern_usage = `mut` may be followed by `variable` and `variable @ pattern`
+
+parse_repeated_mut_in_pattern = `mut` on a binding may not be repeated
+    .suggestion = remove the additional `mut`s
+
+parse_dot_dot_dot_range_to_pattern_not_allowed = range-to patterns with `...` are not allowed
+    .suggestion = use `..=` instead
+
+parse_enum_pattern_instead_of_identifier = expected identifier, found enum pattern
+
+parse_dot_dot_dot_for_remaining_fields = expected field pattern, found `{$token_str}`
+    .suggestion = to omit remaining fields, use `..`
+
+parse_expected_comma_after_pattern_field = expected `,`
+
+parse_return_types_use_thin_arrow = return types are denoted using `->`
+    .suggestion = use `->` instead
+
+parse_need_plus_after_trait_object_lifetime = lifetime in trait object type must be followed by `+`
+
+parse_expected_mut_or_const_in_raw_pointer_type = expected `mut` or `const` keyword in raw pointer type
+    .suggestion = add `mut` or `const` here
+
+parse_lifetime_after_mut = lifetime must precede `mut`
+    .suggestion = place the lifetime before `mut`
+
+parse_dyn_after_mut = `mut` must precede `dyn`
+    .suggestion = place `mut` before `dyn`
+
+parse_fn_pointer_cannot_be_const = an `fn` pointer type cannot be `const`
+    .label = `const` because of this
+    .suggestion = remove the `const` qualifier
+
+parse_fn_pointer_cannot_be_async = an `fn` pointer type cannot be `async`
+    .label = `async` because of this
+    .suggestion = remove the `async` qualifier
+
+parse_nested_c_variadic_type = C-variadic type `...` may not be nested inside another type
+
+parse_invalid_dyn_keyword = invalid `dyn` keyword
+    .help = `dyn` is only needed at the start of a trait `+`-separated list
+    .suggestion = remove this keyword
+
+parse_negative_bounds_not_supported = negative bounds are not supported
+    .label = negative bounds are not supported
+    .suggestion = {$num_bounds ->
+            [one] remove the bound
+           *[other] remove the bounds
+        }
+
+parse_help_set_edition_cargo = set `edition = "{$edition}"` in `Cargo.toml`
+parse_help_set_edition_standalone = pass `--edition {$edition}` to `rustc`
+parse_note_edition_guide = for more on editions, read https://doc.rust-lang.org/edition-guide
+
+parse_unexpected_token_after_dot = unexpected token: `{$actual}`
+
+parse_cannot_be_raw_ident = `{$ident}` cannot be a raw identifier
+
+parse_cr_doc_comment = bare CR not allowed in {$block ->
+    [true] block doc-comment
+    *[false] doc-comment
+}
+
+parse_no_digits_literal = no valid digits found for number
+
+parse_invalid_digit_literal = invalid digit for a base {$base} literal
+
+parse_empty_exponent_float = expected at least one digit in exponent
+
+parse_float_literal_unsupported_base = {$base} float literal is not supported
+
+parse_more_than_one_char = character literal may only contain one codepoint
+    .followed_by = this `{$chr}` is followed by the combining {$len ->
+        [one] mark
+        *[other] marks
+        } `{$escaped_marks}`
+    .non_printing = there are non-printing characters, the full sequence is `{$escaped}`
+    .consider_normalized = consider using the normalized form `{$ch}` of this character
+    .remove_non = consider removing the non-printing characters
+    .use_double_quotes = if you meant to write a {$is_byte ->
+        [true] byte string
+        *[false] `str`
+        } literal, use double quotes
+
+parse_no_brace_unicode_escape = incorrect unicode escape sequence
+    .label = {parse_no_brace_unicode_escape}
+    .use_braces = format of unicode escape sequences uses braces
+    .format_of_unicode = format of unicode escape sequences is `\u{"{...}"}`
+
+parse_invalid_unicode_escape = invalid unicode character escape
+    .label = invalid escape
+    .help = unicode escape must {$surrogate ->
+    [true] not be a surrogate
+    *[false] be at most 10FFFF
+    }
+
+parse_escape_only_char = {$byte ->
+    [true] byte
+    *[false] character
+    } constant must be escaped: `{$escaped_msg}`
+    .escape = escape the character
+
+parse_bare_cr = {$double_quotes ->
+    [true] bare CR not allowed in string, use `\r` instead
+    *[false] character constant must be escaped: `\r`
+    }
+    .escape = escape the character
+
+parse_bare_cr_in_raw_string = bare CR not allowed in raw string
+
+parse_too_short_hex_escape = numeric character escape is too short
+
+parse_invalid_char_in_escape = {parse_invalid_char_in_escape_msg}: `{$ch}`
+    .label = {parse_invalid_char_in_escape_msg}
+
+parse_invalid_char_in_escape_msg = invalid character in {$is_hex ->
+    [true] numeric character
+    *[false] unicode
+    } escape
+
+parse_out_of_range_hex_escape = out of range hex escape
+    .label = must be a character in the range [\x00-\x7f]
+
+parse_leading_underscore_unicode_escape = {parse_leading_underscore_unicode_escape_label}: `_`
+parse_leading_underscore_unicode_escape_label = invalid start of unicode escape
+
+parse_overlong_unicode_escape = overlong unicode escape
+    .label = must have at most 6 hex digits
+
+parse_unclosed_unicode_escape = unterminated unicode escape
+    .label = missing a closing `{"}"}`
+    .terminate = terminate the unicode escape
+
+parse_unicode_escape_in_byte = unicode escape in byte string
+    .label = {parse_unicode_escape_in_byte}
+    .help = unicode escape sequences cannot be used as a byte or in a byte string
+
+parse_empty_unicode_escape = empty unicode escape
+    .label = this escape must have at least 1 hex digit
+
+parse_zero_chars = empty character literal
+    .label = {parse_zero_chars}
+
+parse_lone_slash = invalid trailing slash in literal
+    .label = {parse_lone_slash}
+
+parse_unskipped_whitespace = non-ASCII whitespace symbol '{$ch}' is not skipped
+    .label = {parse_unskipped_whitespace}
+
+parse_multiple_skipped_lines = multiple lines skipped by escaped newline
+    .label = skipping everything up to and including this point
+
+parse_unknown_prefix = prefix `{$prefix}` is unknown
+    .label = unknown prefix
+    .note =  prefixed identifiers and literals are reserved since Rust 2021
+    .suggestion_br = use `br` for a raw byte string
+    .suggestion_whitespace = consider inserting whitespace here
+
+parse_too_many_hashes = too many `#` symbols: raw strings may be delimited by up to 255 `#` symbols, but found {$num}
+
+parse_unknown_start_of_token = unknown start of token: {$escaped}
+    .sugg_quotes = Unicode characters '“' (Left Double Quotation Mark) and '”' (Right Double Quotation Mark) look like '{$ascii_str}' ({$ascii_name}), but are not
+    .sugg_other = Unicode character '{$ch}' ({$u_name}) looks like '{$ascii_str}' ({$ascii_name}), but it is not
+    .help_null = source files must contain UTF-8 encoded text, unexpected null bytes might occur when a different encoding is used
+    .note_repeats = character appears {$repeats ->
+        [one] once more
+        *[other] {$repeats} more times
+    }
diff --git a/compiler/rustc_parse/src/errors.rs b/compiler/rustc_parse/src/errors.rs
index 06b970ad9..1662db36d 100644
--- a/compiler/rustc_parse/src/errors.rs
+++ b/compiler/rustc_parse/src/errors.rs
@@ -1,11 +1,15 @@
+use std::borrow::Cow;
+
 use rustc_ast::token::Token;
-use rustc_ast::Path;
-use rustc_errors::{fluent, AddToDiagnostic, Applicability, EmissionGuarantee, IntoDiagnostic};
+use rustc_ast::{Path, Visibility};
+use rustc_errors::{AddToDiagnostic, Applicability, EmissionGuarantee, IntoDiagnostic};
 use rustc_macros::{Diagnostic, Subdiagnostic};
 use rustc_session::errors::ExprParenthesesNeeded;
+use rustc_span::edition::{Edition, LATEST_STABLE_EDITION};
 use rustc_span::symbol::Ident;
 use rustc_span::{Span, Symbol};
 
+use crate::fluent_generated as fluent;
 use crate::parser::TokenDescription;
 
 #[derive(Diagnostic)]
@@ -75,7 +79,7 @@ pub(crate) struct IncorrectSemicolon<'a> {
 #[diag(parse_incorrect_use_of_await)]
 pub(crate) struct IncorrectUseOfAwait {
     #[primary_span]
-    #[suggestion(parentheses_suggestion, code = "", applicability = "machine-applicable")]
+    #[suggestion(parse_parentheses_suggestion, code = "", applicability = "machine-applicable")]
     pub span: Span,
 }
 
@@ -84,7 +88,7 @@ pub(crate) struct IncorrectUseOfAwait {
 pub(crate) struct IncorrectAwait {
     #[primary_span]
     pub span: Span,
-    #[suggestion(postfix_suggestion, code = "{expr}.await{question_mark}")]
+    #[suggestion(parse_postfix_suggestion, code = "{expr}.await{question_mark}")]
     pub sugg_span: (Span, Applicability),
     pub expr: String,
     pub question_mark: &'static str,
@@ -137,7 +141,7 @@ pub(crate) struct InvalidComparisonOperator {
 #[derive(Subdiagnostic)]
 pub(crate) enum InvalidComparisonOperatorSub {
     #[suggestion(
-        use_instead,
+        parse_use_instead,
         style = "short",
         applicability = "machine-applicable",
         code = "{correct}"
@@ -148,7 +152,7 @@ pub(crate) enum InvalidComparisonOperatorSub {
         invalid: String,
         correct: String,
     },
-    #[label(spaceship_operator_invalid)]
+    #[label(parse_spaceship_operator_invalid)]
     Spaceship(#[primary_span] Span),
 }
 
@@ -166,14 +170,14 @@ pub(crate) struct InvalidLogicalOperator {
 #[derive(Subdiagnostic)]
 pub(crate) enum InvalidLogicalOperatorSub {
     #[suggestion(
-        use_amp_amp_for_conjunction,
+        parse_use_amp_amp_for_conjunction,
         style = "short",
         applicability = "machine-applicable",
         code = "&&"
     )]
     Conjunction(#[primary_span] Span),
     #[suggestion(
-        use_pipe_pipe_for_disjunction,
+        parse_use_pipe_pipe_for_disjunction,
         style = "short",
         applicability = "machine-applicable",
         code = "||"
@@ -259,14 +263,14 @@ pub(crate) struct UnexpectedTokenAfterLabel {
     #[primary_span]
     #[label(parse_unexpected_token_after_label)]
     pub span: Span,
-    #[suggestion(suggestion_remove_label, style = "verbose", code = "")]
+    #[suggestion(parse_suggestion_remove_label, style = "verbose", code = "")]
     pub remove_label: Option<Span>,
     #[subdiagnostic]
     pub enclose_in_block: Option<UnexpectedTokenAfterLabelSugg>,
 }
 
 #[derive(Subdiagnostic)]
-#[multipart_suggestion(suggestion_enclose_in_block, applicability = "machine-applicable")]
+#[multipart_suggestion(parse_suggestion_enclose_in_block, applicability = "machine-applicable")]
 pub(crate) struct UnexpectedTokenAfterLabelSugg {
     #[suggestion_part(code = "{{ ")]
     pub left: Span,
@@ -337,24 +341,33 @@ pub(crate) struct IfExpressionMissingThenBlock {
     #[primary_span]
     pub if_span: Span,
     #[subdiagnostic]
-    pub sub: IfExpressionMissingThenBlockSub,
+    pub missing_then_block_sub: IfExpressionMissingThenBlockSub,
+    #[subdiagnostic]
+    pub let_else_sub: Option<IfExpressionLetSomeSub>,
 }
 
 #[derive(Subdiagnostic)]
 pub(crate) enum IfExpressionMissingThenBlockSub {
-    #[help(condition_possibly_unfinished)]
+    #[help(parse_condition_possibly_unfinished)]
     UnfinishedCondition(#[primary_span] Span),
-    #[help(add_then_block)]
+    #[help(parse_add_then_block)]
     AddThenBlock(#[primary_span] Span),
 }
 
+#[derive(Subdiagnostic)]
+#[suggestion(parse_extra_if_in_let_else, applicability = "maybe-incorrect", code = "")]
+pub(crate) struct IfExpressionLetSomeSub {
+    #[primary_span]
+    pub if_span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(parse_if_expression_missing_condition)]
 pub(crate) struct IfExpressionMissingCondition {
     #[primary_span]
-    #[label(condition_label)]
+    #[label(parse_condition_label)]
     pub if_span: Span,
-    #[label(block_label)]
+    #[label(parse_block_label)]
     pub block_span: Span,
 }
 
@@ -392,10 +405,10 @@ pub(crate) struct OuterAttributeNotAllowedOnIfElse {
     #[primary_span]
     pub last: Span,
 
-    #[label(branch_label)]
+    #[label(parse_branch_label)]
     pub branch_span: Span,
 
-    #[label(ctx_label)]
+    #[label(parse_ctx_label)]
     pub ctx_span: Span,
     pub ctx: String,
 
@@ -415,13 +428,41 @@ pub(crate) struct MissingInInForLoop {
 #[derive(Subdiagnostic)]
 pub(crate) enum MissingInInForLoopSub {
     // Has been misleading, at least in the past (closed Issue #48492), thus maybe-incorrect
-    #[suggestion(use_in_not_of, style = "short", applicability = "maybe-incorrect", code = "in")]
+    #[suggestion(
+        parse_use_in_not_of,
+        style = "short",
+        applicability = "maybe-incorrect",
+        code = "in"
+    )]
     InNotOf(#[primary_span] Span),
-    #[suggestion(add_in, style = "short", applicability = "maybe-incorrect", code = " in ")]
+    #[suggestion(parse_add_in, style = "short", applicability = "maybe-incorrect", code = " in ")]
     AddIn(#[primary_span] Span),
 }
 
 #[derive(Diagnostic)]
+#[diag(parse_missing_expression_in_for_loop)]
+pub(crate) struct MissingExpressionInForLoop {
+    #[primary_span]
+    #[suggestion(
+        code = "/* expression */ ",
+        applicability = "has-placeholders",
+        style = "verbose"
+    )]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_loop_else)]
+#[note]
+pub(crate) struct LoopElseNotSupported {
+    #[primary_span]
+    pub span: Span,
+    pub loop_kind: &'static str,
+    #[label(parse_loop_keyword)]
+    pub loop_kw: Span,
+}
+
+#[derive(Diagnostic)]
 #[diag(parse_missing_comma_after_match_arm)]
 pub(crate) struct MissingCommaAfterMatchArm {
     #[primary_span]
@@ -460,8 +501,8 @@ pub(crate) struct EqFieldInit {
 #[diag(parse_dotdotdot)]
 pub(crate) struct DotDotDot {
     #[primary_span]
-    #[suggestion(suggest_exclusive_range, applicability = "maybe-incorrect", code = "..")]
-    #[suggestion(suggest_inclusive_range, applicability = "maybe-incorrect", code = "..=")]
+    #[suggestion(parse_suggest_exclusive_range, applicability = "maybe-incorrect", code = "..")]
+    #[suggestion(parse_suggest_inclusive_range, applicability = "maybe-incorrect", code = "..=")]
     pub span: Span,
 }
 
@@ -501,10 +542,10 @@ pub(crate) struct UseEmptyBlockNotSemi {
 #[diag(parse_comparison_interpreted_as_generic)]
 pub(crate) struct ComparisonInterpretedAsGeneric {
     #[primary_span]
-    #[label(label_comparison)]
+    #[label(parse_label_comparison)]
     pub comparison: Span,
     pub r#type: Path,
-    #[label(label_args)]
+    #[label(parse_label_args)]
     pub args: Span,
     #[subdiagnostic]
     pub suggestion: ComparisonOrShiftInterpretedAsGenericSugg,
@@ -514,17 +555,17 @@ pub(crate) struct ComparisonInterpretedAsGeneric {
 #[diag(parse_shift_interpreted_as_generic)]
 pub(crate) struct ShiftInterpretedAsGeneric {
     #[primary_span]
-    #[label(label_comparison)]
+    #[label(parse_label_comparison)]
     pub shift: Span,
     pub r#type: Path,
-    #[label(label_args)]
+    #[label(parse_label_args)]
     pub args: Span,
     #[subdiagnostic]
     pub suggestion: ComparisonOrShiftInterpretedAsGenericSugg,
 }
 
 #[derive(Subdiagnostic)]
-#[multipart_suggestion(suggestion, applicability = "machine-applicable")]
+#[multipart_suggestion(parse_suggestion, applicability = "machine-applicable")]
 pub(crate) struct ComparisonOrShiftInterpretedAsGenericSugg {
     #[suggestion_part(code = "(")]
     pub left: Span,
@@ -550,7 +591,7 @@ pub(crate) struct LeadingPlusNotSupported {
     #[label]
     pub span: Span,
     #[suggestion(
-        suggestion_remove_plus,
+        parse_suggestion_remove_plus,
         style = "verbose",
         code = "",
         applicability = "machine-applicable"
@@ -573,7 +614,7 @@ pub(crate) struct ParenthesesWithStructFields {
 }
 
 #[derive(Subdiagnostic)]
-#[multipart_suggestion(suggestion_braces_for_struct, applicability = "maybe-incorrect")]
+#[multipart_suggestion(parse_suggestion_braces_for_struct, applicability = "maybe-incorrect")]
 pub(crate) struct BracesForStructLiteral {
     #[suggestion_part(code = " {{ ")]
     pub first: Span,
@@ -582,7 +623,7 @@ pub(crate) struct BracesForStructLiteral {
 }
 
 #[derive(Subdiagnostic)]
-#[multipart_suggestion(suggestion_no_fields_for_fn, applicability = "maybe-incorrect")]
+#[multipart_suggestion(parse_suggestion_no_fields_for_fn, applicability = "maybe-incorrect")]
 pub(crate) struct NoFieldsForFnCall {
     #[suggestion_part(code = "")]
     pub fields: Vec<Span>,
@@ -619,7 +660,7 @@ pub(crate) struct ArrayBracketsInsteadOfSpaces {
 }
 
 #[derive(Subdiagnostic)]
-#[multipart_suggestion(suggestion, applicability = "maybe-incorrect")]
+#[multipart_suggestion(parse_suggestion, applicability = "maybe-incorrect")]
 pub(crate) struct ArrayBracketsInsteadOfSpacesSugg {
     #[suggestion_part(code = "[")]
     pub left: Span,
@@ -631,18 +672,57 @@ pub(crate) struct ArrayBracketsInsteadOfSpacesSugg {
 #[diag(parse_match_arm_body_without_braces)]
 pub(crate) struct MatchArmBodyWithoutBraces {
     #[primary_span]
-    #[label(label_statements)]
+    #[label(parse_label_statements)]
     pub statements: Span,
-    #[label(label_arrow)]
+    #[label(parse_label_arrow)]
     pub arrow: Span,
     pub num_statements: usize,
     #[subdiagnostic]
     pub sub: MatchArmBodyWithoutBracesSugg,
 }
 
+#[derive(Diagnostic)]
+#[diag(parse_inclusive_range_extra_equals)]
+#[note]
+pub(crate) struct InclusiveRangeExtraEquals {
+    #[primary_span]
+    #[suggestion(
+        parse_suggestion_remove_eq,
+        style = "short",
+        code = "..=",
+        applicability = "maybe-incorrect"
+    )]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_inclusive_range_match_arrow)]
+pub(crate) struct InclusiveRangeMatchArrow {
+    #[primary_span]
+    pub arrow: Span,
+    #[label]
+    pub span: Span,
+    #[suggestion(style = "verbose", code = " ", applicability = "machine-applicable")]
+    pub after_pat: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_inclusive_range_no_end, code = "E0586")]
+#[note]
+pub(crate) struct InclusiveRangeNoEnd {
+    #[primary_span]
+    #[suggestion(
+        parse_suggestion_open_range,
+        code = "..",
+        applicability = "machine-applicable",
+        style = "short"
+    )]
+    pub span: Span,
+}
+
 #[derive(Subdiagnostic)]
 pub(crate) enum MatchArmBodyWithoutBracesSugg {
-    #[multipart_suggestion(suggestion_add_braces, applicability = "machine-applicable")]
+    #[multipart_suggestion(parse_suggestion_add_braces, applicability = "machine-applicable")]
     AddBraces {
         #[suggestion_part(code = "{{ ")]
         left: Span,
@@ -650,7 +730,7 @@ pub(crate) enum MatchArmBodyWithoutBracesSugg {
         right: Span,
     },
     #[suggestion(
-        suggestion_use_comma_not_semicolon,
+        parse_suggestion_use_comma_not_semicolon,
         code = ",",
         applicability = "machine-applicable"
     )]
@@ -670,7 +750,7 @@ pub(crate) struct StructLiteralNotAllowedHere {
 }
 
 #[derive(Subdiagnostic)]
-#[multipart_suggestion(suggestion, applicability = "machine-applicable")]
+#[multipart_suggestion(parse_suggestion, applicability = "machine-applicable")]
 pub(crate) struct StructLiteralNotAllowedHereSugg {
     #[suggestion_part(code = "(")]
     pub left: Span,
@@ -692,9 +772,9 @@ pub(crate) struct InvalidLiteralSuffixOnTupleIndex {
     #[label]
     pub span: Span,
     pub suffix: Symbol,
-    #[help(tuple_exception_line_1)]
-    #[help(tuple_exception_line_2)]
-    #[help(tuple_exception_line_3)]
+    #[help(parse_tuple_exception_line_1)]
+    #[help(parse_tuple_exception_line_2)]
+    #[help(parse_tuple_exception_line_3)]
     pub exception: Option<()>,
 }
 
@@ -712,11 +792,11 @@ pub(crate) struct MismatchedClosingDelimiter {
     #[primary_span]
     pub spans: Vec<Span>,
     pub delimiter: String,
-    #[label(label_unmatched)]
+    #[label(parse_label_unmatched)]
     pub unmatched: Span,
-    #[label(label_opening_candidate)]
+    #[label(parse_label_opening_candidate)]
     pub opening_candidate: Option<Span>,
-    #[label(label_unclosed)]
+    #[label(parse_label_unclosed)]
     pub unclosed: Option<Span>,
 }
 
@@ -867,7 +947,7 @@ impl<'a, G: EmissionGuarantee> IntoDiagnostic<'a, G> for ExpectedIdentifier {
         self,
         handler: &'a rustc_errors::Handler,
     ) -> rustc_errors::DiagnosticBuilder<'a, G> {
-        let token_descr = super::parser::TokenDescription::from_token(&self.token);
+        let token_descr = TokenDescription::from_token(&self.token);
 
         let mut diag = handler.struct_diagnostic(match token_descr {
             Some(TokenDescription::ReservedIdentifier) => {
@@ -913,7 +993,7 @@ impl<'a, G: EmissionGuarantee> IntoDiagnostic<'a, G> for ExpectedSemi {
         self,
         handler: &'a rustc_errors::Handler,
     ) -> rustc_errors::DiagnosticBuilder<'a, G> {
-        let token_descr = super::parser::TokenDescription::from_token(&self.token);
+        let token_descr = TokenDescription::from_token(&self.token);
 
         let mut diag = handler.struct_diagnostic(match token_descr {
             Some(TokenDescription::ReservedIdentifier) => {
@@ -962,7 +1042,7 @@ pub(crate) struct StructLiteralBodyWithoutPath {
 }
 
 #[derive(Subdiagnostic)]
-#[multipart_suggestion(suggestion, applicability = "has-placeholders")]
+#[multipart_suggestion(parse_suggestion, applicability = "has-placeholders")]
 pub(crate) struct StructLiteralBodyWithoutPathSugg {
     #[suggestion_part(code = "{{ SomeStruct ")]
     pub before: Span,
@@ -980,7 +1060,7 @@ pub(crate) struct StructLiteralNeedingParens {
 }
 
 #[derive(Subdiagnostic)]
-#[multipart_suggestion(suggestion, applicability = "machine-applicable")]
+#[multipart_suggestion(parse_suggestion, applicability = "machine-applicable")]
 pub(crate) struct StructLiteralNeedingParensSugg {
     #[suggestion_part(code = "(")]
     pub before: Span,
@@ -1007,7 +1087,7 @@ pub(crate) struct GenericParamsWithoutAngleBrackets {
 }
 
 #[derive(Subdiagnostic)]
-#[multipart_suggestion(suggestion, applicability = "machine-applicable")]
+#[multipart_suggestion(parse_suggestion, applicability = "machine-applicable")]
 pub(crate) struct GenericParamsWithoutAngleBracketsSugg {
     #[suggestion_part(code = "<")]
     pub left: Span,
@@ -1028,7 +1108,7 @@ pub(crate) struct ComparisonOperatorsCannotBeChained {
     )]
     pub suggest_turbofish: Option<Span>,
     #[help(parse_sugg_turbofish_syntax)]
-    #[help(sugg_parentheses_for_function_args)]
+    #[help(parse_sugg_parentheses_for_function_args)]
     pub help_turbofish: Option<()>,
     #[subdiagnostic]
     pub chaining_sugg: Option<ComparisonOperatorsCannotBeChainedSugg>,
@@ -1037,7 +1117,7 @@ pub(crate) struct ComparisonOperatorsCannotBeChained {
 #[derive(Subdiagnostic)]
 pub(crate) enum ComparisonOperatorsCannotBeChainedSugg {
     #[suggestion(
-        sugg_split_comparison,
+        parse_sugg_split_comparison,
         style = "verbose",
         code = " && {middle_term}",
         applicability = "maybe-incorrect"
@@ -1047,7 +1127,7 @@ pub(crate) enum ComparisonOperatorsCannotBeChainedSugg {
         span: Span,
         middle_term: String,
     },
-    #[multipart_suggestion(sugg_parenthesize, applicability = "maybe-incorrect")]
+    #[multipart_suggestion(parse_sugg_parenthesize, applicability = "maybe-incorrect")]
     Parenthesize {
         #[suggestion_part(code = "(")]
         left: Span,
@@ -1067,7 +1147,7 @@ pub(crate) struct QuestionMarkInType {
 }
 
 #[derive(Subdiagnostic)]
-#[multipart_suggestion(suggestion, applicability = "machine-applicable")]
+#[multipart_suggestion(parse_suggestion, applicability = "machine-applicable")]
 pub(crate) struct QuestionMarkInTypeSugg {
     #[suggestion_part(code = "Option<")]
     pub left: Span,
@@ -1085,7 +1165,7 @@ pub(crate) struct ParenthesesInForHead {
 }
 
 #[derive(Subdiagnostic)]
-#[multipart_suggestion(suggestion, applicability = "machine-applicable")]
+#[multipart_suggestion(parse_suggestion, applicability = "machine-applicable")]
 pub(crate) struct ParenthesesInForHeadSugg {
     #[suggestion_part(code = "{left_snippet}")]
     pub left: Span,
@@ -1145,7 +1225,7 @@ pub(crate) struct ConstGenericWithoutBraces {
 }
 
 #[derive(Subdiagnostic)]
-#[multipart_suggestion(suggestion, applicability = "machine-applicable")]
+#[multipart_suggestion(parse_suggestion, applicability = "machine-applicable")]
 pub(crate) struct ConstGenericWithoutBracesSugg {
     #[suggestion_part(code = "{{ ")]
     pub left: Span,
@@ -1165,7 +1245,7 @@ pub(crate) struct UnexpectedConstParamDeclaration {
 
 #[derive(Subdiagnostic)]
 pub(crate) enum UnexpectedConstParamDeclarationSugg {
-    #[multipart_suggestion(suggestion, applicability = "machine-applicable")]
+    #[multipart_suggestion(parse_suggestion, applicability = "machine-applicable")]
     AddParam {
         #[suggestion_part(code = "<{snippet}>")]
         impl_generics: Span,
@@ -1174,7 +1254,7 @@ pub(crate) enum UnexpectedConstParamDeclarationSugg {
         snippet: String,
         ident: String,
     },
-    #[multipart_suggestion(suggestion, applicability = "machine-applicable")]
+    #[multipart_suggestion(parse_suggestion, applicability = "machine-applicable")]
     AppendParam {
         #[suggestion_part(code = ", {snippet}")]
         impl_generics_end: Span,
@@ -1221,7 +1301,7 @@ pub(crate) struct FnPtrWithGenerics {
 }
 
 #[derive(Subdiagnostic)]
-#[multipart_suggestion(suggestion, applicability = "maybe-incorrect")]
+#[multipart_suggestion(parse_suggestion, applicability = "maybe-incorrect")]
 pub(crate) struct FnPtrWithGenericsSugg {
     #[suggestion_part(code = "{snippet}")]
     pub left: Span,
@@ -1262,16 +1342,16 @@ pub(crate) struct WhereClauseBeforeTupleStructBody {
     #[primary_span]
     #[label]
     pub span: Span,
-    #[label(name_label)]
+    #[label(parse_name_label)]
     pub name: Span,
-    #[label(body_label)]
+    #[label(parse_body_label)]
     pub body: Span,
     #[subdiagnostic]
     pub sugg: Option<WhereClauseBeforeTupleStructBodySugg>,
 }
 
 #[derive(Subdiagnostic)]
-#[multipart_suggestion(suggestion, applicability = "machine-applicable")]
+#[multipart_suggestion(parse_suggestion, applicability = "machine-applicable")]
 pub(crate) struct WhereClauseBeforeTupleStructBodySugg {
     #[suggestion_part(code = "{snippet}")]
     pub left: Span,
@@ -1279,3 +1359,943 @@ pub(crate) struct WhereClauseBeforeTupleStructBodySugg {
     #[suggestion_part(code = "")]
     pub right: Span,
 }
+
+#[derive(Diagnostic)]
+#[diag(parse_async_fn_in_2015, code = "E0670")]
+pub(crate) struct AsyncFnIn2015 {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+    #[subdiagnostic]
+    pub help: HelpUseLatestEdition,
+}
+
+#[derive(Subdiagnostic)]
+#[label(parse_async_block_in_2015)]
+pub(crate) struct AsyncBlockIn2015 {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_self_argument_pointer)]
+pub(crate) struct SelfArgumentPointer {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_unexpected_token_after_dot)]
+pub struct UnexpectedTokenAfterDot<'a> {
+    #[primary_span]
+    pub span: Span,
+    pub actual: Cow<'a, str>,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_visibility_not_followed_by_item)]
+#[help]
+pub(crate) struct VisibilityNotFollowedByItem {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+    pub vis: Visibility,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_default_not_followed_by_item)]
+#[note]
+pub(crate) struct DefaultNotFollowedByItem {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
+pub(crate) enum MissingKeywordForItemDefinition {
+    #[diag(parse_missing_struct_for_struct_definition)]
+    Struct {
+        #[primary_span]
+        #[suggestion(style = "short", applicability = "maybe-incorrect", code = " struct ")]
+        span: Span,
+        ident: Ident,
+    },
+    #[diag(parse_missing_fn_for_function_definition)]
+    Function {
+        #[primary_span]
+        #[suggestion(style = "short", applicability = "maybe-incorrect", code = " fn ")]
+        span: Span,
+        ident: Ident,
+    },
+    #[diag(parse_missing_fn_for_method_definition)]
+    Method {
+        #[primary_span]
+        #[suggestion(style = "short", applicability = "maybe-incorrect", code = " fn ")]
+        span: Span,
+        ident: Ident,
+    },
+    #[diag(parse_ambiguous_missing_keyword_for_item_definition)]
+    Ambiguous {
+        #[primary_span]
+        span: Span,
+        #[subdiagnostic]
+        subdiag: Option<AmbiguousMissingKwForItemSub>,
+    },
+}
+
+#[derive(Subdiagnostic)]
+pub(crate) enum AmbiguousMissingKwForItemSub {
+    #[suggestion(parse_suggestion, applicability = "maybe-incorrect", code = "{snippet}!")]
+    SuggestMacro {
+        #[primary_span]
+        span: Span,
+        snippet: String,
+    },
+    #[help(parse_help)]
+    HelpMacro,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_missing_trait_in_trait_impl)]
+pub(crate) struct MissingTraitInTraitImpl {
+    #[primary_span]
+    #[suggestion(parse_suggestion_add_trait, code = " Trait ", applicability = "has-placeholders")]
+    pub span: Span,
+    #[suggestion(parse_suggestion_remove_for, code = "", applicability = "maybe-incorrect")]
+    pub for_span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_missing_for_in_trait_impl)]
+pub(crate) struct MissingForInTraitImpl {
+    #[primary_span]
+    #[suggestion(style = "short", code = " for ", applicability = "machine-applicable")]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_expected_trait_in_trait_impl_found_type)]
+pub(crate) struct ExpectedTraitInTraitImplFoundType {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_bounds_not_allowed_on_trait_aliases)]
+pub(crate) struct BoundsNotAllowedOnTraitAliases {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_trait_alias_cannot_be_auto)]
+pub(crate) struct TraitAliasCannotBeAuto {
+    #[primary_span]
+    #[label(parse_trait_alias_cannot_be_auto)]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_trait_alias_cannot_be_unsafe)]
+pub(crate) struct TraitAliasCannotBeUnsafe {
+    #[primary_span]
+    #[label(parse_trait_alias_cannot_be_unsafe)]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_associated_static_item_not_allowed)]
+pub(crate) struct AssociatedStaticItemNotAllowed {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_extern_crate_name_with_dashes)]
+pub(crate) struct ExternCrateNameWithDashes {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+    #[subdiagnostic]
+    pub sugg: ExternCrateNameWithDashesSugg,
+}
+
+#[derive(Subdiagnostic)]
+#[multipart_suggestion(parse_suggestion, applicability = "machine-applicable")]
+pub(crate) struct ExternCrateNameWithDashesSugg {
+    #[suggestion_part(code = "_")]
+    pub dashes: Vec<Span>,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_extern_item_cannot_be_const)]
+#[note]
+pub(crate) struct ExternItemCannotBeConst {
+    #[primary_span]
+    pub ident_span: Span,
+    #[suggestion(code = "static ", applicability = "machine-applicable")]
+    pub const_span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_const_global_cannot_be_mutable)]
+pub(crate) struct ConstGlobalCannotBeMutable {
+    #[primary_span]
+    #[label]
+    pub ident_span: Span,
+    #[suggestion(code = "static", applicability = "maybe-incorrect")]
+    pub const_span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_missing_const_type)]
+pub(crate) struct MissingConstType {
+    #[primary_span]
+    #[suggestion(code = "{colon} <type>", applicability = "has-placeholders")]
+    pub span: Span,
+
+    pub kind: &'static str,
+    pub colon: &'static str,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_enum_struct_mutually_exclusive)]
+pub(crate) struct EnumStructMutuallyExclusive {
+    #[primary_span]
+    #[suggestion(code = "enum", applicability = "machine-applicable")]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
+pub(crate) enum UnexpectedTokenAfterStructName {
+    #[diag(parse_unexpected_token_after_struct_name_found_reserved_identifier)]
+    ReservedIdentifier {
+        #[primary_span]
+        #[label(parse_unexpected_token_after_struct_name)]
+        span: Span,
+        token: Token,
+    },
+    #[diag(parse_unexpected_token_after_struct_name_found_keyword)]
+    Keyword {
+        #[primary_span]
+        #[label(parse_unexpected_token_after_struct_name)]
+        span: Span,
+        token: Token,
+    },
+    #[diag(parse_unexpected_token_after_struct_name_found_reserved_keyword)]
+    ReservedKeyword {
+        #[primary_span]
+        #[label(parse_unexpected_token_after_struct_name)]
+        span: Span,
+        token: Token,
+    },
+    #[diag(parse_unexpected_token_after_struct_name_found_doc_comment)]
+    DocComment {
+        #[primary_span]
+        #[label(parse_unexpected_token_after_struct_name)]
+        span: Span,
+        token: Token,
+    },
+    #[diag(parse_unexpected_token_after_struct_name_found_other)]
+    Other {
+        #[primary_span]
+        #[label(parse_unexpected_token_after_struct_name)]
+        span: Span,
+        token: Token,
+    },
+}
+
+impl UnexpectedTokenAfterStructName {
+    pub fn new(span: Span, token: Token) -> Self {
+        match TokenDescription::from_token(&token) {
+            Some(TokenDescription::ReservedIdentifier) => Self::ReservedIdentifier { span, token },
+            Some(TokenDescription::Keyword) => Self::Keyword { span, token },
+            Some(TokenDescription::ReservedKeyword) => Self::ReservedKeyword { span, token },
+            Some(TokenDescription::DocComment) => Self::DocComment { span, token },
+            None => Self::Other { span, token },
+        }
+    }
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_unexpected_self_in_generic_parameters)]
+#[note]
+pub(crate) struct UnexpectedSelfInGenericParameters {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_unexpected_default_value_for_lifetime_in_generic_parameters)]
+pub(crate) struct UnexpectedDefaultValueForLifetimeInGenericParameters {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_multiple_where_clauses)]
+pub(crate) struct MultipleWhereClauses {
+    #[primary_span]
+    pub span: Span,
+    #[label]
+    pub previous: Span,
+    #[suggestion(style = "verbose", code = ",", applicability = "maybe-incorrect")]
+    pub between: Span,
+}
+
+#[derive(Diagnostic)]
+pub(crate) enum UnexpectedNonterminal {
+    #[diag(parse_nonterminal_expected_item_keyword)]
+    Item(#[primary_span] Span),
+    #[diag(parse_nonterminal_expected_statement)]
+    Statement(#[primary_span] Span),
+    #[diag(parse_nonterminal_expected_ident)]
+    Ident {
+        #[primary_span]
+        span: Span,
+        token: Token,
+    },
+    #[diag(parse_nonterminal_expected_lifetime)]
+    Lifetime {
+        #[primary_span]
+        span: Span,
+        token: Token,
+    },
+}
+
+#[derive(Diagnostic)]
+pub(crate) enum TopLevelOrPatternNotAllowed {
+    #[diag(parse_or_pattern_not_allowed_in_let_binding)]
+    LetBinding {
+        #[primary_span]
+        span: Span,
+        #[subdiagnostic]
+        sub: Option<TopLevelOrPatternNotAllowedSugg>,
+    },
+    #[diag(parse_or_pattern_not_allowed_in_fn_parameters)]
+    FunctionParameter {
+        #[primary_span]
+        span: Span,
+        #[subdiagnostic]
+        sub: Option<TopLevelOrPatternNotAllowedSugg>,
+    },
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_cannot_be_raw_ident)]
+pub struct CannotBeRawIdent {
+    #[primary_span]
+    pub span: Span,
+    pub ident: Symbol,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_cr_doc_comment)]
+pub struct CrDocComment {
+    #[primary_span]
+    pub span: Span,
+    pub block: bool,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_no_digits_literal, code = "E0768")]
+pub struct NoDigitsLiteral {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_invalid_digit_literal)]
+pub struct InvalidDigitLiteral {
+    #[primary_span]
+    pub span: Span,
+    pub base: u32,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_empty_exponent_float)]
+pub struct EmptyExponentFloat {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_float_literal_unsupported_base)]
+pub struct FloatLiteralUnsupportedBase {
+    #[primary_span]
+    pub span: Span,
+    pub base: &'static str,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_unknown_prefix)]
+#[note]
+pub struct UnknownPrefix<'a> {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+    pub prefix: &'a str,
+    #[subdiagnostic]
+    pub sugg: Option<UnknownPrefixSugg>,
+}
+
+#[derive(Subdiagnostic)]
+pub enum UnknownPrefixSugg {
+    #[suggestion(
+        parse_suggestion_br,
+        code = "br",
+        applicability = "maybe-incorrect",
+        style = "verbose"
+    )]
+    UseBr(#[primary_span] Span),
+    #[suggestion(
+        parse_suggestion_whitespace,
+        code = " ",
+        applicability = "maybe-incorrect",
+        style = "verbose"
+    )]
+    Whitespace(#[primary_span] Span),
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_too_many_hashes)]
+pub struct TooManyHashes {
+    #[primary_span]
+    pub span: Span,
+    pub num: u32,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_unknown_start_of_token)]
+pub struct UnknownTokenStart {
+    #[primary_span]
+    pub span: Span,
+    pub escaped: String,
+    #[subdiagnostic]
+    pub sugg: Option<TokenSubstitution>,
+    #[subdiagnostic]
+    pub null: Option<UnknownTokenNull>,
+    #[subdiagnostic]
+    pub repeat: Option<UnknownTokenRepeat>,
+}
+
+#[derive(Subdiagnostic)]
+pub enum TokenSubstitution {
+    #[suggestion(parse_sugg_quotes, code = "{suggestion}", applicability = "maybe-incorrect")]
+    DirectedQuotes {
+        #[primary_span]
+        span: Span,
+        suggestion: String,
+        ascii_str: &'static str,
+        ascii_name: &'static str,
+    },
+    #[suggestion(parse_sugg_other, code = "{suggestion}", applicability = "maybe-incorrect")]
+    Other {
+        #[primary_span]
+        span: Span,
+        suggestion: String,
+        ch: String,
+        u_name: &'static str,
+        ascii_str: &'static str,
+        ascii_name: &'static str,
+    },
+}
+
+#[derive(Subdiagnostic)]
+#[note(parse_note_repeats)]
+pub struct UnknownTokenRepeat {
+    pub repeats: usize,
+}
+
+#[derive(Subdiagnostic)]
+#[help(parse_help_null)]
+pub struct UnknownTokenNull;
+
+#[derive(Diagnostic)]
+pub enum UnescapeError {
+    #[diag(parse_invalid_unicode_escape)]
+    #[help]
+    InvalidUnicodeEscape {
+        #[primary_span]
+        #[label]
+        span: Span,
+        surrogate: bool,
+    },
+    #[diag(parse_escape_only_char)]
+    EscapeOnlyChar {
+        #[primary_span]
+        span: Span,
+        #[suggestion(parse_escape, applicability = "machine-applicable", code = "{escaped_sugg}")]
+        char_span: Span,
+        escaped_sugg: String,
+        escaped_msg: String,
+        byte: bool,
+    },
+    #[diag(parse_bare_cr)]
+    BareCr {
+        #[primary_span]
+        #[suggestion(parse_escape, applicability = "machine-applicable", code = "\\r")]
+        span: Span,
+        double_quotes: bool,
+    },
+    #[diag(parse_bare_cr_in_raw_string)]
+    BareCrRawString(#[primary_span] Span),
+    #[diag(parse_too_short_hex_escape)]
+    TooShortHexEscape(#[primary_span] Span),
+    #[diag(parse_invalid_char_in_escape)]
+    InvalidCharInEscape {
+        #[primary_span]
+        #[label]
+        span: Span,
+        is_hex: bool,
+        ch: String,
+    },
+    #[diag(parse_out_of_range_hex_escape)]
+    OutOfRangeHexEscape(
+        #[primary_span]
+        #[label]
+        Span,
+    ),
+    #[diag(parse_leading_underscore_unicode_escape)]
+    LeadingUnderscoreUnicodeEscape {
+        #[primary_span]
+        #[label(parse_leading_underscore_unicode_escape_label)]
+        span: Span,
+        ch: String,
+    },
+    #[diag(parse_overlong_unicode_escape)]
+    OverlongUnicodeEscape(
+        #[primary_span]
+        #[label]
+        Span,
+    ),
+    #[diag(parse_unclosed_unicode_escape)]
+    UnclosedUnicodeEscape(
+        #[primary_span]
+        #[label]
+        Span,
+        #[suggestion(
+            parse_terminate,
+            code = "}}",
+            applicability = "maybe-incorrect",
+            style = "verbose"
+        )]
+        Span,
+    ),
+    #[diag(parse_no_brace_unicode_escape)]
+    NoBraceInUnicodeEscape {
+        #[primary_span]
+        span: Span,
+        #[label]
+        label: Option<Span>,
+        #[subdiagnostic]
+        sub: NoBraceUnicodeSub,
+    },
+    #[diag(parse_unicode_escape_in_byte)]
+    #[help]
+    UnicodeEscapeInByte(
+        #[primary_span]
+        #[label]
+        Span,
+    ),
+    #[diag(parse_empty_unicode_escape)]
+    EmptyUnicodeEscape(
+        #[primary_span]
+        #[label]
+        Span,
+    ),
+    #[diag(parse_zero_chars)]
+    ZeroChars(
+        #[primary_span]
+        #[label]
+        Span,
+    ),
+    #[diag(parse_lone_slash)]
+    LoneSlash(
+        #[primary_span]
+        #[label]
+        Span,
+    ),
+    #[diag(parse_unskipped_whitespace)]
+    UnskippedWhitespace {
+        #[primary_span]
+        span: Span,
+        #[label]
+        char_span: Span,
+        ch: String,
+    },
+    #[diag(parse_multiple_skipped_lines)]
+    MultipleSkippedLinesWarning(
+        #[primary_span]
+        #[label]
+        Span,
+    ),
+    #[diag(parse_more_than_one_char)]
+    MoreThanOneChar {
+        #[primary_span]
+        span: Span,
+        #[subdiagnostic]
+        note: Option<MoreThanOneCharNote>,
+        #[subdiagnostic]
+        suggestion: MoreThanOneCharSugg,
+    },
+}
+
+#[derive(Subdiagnostic)]
+pub enum MoreThanOneCharSugg {
+    #[suggestion(
+        parse_consider_normalized,
+        code = "{normalized}",
+        applicability = "machine-applicable"
+    )]
+    NormalizedForm {
+        #[primary_span]
+        span: Span,
+        ch: String,
+        normalized: String,
+    },
+    #[suggestion(parse_remove_non, code = "{ch}", applicability = "maybe-incorrect")]
+    RemoveNonPrinting {
+        #[primary_span]
+        span: Span,
+        ch: String,
+    },
+    #[suggestion(parse_use_double_quotes, code = "{sugg}", applicability = "machine-applicable")]
+    Quotes {
+        #[primary_span]
+        span: Span,
+        is_byte: bool,
+        sugg: String,
+    },
+}
+
+#[derive(Subdiagnostic)]
+pub enum MoreThanOneCharNote {
+    #[note(parse_followed_by)]
+    AllCombining {
+        #[primary_span]
+        span: Span,
+        chr: String,
+        len: usize,
+        escaped_marks: String,
+    },
+    #[note(parse_non_printing)]
+    NonPrinting {
+        #[primary_span]
+        span: Span,
+        escaped: String,
+    },
+}
+
+#[derive(Subdiagnostic)]
+pub enum NoBraceUnicodeSub {
+    #[suggestion(parse_use_braces, code = "{suggestion}", applicability = "maybe-incorrect")]
+    Suggestion {
+        #[primary_span]
+        span: Span,
+        suggestion: String,
+    },
+    #[help(parse_format_of_unicode)]
+    Help,
+}
+
+#[derive(Subdiagnostic)]
+pub(crate) enum TopLevelOrPatternNotAllowedSugg {
+    #[suggestion(
+        parse_sugg_remove_leading_vert_in_pattern,
+        code = "{pat}",
+        applicability = "machine-applicable"
+    )]
+    RemoveLeadingVert {
+        #[primary_span]
+        span: Span,
+        pat: String,
+    },
+    #[suggestion(
+        parse_sugg_wrap_pattern_in_parens,
+        code = "({pat})",
+        applicability = "machine-applicable"
+    )]
+    WrapInParens {
+        #[primary_span]
+        span: Span,
+        pat: String,
+    },
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_unexpected_vert_vert_before_function_parameter)]
+#[note(parse_note_pattern_alternatives_use_single_vert)]
+pub(crate) struct UnexpectedVertVertBeforeFunctionParam {
+    #[primary_span]
+    #[suggestion(code = "", applicability = "machine-applicable")]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_unexpected_vert_vert_in_pattern)]
+pub(crate) struct UnexpectedVertVertInPattern {
+    #[primary_span]
+    #[suggestion(code = "|", applicability = "machine-applicable")]
+    pub span: Span,
+    #[label(parse_label_while_parsing_or_pattern_here)]
+    pub start: Option<Span>,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_trailing_vert_not_allowed)]
+pub(crate) struct TrailingVertNotAllowed {
+    #[primary_span]
+    #[suggestion(code = "", applicability = "machine-applicable")]
+    pub span: Span,
+    #[label(parse_label_while_parsing_or_pattern_here)]
+    pub start: Option<Span>,
+    pub token: Token,
+    #[note(parse_note_pattern_alternatives_use_single_vert)]
+    pub note_double_vert: Option<()>,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_dotdotdot_rest_pattern)]
+pub(crate) struct DotDotDotRestPattern {
+    #[primary_span]
+    #[suggestion(style = "short", code = "..", applicability = "machine-applicable")]
+    #[label]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_pattern_on_wrong_side_of_at)]
+pub(crate) struct PatternOnWrongSideOfAt {
+    #[primary_span]
+    #[suggestion(code = "{whole_pat}", applicability = "machine-applicable")]
+    pub whole_span: Span,
+    pub whole_pat: String,
+    #[label(parse_label_pattern)]
+    pub pattern: Span,
+    #[label(parse_label_binding)]
+    pub binding: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_expected_binding_left_of_at)]
+#[note]
+pub(crate) struct ExpectedBindingLeftOfAt {
+    #[primary_span]
+    pub whole_span: Span,
+    #[label(parse_label_lhs)]
+    pub lhs: Span,
+    #[label(parse_label_rhs)]
+    pub rhs: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_ambiguous_range_pattern)]
+pub(crate) struct AmbiguousRangePattern {
+    #[primary_span]
+    #[suggestion(code = "({pat})", applicability = "maybe-incorrect")]
+    pub span: Span,
+    pub pat: String,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_unexpected_lifetime_in_pattern)]
+pub(crate) struct UnexpectedLifetimeInPattern {
+    #[primary_span]
+    #[suggestion(code = "", applicability = "machine-applicable")]
+    pub span: Span,
+    pub symbol: Symbol,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_ref_mut_order_incorrect)]
+pub(crate) struct RefMutOrderIncorrect {
+    #[primary_span]
+    #[suggestion(code = "ref mut", applicability = "machine-applicable")]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
+pub(crate) enum InvalidMutInPattern {
+    #[diag(parse_mut_on_nested_ident_pattern)]
+    #[note(parse_note_mut_pattern_usage)]
+    NestedIdent {
+        #[primary_span]
+        #[suggestion(code = "{pat}", applicability = "machine-applicable")]
+        span: Span,
+        pat: String,
+    },
+    #[diag(parse_mut_on_non_ident_pattern)]
+    #[note(parse_note_mut_pattern_usage)]
+    NonIdent {
+        #[primary_span]
+        #[suggestion(code = "{pat}", applicability = "machine-applicable")]
+        span: Span,
+        pat: String,
+    },
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_repeated_mut_in_pattern)]
+pub(crate) struct RepeatedMutInPattern {
+    #[primary_span]
+    #[suggestion(code = "", applicability = "machine-applicable")]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_dot_dot_dot_range_to_pattern_not_allowed)]
+pub(crate) struct DotDotDotRangeToPatternNotAllowed {
+    #[primary_span]
+    #[suggestion(style = "short", code = "..=", applicability = "machine-applicable")]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_enum_pattern_instead_of_identifier)]
+pub(crate) struct EnumPatternInsteadOfIdentifier {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_dot_dot_dot_for_remaining_fields)]
+pub(crate) struct DotDotDotForRemainingFields {
+    #[primary_span]
+    #[suggestion(code = "..", style = "verbose", applicability = "machine-applicable")]
+    pub span: Span,
+    pub token_str: Cow<'static, str>,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_expected_comma_after_pattern_field)]
+pub(crate) struct ExpectedCommaAfterPatternField {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_return_types_use_thin_arrow)]
+pub(crate) struct ReturnTypesUseThinArrow {
+    #[primary_span]
+    #[suggestion(style = "short", code = "->", applicability = "machine-applicable")]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_need_plus_after_trait_object_lifetime)]
+pub(crate) struct NeedPlusAfterTraitObjectLifetime {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_expected_mut_or_const_in_raw_pointer_type)]
+pub(crate) struct ExpectedMutOrConstInRawPointerType {
+    #[primary_span]
+    pub span: Span,
+    #[suggestion(code("mut ", "const "), applicability = "has-placeholders")]
+    pub after_asterisk: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_lifetime_after_mut)]
+pub(crate) struct LifetimeAfterMut {
+    #[primary_span]
+    pub span: Span,
+    #[suggestion(code = "&{snippet} mut", applicability = "maybe-incorrect")]
+    pub suggest_lifetime: Option<Span>,
+    pub snippet: String,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_dyn_after_mut)]
+pub(crate) struct DynAfterMut {
+    #[primary_span]
+    #[suggestion(code = "&mut dyn", applicability = "machine-applicable")]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_fn_pointer_cannot_be_const)]
+pub(crate) struct FnPointerCannotBeConst {
+    #[primary_span]
+    pub span: Span,
+    #[suggestion(code = "", applicability = "maybe-incorrect")]
+    #[label]
+    pub qualifier: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_fn_pointer_cannot_be_async)]
+pub(crate) struct FnPointerCannotBeAsync {
+    #[primary_span]
+    pub span: Span,
+    #[suggestion(code = "", applicability = "maybe-incorrect")]
+    #[label]
+    pub qualifier: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_nested_c_variadic_type, code = "E0743")]
+pub(crate) struct NestedCVariadicType {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_invalid_dyn_keyword)]
+#[help]
+pub(crate) struct InvalidDynKeyword {
+    #[primary_span]
+    #[suggestion(code = "", applicability = "machine-applicable")]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_negative_bounds_not_supported)]
+pub(crate) struct NegativeBoundsNotSupported {
+    #[primary_span]
+    pub negative_bounds: Vec<Span>,
+    #[label]
+    pub last_span: Span,
+    #[subdiagnostic]
+    pub sub: Option<NegativeBoundsNotSupportedSugg>,
+}
+
+#[derive(Subdiagnostic)]
+#[suggestion(
+    parse_suggestion,
+    style = "tool-only",
+    code = "{fixed}",
+    applicability = "machine-applicable"
+)]
+pub(crate) struct NegativeBoundsNotSupportedSugg {
+    #[primary_span]
+    pub bound_list: Span,
+    pub num_bounds: usize,
+    pub fixed: String,
+}
+
+#[derive(Subdiagnostic)]
+pub enum HelpUseLatestEdition {
+    #[help(parse_help_set_edition_cargo)]
+    #[note(parse_note_edition_guide)]
+    Cargo { edition: Edition },
+    #[help(parse_help_set_edition_standalone)]
+    #[note(parse_note_edition_guide)]
+    Standalone { edition: Edition },
+}
+
+impl HelpUseLatestEdition {
+    pub fn new() -> Self {
+        let edition = LATEST_STABLE_EDITION;
+        if std::env::var_os("CARGO").is_some() {
+            Self::Cargo { edition }
+        } else {
+            Self::Standalone { edition }
+        }
+    }
+}
diff --git a/compiler/rustc_parse/src/lexer/diagnostics.rs b/compiler/rustc_parse/src/lexer/diagnostics.rs
new file mode 100644
index 000000000..27f4428d3
--- /dev/null
+++ b/compiler/rustc_parse/src/lexer/diagnostics.rs
@@ -0,0 +1,119 @@
+use super::UnmatchedDelim;
+use rustc_ast::token::Delimiter;
+use rustc_errors::Diagnostic;
+use rustc_span::source_map::SourceMap;
+use rustc_span::Span;
+
+#[derive(Default)]
+pub struct TokenTreeDiagInfo {
+    /// Stack of open delimiters and their spans. Used for error message.
+    pub open_braces: Vec<(Delimiter, Span)>,
+    pub unmatched_delims: Vec<UnmatchedDelim>,
+
+    /// Used only for error recovery when arriving to EOF with mismatched braces.
+    pub last_unclosed_found_span: Option<Span>,
+
+    /// Collect empty block spans that might have been auto-inserted by editors.
+    pub empty_block_spans: Vec<Span>,
+
+    /// Collect the spans of braces (Open, Close). Used only
+    /// for detecting if blocks are empty and only braces.
+    pub matching_block_spans: Vec<(Span, Span)>,
+}
+
+pub fn same_identation_level(sm: &SourceMap, open_sp: Span, close_sp: Span) -> bool {
+    match (sm.span_to_margin(open_sp), sm.span_to_margin(close_sp)) {
+        (Some(open_padding), Some(close_padding)) => open_padding == close_padding,
+        _ => false,
+    }
+}
+
+// When we get a `)` or `]` for `{`, we should emit help message here
+// it's more friendly compared to report `unmatched error` in later phase
+pub fn report_missing_open_delim(
+    err: &mut Diagnostic,
+    unmatched_delims: &[UnmatchedDelim],
+) -> bool {
+    let mut reported_missing_open = false;
+    for unmatch_brace in unmatched_delims.iter() {
+        if let Some(delim) = unmatch_brace.found_delim
+            && matches!(delim, Delimiter::Parenthesis | Delimiter::Bracket)
+        {
+            let missed_open = match delim {
+                Delimiter::Parenthesis => "(",
+                Delimiter::Bracket => "[",
+                _ => unreachable!(),
+            };
+            err.span_label(
+                unmatch_brace.found_span.shrink_to_lo(),
+                format!("missing open `{}` for this delimiter", missed_open),
+            );
+            reported_missing_open = true;
+        }
+    }
+    reported_missing_open
+}
+
+pub fn report_suspicious_mismatch_block(
+    err: &mut Diagnostic,
+    diag_info: &TokenTreeDiagInfo,
+    sm: &SourceMap,
+    delim: Delimiter,
+) {
+    if report_missing_open_delim(err, &diag_info.unmatched_delims) {
+        return;
+    }
+
+    let mut matched_spans: Vec<(Span, bool)> = diag_info
+        .matching_block_spans
+        .iter()
+        .map(|&(open, close)| (open.with_hi(close.lo()), same_identation_level(sm, open, close)))
+        .collect();
+
+    // sort by `lo`, so the large block spans in the front
+    matched_spans.sort_by(|a, b| a.0.lo().cmp(&b.0.lo()));
+
+    // We use larger block whose identation is well to cover those inner mismatched blocks
+    // O(N^2) here, but we are on error reporting path, so it is fine
+    for i in 0..matched_spans.len() {
+        let (block_span, same_ident) = matched_spans[i];
+        if same_ident {
+            for j in i + 1..matched_spans.len() {
+                let (inner_block, inner_same_ident) = matched_spans[j];
+                if block_span.contains(inner_block) && !inner_same_ident {
+                    matched_spans[j] = (inner_block, true);
+                }
+            }
+        }
+    }
+
+    // Find the inner-most span candidate for final report
+    let candidate_span =
+        matched_spans.into_iter().rev().find(|&(_, same_ident)| !same_ident).map(|(span, _)| span);
+
+    if let Some(block_span) = candidate_span {
+        err.span_label(block_span.shrink_to_lo(), "this delimiter might not be properly closed...");
+        err.span_label(
+            block_span.shrink_to_hi(),
+            "...as it matches this but it has different indentation",
+        );
+
+        // If there is a empty block in the mismatched span, note it
+        if delim == Delimiter::Brace {
+            for span in diag_info.empty_block_spans.iter() {
+                if block_span.contains(*span) {
+                    err.span_label(*span, "block is empty, you might have not meant to close it");
+                    break;
+                }
+            }
+        }
+    } else {
+        // If there is no suspicious span, give the last properly closed block may help
+        if let Some(parent) = diag_info.matching_block_spans.last()
+            && diag_info.open_braces.last().is_none()
+            && diag_info.empty_block_spans.iter().all(|&sp| sp != parent.0.to(parent.1)) {
+                err.span_label(parent.0, "this opening brace...");
+                err.span_label(parent.1, "...matches this closing brace");
+        }
+    }
+}
diff --git a/compiler/rustc_parse/src/lexer/mod.rs b/compiler/rustc_parse/src/lexer/mod.rs
index 9fe8d9836..59958a309 100644
--- a/compiler/rustc_parse/src/lexer/mod.rs
+++ b/compiler/rustc_parse/src/lexer/mod.rs
@@ -1,11 +1,11 @@
+use crate::errors;
 use crate::lexer::unicode_chars::UNICODE_ARRAY;
+use crate::make_unclosed_delims_error;
 use rustc_ast::ast::{self, AttrStyle};
 use rustc_ast::token::{self, CommentKind, Delimiter, Token, TokenKind};
 use rustc_ast::tokenstream::TokenStream;
 use rustc_ast::util::unicode::contains_text_flow_control_chars;
-use rustc_errors::{
-    error_code, Applicability, DiagnosticBuilder, ErrorGuaranteed, PResult, StashKey,
-};
+use rustc_errors::{error_code, Applicability, Diagnostic, DiagnosticBuilder, StashKey};
 use rustc_lexer::unescape::{self, Mode};
 use rustc_lexer::Cursor;
 use rustc_lexer::{Base, DocStyle, RawStrError};
@@ -17,6 +17,7 @@ use rustc_session::parse::ParseSess;
 use rustc_span::symbol::{sym, Symbol};
 use rustc_span::{edition::Edition, BytePos, Pos, Span};
 
+mod diagnostics;
 mod tokentrees;
 mod unescape_error_reporting;
 mod unicode_chars;
@@ -31,7 +32,7 @@ use unescape_error_reporting::{emit_unescape_error, escaped_char};
 rustc_data_structures::static_assert_size!(rustc_lexer::Token, 12);
 
 #[derive(Clone, Debug)]
-pub struct UnmatchedBrace {
+pub struct UnmatchedDelim {
     pub expected_delim: Delimiter,
     pub found_delim: Option<Delimiter>,
     pub found_span: Span,
@@ -44,7 +45,7 @@ pub(crate) fn parse_token_trees<'a>(
     mut src: &'a str,
     mut start_pos: BytePos,
     override_span: Option<Span>,
-) -> (PResult<'a, TokenStream>, Vec<UnmatchedBrace>) {
+) -> Result<TokenStream, Vec<Diagnostic>> {
     // Skip `#!`, if present.
     if let Some(shebang_len) = rustc_lexer::strip_shebang(src) {
         src = &src[shebang_len..];
@@ -61,7 +62,29 @@ pub(crate) fn parse_token_trees<'a>(
         override_span,
         nbsp_is_whitespace: false,
     };
-    tokentrees::TokenTreesReader::parse_all_token_trees(string_reader)
+    let (token_trees, unmatched_delims) =
+        tokentrees::TokenTreesReader::parse_all_token_trees(string_reader);
+    match token_trees {
+        Ok(stream) if unmatched_delims.is_empty() => Ok(stream),
+        _ => {
+            // Return error if there are unmatched delimiters or unclosng delimiters.
+            // We emit delimiter mismatch errors first, then emit the unclosing delimiter mismatch
+            // because the delimiter mismatch is more likely to be the root cause of error
+
+            let mut buffer = Vec::with_capacity(1);
+            // Not using `emit_unclosed_delims` to use `db.buffer`
+            for unmatched in unmatched_delims {
+                if let Some(err) = make_unclosed_delims_error(unmatched, &sess) {
+                    err.buffer(&mut buffer);
+                }
+            }
+            if let Err(err) = token_trees {
+                // Add unclosing delimiter error
+                err.buffer(&mut buffer);
+            }
+            Err(buffer)
+        }
+    }
 }
 
 struct StringReader<'a> {
@@ -150,7 +173,7 @@ impl<'a> StringReader<'a> {
                     let span = self.mk_sp(start, self.pos);
                     self.sess.symbol_gallery.insert(sym, span);
                     if !sym.can_be_raw() {
-                        self.err_span(span, &format!("`{}` cannot be a raw identifier", sym));
+                        self.sess.emit_err(errors::CannotBeRawIdent { span, ident: sym });
                     }
                     self.sess.raw_identifier_spans.borrow_mut().push(span);
                     token::Ident(sym, true)
@@ -261,27 +284,24 @@ impl<'a> StringReader<'a> {
                         self.nbsp_is_whitespace = true;
                     }
                     let repeats = it.take_while(|c1| *c1 == c).count();
-                    let mut err =
-                        self.struct_err_span_char(start, self.pos + Pos::from_usize(repeats * c.len_utf8()), "unknown start of token", c);
                     // FIXME: the lexer could be used to turn the ASCII version of unicode
                     // homoglyphs, instead of keeping a table in `check_for_substitution`into the
                     // token. Ideally, this should be inside `rustc_lexer`. However, we should
                     // first remove compound tokens like `<<` from `rustc_lexer`, and then add
                     // fancier error recovery to it, as there will be less overall work to do this
                     // way.
-                    let token = unicode_chars::check_for_substitution(self, start, c, &mut err, repeats+1);
-                    if c == '\x00' {
-                        err.help("source files must contain UTF-8 encoded text, unexpected null bytes might occur when a different encoding is used");
-                    }
-                    if repeats > 0 {
-                        if repeats == 1 {
-                            err.note(format!("character appears once more"));
-                        } else {
-                            err.note(format!("character appears {repeats} more times"));
-                        }
-                        swallow_next_invalid = repeats;
-                    }
-                    err.emit();
+                    let (token, sugg) = unicode_chars::check_for_substitution(self, start, c, repeats+1);
+                    self.sess.emit_err(errors::UnknownTokenStart {
+                        span: self.mk_sp(start, self.pos + Pos::from_usize(repeats * c.len_utf8())),
+                        escaped: escaped_char(c),
+                        sugg,
+                        null: if c == '\x00' {Some(errors::UnknownTokenNull)} else {None},
+                        repeat: if repeats > 0 {
+                            swallow_next_invalid = repeats;
+                            Some(errors::UnknownTokenRepeat { repeats })
+                        } else {None}
+                    });
+
                     if let Some(token) = token {
                         token
                     } else {
@@ -296,26 +316,6 @@ impl<'a> StringReader<'a> {
         }
     }
 
-    /// Report a fatal lexical error with a given span.
-    fn fatal_span(&self, sp: Span, m: &str) -> ! {
-        self.sess.span_diagnostic.span_fatal(sp, m)
-    }
-
-    /// Report a lexical error with a given span.
-    fn err_span(&self, sp: Span, m: &str) {
-        self.sess.span_diagnostic.struct_span_err(sp, m).emit();
-    }
-
-    /// Report a fatal error spanning [`from_pos`, `to_pos`).
-    fn fatal_span_(&self, from_pos: BytePos, to_pos: BytePos, m: &str) -> ! {
-        self.fatal_span(self.mk_sp(from_pos, to_pos), m)
-    }
-
-    /// Report a lexical error spanning [`from_pos`, `to_pos`).
-    fn err_span_(&self, from_pos: BytePos, to_pos: BytePos, m: &str) {
-        self.err_span(self.mk_sp(from_pos, to_pos), m)
-    }
-
     fn struct_fatal_span_char(
         &self,
         from_pos: BytePos,
@@ -328,18 +328,6 @@ impl<'a> StringReader<'a> {
             .struct_span_fatal(self.mk_sp(from_pos, to_pos), &format!("{}: {}", m, escaped_char(c)))
     }
 
-    fn struct_err_span_char(
-        &self,
-        from_pos: BytePos,
-        to_pos: BytePos,
-        m: &str,
-        c: char,
-    ) -> DiagnosticBuilder<'a, ErrorGuaranteed> {
-        self.sess
-            .span_diagnostic
-            .struct_span_err(self.mk_sp(from_pos, to_pos), &format!("{}: {}", m, escaped_char(c)))
-    }
-
     /// Detect usages of Unicode codepoints changing the direction of the text on screen and loudly
     /// complain about it.
     fn lint_unicode_text_flow(&self, start: BytePos) {
@@ -367,14 +355,12 @@ impl<'a> StringReader<'a> {
     ) -> TokenKind {
         if content.contains('\r') {
             for (idx, _) in content.char_indices().filter(|&(_, c)| c == '\r') {
-                self.err_span_(
+                let span = self.mk_sp(
                     content_start + BytePos(idx as u32),
                     content_start + BytePos(idx as u32 + 1),
-                    match comment_kind {
-                        CommentKind::Line => "bare CR not allowed in doc-comment",
-                        CommentKind::Block => "bare CR not allowed in block doc-comment",
-                    },
                 );
+                let block = matches!(comment_kind, CommentKind::Block);
+                self.sess.emit_err(errors::CrDocComment { span, block });
             }
         }
 
@@ -453,26 +439,20 @@ impl<'a> StringReader<'a> {
             }
             rustc_lexer::LiteralKind::Int { base, empty_int } => {
                 if empty_int {
-                    self.sess
-                        .span_diagnostic
-                        .struct_span_err_with_code(
-                            self.mk_sp(start, end),
-                            "no valid digits found for number",
-                            error_code!(E0768),
-                        )
-                        .emit();
+                    let span = self.mk_sp(start, end);
+                    self.sess.emit_err(errors::NoDigitsLiteral { span });
                     (token::Integer, sym::integer(0))
                 } else {
                     if matches!(base, Base::Binary | Base::Octal) {
                         let base = base as u32;
                         let s = self.str_from_to(start + BytePos(2), end);
                         for (idx, c) in s.char_indices() {
+                            let span = self.mk_sp(
+                                start + BytePos::from_usize(2 + idx),
+                                start + BytePos::from_usize(2 + idx + c.len_utf8()),
+                            );
                             if c != '_' && c.to_digit(base).is_none() {
-                                self.err_span_(
-                                    start + BytePos::from_usize(2 + idx),
-                                    start + BytePos::from_usize(2 + idx + c.len_utf8()),
-                                    &format!("invalid digit for a base {} literal", base),
-                                );
+                                self.sess.emit_err(errors::InvalidDigitLiteral { span, base });
                             }
                         }
                     }
@@ -481,19 +461,18 @@ impl<'a> StringReader<'a> {
             }
             rustc_lexer::LiteralKind::Float { base, empty_exponent } => {
                 if empty_exponent {
-                    self.err_span_(start, self.pos, "expected at least one digit in exponent");
+                    let span = self.mk_sp(start, self.pos);
+                    self.sess.emit_err(errors::EmptyExponentFloat { span });
                 }
-                match base {
-                    Base::Hexadecimal => {
-                        self.err_span_(start, end, "hexadecimal float literal is not supported")
-                    }
-                    Base::Octal => {
-                        self.err_span_(start, end, "octal float literal is not supported")
-                    }
-                    Base::Binary => {
-                        self.err_span_(start, end, "binary float literal is not supported")
-                    }
-                    _ => {}
+                let base = match base {
+                    Base::Hexadecimal => Some("hexadecimal"),
+                    Base::Octal => Some("octal"),
+                    Base::Binary => Some("binary"),
+                    _ => None,
+                };
+                if let Some(base) = base {
+                    let span = self.mk_sp(start, end);
+                    self.sess.emit_err(errors::FloatLiteralUnsupportedBase { span, base });
                 }
                 (token::Float, self.symbol_from_to(start, end))
             }
@@ -643,54 +622,34 @@ impl<'a> StringReader<'a> {
     // identifier tokens.
     fn report_unknown_prefix(&self, start: BytePos) {
         let prefix_span = self.mk_sp(start, self.pos);
-        let prefix_str = self.str_from_to(start, self.pos);
-        let msg = format!("prefix `{}` is unknown", prefix_str);
+        let prefix = self.str_from_to(start, self.pos);
 
         let expn_data = prefix_span.ctxt().outer_expn_data();
 
         if expn_data.edition >= Edition::Edition2021 {
             // In Rust 2021, this is a hard error.
-            let mut err = self.sess.span_diagnostic.struct_span_err(prefix_span, &msg);
-            err.span_label(prefix_span, "unknown prefix");
-            if prefix_str == "rb" {
-                err.span_suggestion_verbose(
-                    prefix_span,
-                    "use `br` for a raw byte string",
-                    "br",
-                    Applicability::MaybeIncorrect,
-                );
+            let sugg = if prefix == "rb" {
+                Some(errors::UnknownPrefixSugg::UseBr(prefix_span))
             } else if expn_data.is_root() {
-                err.span_suggestion_verbose(
-                    prefix_span.shrink_to_hi(),
-                    "consider inserting whitespace here",
-                    " ",
-                    Applicability::MaybeIncorrect,
-                );
-            }
-            err.note("prefixed identifiers and literals are reserved since Rust 2021");
-            err.emit();
+                Some(errors::UnknownPrefixSugg::Whitespace(prefix_span.shrink_to_hi()))
+            } else {
+                None
+            };
+            self.sess.emit_err(errors::UnknownPrefix { span: prefix_span, prefix, sugg });
         } else {
             // Before Rust 2021, only emit a lint for migration.
             self.sess.buffer_lint_with_diagnostic(
                 &RUST_2021_PREFIXES_INCOMPATIBLE_SYNTAX,
                 prefix_span,
                 ast::CRATE_NODE_ID,
-                &msg,
+                &format!("prefix `{prefix}` is unknown"),
                 BuiltinLintDiagnostics::ReservedPrefix(prefix_span),
             );
         }
     }
 
-    fn report_too_many_hashes(&self, start: BytePos, found: u32) -> ! {
-        self.fatal_span_(
-            start,
-            self.pos,
-            &format!(
-                "too many `#` symbols: raw strings may be delimited \
-                by up to 255 `#` symbols, but found {}",
-                found
-            ),
-        )
+    fn report_too_many_hashes(&self, start: BytePos, num: u32) -> ! {
+        self.sess.emit_fatal(errors::TooManyHashes { span: self.mk_sp(start, self.pos), num });
     }
 
     fn cook_quoted(
diff --git a/compiler/rustc_parse/src/lexer/tokentrees.rs b/compiler/rustc_parse/src/lexer/tokentrees.rs
index b2701817d..36fd1e37d 100644
--- a/compiler/rustc_parse/src/lexer/tokentrees.rs
+++ b/compiler/rustc_parse/src/lexer/tokentrees.rs
@@ -1,47 +1,31 @@
-use super::{StringReader, UnmatchedBrace};
+use super::diagnostics::report_suspicious_mismatch_block;
+use super::diagnostics::same_identation_level;
+use super::diagnostics::TokenTreeDiagInfo;
+use super::{StringReader, UnmatchedDelim};
 use rustc_ast::token::{self, Delimiter, Token};
 use rustc_ast::tokenstream::{DelimSpan, Spacing, TokenStream, TokenTree};
 use rustc_ast_pretty::pprust::token_to_string;
-use rustc_data_structures::fx::FxHashMap;
 use rustc_errors::{PErr, PResult};
-use rustc_span::Span;
 
 pub(super) struct TokenTreesReader<'a> {
     string_reader: StringReader<'a>,
     /// The "next" token, which has been obtained from the `StringReader` but
     /// not yet handled by the `TokenTreesReader`.
     token: Token,
-    /// Stack of open delimiters and their spans. Used for error message.
-    open_braces: Vec<(Delimiter, Span)>,
-    unmatched_braces: Vec<UnmatchedBrace>,
-    /// The type and spans for all braces
-    ///
-    /// Used only for error recovery when arriving to EOF with mismatched braces.
-    matching_delim_spans: Vec<(Delimiter, Span, Span)>,
-    last_unclosed_found_span: Option<Span>,
-    /// Collect empty block spans that might have been auto-inserted by editors.
-    last_delim_empty_block_spans: FxHashMap<Delimiter, Span>,
-    /// Collect the spans of braces (Open, Close). Used only
-    /// for detecting if blocks are empty and only braces.
-    matching_block_spans: Vec<(Span, Span)>,
+    diag_info: TokenTreeDiagInfo,
 }
 
 impl<'a> TokenTreesReader<'a> {
     pub(super) fn parse_all_token_trees(
         string_reader: StringReader<'a>,
-    ) -> (PResult<'a, TokenStream>, Vec<UnmatchedBrace>) {
+    ) -> (PResult<'a, TokenStream>, Vec<UnmatchedDelim>) {
         let mut tt_reader = TokenTreesReader {
             string_reader,
             token: Token::dummy(),
-            open_braces: Vec::new(),
-            unmatched_braces: Vec::new(),
-            matching_delim_spans: Vec::new(),
-            last_unclosed_found_span: None,
-            last_delim_empty_block_spans: FxHashMap::default(),
-            matching_block_spans: Vec::new(),
+            diag_info: TokenTreeDiagInfo::default(),
         };
         let res = tt_reader.parse_token_trees(/* is_delimited */ false);
-        (res, tt_reader.unmatched_braces)
+        (res, tt_reader.diag_info.unmatched_delims)
     }
 
     // Parse a stream of tokens into a list of `TokenTree`s.
@@ -50,7 +34,7 @@ impl<'a> TokenTreesReader<'a> {
         let mut buf = Vec::new();
         loop {
             match self.token.kind {
-                token::OpenDelim(delim) => buf.push(self.parse_token_tree_open_delim(delim)),
+                token::OpenDelim(delim) => buf.push(self.parse_token_tree_open_delim(delim)?),
                 token::CloseDelim(delim) => {
                     return if is_delimited {
                         Ok(TokenStream::new(buf))
@@ -59,10 +43,11 @@ impl<'a> TokenTreesReader<'a> {
                     };
                 }
                 token::Eof => {
-                    if is_delimited {
-                        self.eof_err().emit();
-                    }
-                    return Ok(TokenStream::new(buf));
+                    return if is_delimited {
+                        Err(self.eof_err())
+                    } else {
+                        Ok(TokenStream::new(buf))
+                    };
                 }
                 _ => {
                     // Get the next normal token. This might require getting multiple adjacent
@@ -92,9 +77,9 @@ impl<'a> TokenTreesReader<'a> {
     fn eof_err(&mut self) -> PErr<'a> {
         let msg = "this file contains an unclosed delimiter";
         let mut err = self.string_reader.sess.span_diagnostic.struct_span_err(self.token.span, msg);
-        for &(_, sp) in &self.open_braces {
+        for &(_, sp) in &self.diag_info.open_braces {
             err.span_label(sp, "unclosed delimiter");
-            self.unmatched_braces.push(UnmatchedBrace {
+            self.diag_info.unmatched_delims.push(UnmatchedDelim {
                 expected_delim: Delimiter::Brace,
                 found_delim: None,
                 found_span: self.token.span,
@@ -103,69 +88,53 @@ impl<'a> TokenTreesReader<'a> {
             });
         }
 
-        if let Some((delim, _)) = self.open_braces.last() {
-            if let Some((_, open_sp, close_sp)) =
-                self.matching_delim_spans.iter().find(|(d, open_sp, close_sp)| {
-                    let sm = self.string_reader.sess.source_map();
-                    if let Some(close_padding) = sm.span_to_margin(*close_sp) {
-                        if let Some(open_padding) = sm.span_to_margin(*open_sp) {
-                            return delim == d && close_padding != open_padding;
-                        }
-                    }
-                    false
-                })
-            // these are in reverse order as they get inserted on close, but
-            {
-                // we want the last open/first close
-                err.span_label(*open_sp, "this delimiter might not be properly closed...");
-                err.span_label(*close_sp, "...as it matches this but it has different indentation");
-            }
+        if let Some((delim, _)) = self.diag_info.open_braces.last() {
+            report_suspicious_mismatch_block(
+                &mut err,
+                &self.diag_info,
+                &self.string_reader.sess.source_map(),
+                *delim,
+            )
         }
         err
     }
 
-    fn parse_token_tree_open_delim(&mut self, open_delim: Delimiter) -> TokenTree {
+    fn parse_token_tree_open_delim(&mut self, open_delim: Delimiter) -> PResult<'a, TokenTree> {
         // The span for beginning of the delimited section
         let pre_span = self.token.span;
 
-        self.open_braces.push((open_delim, self.token.span));
+        self.diag_info.open_braces.push((open_delim, self.token.span));
 
         // Parse the token trees within the delimiters.
         // We stop at any delimiter so we can try to recover if the user
         // uses an incorrect delimiter.
-        let tts = self.parse_token_trees(/* is_delimited */ true).unwrap();
+        let tts = self.parse_token_trees(/* is_delimited */ true)?;
 
         // Expand to cover the entire delimited token tree
         let delim_span = DelimSpan::from_pair(pre_span, self.token.span);
+        let sm = self.string_reader.sess.source_map();
 
         match self.token.kind {
             // Correct delimiter.
             token::CloseDelim(close_delim) if close_delim == open_delim => {
-                let (open_brace, open_brace_span) = self.open_braces.pop().unwrap();
+                let (open_brace, open_brace_span) = self.diag_info.open_braces.pop().unwrap();
                 let close_brace_span = self.token.span;
 
-                if tts.is_empty() {
+                if tts.is_empty() && close_delim == Delimiter::Brace {
                     let empty_block_span = open_brace_span.to(close_brace_span);
-                    let sm = self.string_reader.sess.source_map();
                     if !sm.is_multiline(empty_block_span) {
                         // Only track if the block is in the form of `{}`, otherwise it is
                         // likely that it was written on purpose.
-                        self.last_delim_empty_block_spans.insert(open_delim, empty_block_span);
+                        self.diag_info.empty_block_spans.push(empty_block_span);
                     }
                 }
 
-                //only add braces
+                // only add braces
                 if let (Delimiter::Brace, Delimiter::Brace) = (open_brace, open_delim) {
-                    self.matching_block_spans.push((open_brace_span, close_brace_span));
+                    // Add all the matching spans, we will sort by span later
+                    self.diag_info.matching_block_spans.push((open_brace_span, close_brace_span));
                 }
 
-                if self.open_braces.is_empty() {
-                    // Clear up these spans to avoid suggesting them as we've found
-                    // properly matched delimiters so far for an entire block.
-                    self.matching_delim_spans.clear();
-                } else {
-                    self.matching_delim_spans.push((open_brace, open_brace_span, close_brace_span));
-                }
                 // Move past the closing delimiter.
                 self.token = self.string_reader.next_token().0;
             }
@@ -174,28 +143,25 @@ impl<'a> TokenTreesReader<'a> {
                 let mut unclosed_delimiter = None;
                 let mut candidate = None;
 
-                if self.last_unclosed_found_span != Some(self.token.span) {
+                if self.diag_info.last_unclosed_found_span != Some(self.token.span) {
                     // do not complain about the same unclosed delimiter multiple times
-                    self.last_unclosed_found_span = Some(self.token.span);
+                    self.diag_info.last_unclosed_found_span = Some(self.token.span);
                     // This is a conservative error: only report the last unclosed
                     // delimiter. The previous unclosed delimiters could actually be
                     // closed! The parser just hasn't gotten to them yet.
-                    if let Some(&(_, sp)) = self.open_braces.last() {
+                    if let Some(&(_, sp)) = self.diag_info.open_braces.last() {
                         unclosed_delimiter = Some(sp);
                     };
-                    let sm = self.string_reader.sess.source_map();
-                    if let Some(current_padding) = sm.span_to_margin(self.token.span) {
-                        for (brace, brace_span) in &self.open_braces {
-                            if let Some(padding) = sm.span_to_margin(*brace_span) {
-                                // high likelihood of these two corresponding
-                                if current_padding == padding && brace == &close_delim {
-                                    candidate = Some(*brace_span);
-                                }
-                            }
+                    for (brace, brace_span) in &self.diag_info.open_braces {
+                        if same_identation_level(&sm, self.token.span, *brace_span)
+                            && brace == &close_delim
+                        {
+                            // high likelihood of these two corresponding
+                            candidate = Some(*brace_span);
                         }
                     }
-                    let (tok, _) = self.open_braces.pop().unwrap();
-                    self.unmatched_braces.push(UnmatchedBrace {
+                    let (tok, _) = self.diag_info.open_braces.pop().unwrap();
+                    self.diag_info.unmatched_delims.push(UnmatchedDelim {
                         expected_delim: tok,
                         found_delim: Some(close_delim),
                         found_span: self.token.span,
@@ -203,7 +169,7 @@ impl<'a> TokenTreesReader<'a> {
                         candidate_span: candidate,
                     });
                 } else {
-                    self.open_braces.pop();
+                    self.diag_info.open_braces.pop();
                 }
 
                 // If the incorrect delimiter matches an earlier opening
@@ -213,7 +179,7 @@ impl<'a> TokenTreesReader<'a> {
                 // fn foo() {
                 //     bar(baz(
                 // }  // Incorrect delimiter but matches the earlier `{`
-                if !self.open_braces.iter().any(|&(b, _)| b == close_delim) {
+                if !self.diag_info.open_braces.iter().any(|&(b, _)| b == close_delim) {
                     self.token = self.string_reader.next_token().0;
                 }
             }
@@ -225,7 +191,7 @@ impl<'a> TokenTreesReader<'a> {
             _ => unreachable!(),
         }
 
-        TokenTree::Delimited(delim_span, open_delim, tts)
+        Ok(TokenTree::Delimited(delim_span, open_delim, tts))
     }
 
     fn close_delim_err(&mut self, delim: Delimiter) -> PErr<'a> {
@@ -236,22 +202,12 @@ impl<'a> TokenTreesReader<'a> {
         let mut err =
             self.string_reader.sess.span_diagnostic.struct_span_err(self.token.span, &msg);
 
-        // Braces are added at the end, so the last element is the biggest block
-        if let Some(parent) = self.matching_block_spans.last() {
-            if let Some(span) = self.last_delim_empty_block_spans.remove(&delim) {
-                // Check if the (empty block) is in the last properly closed block
-                if (parent.0.to(parent.1)).contains(span) {
-                    err.span_label(span, "block is empty, you might have not meant to close it");
-                } else {
-                    err.span_label(parent.0, "this opening brace...");
-                    err.span_label(parent.1, "...matches this closing brace");
-                }
-            } else {
-                err.span_label(parent.0, "this opening brace...");
-                err.span_label(parent.1, "...matches this closing brace");
-            }
-        }
-
+        report_suspicious_mismatch_block(
+            &mut err,
+            &self.diag_info,
+            &self.string_reader.sess.source_map(),
+            delim,
+        );
         err.span_label(self.token.span, "unexpected closing delimiter");
         err
     }
diff --git a/compiler/rustc_parse/src/lexer/unescape_error_reporting.rs b/compiler/rustc_parse/src/lexer/unescape_error_reporting.rs
index 6373f5b4f..0d12ec608 100644
--- a/compiler/rustc_parse/src/lexer/unescape_error_reporting.rs
+++ b/compiler/rustc_parse/src/lexer/unescape_error_reporting.rs
@@ -3,10 +3,12 @@
 use std::iter::once;
 use std::ops::Range;
 
-use rustc_errors::{pluralize, Applicability, Handler};
+use rustc_errors::{Applicability, Handler};
 use rustc_lexer::unescape::{EscapeError, Mode};
 use rustc_span::{BytePos, Span};
 
+use crate::errors::{MoreThanOneCharNote, MoreThanOneCharSugg, NoBraceUnicodeSub, UnescapeError};
+
 pub(crate) fn emit_unescape_error(
     handler: &Handler,
     // interior part of the literal, without quotes
@@ -31,53 +33,32 @@ pub(crate) fn emit_unescape_error(
     };
     match error {
         EscapeError::LoneSurrogateUnicodeEscape => {
-            handler
-                .struct_span_err(span, "invalid unicode character escape")
-                .span_label(span, "invalid escape")
-                .help("unicode escape must not be a surrogate")
-                .emit();
+            handler.emit_err(UnescapeError::InvalidUnicodeEscape { span, surrogate: true });
         }
         EscapeError::OutOfRangeUnicodeEscape => {
-            handler
-                .struct_span_err(span, "invalid unicode character escape")
-                .span_label(span, "invalid escape")
-                .help("unicode escape must be at most 10FFFF")
-                .emit();
+            handler.emit_err(UnescapeError::InvalidUnicodeEscape { span, surrogate: false });
         }
         EscapeError::MoreThanOneChar => {
             use unicode_normalization::{char::is_combining_mark, UnicodeNormalization};
+            let mut sugg = None;
+            let mut note = None;
 
-            let mut has_help = false;
-            let mut handler = handler.struct_span_err(
-                span_with_quotes,
-                "character literal may only contain one codepoint",
-            );
-
-            if lit.chars().skip(1).all(|c| is_combining_mark(c)) {
-                let escaped_marks =
-                    lit.chars().skip(1).map(|c| c.escape_default().to_string()).collect::<Vec<_>>();
-                handler.span_note(
-                    span,
-                    &format!(
-                        "this `{}` is followed by the combining mark{} `{}`",
-                        lit.chars().next().unwrap(),
-                        pluralize!(escaped_marks.len()),
-                        escaped_marks.join(""),
-                    ),
-                );
+            let lit_chars = lit.chars().collect::<Vec<_>>();
+            let (first, rest) = lit_chars.split_first().unwrap();
+            if rest.iter().copied().all(is_combining_mark) {
                 let normalized = lit.nfc().to_string();
                 if normalized.chars().count() == 1 {
-                    has_help = true;
-                    handler.span_suggestion(
-                        span,
-                        &format!(
-                            "consider using the normalized form `{}` of this character",
-                            normalized.chars().next().unwrap().escape_default()
-                        ),
-                        normalized,
-                        Applicability::MachineApplicable,
-                    );
+                    let ch = normalized.chars().next().unwrap().escape_default().to_string();
+                    sugg = Some(MoreThanOneCharSugg::NormalizedForm { span, ch, normalized });
                 }
+                let escaped_marks =
+                    rest.iter().map(|c| c.escape_default().to_string()).collect::<Vec<_>>();
+                note = Some(MoreThanOneCharNote::AllCombining {
+                    span,
+                    chr: format!("{first}"),
+                    len: escaped_marks.len(),
+                    escaped_marks: escaped_marks.join(""),
+                });
             } else {
                 let printable: Vec<char> = lit
                     .chars()
@@ -87,32 +68,18 @@ pub(crate) fn emit_unescape_error(
                     })
                     .collect();
 
-                if let [ch] = printable.as_slice() {
-                    has_help = true;
-
-                    handler.span_note(
+                if let &[ch] = printable.as_slice() {
+                    sugg =
+                        Some(MoreThanOneCharSugg::RemoveNonPrinting { span, ch: ch.to_string() });
+                    note = Some(MoreThanOneCharNote::NonPrinting {
                         span,
-                        &format!(
-                            "there are non-printing characters, the full sequence is `{}`",
-                            lit.escape_default(),
-                        ),
-                    );
-
-                    handler.span_suggestion(
-                        span,
-                        "consider removing the non-printing characters",
-                        ch,
-                        Applicability::MaybeIncorrect,
-                    );
+                        escaped: lit.escape_default().to_string(),
+                    });
                 }
-            }
-
-            if !has_help {
-                let (prefix, msg) = if mode.is_byte() {
-                    ("b", "if you meant to write a byte string literal, use double quotes")
-                } else {
-                    ("", "if you meant to write a `str` literal, use double quotes")
-                };
+            };
+            let sugg = sugg.unwrap_or_else(|| {
+                let is_byte = mode.is_byte();
+                let prefix = if is_byte { "b" } else { "" };
                 let mut escaped = String::with_capacity(lit.len());
                 let mut chrs = lit.chars().peekable();
                 while let Some(first) = chrs.next() {
@@ -129,54 +96,32 @@ pub(crate) fn emit_unescape_error(
                         (c, _) => escaped.push(c),
                     };
                 }
-                handler.span_suggestion(
-                    span_with_quotes,
-                    msg,
-                    format!("{prefix}\"{escaped}\""),
-                    Applicability::MachineApplicable,
-                );
-            }
-
-            handler.emit();
+                let sugg = format!("{prefix}\"{escaped}\"");
+                MoreThanOneCharSugg::Quotes { span: span_with_quotes, is_byte, sugg }
+            });
+            handler.emit_err(UnescapeError::MoreThanOneChar {
+                span: span_with_quotes,
+                note,
+                suggestion: sugg,
+            });
         }
         EscapeError::EscapeOnlyChar => {
             let (c, char_span) = last_char();
-
-            let msg = if mode.is_byte() {
-                "byte constant must be escaped"
-            } else {
-                "character constant must be escaped"
-            };
-            handler
-                .struct_span_err(span, &format!("{}: `{}`", msg, escaped_char(c)))
-                .span_suggestion(
-                    char_span,
-                    "escape the character",
-                    c.escape_default(),
-                    Applicability::MachineApplicable,
-                )
-                .emit();
+            handler.emit_err(UnescapeError::EscapeOnlyChar {
+                span,
+                char_span,
+                escaped_sugg: c.escape_default().to_string(),
+                escaped_msg: escaped_char(c),
+                byte: mode.is_byte(),
+            });
         }
         EscapeError::BareCarriageReturn => {
-            let msg = if mode.in_double_quotes() {
-                "bare CR not allowed in string, use `\\r` instead"
-            } else {
-                "character constant must be escaped: `\\r`"
-            };
-            handler
-                .struct_span_err(span, msg)
-                .span_suggestion(
-                    span,
-                    "escape the character",
-                    "\\r",
-                    Applicability::MachineApplicable,
-                )
-                .emit();
+            let double_quotes = mode.in_double_quotes();
+            handler.emit_err(UnescapeError::BareCr { span, double_quotes });
         }
         EscapeError::BareCarriageReturnInRawString => {
             assert!(mode.in_double_quotes());
-            let msg = "bare CR not allowed in raw string";
-            handler.span_err(span, msg);
+            handler.emit_err(UnescapeError::BareCrRawString(span));
         }
         EscapeError::InvalidEscape => {
             let (c, span) = last_char();
@@ -213,22 +158,13 @@ pub(crate) fn emit_unescape_error(
             diag.emit();
         }
         EscapeError::TooShortHexEscape => {
-            handler.span_err(span, "numeric character escape is too short");
+            handler.emit_err(UnescapeError::TooShortHexEscape(span));
         }
         EscapeError::InvalidCharInHexEscape | EscapeError::InvalidCharInUnicodeEscape => {
             let (c, span) = last_char();
-
-            let msg = if error == EscapeError::InvalidCharInHexEscape {
-                "invalid character in numeric character escape"
-            } else {
-                "invalid character in unicode escape"
-            };
-            let c = escaped_char(c);
-
-            handler
-                .struct_span_err(span, &format!("{}: `{}`", msg, c))
-                .span_label(span, msg)
-                .emit();
+            let is_hex = error == EscapeError::InvalidCharInHexEscape;
+            let ch = escaped_char(c);
+            handler.emit_err(UnescapeError::InvalidCharInEscape { span, is_hex, ch });
         }
         EscapeError::NonAsciiCharInByte => {
             let (c, span) = last_char();
@@ -278,41 +214,22 @@ pub(crate) fn emit_unescape_error(
             err.emit();
         }
         EscapeError::OutOfRangeHexEscape => {
-            handler
-                .struct_span_err(span, "out of range hex escape")
-                .span_label(span, "must be a character in the range [\\x00-\\x7f]")
-                .emit();
+            handler.emit_err(UnescapeError::OutOfRangeHexEscape(span));
         }
         EscapeError::LeadingUnderscoreUnicodeEscape => {
             let (c, span) = last_char();
-            let msg = "invalid start of unicode escape";
-            handler
-                .struct_span_err(span, &format!("{}: `{}`", msg, c))
-                .span_label(span, msg)
-                .emit();
+            handler.emit_err(UnescapeError::LeadingUnderscoreUnicodeEscape {
+                span,
+                ch: escaped_char(c),
+            });
         }
         EscapeError::OverlongUnicodeEscape => {
-            handler
-                .struct_span_err(span, "overlong unicode escape")
-                .span_label(span, "must have at most 6 hex digits")
-                .emit();
+            handler.emit_err(UnescapeError::OverlongUnicodeEscape(span));
         }
         EscapeError::UnclosedUnicodeEscape => {
-            handler
-                .struct_span_err(span, "unterminated unicode escape")
-                .span_label(span, "missing a closing `}`")
-                .span_suggestion_verbose(
-                    span.shrink_to_hi(),
-                    "terminate the unicode escape",
-                    "}",
-                    Applicability::MaybeIncorrect,
-                )
-                .emit();
+            handler.emit_err(UnescapeError::UnclosedUnicodeEscape(span, span.shrink_to_hi()));
         }
         EscapeError::NoBraceInUnicodeEscape => {
-            let msg = "incorrect unicode escape sequence";
-            let mut diag = handler.struct_span_err(span, msg);
-
             let mut suggestion = "\\u{".to_owned();
             let mut suggestion_len = 0;
             let (c, char_span) = last_char();
@@ -322,54 +239,37 @@ pub(crate) fn emit_unescape_error(
                 suggestion_len += c.len_utf8();
             }
 
-            if suggestion_len > 0 {
+            let (label, sub) = if suggestion_len > 0 {
                 suggestion.push('}');
                 let hi = char_span.lo() + BytePos(suggestion_len as u32);
-                diag.span_suggestion(
-                    span.with_hi(hi),
-                    "format of unicode escape sequences uses braces",
-                    suggestion,
-                    Applicability::MaybeIncorrect,
-                );
+                (None, NoBraceUnicodeSub::Suggestion { span: span.with_hi(hi), suggestion })
             } else {
-                diag.span_label(span, msg);
-                diag.help("format of unicode escape sequences is `\\u{...}`");
-            }
-
-            diag.emit();
+                (Some(span), NoBraceUnicodeSub::Help)
+            };
+            handler.emit_err(UnescapeError::NoBraceInUnicodeEscape { span, label, sub });
         }
         EscapeError::UnicodeEscapeInByte => {
-            let msg = "unicode escape in byte string";
-            handler
-                .struct_span_err(span, msg)
-                .span_label(span, msg)
-                .help("unicode escape sequences cannot be used as a byte or in a byte string")
-                .emit();
+            handler.emit_err(UnescapeError::UnicodeEscapeInByte(span));
         }
         EscapeError::EmptyUnicodeEscape => {
-            handler
-                .struct_span_err(span, "empty unicode escape")
-                .span_label(span, "this escape must have at least 1 hex digit")
-                .emit();
+            handler.emit_err(UnescapeError::EmptyUnicodeEscape(span));
         }
         EscapeError::ZeroChars => {
-            let msg = "empty character literal";
-            handler.struct_span_err(span, msg).span_label(span, msg).emit();
+            handler.emit_err(UnescapeError::ZeroChars(span));
         }
         EscapeError::LoneSlash => {
-            let msg = "invalid trailing slash in literal";
-            handler.struct_span_err(span, msg).span_label(span, msg).emit();
+            handler.emit_err(UnescapeError::LoneSlash(span));
         }
         EscapeError::UnskippedWhitespaceWarning => {
             let (c, char_span) = last_char();
-            let msg =
-                format!("non-ASCII whitespace symbol '{}' is not skipped", c.escape_unicode());
-            handler.struct_span_warn(span, &msg).span_label(char_span, &msg).emit();
+            handler.emit_warning(UnescapeError::UnskippedWhitespace {
+                span,
+                ch: escaped_char(c),
+                char_span,
+            });
         }
         EscapeError::MultipleSkippedLinesWarning => {
-            let msg = "multiple lines skipped by escaped newline";
-            let bottom_msg = "skipping everything up to and including this point";
-            handler.struct_span_warn(span, msg).span_label(span, bottom_msg).emit();
+            handler.emit_warning(UnescapeError::MultipleSkippedLinesWarning(span));
         }
     }
 }
diff --git a/compiler/rustc_parse/src/lexer/unicode_chars.rs b/compiler/rustc_parse/src/lexer/unicode_chars.rs
index 34d003ccf..d4f971d5b 100644
--- a/compiler/rustc_parse/src/lexer/unicode_chars.rs
+++ b/compiler/rustc_parse/src/lexer/unicode_chars.rs
@@ -2,8 +2,10 @@
 //! <https://www.unicode.org/Public/security/10.0.0/confusables.txt>
 
 use super::StringReader;
-use crate::token::{self, Delimiter};
-use rustc_errors::{Applicability, Diagnostic};
+use crate::{
+    errors::TokenSubstitution,
+    token::{self, Delimiter},
+};
 use rustc_span::{symbol::kw, BytePos, Pos, Span};
 
 #[rustfmt::skip] // for line breaks
@@ -338,48 +340,44 @@ pub(super) fn check_for_substitution<'a>(
     reader: &StringReader<'a>,
     pos: BytePos,
     ch: char,
-    err: &mut Diagnostic,
     count: usize,
-) -> Option<token::TokenKind> {
-    let &(_, u_name, ascii_str) = UNICODE_ARRAY.iter().find(|&&(c, _, _)| c == ch)?;
+) -> (Option<token::TokenKind>, Option<TokenSubstitution>) {
+    let Some(&(_, u_name, ascii_str)) = UNICODE_ARRAY.iter().find(|&&(c, _, _)| c == ch) else {
+        return (None, None);
+    };
 
     let span = Span::with_root_ctxt(pos, pos + Pos::from_usize(ch.len_utf8() * count));
 
     let Some((_, ascii_name, token)) = ASCII_ARRAY.iter().find(|&&(s, _, _)| s == ascii_str) else {
         let msg = format!("substitution character not found for '{}'", ch);
         reader.sess.span_diagnostic.span_bug_no_panic(span, &msg);
-        return None;
+        return (None, None);
     };
 
     // special help suggestion for "directed" double quotes
-    if let Some(s) = peek_delimited(&reader.src[reader.src_index(pos)..], '“', '”') {
-        let msg = format!(
-            "Unicode characters '“' (Left Double Quotation Mark) and \
-             '”' (Right Double Quotation Mark) look like '{}' ({}), but are not",
-            ascii_str, ascii_name
-        );
-        err.span_suggestion(
-            Span::with_root_ctxt(
-                pos,
-                pos + Pos::from_usize('“'.len_utf8() + s.len() + '”'.len_utf8()),
-            ),
-            &msg,
-            format!("\"{}\"", s),
-            Applicability::MaybeIncorrect,
+    let sugg = if let Some(s) = peek_delimited(&reader.src[reader.src_index(pos)..], '“', '”') {
+        let span = Span::with_root_ctxt(
+            pos,
+            pos + Pos::from_usize('“'.len_utf8() + s.len() + '”'.len_utf8()),
         );
+        Some(TokenSubstitution::DirectedQuotes {
+            span,
+            suggestion: format!("\"{s}\""),
+            ascii_str,
+            ascii_name,
+        })
     } else {
-        let msg = format!(
-            "Unicode character '{}' ({}) looks like '{}' ({}), but it is not",
-            ch, u_name, ascii_str, ascii_name
-        );
-        err.span_suggestion(
+        let suggestion = ascii_str.to_string().repeat(count);
+        Some(TokenSubstitution::Other {
             span,
-            &msg,
-            ascii_str.to_string().repeat(count),
-            Applicability::MaybeIncorrect,
-        );
-    }
-    token.clone()
+            suggestion,
+            ch: ch.to_string(),
+            u_name,
+            ascii_str,
+            ascii_name,
+        })
+    };
+    (token.clone(), sugg)
 }
 
 /// Extract string if found at current position with given delimiters
diff --git a/compiler/rustc_parse/src/lib.rs b/compiler/rustc_parse/src/lib.rs
index b49a01d75..d1c3fd0cd 100644
--- a/compiler/rustc_parse/src/lib.rs
+++ b/compiler/rustc_parse/src/lib.rs
@@ -19,6 +19,8 @@ use rustc_ast::{AttrItem, Attribute, MetaItem};
 use rustc_ast_pretty::pprust;
 use rustc_data_structures::sync::Lrc;
 use rustc_errors::{Applicability, Diagnostic, FatalError, Level, PResult};
+use rustc_errors::{DiagnosticMessage, SubdiagnosticMessage};
+use rustc_macros::fluent_messages;
 use rustc_session::parse::ParseSess;
 use rustc_span::{FileName, SourceFile, Span};
 
@@ -28,12 +30,14 @@ pub const MACRO_ARGUMENTS: Option<&str> = Some("macro arguments");
 
 #[macro_use]
 pub mod parser;
-use parser::{emit_unclosed_delims, make_unclosed_delims_error, Parser};
+use parser::{make_unclosed_delims_error, Parser};
 pub mod lexer;
 pub mod validate_attr;
 
 mod errors;
 
+fluent_messages! { "../locales/en-US.ftl" }
+
 // A bunch of utility functions of the form `parse_<thing>_from_<source>`
 // where <thing> includes crate, expr, item, stmt, tts, and one that
 // uses a HOF to parse anything, and <source> includes file and
@@ -92,10 +96,7 @@ pub fn parse_stream_from_source_str(
     sess: &ParseSess,
     override_span: Option<Span>,
 ) -> TokenStream {
-    let (stream, mut errors) =
-        source_file_to_stream(sess, sess.source_map().new_source_file(name, source), override_span);
-    emit_unclosed_delims(&mut errors, &sess);
-    stream
+    source_file_to_stream(sess, sess.source_map().new_source_file(name, source), override_span)
 }
 
 /// Creates a new parser from a source string.
@@ -131,9 +132,8 @@ fn maybe_source_file_to_parser(
     source_file: Lrc<SourceFile>,
 ) -> Result<Parser<'_>, Vec<Diagnostic>> {
     let end_pos = source_file.end_pos;
-    let (stream, unclosed_delims) = maybe_file_to_stream(sess, source_file, None)?;
+    let stream = maybe_file_to_stream(sess, source_file, None)?;
     let mut parser = stream_to_parser(sess, stream, None);
-    parser.unclosed_delims = unclosed_delims;
     if parser.token == token::Eof {
         parser.token.span = Span::new(end_pos, end_pos, parser.token.span.ctxt(), None);
     }
@@ -178,7 +178,7 @@ pub fn source_file_to_stream(
     sess: &ParseSess,
     source_file: Lrc<SourceFile>,
     override_span: Option<Span>,
-) -> (TokenStream, Vec<lexer::UnmatchedBrace>) {
+) -> TokenStream {
     panictry_buffer!(&sess.span_diagnostic, maybe_file_to_stream(sess, source_file, override_span))
 }
 
@@ -188,7 +188,7 @@ pub fn maybe_file_to_stream(
     sess: &ParseSess,
     source_file: Lrc<SourceFile>,
     override_span: Option<Span>,
-) -> Result<(TokenStream, Vec<lexer::UnmatchedBrace>), Vec<Diagnostic>> {
+) -> Result<TokenStream, Vec<Diagnostic>> {
     let src = source_file.src.as_ref().unwrap_or_else(|| {
         sess.span_diagnostic.bug(&format!(
             "cannot lex `source_file` without source: {}",
@@ -196,23 +196,7 @@ pub fn maybe_file_to_stream(
         ));
     });
 
-    let (token_trees, unmatched_braces) =
-        lexer::parse_token_trees(sess, src.as_str(), source_file.start_pos, override_span);
-
-    match token_trees {
-        Ok(stream) => Ok((stream, unmatched_braces)),
-        Err(err) => {
-            let mut buffer = Vec::with_capacity(1);
-            err.buffer(&mut buffer);
-            // Not using `emit_unclosed_delims` to use `db.buffer`
-            for unmatched in unmatched_braces {
-                if let Some(err) = make_unclosed_delims_error(unmatched, &sess) {
-                    err.buffer(&mut buffer);
-                }
-            }
-            Err(buffer)
-        }
-    }
+    lexer::parse_token_trees(sess, src.as_str(), source_file.start_pos, override_span)
 }
 
 /// Given a stream and the `ParseSess`, produces a parser.
diff --git a/compiler/rustc_parse/src/parser/attr.rs b/compiler/rustc_parse/src/parser/attr.rs
index 686454a8f..e3e7c63e3 100644
--- a/compiler/rustc_parse/src/parser/attr.rs
+++ b/compiler/rustc_parse/src/parser/attr.rs
@@ -1,11 +1,15 @@
 use crate::errors::{InvalidMetaItem, SuffixedLiteralInAttribute};
+use crate::fluent_generated as fluent;
 
 use super::{AttrWrapper, Capturing, FnParseMode, ForceCollect, Parser, PathStyle};
 use rustc_ast as ast;
 use rustc_ast::attr;
 use rustc_ast::token::{self, Delimiter, Nonterminal};
-use rustc_errors::{error_code, fluent, Diagnostic, IntoDiagnostic, PResult};
+use rustc_errors::{error_code, Diagnostic, IntoDiagnostic, PResult};
 use rustc_span::{sym, BytePos, Span};
+use std::convert::TryInto;
+use thin_vec::ThinVec;
+use tracing::debug;
 
 // Public for rustfmt usage
 #[derive(Debug)]
@@ -65,10 +69,10 @@ impl<'a> Parser<'a> {
                             token::CommentKind::Block => OuterAttributeType::DocBlockComment,
                         },
                     ) {
-                        err.note(fluent::note);
+                        err.note(fluent::parse_note);
                         err.span_suggestion_verbose(
                             replacement_span,
-                            fluent::suggestion,
+                            fluent::parse_suggestion,
                             "",
                             rustc_errors::Applicability::MachineApplicable,
                         );
@@ -172,10 +176,10 @@ impl<'a> Parser<'a> {
             Ok(Some(item)) => {
                 // FIXME(#100717)
                 err.set_arg("item", item.kind.descr());
-                err.span_label(item.span, fluent::label_does_not_annotate_this);
+                err.span_label(item.span, fluent::parse_label_does_not_annotate_this);
                 err.span_suggestion_verbose(
                     replacement_span,
-                    fluent::sugg_change_inner_to_outer,
+                    fluent::parse_sugg_change_inner_to_outer,
                     match attr_type {
                         OuterAttributeType::Attribute => "",
                         OuterAttributeType::DocBlockComment => "*",
@@ -201,8 +205,8 @@ impl<'a> Parser<'a> {
                         attr_sp,
                         fluent::parse_inner_attr_not_permitted_after_outer_doc_comment,
                     );
-                    diag.span_label(attr_sp, fluent::label_attr)
-                        .span_label(prev_doc_comment_span, fluent::label_prev_doc_comment);
+                    diag.span_label(attr_sp, fluent::parse_label_attr)
+                        .span_label(prev_doc_comment_span, fluent::parse_label_prev_doc_comment);
                     diag
                 }
                 Some(InnerAttrForbiddenReason::AfterOuterAttribute { prev_outer_attr_sp }) => {
@@ -210,8 +214,8 @@ impl<'a> Parser<'a> {
                         attr_sp,
                         fluent::parse_inner_attr_not_permitted_after_outer_attr,
                     );
-                    diag.span_label(attr_sp, fluent::label_attr)
-                        .span_label(prev_outer_attr_sp, fluent::label_prev_attr);
+                    diag.span_label(attr_sp, fluent::parse_label_attr)
+                        .span_label(prev_outer_attr_sp, fluent::parse_label_prev_attr);
                     diag
                 }
                 Some(InnerAttrForbiddenReason::InCodeBlock) | None => {
@@ -346,9 +350,9 @@ impl<'a> Parser<'a> {
     }
 
     /// Matches `COMMASEP(meta_item_inner)`.
-    pub(crate) fn parse_meta_seq_top(&mut self) -> PResult<'a, Vec<ast::NestedMetaItem>> {
+    pub(crate) fn parse_meta_seq_top(&mut self) -> PResult<'a, ThinVec<ast::NestedMetaItem>> {
         // Presumably, the majority of the time there will only be one attr.
-        let mut nmis = Vec::with_capacity(1);
+        let mut nmis = ThinVec::with_capacity(1);
         while self.token.kind != token::Eof {
             nmis.push(self.parse_meta_item_inner()?);
             if !self.eat(&token::Comma) {
diff --git a/compiler/rustc_parse/src/parser/attr_wrapper.rs b/compiler/rustc_parse/src/parser/attr_wrapper.rs
index b97f22417..b0ab0f106 100644
--- a/compiler/rustc_parse/src/parser/attr_wrapper.rs
+++ b/compiler/rustc_parse/src/parser/attr_wrapper.rs
@@ -134,11 +134,11 @@ impl ToAttrTokenStream for LazyAttrTokenStreamImpl {
             // Process the replace ranges, starting from the highest start
             // position and working our way back. If have tokens like:
             //
-            // `#[cfg(FALSE)]` struct Foo { #[cfg(FALSE)] field: bool }`
+            // `#[cfg(FALSE)] struct Foo { #[cfg(FALSE)] field: bool }`
             //
             // Then we will generate replace ranges for both
             // the `#[cfg(FALSE)] field: bool` and the entire
-            // `#[cfg(FALSE)]` struct Foo { #[cfg(FALSE)] field: bool }`
+            // `#[cfg(FALSE)] struct Foo { #[cfg(FALSE)] field: bool }`
             //
             // By starting processing from the replace range with the greatest
             // start position, we ensure that any replace range which encloses
@@ -469,6 +469,6 @@ mod size_asserts {
     use rustc_data_structures::static_assert_size;
     // tidy-alphabetical-start
     static_assert_size!(AttrWrapper, 16);
-    static_assert_size!(LazyAttrTokenStreamImpl, 144);
+    static_assert_size!(LazyAttrTokenStreamImpl, 120);
     // tidy-alphabetical-end
 }
diff --git a/compiler/rustc_parse/src/parser/diagnostics.rs b/compiler/rustc_parse/src/parser/diagnostics.rs
index 4c918c670..a051dbe9f 100644
--- a/compiler/rustc_parse/src/parser/diagnostics.rs
+++ b/compiler/rustc_parse/src/parser/diagnostics.rs
@@ -18,7 +18,8 @@ use crate::errors::{
     UseEqInstead,
 };
 
-use crate::lexer::UnmatchedBrace;
+use crate::fluent_generated as fluent;
+use crate::lexer::UnmatchedDelim;
 use crate::parser;
 use rustc_ast as ast;
 use rustc_ast::ptr::P;
@@ -32,10 +33,9 @@ use rustc_ast::{
 use rustc_ast_pretty::pprust;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_errors::{
-    fluent, Applicability, DiagnosticBuilder, DiagnosticMessage, FatalError, Handler, MultiSpan,
-    PResult,
+    pluralize, Applicability, Diagnostic, DiagnosticBuilder, DiagnosticMessage, ErrorGuaranteed,
+    FatalError, Handler, IntoDiagnostic, MultiSpan, PResult,
 };
-use rustc_errors::{pluralize, Diagnostic, ErrorGuaranteed, IntoDiagnostic};
 use rustc_session::errors::ExprParenthesesNeeded;
 use rustc_span::source_map::Spanned;
 use rustc_span::symbol::{kw, sym, Ident};
@@ -165,8 +165,6 @@ enum IsStandalone {
 #[derive(Debug, Copy, Clone, PartialEq, Eq)]
 enum IncOrDec {
     Inc,
-    // FIXME: `i--` recovery isn't implemented yet
-    #[allow(dead_code)]
     Dec,
 }
 
@@ -222,7 +220,7 @@ impl MultiSugg {
 /// is dropped.
 pub struct SnapshotParser<'a> {
     parser: Parser<'a>,
-    unclosed_delims: Vec<UnmatchedBrace>,
+    unclosed_delims: Vec<UnmatchedDelim>,
 }
 
 impl<'a> Deref for SnapshotParser<'a> {
@@ -264,7 +262,7 @@ impl<'a> Parser<'a> {
         self.unclosed_delims.extend(snapshot.unclosed_delims);
     }
 
-    pub fn unclosed_delims(&self) -> &[UnmatchedBrace] {
+    pub fn unclosed_delims(&self) -> &[UnmatchedDelim] {
         &self.unclosed_delims
     }
 
@@ -284,7 +282,7 @@ impl<'a> Parser<'a> {
         self.sess.source_map().span_to_snippet(span)
     }
 
-    pub(super) fn expected_ident_found(&self) -> DiagnosticBuilder<'a, ErrorGuaranteed> {
+    pub(super) fn expected_ident_found(&mut self) -> DiagnosticBuilder<'a, ErrorGuaranteed> {
         let valid_follow = &[
             TokenKind::Eq,
             TokenKind::Colon,
@@ -324,7 +322,61 @@ impl<'a> Parser<'a> {
             suggest_raw,
             suggest_remove_comma,
         };
-        err.into_diagnostic(&self.sess.span_diagnostic)
+        let mut err = err.into_diagnostic(&self.sess.span_diagnostic);
+
+        // if the token we have is a `<`
+        // it *might* be a misplaced generic
+        if self.token == token::Lt {
+            // all keywords that could have generic applied
+            let valid_prev_keywords =
+                [kw::Fn, kw::Type, kw::Struct, kw::Enum, kw::Union, kw::Trait];
+
+            // If we've expected an identifier,
+            // and the current token is a '<'
+            // if the previous token is a valid keyword
+            // that might use a generic, then suggest a correct
+            // generic placement (later on)
+            let maybe_keyword = self.prev_token.clone();
+            if valid_prev_keywords.into_iter().any(|x| maybe_keyword.is_keyword(x)) {
+                // if we have a valid keyword, attempt to parse generics
+                // also obtain the keywords symbol
+                match self.parse_generics() {
+                    Ok(generic) => {
+                        if let TokenKind::Ident(symbol, _) = maybe_keyword.kind {
+                            let ident_name = symbol;
+                            // at this point, we've found something like
+                            // `fn <T>id`
+                            // and current token should be Ident with the item name (i.e. the function name)
+                            // if there is a `<` after the fn name, then don't show a suggestion, show help
+
+                            if !self.look_ahead(1, |t| *t == token::Lt) &&
+                                let Ok(snippet) = self.sess.source_map().span_to_snippet(generic.span) {
+                                    err.multipart_suggestion_verbose(
+                                        format!("place the generic parameter name after the {ident_name} name"),
+                                        vec![
+                                            (self.token.span.shrink_to_hi(), snippet),
+                                            (generic.span, String::new())
+                                        ],
+                                        Applicability::MaybeIncorrect,
+                                    );
+                                } else {
+                                    err.help(format!(
+                                        "place the generic parameter name after the {ident_name} name"
+                                    ));
+                                }
+                        }
+                    }
+                    Err(err) => {
+                        // if there's an error parsing the generics,
+                        // then don't do a misplaced generics suggestion
+                        // and emit the expected ident error instead;
+                        err.cancel();
+                    }
+                }
+            }
+        }
+
+        err
     }
 
     pub(super) fn expected_one_of_not_found(
@@ -639,7 +691,7 @@ impl<'a> Parser<'a> {
                 span: self.prev_token.span.shrink_to_lo(),
                 tokens: None,
             };
-            let struct_expr = snapshot.parse_struct_expr(None, path, false);
+            let struct_expr = snapshot.parse_expr_struct(None, path, false);
             let block_tail = self.parse_block_tail(lo, s, AttemptLocalParseRecovery::No);
             return Some(match (struct_expr, block_tail) {
                 (Ok(expr), Err(mut err)) => {
@@ -654,7 +706,7 @@ impl<'a> Parser<'a> {
                     err.delay_as_bug();
                     self.restore_snapshot(snapshot);
                     let mut tail = self.mk_block(
-                        vec![self.mk_stmt_err(expr.span)],
+                        thin_vec![self.mk_stmt_err(expr.span)],
                         s,
                         lo.to(self.prev_token.span),
                     );
@@ -1303,6 +1355,20 @@ impl<'a> Parser<'a> {
         self.recover_from_inc_dec(operand_expr, kind, op_span)
     }
 
+    pub(super) fn recover_from_postfix_decrement(
+        &mut self,
+        operand_expr: P<Expr>,
+        op_span: Span,
+        start_stmt: bool,
+    ) -> PResult<'a, P<Expr>> {
+        let kind = IncDecRecovery {
+            standalone: if start_stmt { IsStandalone::Standalone } else { IsStandalone::Subexpr },
+            op: IncOrDec::Dec,
+            fixity: UnaryFixity::Post,
+        };
+        self.recover_from_inc_dec(operand_expr, kind, op_span)
+    }
+
     fn recover_from_inc_dec(
         &mut self,
         base: P<Expr>,
@@ -1570,7 +1636,7 @@ impl<'a> Parser<'a> {
             // Handle `await { <expr> }`.
             // This needs to be handled separately from the next arm to avoid
             // interpreting `await { <expr> }?` as `<expr>?.await`.
-            self.parse_block_expr(None, self.token.span, BlockCheckMode::Default)
+            self.parse_expr_block(None, self.token.span, BlockCheckMode::Default)
         } else {
             self.parse_expr()
         }
@@ -2030,7 +2096,7 @@ impl<'a> Parser<'a> {
     }
 
     pub(super) fn recover_arg_parse(&mut self) -> PResult<'a, (P<ast::Pat>, P<ast::Ty>)> {
-        let pat = self.parse_pat_no_top_alt(Some("argument name"))?;
+        let pat = self.parse_pat_no_top_alt(Some(Expected::ArgumentName))?;
         self.expect(&token::Colon)?;
         let ty = self.parse_ty()?;
 
@@ -2121,7 +2187,7 @@ impl<'a> Parser<'a> {
     /// the parameters are *names* (so we don't emit errors about not being able to find `b` in
     /// the local scope), but if we find the same name multiple times, like in `fn foo(i8, i8)`,
     /// we deduplicate them to not complain about duplicated parameter names.
-    pub(super) fn deduplicate_recovered_params_names(&self, fn_inputs: &mut Vec<Param>) {
+    pub(super) fn deduplicate_recovered_params_names(&self, fn_inputs: &mut ThinVec<Param>) {
         let mut seen_inputs = FxHashSet::default();
         for input in fn_inputs.iter_mut() {
             let opt_ident = if let (PatKind::Ident(_, ident, _), TyKind::Err) =
@@ -2145,7 +2211,7 @@ impl<'a> Parser<'a> {
     /// like the user has forgotten them.
     pub fn handle_ambiguous_unbraced_const_arg(
         &mut self,
-        args: &mut Vec<AngleBracketedArg>,
+        args: &mut ThinVec<AngleBracketedArg>,
     ) -> PResult<'a, bool> {
         // If we haven't encountered a closing `>`, then the argument is malformed.
         // It's likely that the user has written a const expression without enclosing it
@@ -2353,6 +2419,28 @@ impl<'a> Parser<'a> {
         Err(err)
     }
 
+    /// Try to recover from an unbraced const argument whose first token [could begin a type][ty].
+    ///
+    /// [ty]: token::Token::can_begin_type
+    pub(crate) fn recover_unbraced_const_arg_that_can_begin_ty(
+        &mut self,
+        mut snapshot: SnapshotParser<'a>,
+    ) -> Option<P<ast::Expr>> {
+        match snapshot.parse_expr_res(Restrictions::CONST_EXPR, None) {
+            // Since we don't know the exact reason why we failed to parse the type or the
+            // expression, employ a simple heuristic to weed out some pathological cases.
+            Ok(expr) if let token::Comma | token::Gt = snapshot.token.kind => {
+                self.restore_snapshot(snapshot);
+                Some(expr)
+            }
+            Ok(_) => None,
+            Err(err) => {
+                err.cancel();
+                None
+            }
+        }
+    }
+
     /// Creates a dummy const argument, and reports that the expression must be enclosed in braces
     pub fn dummy_const_arg_needs_braces(
         &self,
@@ -2375,7 +2463,7 @@ impl<'a> Parser<'a> {
     pub(crate) fn maybe_recover_colon_colon_in_pat_typo(
         &mut self,
         mut first_pat: P<Pat>,
-        expected: Expected,
+        expected: Option<Expected>,
     ) -> P<Pat> {
         if token::Colon != self.token.kind {
             return first_pat;
@@ -2383,26 +2471,42 @@ impl<'a> Parser<'a> {
         if !matches!(first_pat.kind, PatKind::Ident(_, _, None) | PatKind::Path(..))
             || !self.look_ahead(1, |token| token.is_ident() && !token.is_reserved_ident())
         {
+            let mut snapshot_type = self.create_snapshot_for_diagnostic();
+            snapshot_type.bump(); // `:`
+            match snapshot_type.parse_ty() {
+                Err(inner_err) => {
+                    inner_err.cancel();
+                }
+                Ok(ty) => {
+                    let Err(mut err) = self.expected_one_of_not_found(&[], &[]) else {
+                        return first_pat;
+                    };
+                    err.span_label(ty.span, "specifying the type of a pattern isn't supported");
+                    self.restore_snapshot(snapshot_type);
+                    let span = first_pat.span.to(ty.span);
+                    first_pat = self.mk_pat(span, PatKind::Wild);
+                    err.emit();
+                }
+            }
             return first_pat;
         }
         // The pattern looks like it might be a path with a `::` -> `:` typo:
         // `match foo { bar:baz => {} }`
-        let span = self.token.span;
+        let colon_span = self.token.span;
         // We only emit "unexpected `:`" error here if we can successfully parse the
         // whole pattern correctly in that case.
-        let snapshot = self.create_snapshot_for_diagnostic();
+        let mut snapshot_pat = self.create_snapshot_for_diagnostic();
+        let mut snapshot_type = self.create_snapshot_for_diagnostic();
 
         // Create error for "unexpected `:`".
         match self.expected_one_of_not_found(&[], &[]) {
             Err(mut err) => {
-                self.bump(); // Skip the `:`.
-                match self.parse_pat_no_top_alt(expected) {
+                // Skip the `:`.
+                snapshot_pat.bump();
+                snapshot_type.bump();
+                match snapshot_pat.parse_pat_no_top_alt(expected) {
                     Err(inner_err) => {
-                        // Carry on as if we had not done anything, callers will emit a
-                        // reasonable error.
                         inner_err.cancel();
-                        err.cancel();
-                        self.restore_snapshot(snapshot);
                     }
                     Ok(mut pat) => {
                         // We've parsed the rest of the pattern.
@@ -2466,8 +2570,8 @@ impl<'a> Parser<'a> {
                             _ => {}
                         }
                         if show_sugg {
-                            err.span_suggestion(
-                                span,
+                            err.span_suggestion_verbose(
+                                colon_span.until(self.look_ahead(1, |t| t.span)),
                                 "maybe write a path separator here",
                                 "::",
                                 Applicability::MaybeIncorrect,
@@ -2475,13 +2579,24 @@ impl<'a> Parser<'a> {
                         } else {
                             first_pat = self.mk_pat(new_span, PatKind::Wild);
                         }
-                        err.emit();
+                        self.restore_snapshot(snapshot_pat);
+                    }
+                }
+                match snapshot_type.parse_ty() {
+                    Err(inner_err) => {
+                        inner_err.cancel();
+                    }
+                    Ok(ty) => {
+                        err.span_label(ty.span, "specifying the type of a pattern isn't supported");
+                        self.restore_snapshot(snapshot_type);
+                        let new_span = first_pat.span.to(ty.span);
+                        first_pat = self.mk_pat(new_span, PatKind::Wild);
                     }
                 }
+                err.emit();
             }
             _ => {
                 // Carry on as if we had not done anything. This should be unreachable.
-                self.restore_snapshot(snapshot);
             }
         };
         first_pat
diff --git a/compiler/rustc_parse/src/parser/expr.rs b/compiler/rustc_parse/src/parser/expr.rs
index bf93a89f0..95a7ca80d 100644
--- a/compiler/rustc_parse/src/parser/expr.rs
+++ b/compiler/rustc_parse/src/parser/expr.rs
@@ -1,29 +1,12 @@
 use super::diagnostics::SnapshotParser;
-use super::pat::{CommaRecoveryMode, RecoverColon, RecoverComma, PARAM_EXPECTED};
+use super::pat::{CommaRecoveryMode, Expected, RecoverColon, RecoverComma};
 use super::ty::{AllowPlus, RecoverQPath, RecoverReturnSign};
 use super::{
     AttrWrapper, BlockMode, ClosureSpans, ForceCollect, Parser, PathStyle, Restrictions,
     SemiColonMode, SeqSep, TokenExpectType, TokenType, TrailingToken,
 };
-use crate::errors::{
-    ArrayBracketsInsteadOfSpaces, ArrayBracketsInsteadOfSpacesSugg, AsyncMoveOrderIncorrect,
-    BracesForStructLiteral, CatchAfterTry, CommaAfterBaseStruct, ComparisonInterpretedAsGeneric,
-    ComparisonOrShiftInterpretedAsGenericSugg, DoCatchSyntaxRemoved, DotDotDot, EqFieldInit,
-    ExpectedElseBlock, ExpectedEqForLetExpr, ExpectedExpressionFoundLet,
-    FieldExpressionWithGeneric, FloatLiteralRequiresIntegerPart, FoundExprWouldBeStmt,
-    IfExpressionMissingCondition, IfExpressionMissingThenBlock, IfExpressionMissingThenBlockSub,
-    InvalidBlockMacroSegment, InvalidComparisonOperator, InvalidComparisonOperatorSub,
-    InvalidInterpolatedExpression, InvalidLiteralSuffixOnTupleIndex, InvalidLogicalOperator,
-    InvalidLogicalOperatorSub, LabeledLoopInBreak, LeadingPlusNotSupported, LeftArrowOperator,
-    LifetimeInBorrowExpression, MacroInvocationWithQualifiedPath, MalformedLoopLabel,
-    MatchArmBodyWithoutBraces, MatchArmBodyWithoutBracesSugg, MissingCommaAfterMatchArm,
-    MissingDotDot, MissingInInForLoop, MissingInInForLoopSub, MissingSemicolonBeforeArray,
-    NoFieldsForFnCall, NotAsNegationOperator, NotAsNegationOperatorSub,
-    OuterAttributeNotAllowedOnIfElse, ParenthesesWithStructFields,
-    RequireColonAfterLabeledExpression, ShiftInterpretedAsGeneric, StructLiteralNotAllowedHere,
-    StructLiteralNotAllowedHereSugg, TildeAsUnaryOperator, UnexpectedIfWithIf,
-    UnexpectedTokenAfterLabel, UnexpectedTokenAfterLabelSugg, WrapExpressionInParentheses,
-};
+
+use crate::errors;
 use crate::maybe_recover_from_interpolated_ty_qpath;
 use core::mem;
 use rustc_ast::ptr::P;
@@ -39,8 +22,8 @@ use rustc_ast::{Arm, Async, BlockCheckMode, Expr, ExprKind, Label, Movability, R
 use rustc_ast::{ClosureBinder, MetaItemLit, StmtKind};
 use rustc_ast_pretty::pprust;
 use rustc_errors::{
-    Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed, IntoDiagnostic, PResult,
-    StashKey,
+    AddToDiagnostic, Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed, IntoDiagnostic,
+    PResult, StashKey,
 };
 use rustc_session::errors::{report_lit_error, ExprParenthesesNeeded};
 use rustc_session::lint::builtin::BREAK_WITH_LABEL_AND_LOOP;
@@ -48,6 +31,7 @@ use rustc_session::lint::BuiltinLintDiagnostics;
 use rustc_span::source_map::{self, Span, Spanned};
 use rustc_span::symbol::{kw, sym, Ident, Symbol};
 use rustc_span::{BytePos, Pos};
+use thin_vec::{thin_vec, ThinVec};
 
 /// Possibly accepts an `token::Interpolated` expression (a pre-parsed expression
 /// dropped into the token stream, which happens while parsing the result of
@@ -119,7 +103,7 @@ impl<'a> Parser<'a> {
         self.collect_tokens_no_attrs(|this| this.parse_expr())
     }
 
-    pub fn parse_anon_const_expr(&mut self) -> PResult<'a, AnonConst> {
+    pub fn parse_expr_anon_const(&mut self) -> PResult<'a, AnonConst> {
         self.parse_expr().map(|value| AnonConst { id: DUMMY_NODE_ID, value })
     }
 
@@ -141,7 +125,7 @@ impl<'a> Parser<'a> {
     }
 
     /// Parses a sequence of expressions delimited by parentheses.
-    fn parse_paren_expr_seq(&mut self) -> PResult<'a, Vec<P<Expr>>> {
+    fn parse_expr_paren_seq(&mut self) -> PResult<'a, ThinVec<P<Expr>>> {
         self.parse_paren_comma_seq(|p| p.parse_expr_catch_underscore()).map(|(r, _)| r)
     }
 
@@ -152,7 +136,7 @@ impl<'a> Parser<'a> {
         r: Restrictions,
         already_parsed_attrs: Option<AttrWrapper>,
     ) -> PResult<'a, P<Expr>> {
-        self.with_res(r, |this| this.parse_assoc_expr(already_parsed_attrs))
+        self.with_res(r, |this| this.parse_expr_assoc(already_parsed_attrs))
     }
 
     /// Parses an associative expression.
@@ -160,15 +144,15 @@ impl<'a> Parser<'a> {
     /// This parses an expression accounting for associativity and precedence of the operators in
     /// the expression.
     #[inline]
-    fn parse_assoc_expr(
+    fn parse_expr_assoc(
         &mut self,
         already_parsed_attrs: Option<AttrWrapper>,
     ) -> PResult<'a, P<Expr>> {
-        self.parse_assoc_expr_with(0, already_parsed_attrs.into())
+        self.parse_expr_assoc_with(0, already_parsed_attrs.into())
     }
 
     /// Parses an associative expression with operators of at least `min_prec` precedence.
-    pub(super) fn parse_assoc_expr_with(
+    pub(super) fn parse_expr_assoc_with(
         &mut self,
         min_prec: usize,
         lhs: LhsExpr,
@@ -183,9 +167,9 @@ impl<'a> Parser<'a> {
                 _ => None,
             };
             if self.token.is_range_separator() {
-                return self.parse_prefix_range_expr(attrs);
+                return self.parse_expr_prefix_range(attrs);
             } else {
-                self.parse_prefix_expr(attrs)?
+                self.parse_expr_prefix(attrs)?
             }
         };
         let last_type_ascription_set = self.last_type_ascription.is_some();
@@ -243,10 +227,10 @@ impl<'a> Parser<'a> {
                 }
                 .into();
                 let invalid = format!("{}=", &sugg);
-                self.sess.emit_err(InvalidComparisonOperator {
+                self.sess.emit_err(errors::InvalidComparisonOperator {
                     span: sp,
                     invalid: invalid.clone(),
-                    sub: InvalidComparisonOperatorSub::Correctable {
+                    sub: errors::InvalidComparisonOperatorSub::Correctable {
                         span: sp,
                         invalid,
                         correct: sugg,
@@ -261,10 +245,10 @@ impl<'a> Parser<'a> {
                 && self.prev_token.span.hi() == self.token.span.lo()
             {
                 let sp = op.span.to(self.token.span);
-                self.sess.emit_err(InvalidComparisonOperator {
+                self.sess.emit_err(errors::InvalidComparisonOperator {
                     span: sp,
                     invalid: "<>".into(),
-                    sub: InvalidComparisonOperatorSub::Correctable {
+                    sub: errors::InvalidComparisonOperatorSub::Correctable {
                         span: sp,
                         invalid: "<>".into(),
                         correct: "!=".into(),
@@ -279,10 +263,10 @@ impl<'a> Parser<'a> {
                 && self.prev_token.span.hi() == self.token.span.lo()
             {
                 let sp = op.span.to(self.token.span);
-                self.sess.emit_err(InvalidComparisonOperator {
+                self.sess.emit_err(errors::InvalidComparisonOperator {
                     span: sp,
                     invalid: "<=>".into(),
-                    sub: InvalidComparisonOperatorSub::Spaceship(sp),
+                    sub: errors::InvalidComparisonOperatorSub::Spaceship(sp),
                 });
                 self.bump();
             }
@@ -298,6 +282,18 @@ impl<'a> Parser<'a> {
                 continue;
             }
 
+            if self.prev_token == token::BinOp(token::Minus)
+                && self.token == token::BinOp(token::Minus)
+                && self.prev_token.span.between(self.token.span).is_empty()
+                && !self.look_ahead(1, |tok| tok.can_begin_expr())
+            {
+                let op_span = self.prev_token.span.to(self.token.span);
+                // Eat the second `-`
+                self.bump();
+                lhs = self.recover_from_postfix_decrement(lhs, op_span, starts_stmt)?;
+                continue;
+            }
+
             let op = op.node;
             // Special cases:
             if op == AssocOp::As {
@@ -309,7 +305,7 @@ impl<'a> Parser<'a> {
             } else if op == AssocOp::DotDot || op == AssocOp::DotDotEq {
                 // If we didn't have to handle `x..`/`x..=`, it would be pretty easy to
                 // generalise it to the Fixity::None code.
-                lhs = self.parse_range_expr(prec, lhs, op, cur_op_span)?;
+                lhs = self.parse_expr_range(prec, lhs, op, cur_op_span)?;
                 break;
             }
 
@@ -322,7 +318,7 @@ impl<'a> Parser<'a> {
                 Fixity::None => 1,
             };
             let rhs = self.with_res(restrictions - Restrictions::STMT_EXPR, |this| {
-                this.parse_assoc_expr_with(prec + prec_adjustment, LhsExpr::NotYetParsed)
+                this.parse_expr_assoc_with(prec + prec_adjustment, LhsExpr::NotYetParsed)
             })?;
 
             let span = self.mk_expr_sp(&lhs, lhs_span, rhs.span);
@@ -419,7 +415,7 @@ impl<'a> Parser<'a> {
     /// but the next token implies this should be parsed as an expression.
     /// For example: `if let Some(x) = x { x } else { 0 } / 2`.
     fn error_found_expr_would_be_stmt(&self, lhs: &Expr) {
-        self.sess.emit_err(FoundExprWouldBeStmt {
+        self.sess.emit_err(errors::FoundExprWouldBeStmt {
             span: self.token.span,
             token: self.token.clone(),
             suggestion: ExprParenthesesNeeded::surrounding(lhs.span),
@@ -446,18 +442,18 @@ impl<'a> Parser<'a> {
             }
             (Some(op), _) => (op, self.token.span),
             (None, Some((Ident { name: sym::and, span }, false))) if self.may_recover() => {
-                self.sess.emit_err(InvalidLogicalOperator {
+                self.sess.emit_err(errors::InvalidLogicalOperator {
                     span: self.token.span,
                     incorrect: "and".into(),
-                    sub: InvalidLogicalOperatorSub::Conjunction(self.token.span),
+                    sub: errors::InvalidLogicalOperatorSub::Conjunction(self.token.span),
                 });
                 (AssocOp::LAnd, span)
             }
             (None, Some((Ident { name: sym::or, span }, false))) if self.may_recover() => {
-                self.sess.emit_err(InvalidLogicalOperator {
+                self.sess.emit_err(errors::InvalidLogicalOperator {
                     span: self.token.span,
                     incorrect: "or".into(),
-                    sub: InvalidLogicalOperatorSub::Disjunction(self.token.span),
+                    sub: errors::InvalidLogicalOperatorSub::Disjunction(self.token.span),
                 });
                 (AssocOp::LOr, span)
             }
@@ -474,7 +470,7 @@ impl<'a> Parser<'a> {
 
     /// Parses `x..y`, `x..=y`, and `x..`/`x..=`.
     /// The other two variants are handled in `parse_prefix_range_expr` below.
-    fn parse_range_expr(
+    fn parse_expr_range(
         &mut self,
         prec: usize,
         lhs: P<Expr>,
@@ -482,7 +478,7 @@ impl<'a> Parser<'a> {
         cur_op_span: Span,
     ) -> PResult<'a, P<Expr>> {
         let rhs = if self.is_at_start_of_range_notation_rhs() {
-            Some(self.parse_assoc_expr_with(prec + 1, LhsExpr::NotYetParsed)?)
+            Some(self.parse_expr_assoc_with(prec + 1, LhsExpr::NotYetParsed)?)
         } else {
             None
         };
@@ -507,7 +503,7 @@ impl<'a> Parser<'a> {
     }
 
     /// Parses prefix-forms of range notation: `..expr`, `..`, `..=expr`.
-    fn parse_prefix_range_expr(&mut self, attrs: Option<AttrWrapper>) -> PResult<'a, P<Expr>> {
+    fn parse_expr_prefix_range(&mut self, attrs: Option<AttrWrapper>) -> PResult<'a, P<Expr>> {
         // Check for deprecated `...` syntax.
         if self.token == token::DotDotDot {
             self.err_dotdotdot_syntax(self.token.span);
@@ -534,7 +530,7 @@ impl<'a> Parser<'a> {
             this.bump();
             let (span, opt_end) = if this.is_at_start_of_range_notation_rhs() {
                 // RHS must be parsed with more associativity than the dots.
-                this.parse_assoc_expr_with(op.unwrap().precedence() + 1, LhsExpr::NotYetParsed)
+                this.parse_expr_assoc_with(op.unwrap().precedence() + 1, LhsExpr::NotYetParsed)
                     .map(|x| (lo.to(x.span), Some(x)))?
             } else {
                 (lo, None)
@@ -545,7 +541,7 @@ impl<'a> Parser<'a> {
     }
 
     /// Parses a prefix-unary-operator expr.
-    fn parse_prefix_expr(&mut self, attrs: Option<AttrWrapper>) -> PResult<'a, P<Expr>> {
+    fn parse_expr_prefix(&mut self, attrs: Option<AttrWrapper>) -> PResult<'a, P<Expr>> {
         let attrs = self.parse_or_use_outer_attributes(attrs)?;
         let lo = self.token.span;
 
@@ -563,25 +559,28 @@ impl<'a> Parser<'a> {
         // Note: when adding new unary operators, don't forget to adjust TokenKind::can_begin_expr()
         match this.token.uninterpolate().kind {
             // `!expr`
-            token::Not => make_it!(this, attrs, |this, _| this.parse_unary_expr(lo, UnOp::Not)),
+            token::Not => make_it!(this, attrs, |this, _| this.parse_expr_unary(lo, UnOp::Not)),
             // `~expr`
             token::Tilde => make_it!(this, attrs, |this, _| this.recover_tilde_expr(lo)),
             // `-expr`
             token::BinOp(token::Minus) => {
-                make_it!(this, attrs, |this, _| this.parse_unary_expr(lo, UnOp::Neg))
+                make_it!(this, attrs, |this, _| this.parse_expr_unary(lo, UnOp::Neg))
             }
             // `*expr`
             token::BinOp(token::Star) => {
-                make_it!(this, attrs, |this, _| this.parse_unary_expr(lo, UnOp::Deref))
+                make_it!(this, attrs, |this, _| this.parse_expr_unary(lo, UnOp::Deref))
             }
             // `&expr` and `&&expr`
             token::BinOp(token::And) | token::AndAnd => {
-                make_it!(this, attrs, |this, _| this.parse_borrow_expr(lo))
+                make_it!(this, attrs, |this, _| this.parse_expr_borrow(lo))
             }
             // `+lit`
             token::BinOp(token::Plus) if this.look_ahead(1, |tok| tok.is_numeric_lit()) => {
-                let mut err =
-                    LeadingPlusNotSupported { span: lo, remove_plus: None, add_parentheses: None };
+                let mut err = errors::LeadingPlusNotSupported {
+                    span: lo,
+                    remove_plus: None,
+                    add_parentheses: None,
+                };
 
                 // a block on the LHS might have been intended to be an expression instead
                 if let Some(sp) = this.sess.ambiguous_block_expr_parse.borrow().get(&lo) {
@@ -592,7 +591,7 @@ impl<'a> Parser<'a> {
                 this.sess.emit_err(err);
 
                 this.bump();
-                this.parse_prefix_expr(None)
+                this.parse_expr_prefix(None)
             }
             // Recover from `++x`:
             token::BinOp(token::Plus)
@@ -605,41 +604,41 @@ impl<'a> Parser<'a> {
                 this.bump();
                 this.bump();
 
-                let operand_expr = this.parse_dot_or_call_expr(Default::default())?;
+                let operand_expr = this.parse_expr_dot_or_call(Default::default())?;
                 this.recover_from_prefix_increment(operand_expr, pre_span, starts_stmt)
             }
             token::Ident(..) if this.token.is_keyword(kw::Box) => {
-                make_it!(this, attrs, |this, _| this.parse_box_expr(lo))
+                make_it!(this, attrs, |this, _| this.parse_expr_box(lo))
             }
             token::Ident(..) if this.may_recover() && this.is_mistaken_not_ident_negation() => {
                 make_it!(this, attrs, |this, _| this.recover_not_expr(lo))
             }
-            _ => return this.parse_dot_or_call_expr(Some(attrs)),
+            _ => return this.parse_expr_dot_or_call(Some(attrs)),
         }
     }
 
-    fn parse_prefix_expr_common(&mut self, lo: Span) -> PResult<'a, (Span, P<Expr>)> {
+    fn parse_expr_prefix_common(&mut self, lo: Span) -> PResult<'a, (Span, P<Expr>)> {
         self.bump();
-        let expr = self.parse_prefix_expr(None);
+        let expr = self.parse_expr_prefix(None);
         let (span, expr) = self.interpolated_or_expr_span(expr)?;
         Ok((lo.to(span), expr))
     }
 
-    fn parse_unary_expr(&mut self, lo: Span, op: UnOp) -> PResult<'a, (Span, ExprKind)> {
-        let (span, expr) = self.parse_prefix_expr_common(lo)?;
+    fn parse_expr_unary(&mut self, lo: Span, op: UnOp) -> PResult<'a, (Span, ExprKind)> {
+        let (span, expr) = self.parse_expr_prefix_common(lo)?;
         Ok((span, self.mk_unary(op, expr)))
     }
 
     /// Recover on `~expr` in favor of `!expr`.
     fn recover_tilde_expr(&mut self, lo: Span) -> PResult<'a, (Span, ExprKind)> {
-        self.sess.emit_err(TildeAsUnaryOperator(lo));
+        self.sess.emit_err(errors::TildeAsUnaryOperator(lo));
 
-        self.parse_unary_expr(lo, UnOp::Not)
+        self.parse_expr_unary(lo, UnOp::Not)
     }
 
     /// Parse `box expr`.
-    fn parse_box_expr(&mut self, lo: Span) -> PResult<'a, (Span, ExprKind)> {
-        let (span, expr) = self.parse_prefix_expr_common(lo)?;
+    fn parse_expr_box(&mut self, lo: Span) -> PResult<'a, (Span, ExprKind)> {
+        let (span, expr) = self.parse_expr_prefix_common(lo)?;
         self.sess.gated_spans.gate(sym::box_syntax, span);
         Ok((span, ExprKind::Box(expr)))
     }
@@ -660,14 +659,14 @@ impl<'a> Parser<'a> {
         let negated_token = self.look_ahead(1, |t| t.clone());
 
         let sub_diag = if negated_token.is_numeric_lit() {
-            NotAsNegationOperatorSub::SuggestNotBitwise
+            errors::NotAsNegationOperatorSub::SuggestNotBitwise
         } else if negated_token.is_bool_lit() {
-            NotAsNegationOperatorSub::SuggestNotLogical
+            errors::NotAsNegationOperatorSub::SuggestNotLogical
         } else {
-            NotAsNegationOperatorSub::SuggestNotDefault
+            errors::NotAsNegationOperatorSub::SuggestNotDefault
         };
 
-        self.sess.emit_err(NotAsNegationOperator {
+        self.sess.emit_err(errors::NotAsNegationOperator {
             negated: negated_token.span,
             negated_desc: super::token_descr(&negated_token),
             // Span the `not` plus trailing whitespace to avoid
@@ -677,7 +676,7 @@ impl<'a> Parser<'a> {
             ),
         });
 
-        self.parse_unary_expr(lo, UnOp::Not)
+        self.parse_expr_unary(lo, UnOp::Not)
     }
 
     /// Returns the span of expr, if it was not interpolated or the span of the interpolated token.
@@ -735,10 +734,10 @@ impl<'a> Parser<'a> {
                                 segments[0].ident.span,
                             ),
                         };
-                        match self.parse_labeled_expr(label, false) {
+                        match self.parse_expr_labeled(label, false) {
                             Ok(expr) => {
                                 type_err.cancel();
-                                self.sess.emit_err(MalformedLoopLabel {
+                                self.sess.emit_err(errors::MalformedLoopLabel {
                                     span: label.ident.span,
                                     correct_label: label.ident,
                                 });
@@ -763,20 +762,22 @@ impl<'a> Parser<'a> {
                         );
 
                         let args_span = self.look_ahead(1, |t| t.span).to(span_after_type);
-                        let suggestion = ComparisonOrShiftInterpretedAsGenericSugg {
+                        let suggestion = errors::ComparisonOrShiftInterpretedAsGenericSugg {
                             left: expr.span.shrink_to_lo(),
                             right: expr.span.shrink_to_hi(),
                         };
 
                         match self.token.kind {
-                            token::Lt => self.sess.emit_err(ComparisonInterpretedAsGeneric {
-                                comparison: self.token.span,
-                                r#type: path,
-                                args: args_span,
-                                suggestion,
-                            }),
+                            token::Lt => {
+                                self.sess.emit_err(errors::ComparisonInterpretedAsGeneric {
+                                    comparison: self.token.span,
+                                    r#type: path,
+                                    args: args_span,
+                                    suggestion,
+                                })
+                            }
                             token::BinOp(token::Shl) => {
-                                self.sess.emit_err(ShiftInterpretedAsGeneric {
+                                self.sess.emit_err(errors::ShiftInterpretedAsGeneric {
                                     shift: self.token.span,
                                     r#type: path,
                                     args: args_span,
@@ -827,7 +828,7 @@ impl<'a> Parser<'a> {
                 ("cast", None)
             };
 
-        let with_postfix = self.parse_dot_or_call_expr_with_(cast_expr, span)?;
+        let with_postfix = self.parse_expr_dot_or_call_with_(cast_expr, span)?;
 
         // Check if an illegal postfix operator has been added after the cast.
         // If the resulting expression is not a cast, it is an illegal postfix operator.
@@ -898,15 +899,15 @@ impl<'a> Parser<'a> {
     }
 
     /// Parse `& mut? <expr>` or `& raw [ const | mut ] <expr>`.
-    fn parse_borrow_expr(&mut self, lo: Span) -> PResult<'a, (Span, ExprKind)> {
+    fn parse_expr_borrow(&mut self, lo: Span) -> PResult<'a, (Span, ExprKind)> {
         self.expect_and()?;
         let has_lifetime = self.token.is_lifetime() && self.look_ahead(1, |t| t != &token::Colon);
         let lifetime = has_lifetime.then(|| self.expect_lifetime()); // For recovery, see below.
         let (borrow_kind, mutbl) = self.parse_borrow_modifiers(lo);
         let expr = if self.token.is_range_separator() {
-            self.parse_prefix_range_expr(None)
+            self.parse_expr_prefix_range(None)
         } else {
-            self.parse_prefix_expr(None)
+            self.parse_expr_prefix(None)
         };
         let (hi, expr) = self.interpolated_or_expr_span(expr)?;
         let span = lo.to(hi);
@@ -917,7 +918,7 @@ impl<'a> Parser<'a> {
     }
 
     fn error_remove_borrow_lifetime(&self, span: Span, lt_span: Span) {
-        self.sess.emit_err(LifetimeInBorrowExpression { span, lifetime_span: lt_span });
+        self.sess.emit_err(errors::LifetimeInBorrowExpression { span, lifetime_span: lt_span });
     }
 
     /// Parse `mut?` or `raw [ const | mut ]`.
@@ -936,16 +937,16 @@ impl<'a> Parser<'a> {
     }
 
     /// Parses `a.b` or `a(13)` or `a[4]` or just `a`.
-    fn parse_dot_or_call_expr(&mut self, attrs: Option<AttrWrapper>) -> PResult<'a, P<Expr>> {
+    fn parse_expr_dot_or_call(&mut self, attrs: Option<AttrWrapper>) -> PResult<'a, P<Expr>> {
         let attrs = self.parse_or_use_outer_attributes(attrs)?;
         self.collect_tokens_for_expr(attrs, |this, attrs| {
-            let base = this.parse_bottom_expr();
+            let base = this.parse_expr_bottom();
             let (span, base) = this.interpolated_or_expr_span(base)?;
-            this.parse_dot_or_call_expr_with(base, span, attrs)
+            this.parse_expr_dot_or_call_with(base, span, attrs)
         })
     }
 
-    pub(super) fn parse_dot_or_call_expr_with(
+    pub(super) fn parse_expr_dot_or_call_with(
         &mut self,
         e0: P<Expr>,
         lo: Span,
@@ -954,7 +955,7 @@ impl<'a> Parser<'a> {
         // Stitch the list of outer attributes onto the return value.
         // A little bit ugly, but the best way given the current code
         // structure
-        let res = self.parse_dot_or_call_expr_with_(e0, lo);
+        let res = self.parse_expr_dot_or_call_with_(e0, lo);
         if attrs.is_empty() {
             res
         } else {
@@ -968,7 +969,7 @@ impl<'a> Parser<'a> {
         }
     }
 
-    fn parse_dot_or_call_expr_with_(&mut self, mut e: P<Expr>, lo: Span) -> PResult<'a, P<Expr>> {
+    fn parse_expr_dot_or_call_with_(&mut self, mut e: P<Expr>, lo: Span) -> PResult<'a, P<Expr>> {
         loop {
             let has_question = if self.prev_token.kind == TokenKind::Ident(kw::Return, false) {
                 // we are using noexpect here because we don't expect a `?` directly after a `return`
@@ -991,15 +992,15 @@ impl<'a> Parser<'a> {
             };
             if has_dot {
                 // expr.f
-                e = self.parse_dot_suffix_expr(lo, e)?;
+                e = self.parse_expr_dot_suffix(lo, e)?;
                 continue;
             }
             if self.expr_is_complete(&e) {
                 return Ok(e);
             }
             e = match self.token.kind {
-                token::OpenDelim(Delimiter::Parenthesis) => self.parse_fn_call_expr(lo, e),
-                token::OpenDelim(Delimiter::Bracket) => self.parse_index_expr(lo, e)?,
+                token::OpenDelim(Delimiter::Parenthesis) => self.parse_expr_fn_call(lo, e),
+                token::OpenDelim(Delimiter::Bracket) => self.parse_expr_index(lo, e)?,
                 _ => return Ok(e),
             }
         }
@@ -1011,14 +1012,14 @@ impl<'a> Parser<'a> {
             && self.look_ahead(3, |t| t.can_begin_expr())
     }
 
-    fn parse_dot_suffix_expr(&mut self, lo: Span, base: P<Expr>) -> PResult<'a, P<Expr>> {
+    fn parse_expr_dot_suffix(&mut self, lo: Span, base: P<Expr>) -> PResult<'a, P<Expr>> {
         match self.token.uninterpolate().kind {
             token::Ident(..) => self.parse_dot_suffix(base, lo),
             token::Literal(token::Lit { kind: token::Integer, symbol, suffix }) => {
-                Ok(self.parse_tuple_field_access_expr(lo, base, symbol, suffix, None))
+                Ok(self.parse_expr_tuple_field_access(lo, base, symbol, suffix, None))
             }
             token::Literal(token::Lit { kind: token::Float, symbol, suffix }) => {
-                Ok(self.parse_tuple_field_access_expr_float(lo, base, symbol, suffix))
+                Ok(self.parse_expr_tuple_field_access_float(lo, base, symbol, suffix))
             }
             _ => {
                 self.error_unexpected_after_dot();
@@ -1030,7 +1031,7 @@ impl<'a> Parser<'a> {
     fn error_unexpected_after_dot(&self) {
         // FIXME Could factor this out into non_fatal_unexpected or something.
         let actual = pprust::token_to_string(&self.token);
-        self.struct_span_err(self.token.span, &format!("unexpected token: `{actual}`")).emit();
+        self.sess.emit_err(errors::UnexpectedTokenAfterDot { span: self.token.span, actual });
     }
 
     // We need an identifier or integer, but the next token is a float.
@@ -1040,7 +1041,7 @@ impl<'a> Parser<'a> {
     // support pushing "future tokens" (would be also helpful to `break_and_eat`), or
     // we should break everything including floats into more basic proc-macro style
     // tokens in the lexer (probably preferable).
-    fn parse_tuple_field_access_expr_float(
+    fn parse_expr_tuple_field_access_float(
         &mut self,
         lo: Span,
         base: P<Expr>,
@@ -1083,7 +1084,7 @@ impl<'a> Parser<'a> {
         match &*components {
             // 1e2
             [IdentLike(i)] => {
-                self.parse_tuple_field_access_expr(lo, base, Symbol::intern(&i), suffix, None)
+                self.parse_expr_tuple_field_access(lo, base, Symbol::intern(&i), suffix, None)
             }
             // 1.
             [IdentLike(i), Punct('.')] => {
@@ -1099,7 +1100,7 @@ impl<'a> Parser<'a> {
                 let symbol = Symbol::intern(&i);
                 self.token = Token::new(token::Ident(symbol, false), ident_span);
                 let next_token = (Token::new(token::Dot, dot_span), self.token_spacing);
-                self.parse_tuple_field_access_expr(lo, base, symbol, None, Some(next_token))
+                self.parse_expr_tuple_field_access(lo, base, symbol, None, Some(next_token))
             }
             // 1.2 | 1.2e3
             [IdentLike(i1), Punct('.'), IdentLike(i2)] => {
@@ -1120,11 +1121,11 @@ impl<'a> Parser<'a> {
                 // See issue #76399 and PR #76285 for more details
                 let next_token1 = (Token::new(token::Dot, dot_span), Spacing::Alone);
                 let base1 =
-                    self.parse_tuple_field_access_expr(lo, base, symbol1, None, Some(next_token1));
+                    self.parse_expr_tuple_field_access(lo, base, symbol1, None, Some(next_token1));
                 let symbol2 = Symbol::intern(&i2);
                 let next_token2 = Token::new(token::Ident(symbol2, false), ident2_span);
                 self.bump_with((next_token2, self.token_spacing)); // `.`
-                self.parse_tuple_field_access_expr(lo, base1, symbol2, suffix, None)
+                self.parse_expr_tuple_field_access(lo, base1, symbol2, suffix, None)
             }
             // 1e+ | 1e- (recovered)
             [IdentLike(_), Punct('+' | '-')] |
@@ -1142,7 +1143,7 @@ impl<'a> Parser<'a> {
         }
     }
 
-    fn parse_tuple_field_access_expr(
+    fn parse_expr_tuple_field_access(
         &mut self,
         lo: Span,
         base: P<Expr>,
@@ -1163,7 +1164,7 @@ impl<'a> Parser<'a> {
     }
 
     /// Parse a function call expression, `expr(...)`.
-    fn parse_fn_call_expr(&mut self, lo: Span, fun: P<Expr>) -> P<Expr> {
+    fn parse_expr_fn_call(&mut self, lo: Span, fun: P<Expr>) -> P<Expr> {
         let snapshot = if self.token.kind == token::OpenDelim(Delimiter::Parenthesis)
             && self.look_ahead_type_ascription_as_field()
         {
@@ -1174,7 +1175,7 @@ impl<'a> Parser<'a> {
         let open_paren = self.token.span;
 
         let mut seq = self
-            .parse_paren_expr_seq()
+            .parse_expr_paren_seq()
             .map(|args| self.mk_expr(lo.to(self.prev_token.span), self.mk_call(fun, args)));
         if let Some(expr) =
             self.maybe_recover_struct_lit_bad_delims(lo, open_paren, &mut seq, snapshot)
@@ -1209,16 +1210,21 @@ impl<'a> Parser<'a> {
                         // `Enum::Foo { a: 3, b: 4 }` or `Enum::Foo(3, 4)`.
                         self.restore_snapshot(snapshot);
                         let close_paren = self.prev_token.span;
-                        let span = lo.to(self.prev_token.span);
-                        if !fields.is_empty() {
-                            let mut replacement_err = ParenthesesWithStructFields {
+                        let span = lo.to(close_paren);
+                        if !fields.is_empty() &&
+                            // `token.kind` should not be compared here.
+                            // This is because the `snapshot.token.kind` is treated as the same as
+                            // that of the open delim in `TokenTreesReader::parse_token_tree`, even if they are different.
+                            self.span_to_snippet(close_paren).map_or(false, |snippet| snippet == ")")
+                        {
+                            let mut replacement_err = errors::ParenthesesWithStructFields {
                                 span,
                                 r#type: path,
-                                braces_for_struct: BracesForStructLiteral {
+                                braces_for_struct: errors::BracesForStructLiteral {
                                     first: open_paren,
                                     second: close_paren,
                                 },
-                                no_fields_for_fn: NoFieldsForFnCall {
+                                no_fields_for_fn: errors::NoFieldsForFnCall {
                                     fields: fields
                                         .into_iter()
                                         .map(|field| field.span.until(field.expr.span))
@@ -1247,7 +1253,7 @@ impl<'a> Parser<'a> {
     }
 
     /// Parse an indexing expression `expr[...]`.
-    fn parse_index_expr(&mut self, lo: Span, base: P<Expr>) -> PResult<'a, P<Expr>> {
+    fn parse_expr_index(&mut self, lo: Span, base: P<Expr>) -> PResult<'a, P<Expr>> {
         let prev_span = self.prev_token.span;
         let open_delim_span = self.token.span;
         self.bump(); // `[`
@@ -1270,7 +1276,7 @@ impl<'a> Parser<'a> {
 
         if self.check(&token::OpenDelim(Delimiter::Parenthesis)) {
             // Method call `expr.f()`
-            let args = self.parse_paren_expr_seq()?;
+            let args = self.parse_expr_paren_seq()?;
             let fn_span = fn_span_lo.to(self.prev_token.span);
             let span = lo.to(self.prev_token.span);
             Ok(self.mk_expr(
@@ -1285,7 +1291,7 @@ impl<'a> Parser<'a> {
         } else {
             // Field access `expr.f`
             if let Some(args) = seg.args {
-                self.sess.emit_err(FieldExpressionWithGeneric(args.span()));
+                self.sess.emit_err(errors::FieldExpressionWithGeneric(args.span()));
             }
 
             let span = lo.to(self.prev_token.span);
@@ -1298,7 +1304,7 @@ impl<'a> Parser<'a> {
     ///
     /// N.B., this does not parse outer attributes, and is private because it only works
     /// correctly if called from `parse_dot_or_call_expr()`.
-    fn parse_bottom_expr(&mut self) -> PResult<'a, P<Expr>> {
+    fn parse_expr_bottom(&mut self) -> PResult<'a, P<Expr>> {
         maybe_recover_from_interpolated_ty_qpath!(self, true);
         maybe_whole_expr!(self);
 
@@ -1311,13 +1317,13 @@ impl<'a> Parser<'a> {
             // This match arm is a special-case of the `_` match arm below and
             // could be removed without changing functionality, but it's faster
             // to have it here, especially for programs with large constants.
-            self.parse_lit_expr()
+            self.parse_expr_lit()
         } else if self.check(&token::OpenDelim(Delimiter::Parenthesis)) {
-            self.parse_tuple_parens_expr()
+            self.parse_expr_tuple_parens()
         } else if self.check(&token::OpenDelim(Delimiter::Brace)) {
-            self.parse_block_expr(None, lo, BlockCheckMode::Default)
+            self.parse_expr_block(None, lo, BlockCheckMode::Default)
         } else if self.check(&token::BinOp(token::Or)) || self.check(&token::OrOr) {
-            self.parse_closure_expr().map_err(|mut err| {
+            self.parse_expr_closure().map_err(|mut err| {
                 // If the input is something like `if a { 1 } else { 2 } | if a { 3 } else { 4 }`
                 // then suggest parens around the lhs.
                 if let Some(sp) = self.sess.ambiguous_block_expr_parse.borrow().get(&lo) {
@@ -1326,42 +1332,42 @@ impl<'a> Parser<'a> {
                 err
             })
         } else if self.check(&token::OpenDelim(Delimiter::Bracket)) {
-            self.parse_array_or_repeat_expr(Delimiter::Bracket)
+            self.parse_expr_array_or_repeat(Delimiter::Bracket)
         } else if self.check_path() {
-            self.parse_path_start_expr()
+            self.parse_expr_path_start()
         } else if self.check_keyword(kw::Move)
             || self.check_keyword(kw::Static)
             || self.check_const_closure()
         {
-            self.parse_closure_expr()
+            self.parse_expr_closure()
         } else if self.eat_keyword(kw::If) {
-            self.parse_if_expr()
+            self.parse_expr_if()
         } else if self.check_keyword(kw::For) {
             if self.choose_generics_over_qpath(1) {
-                self.parse_closure_expr()
+                self.parse_expr_closure()
             } else {
                 assert!(self.eat_keyword(kw::For));
-                self.parse_for_expr(None, self.prev_token.span)
+                self.parse_expr_for(None, self.prev_token.span)
             }
         } else if self.eat_keyword(kw::While) {
-            self.parse_while_expr(None, self.prev_token.span)
+            self.parse_expr_while(None, self.prev_token.span)
         } else if let Some(label) = self.eat_label() {
-            self.parse_labeled_expr(label, true)
+            self.parse_expr_labeled(label, true)
         } else if self.eat_keyword(kw::Loop) {
             let sp = self.prev_token.span;
-            self.parse_loop_expr(None, self.prev_token.span).map_err(|mut err| {
+            self.parse_expr_loop(None, self.prev_token.span).map_err(|mut err| {
                 err.span_label(sp, "while parsing this `loop` expression");
                 err
             })
         } else if self.eat_keyword(kw::Match) {
             let match_sp = self.prev_token.span;
-            self.parse_match_expr().map_err(|mut err| {
+            self.parse_expr_match().map_err(|mut err| {
                 err.span_label(match_sp, "while parsing this `match` expression");
                 err
             })
         } else if self.eat_keyword(kw::Unsafe) {
             let sp = self.prev_token.span;
-            self.parse_block_expr(None, lo, BlockCheckMode::Unsafe(ast::UserProvided)).map_err(
+            self.parse_expr_block(None, lo, BlockCheckMode::Unsafe(ast::UserProvided)).map_err(
                 |mut err| {
                     err.span_label(sp, "while parsing this `unsafe` expression");
                     err
@@ -1375,17 +1381,17 @@ impl<'a> Parser<'a> {
             self.expect_keyword(kw::Try)?;
             self.parse_try_block(lo)
         } else if self.eat_keyword(kw::Return) {
-            self.parse_return_expr()
+            self.parse_expr_return()
         } else if self.eat_keyword(kw::Continue) {
-            self.parse_continue_expr(lo)
+            self.parse_expr_continue(lo)
         } else if self.eat_keyword(kw::Break) {
-            self.parse_break_expr()
+            self.parse_expr_break()
         } else if self.eat_keyword(kw::Yield) {
-            self.parse_yield_expr()
+            self.parse_expr_yield()
         } else if self.is_do_yeet() {
-            self.parse_yeet_expr()
+            self.parse_expr_yeet()
         } else if self.check_keyword(kw::Let) {
-            self.parse_let_expr()
+            self.parse_expr_let()
         } else if self.eat_keyword(kw::Underscore) {
             Ok(self.mk_expr(self.prev_token.span, ExprKind::Underscore))
         } else if !self.unclosed_delims.is_empty() && self.check(&token::Semi) {
@@ -1408,19 +1414,19 @@ impl<'a> Parser<'a> {
                     // Check for `async {` and `async move {`.
                     self.parse_async_block()
                 } else {
-                    self.parse_closure_expr()
+                    self.parse_expr_closure()
                 }
             } else if self.eat_keyword(kw::Await) {
                 self.recover_incorrect_await_syntax(lo, self.prev_token.span)
             } else {
-                self.parse_lit_expr()
+                self.parse_expr_lit()
             }
         } else {
-            self.parse_lit_expr()
+            self.parse_expr_lit()
         }
     }
 
-    fn parse_lit_expr(&mut self) -> PResult<'a, P<Expr>> {
+    fn parse_expr_lit(&mut self) -> PResult<'a, P<Expr>> {
         let lo = self.token.span;
         match self.parse_opt_token_lit() {
             Some((token_lit, _)) => {
@@ -1431,7 +1437,7 @@ impl<'a> Parser<'a> {
         }
     }
 
-    fn parse_tuple_parens_expr(&mut self) -> PResult<'a, P<Expr>> {
+    fn parse_expr_tuple_parens(&mut self) -> PResult<'a, P<Expr>> {
         let lo = self.token.span;
         self.expect(&token::OpenDelim(Delimiter::Parenthesis))?;
         let (es, trailing_comma) = match self.parse_seq_to_end(
@@ -1455,20 +1461,20 @@ impl<'a> Parser<'a> {
         self.maybe_recover_from_bad_qpath(expr)
     }
 
-    fn parse_array_or_repeat_expr(&mut self, close_delim: Delimiter) -> PResult<'a, P<Expr>> {
+    fn parse_expr_array_or_repeat(&mut self, close_delim: Delimiter) -> PResult<'a, P<Expr>> {
         let lo = self.token.span;
         self.bump(); // `[` or other open delim
 
         let close = &token::CloseDelim(close_delim);
         let kind = if self.eat(close) {
             // Empty vector
-            ExprKind::Array(Vec::new())
+            ExprKind::Array(ThinVec::new())
         } else {
             // Non-empty vector
             let first_expr = self.parse_expr()?;
             if self.eat(&token::Semi) {
                 // Repeating array syntax: `[ 0; 512 ]`
-                let count = self.parse_anon_const_expr()?;
+                let count = self.parse_expr_anon_const()?;
                 self.expect(close)?;
                 ExprKind::Repeat(first_expr, count)
             } else if self.eat(&token::Comma) {
@@ -1480,14 +1486,14 @@ impl<'a> Parser<'a> {
             } else {
                 // Vector with one element
                 self.expect(close)?;
-                ExprKind::Array(vec![first_expr])
+                ExprKind::Array(thin_vec![first_expr])
             }
         };
         let expr = self.mk_expr(lo.to(self.prev_token.span), kind);
         self.maybe_recover_from_bad_qpath(expr)
     }
 
-    fn parse_path_start_expr(&mut self) -> PResult<'a, P<Expr>> {
+    fn parse_expr_path_start(&mut self) -> PResult<'a, P<Expr>> {
         let (qself, path) = if self.eat_lt() {
             let (qself, path) = self.parse_qpath(PathStyle::Expr)?;
             (Some(qself), path)
@@ -1499,7 +1505,7 @@ impl<'a> Parser<'a> {
         let (span, kind) = if self.eat(&token::Not) {
             // MACRO INVOCATION expression
             if qself.is_some() {
-                self.sess.emit_err(MacroInvocationWithQualifiedPath(path.span));
+                self.sess.emit_err(errors::MacroInvocationWithQualifiedPath(path.span));
             }
             let lo = path.span;
             let mac = P(MacCall {
@@ -1524,7 +1530,7 @@ impl<'a> Parser<'a> {
     }
 
     /// Parse `'label: $expr`. The label is already parsed.
-    fn parse_labeled_expr(
+    fn parse_expr_labeled(
         &mut self,
         label_: Label,
         mut consume_colon: bool,
@@ -1533,15 +1539,15 @@ impl<'a> Parser<'a> {
         let label = Some(label_);
         let ate_colon = self.eat(&token::Colon);
         let expr = if self.eat_keyword(kw::While) {
-            self.parse_while_expr(label, lo)
+            self.parse_expr_while(label, lo)
         } else if self.eat_keyword(kw::For) {
-            self.parse_for_expr(label, lo)
+            self.parse_expr_for(label, lo)
         } else if self.eat_keyword(kw::Loop) {
-            self.parse_loop_expr(label, lo)
+            self.parse_expr_loop(label, lo)
         } else if self.check_noexpect(&token::OpenDelim(Delimiter::Brace))
             || self.token.is_whole_block()
         {
-            self.parse_block_expr(label, lo, BlockCheckMode::Default)
+            self.parse_expr_block(label, lo, BlockCheckMode::Default)
         } else if !ate_colon
             && self.may_recover()
             && (matches!(self.token.kind, token::CloseDelim(_) | token::Comma)
@@ -1549,7 +1555,7 @@ impl<'a> Parser<'a> {
         {
             let (lit, _) =
                 self.recover_unclosed_char(label_.ident, Parser::mk_token_lit_char, |self_| {
-                    self_.sess.create_err(UnexpectedTokenAfterLabel {
+                    self_.sess.create_err(errors::UnexpectedTokenAfterLabel {
                         span: self_.token.span,
                         remove_label: None,
                         enclose_in_block: None,
@@ -1561,7 +1567,7 @@ impl<'a> Parser<'a> {
             && (self.check_noexpect(&TokenKind::Comma) || self.check_noexpect(&TokenKind::Gt))
         {
             // We're probably inside of a `Path<'a>` that needs a turbofish
-            self.sess.emit_err(UnexpectedTokenAfterLabel {
+            self.sess.emit_err(errors::UnexpectedTokenAfterLabel {
                 span: self.token.span,
                 remove_label: None,
                 enclose_in_block: None,
@@ -1569,7 +1575,7 @@ impl<'a> Parser<'a> {
             consume_colon = false;
             Ok(self.mk_expr_err(lo))
         } else {
-            let mut err = UnexpectedTokenAfterLabel {
+            let mut err = errors::UnexpectedTokenAfterLabel {
                 span: self.token.span,
                 remove_label: None,
                 enclose_in_block: None,
@@ -1605,14 +1611,14 @@ impl<'a> Parser<'a> {
                     return expr;
                 }
 
-                err.enclose_in_block = Some(UnexpectedTokenAfterLabelSugg {
+                err.enclose_in_block = Some(errors::UnexpectedTokenAfterLabelSugg {
                     left: span.shrink_to_lo(),
                     right: span.shrink_to_hi(),
                 });
 
                 // Replace `'label: non_block_expr` with `'label: {non_block_expr}` in order to suppress future errors about `break 'label`.
                 let stmt = self.mk_stmt(span, StmtKind::Expr(expr));
-                let blk = self.mk_block(vec![stmt], BlockCheckMode::Default, span);
+                let blk = self.mk_block(thin_vec![stmt], BlockCheckMode::Default, span);
                 self.mk_expr(span, ExprKind::Block(blk, label))
             });
 
@@ -1621,7 +1627,7 @@ impl<'a> Parser<'a> {
         }?;
 
         if !ate_colon && consume_colon {
-            self.sess.emit_err(RequireColonAfterLabeledExpression {
+            self.sess.emit_err(errors::RequireColonAfterLabeledExpression {
                 span: expr.span,
                 label: lo,
                 label_end: lo.shrink_to_hi(),
@@ -1670,7 +1676,7 @@ impl<'a> Parser<'a> {
         self.bump(); // `catch`
 
         let span = lo.to(self.prev_token.span);
-        self.sess.emit_err(DoCatchSyntaxRemoved { span });
+        self.sess.emit_err(errors::DoCatchSyntaxRemoved { span });
 
         self.parse_try_block(lo)
     }
@@ -1681,7 +1687,7 @@ impl<'a> Parser<'a> {
     }
 
     /// Parse `"return" expr?`.
-    fn parse_return_expr(&mut self) -> PResult<'a, P<Expr>> {
+    fn parse_expr_return(&mut self) -> PResult<'a, P<Expr>> {
         let lo = self.prev_token.span;
         let kind = ExprKind::Ret(self.parse_expr_opt()?);
         let expr = self.mk_expr(lo.to(self.prev_token.span), kind);
@@ -1689,7 +1695,7 @@ impl<'a> Parser<'a> {
     }
 
     /// Parse `"do" "yeet" expr?`.
-    fn parse_yeet_expr(&mut self) -> PResult<'a, P<Expr>> {
+    fn parse_expr_yeet(&mut self) -> PResult<'a, P<Expr>> {
         let lo = self.token.span;
 
         self.bump(); // `do`
@@ -1711,16 +1717,16 @@ impl<'a> Parser<'a> {
     /// `break 'lbl: loop {}`); a labeled break with an unlabeled loop as its value
     /// expression only gets a warning for compatibility reasons; and a labeled break
     /// with a labeled loop does not even get a warning because there is no ambiguity.
-    fn parse_break_expr(&mut self) -> PResult<'a, P<Expr>> {
+    fn parse_expr_break(&mut self) -> PResult<'a, P<Expr>> {
         let lo = self.prev_token.span;
         let mut label = self.eat_label();
         let kind = if self.token == token::Colon && let Some(label) = label.take() {
             // The value expression can be a labeled loop, see issue #86948, e.g.:
             // `loop { break 'label: loop { break 'label 42; }; }`
-            let lexpr = self.parse_labeled_expr(label, true)?;
-            self.sess.emit_err(LabeledLoopInBreak {
+            let lexpr = self.parse_expr_labeled(label, true)?;
+            self.sess.emit_err(errors::LabeledLoopInBreak {
                 span: lexpr.span,
-                sub: WrapExpressionInParentheses {
+                sub: errors::WrapExpressionInParentheses {
                     left: lexpr.span.shrink_to_lo(),
                     right: lexpr.span.shrink_to_hi(),
                 },
@@ -1770,7 +1776,7 @@ impl<'a> Parser<'a> {
     }
 
     /// Parse `"continue" label?`.
-    fn parse_continue_expr(&mut self, lo: Span) -> PResult<'a, P<Expr>> {
+    fn parse_expr_continue(&mut self, lo: Span) -> PResult<'a, P<Expr>> {
         let mut label = self.eat_label();
 
         // Recover `continue label` -> `continue 'label`
@@ -1787,7 +1793,7 @@ impl<'a> Parser<'a> {
     }
 
     /// Parse `"yield" expr?`.
-    fn parse_yield_expr(&mut self) -> PResult<'a, P<Expr>> {
+    fn parse_expr_yield(&mut self) -> PResult<'a, P<Expr>> {
         let lo = self.prev_token.span;
         let kind = ExprKind::Yield(self.parse_expr_opt()?);
         let span = lo.to(self.prev_token.span);
@@ -1840,7 +1846,7 @@ impl<'a> Parser<'a> {
             };
             if let Some(expr) = expr {
                 if matches!(expr.kind, ExprKind::Err) {
-                    let mut err = InvalidInterpolatedExpression { span: self.token.span }
+                    let mut err = errors::InvalidInterpolatedExpression { span: self.token.span }
                         .into_diagnostic(&self.sess.span_diagnostic);
                     err.downgrade_to_delayed_bug();
                     return Err(err);
@@ -1882,7 +1888,16 @@ impl<'a> Parser<'a> {
                 if let token::Literal(token::Lit { kind: token::Integer, symbol, suffix }) =
                     next_token.kind
                 {
-                    if self.token.span.hi() == next_token.span.lo() {
+                    // If this integer looks like a float, then recover as such.
+                    //
+                    // We will never encounter the exponent part of a floating
+                    // point literal here, since there's no use of the exponent
+                    // syntax that also constitutes a valid integer, so we need
+                    // not check for that.
+                    if suffix.map_or(true, |s| s == sym::f32 || s == sym::f64)
+                        && symbol.as_str().chars().all(|c| c.is_numeric() || c == '_')
+                        && self.token.span.hi() == next_token.span.lo()
+                    {
                         let s = String::from("0.") + symbol.as_str();
                         let kind = TokenKind::lit(token::Float, Symbol::intern(&s), suffix);
                         return Some(Token::new(kind, self.token.span.to(next_token.span)));
@@ -1892,7 +1907,7 @@ impl<'a> Parser<'a> {
             });
             if let Some(token) = &recovered {
                 self.bump();
-                self.sess.emit_err(FloatLiteralRequiresIntegerPart {
+                self.sess.emit_err(errors::FloatLiteralRequiresIntegerPart {
                     span: token.span,
                     correct: pprust::token_to_string(token).into_owned(),
                 });
@@ -1953,13 +1968,17 @@ impl<'a> Parser<'a> {
         if [sym::i32, sym::u32, sym::isize, sym::usize].contains(&suffix) {
             // #59553: warn instead of reject out of hand to allow the fix to percolate
             // through the ecosystem when people fix their macros
-            self.sess.emit_warning(InvalidLiteralSuffixOnTupleIndex {
+            self.sess.emit_warning(errors::InvalidLiteralSuffixOnTupleIndex {
                 span,
                 suffix,
                 exception: Some(()),
             });
         } else {
-            self.sess.emit_err(InvalidLiteralSuffixOnTupleIndex { span, suffix, exception: None });
+            self.sess.emit_err(errors::InvalidLiteralSuffixOnTupleIndex {
+                span,
+                suffix,
+                exception: None,
+            });
         }
     }
 
@@ -1991,11 +2010,11 @@ impl<'a> Parser<'a> {
     /// expression.
     fn maybe_suggest_brackets_instead_of_braces(&mut self, lo: Span) -> Option<P<Expr>> {
         let mut snapshot = self.create_snapshot_for_diagnostic();
-        match snapshot.parse_array_or_repeat_expr(Delimiter::Brace) {
+        match snapshot.parse_expr_array_or_repeat(Delimiter::Brace) {
             Ok(arr) => {
-                self.sess.emit_err(ArrayBracketsInsteadOfSpaces {
+                self.sess.emit_err(errors::ArrayBracketsInsteadOfSpaces {
                     span: arr.span,
-                    sub: ArrayBracketsInsteadOfSpacesSugg {
+                    sub: errors::ArrayBracketsInsteadOfSpacesSugg {
                         left: lo,
                         right: snapshot.prev_token.span,
                     },
@@ -2041,7 +2060,7 @@ impl<'a> Parser<'a> {
                         .span_to_snippet(snapshot.token.span)
                         .map_or(false, |snippet| snippet == "]") =>
                 {
-                    return Err(MissingSemicolonBeforeArray {
+                    return Err(errors::MissingSemicolonBeforeArray {
                         open_delim: open_delim_span,
                         semicolon: prev_span.shrink_to_hi(),
                     }.into_diagnostic(&self.sess.span_diagnostic));
@@ -2054,7 +2073,7 @@ impl<'a> Parser<'a> {
     }
 
     /// Parses a block or unsafe block.
-    pub(super) fn parse_block_expr(
+    pub(super) fn parse_expr_block(
         &mut self,
         opt_label: Option<Label>,
         lo: Span,
@@ -2067,7 +2086,7 @@ impl<'a> Parser<'a> {
         }
 
         if self.token.is_whole_block() {
-            self.sess.emit_err(InvalidBlockMacroSegment {
+            self.sess.emit_err(errors::InvalidBlockMacroSegment {
                 span: self.token.span,
                 context: lo.to(self.token.span),
             });
@@ -2084,7 +2103,7 @@ impl<'a> Parser<'a> {
     }
 
     /// Parses a closure expression (e.g., `move |args| expr`).
-    fn parse_closure_expr(&mut self) -> PResult<'a, P<Expr>> {
+    fn parse_expr_closure(&mut self) -> PResult<'a, P<Expr>> {
         let lo = self.token.span;
 
         let binder = if self.check_keyword(kw::For) {
@@ -2094,12 +2113,12 @@ impl<'a> Parser<'a> {
 
             self.sess.gated_spans.gate(sym::closure_lifetime_binder, span);
 
-            ClosureBinder::For { span, generic_params: P::from_vec(lifetime_defs) }
+            ClosureBinder::For { span, generic_params: lifetime_defs }
         } else {
             ClosureBinder::NotPresent
         };
 
-        let constness = self.parse_constness(Case::Sensitive);
+        let constness = self.parse_closure_constness(Case::Sensitive);
 
         let movability =
             if self.eat_keyword(kw::Static) { Movability::Static } else { Movability::Movable };
@@ -2121,7 +2140,7 @@ impl<'a> Parser<'a> {
             _ => {
                 // If an explicit return type is given, require a block to appear (RFC 968).
                 let body_lo = self.token.span;
-                self.parse_block_expr(None, body_lo, BlockCheckMode::Default)?
+                self.parse_expr_block(None, body_lo, BlockCheckMode::Default)?
             }
         };
 
@@ -2131,7 +2150,7 @@ impl<'a> Parser<'a> {
         }
 
         if self.token.kind == TokenKind::Semi
-            && matches!(self.token_cursor.frame.delim_sp, Some((Delimiter::Parenthesis, _)))
+            && matches!(self.token_cursor.stack.last(), Some((_, Delimiter::Parenthesis, _)))
             && self.may_recover()
         {
             // It is likely that the closure body is a block but where the
@@ -2171,7 +2190,7 @@ impl<'a> Parser<'a> {
             // Check for `move async` and recover
             if self.check_keyword(kw::Async) {
                 let move_async_span = self.token.span.with_lo(self.prev_token.span.data().lo);
-                Err(AsyncMoveOrderIncorrect { span: move_async_span }
+                Err(errors::AsyncMoveOrderIncorrect { span: move_async_span }
                     .into_diagnostic(&self.sess.span_diagnostic))
             } else {
                 Ok(CaptureBy::Value)
@@ -2186,7 +2205,7 @@ impl<'a> Parser<'a> {
         let arg_start = self.token.span.lo();
 
         let inputs = if self.eat(&token::OrOr) {
-            Vec::new()
+            ThinVec::new()
         } else {
             self.expect(&token::BinOp(token::Or))?;
             let args = self
@@ -2212,7 +2231,7 @@ impl<'a> Parser<'a> {
         let lo = self.token.span;
         let attrs = self.parse_outer_attributes()?;
         self.collect_tokens_trailing_token(attrs, ForceCollect::No, |this, attrs| {
-            let pat = this.parse_pat_no_top_alt(PARAM_EXPECTED)?;
+            let pat = this.parse_pat_no_top_alt(Some(Expected::ParameterName))?;
             let ty = if this.eat(&token::Colon) {
                 this.parse_ty()?
             } else {
@@ -2234,9 +2253,9 @@ impl<'a> Parser<'a> {
     }
 
     /// Parses an `if` expression (`if` token already eaten).
-    fn parse_if_expr(&mut self) -> PResult<'a, P<Expr>> {
+    fn parse_expr_if(&mut self) -> PResult<'a, P<Expr>> {
         let lo = self.prev_token.span;
-        let cond = self.parse_cond_expr()?;
+        let cond = self.parse_expr_cond()?;
         self.parse_if_after_cond(lo, cond)
     }
 
@@ -2249,16 +2268,17 @@ impl<'a> Parser<'a> {
             let block = match &mut cond.kind {
                 ExprKind::Binary(Spanned { span: binop_span, .. }, _, right)
                     if let ExprKind::Block(_, None) = right.kind => {
-                        self.sess.emit_err(IfExpressionMissingThenBlock {
+                        self.sess.emit_err(errors::IfExpressionMissingThenBlock {
                             if_span: lo,
-                            sub: IfExpressionMissingThenBlockSub::UnfinishedCondition(
-                                cond_span.shrink_to_lo().to(*binop_span)
-                            ),
+                            missing_then_block_sub:
+                                errors::IfExpressionMissingThenBlockSub::UnfinishedCondition(cond_span.shrink_to_lo().to(*binop_span)),
+                                let_else_sub: None,
+
                         });
                         std::mem::replace(right, this.mk_expr_err(binop_span.shrink_to_hi()))
                     },
                 ExprKind::Block(_, None) => {
-                    self.sess.emit_err(IfExpressionMissingCondition {
+                    self.sess.emit_err(errors::IfExpressionMissingCondition {
                         if_span: lo.shrink_to_hi(),
                         block_span: self.sess.source_map().start_point(cond_span),
                     });
@@ -2279,9 +2299,15 @@ impl<'a> Parser<'a> {
             if let Some(block) = recover_block_from_condition(self) {
                 block
             } else {
-                self.sess.emit_err(IfExpressionMissingThenBlock {
+                let let_else_sub = matches!(cond.kind, ExprKind::Let(..))
+                    .then(|| errors::IfExpressionLetSomeSub { if_span: lo.until(cond_span) });
+
+                self.sess.emit_err(errors::IfExpressionMissingThenBlock {
                     if_span: lo,
-                    sub: IfExpressionMissingThenBlockSub::AddThenBlock(cond_span.shrink_to_hi()),
+                    missing_then_block_sub: errors::IfExpressionMissingThenBlockSub::AddThenBlock(
+                        cond_span.shrink_to_hi(),
+                    ),
+                    let_else_sub,
                 });
                 self.mk_block_err(cond_span.shrink_to_hi())
             }
@@ -2307,12 +2333,12 @@ impl<'a> Parser<'a> {
             self.error_on_if_block_attrs(lo, false, block.span, attrs);
             block
         };
-        let els = if self.eat_keyword(kw::Else) { Some(self.parse_else_expr()?) } else { None };
+        let els = if self.eat_keyword(kw::Else) { Some(self.parse_expr_else()?) } else { None };
         Ok(self.mk_expr(lo.to(self.prev_token.span), ExprKind::If(cond, thn, els)))
     }
 
     /// Parses the condition of a `if` or `while` expression.
-    fn parse_cond_expr(&mut self) -> PResult<'a, P<Expr>> {
+    fn parse_expr_cond(&mut self) -> PResult<'a, P<Expr>> {
         let cond =
             self.parse_expr_res(Restrictions::NO_STRUCT_LITERAL | Restrictions::ALLOW_LET, None)?;
 
@@ -2325,7 +2351,7 @@ impl<'a> Parser<'a> {
     }
 
     /// Parses a `let $pat = $expr` pseudo-expression.
-    fn parse_let_expr(&mut self) -> PResult<'a, P<Expr>> {
+    fn parse_expr_let(&mut self) -> PResult<'a, P<Expr>> {
         // This is a *approximate* heuristic that detects if `let` chains are
         // being parsed in the right position. It's approximate because it
         // doesn't deny all invalid `let` expressions, just completely wrong usages.
@@ -2334,7 +2360,7 @@ impl<'a> Parser<'a> {
             TokenKind::AndAnd | TokenKind::Ident(kw::If, _) | TokenKind::Ident(kw::While, _)
         );
         if !self.restrictions.contains(Restrictions::ALLOW_LET) || not_in_chain {
-            self.sess.emit_err(ExpectedExpressionFoundLet { span: self.token.span });
+            self.sess.emit_err(errors::ExpectedExpressionFoundLet { span: self.token.span });
         }
 
         self.bump(); // Eat `let` token
@@ -2346,7 +2372,7 @@ impl<'a> Parser<'a> {
             CommaRecoveryMode::LikelyTuple,
         )?;
         if self.token == token::EqEq {
-            self.sess.emit_err(ExpectedEqForLetExpr {
+            self.sess.emit_err(errors::ExpectedEqForLetExpr {
                 span: self.token.span,
                 sugg_span: self.token.span,
             });
@@ -2355,7 +2381,7 @@ impl<'a> Parser<'a> {
             self.expect(&token::Eq)?;
         }
         let expr = self.with_res(self.restrictions | Restrictions::NO_STRUCT_LITERAL, |this| {
-            this.parse_assoc_expr_with(1 + prec_let_scrutinee_needs_par(), None.into())
+            this.parse_expr_assoc_with(1 + prec_let_scrutinee_needs_par(), None.into())
         })?;
         let span = lo.to(expr.span);
         self.sess.gated_spans.gate(sym::let_chains, span);
@@ -2363,11 +2389,11 @@ impl<'a> Parser<'a> {
     }
 
     /// Parses an `else { ... }` expression (`else` token already eaten).
-    fn parse_else_expr(&mut self) -> PResult<'a, P<Expr>> {
+    fn parse_expr_else(&mut self) -> PResult<'a, P<Expr>> {
         let else_span = self.prev_token.span; // `else`
         let attrs = self.parse_outer_attributes()?; // For recovery.
         let expr = if self.eat_keyword(kw::If) {
-            self.parse_if_expr()?
+            self.parse_expr_if()?
         } else if self.check(&TokenKind::OpenDelim(Delimiter::Brace)) {
             self.parse_simple_block()?
         } else {
@@ -2381,7 +2407,7 @@ impl<'a> Parser<'a> {
                     if self.check(&TokenKind::OpenDelim(Delimiter::Brace))
                         && classify::expr_requires_semi_to_be_stmt(&cond) =>
                 {
-                    self.sess.emit_err(ExpectedElseBlock {
+                    self.sess.emit_err(errors::ExpectedElseBlock {
                         first_tok_span,
                         first_tok,
                         else_span,
@@ -2421,7 +2447,7 @@ impl<'a> Parser<'a> {
             [x0 @ xn] | [x0, .., xn] => (x0.span.to(xn.span), xn.span),
         };
         let ctx = if is_ctx_else { "else" } else { "if" };
-        self.sess.emit_err(OuterAttributeNotAllowedOnIfElse {
+        self.sess.emit_err(errors::OuterAttributeNotAllowedOnIfElse {
             last,
             branch_span,
             ctx_span,
@@ -2434,14 +2460,14 @@ impl<'a> Parser<'a> {
         if let ExprKind::Binary(Spanned { span: binop_span, node: binop}, _, right) = &cond.kind &&
             let BinOpKind::And = binop &&
             let ExprKind::If(cond, ..) = &right.kind {
-                    Err(self.sess.create_err(UnexpectedIfWithIf(binop_span.shrink_to_hi().to(cond.span.shrink_to_lo()))))
+                    Err(self.sess.create_err(errors::UnexpectedIfWithIf(binop_span.shrink_to_hi().to(cond.span.shrink_to_lo()))))
             } else {
                 Ok(())
             }
     }
 
     /// Parses `for <src_pat> in <src_expr> <src_loop_block>` (`for` token already eaten).
-    fn parse_for_expr(&mut self, opt_label: Option<Label>, lo: Span) -> PResult<'a, P<Expr>> {
+    fn parse_expr_for(&mut self, opt_label: Option<Label>, lo: Span) -> PResult<'a, P<Expr>> {
         // Record whether we are about to parse `for (`.
         // This is used below for recovery in case of `for ( $stuff ) $block`
         // in which case we will suggest `for $stuff $block`.
@@ -2464,28 +2490,61 @@ impl<'a> Parser<'a> {
 
         let pat = self.recover_parens_around_for_head(pat, begin_paren);
 
+        // Recover from missing expression in `for` loop
+        if matches!(expr.kind, ExprKind::Block(..))
+            && !matches!(self.token.kind, token::OpenDelim(token::Delimiter::Brace))
+            && self.may_recover()
+        {
+            self.sess
+                .emit_err(errors::MissingExpressionInForLoop { span: expr.span.shrink_to_lo() });
+            let err_expr = self.mk_expr(expr.span, ExprKind::Err);
+            let block = self.mk_block(thin_vec![], BlockCheckMode::Default, self.prev_token.span);
+            return Ok(self.mk_expr(
+                lo.to(self.prev_token.span),
+                ExprKind::ForLoop(pat, err_expr, block, opt_label),
+            ));
+        }
+
         let (attrs, loop_block) = self.parse_inner_attrs_and_block()?;
 
         let kind = ExprKind::ForLoop(pat, expr, loop_block, opt_label);
+
+        self.recover_loop_else("for", lo)?;
+
         Ok(self.mk_expr_with_attrs(lo.to(self.prev_token.span), kind, attrs))
     }
 
+    /// Recovers from an `else` clause after a loop (`for...else`, `while...else`)
+    fn recover_loop_else(&mut self, loop_kind: &'static str, loop_kw: Span) -> PResult<'a, ()> {
+        if self.token.is_keyword(kw::Else) && self.may_recover() {
+            let else_span = self.token.span;
+            self.bump();
+            let else_clause = self.parse_expr_else()?;
+            self.sess.emit_err(errors::LoopElseNotSupported {
+                span: else_span.to(else_clause.span),
+                loop_kind,
+                loop_kw,
+            });
+        }
+        Ok(())
+    }
+
     fn error_missing_in_for_loop(&mut self) {
         let (span, sub): (_, fn(_) -> _) = if self.token.is_ident_named(sym::of) {
             // Possibly using JS syntax (#75311).
             let span = self.token.span;
             self.bump();
-            (span, MissingInInForLoopSub::InNotOf)
+            (span, errors::MissingInInForLoopSub::InNotOf)
         } else {
-            (self.prev_token.span.between(self.token.span), MissingInInForLoopSub::AddIn)
+            (self.prev_token.span.between(self.token.span), errors::MissingInInForLoopSub::AddIn)
         };
 
-        self.sess.emit_err(MissingInInForLoop { span, sub: sub(span) });
+        self.sess.emit_err(errors::MissingInInForLoop { span, sub: sub(span) });
     }
 
     /// Parses a `while` or `while let` expression (`while` token already eaten).
-    fn parse_while_expr(&mut self, opt_label: Option<Label>, lo: Span) -> PResult<'a, P<Expr>> {
-        let cond = self.parse_cond_expr().map_err(|mut err| {
+    fn parse_expr_while(&mut self, opt_label: Option<Label>, lo: Span) -> PResult<'a, P<Expr>> {
+        let cond = self.parse_expr_cond().map_err(|mut err| {
             err.span_label(lo, "while parsing the condition of this `while` expression");
             err
         })?;
@@ -2494,6 +2553,9 @@ impl<'a> Parser<'a> {
             err.span_label(cond.span, "this `while` condition successfully parsed");
             err
         })?;
+
+        self.recover_loop_else("while", lo)?;
+
         Ok(self.mk_expr_with_attrs(
             lo.to(self.prev_token.span),
             ExprKind::While(cond, body, opt_label),
@@ -2502,9 +2564,10 @@ impl<'a> Parser<'a> {
     }
 
     /// Parses `loop { ... }` (`loop` token already eaten).
-    fn parse_loop_expr(&mut self, opt_label: Option<Label>, lo: Span) -> PResult<'a, P<Expr>> {
+    fn parse_expr_loop(&mut self, opt_label: Option<Label>, lo: Span) -> PResult<'a, P<Expr>> {
         let loop_span = self.prev_token.span;
         let (attrs, body) = self.parse_inner_attrs_and_block()?;
+        self.recover_loop_else("loop", lo)?;
         Ok(self.mk_expr_with_attrs(
             lo.to(self.prev_token.span),
             ExprKind::Loop(body, opt_label, loop_span),
@@ -2520,7 +2583,7 @@ impl<'a> Parser<'a> {
     }
 
     /// Parses a `match ... { ... }` expression (`match` token already eaten).
-    fn parse_match_expr(&mut self) -> PResult<'a, P<Expr>> {
+    fn parse_expr_match(&mut self) -> PResult<'a, P<Expr>> {
         let match_span = self.prev_token.span;
         let lo = self.prev_token.span;
         let scrutinee = self.parse_expr_res(Restrictions::NO_STRUCT_LITERAL, None)?;
@@ -2542,7 +2605,7 @@ impl<'a> Parser<'a> {
         }
         let attrs = self.parse_inner_attributes()?;
 
-        let mut arms: Vec<Arm> = Vec::new();
+        let mut arms = ThinVec::new();
         while self.token != token::CloseDelim(Delimiter::Brace) {
             match self.parse_arm() {
                 Ok(arm) => arms.push(arm),
@@ -2584,17 +2647,17 @@ impl<'a> Parser<'a> {
         let err = |this: &Parser<'_>, stmts: Vec<ast::Stmt>| {
             let span = stmts[0].span.to(stmts[stmts.len() - 1].span);
 
-            this.sess.emit_err(MatchArmBodyWithoutBraces {
+            this.sess.emit_err(errors::MatchArmBodyWithoutBraces {
                 statements: span,
                 arrow: arrow_span,
                 num_statements: stmts.len(),
                 sub: if stmts.len() > 1 {
-                    MatchArmBodyWithoutBracesSugg::AddBraces {
+                    errors::MatchArmBodyWithoutBracesSugg::AddBraces {
                         left: span.shrink_to_lo(),
                         right: span.shrink_to_hi(),
                     }
                 } else {
-                    MatchArmBodyWithoutBracesSugg::UseComma { semicolon: semi_sp }
+                    errors::MatchArmBodyWithoutBracesSugg::UseComma { semicolon: semi_sp }
                 },
             });
             this.mk_expr_err(span)
@@ -2700,6 +2763,14 @@ impl<'a> Parser<'a> {
                     );
                     err.emit();
                     this.bump();
+                } else if matches!(
+                    (&this.prev_token.kind, &this.token.kind),
+                    (token::DotDotEq, token::Gt)
+                ) {
+                    // `error_inclusive_range_match_arrow` handles cases like `0..=> {}`,
+                    // so we supress the error here
+                    err.delay_as_bug();
+                    this.bump();
                 } else {
                     return Err(err);
                 }
@@ -2777,7 +2848,7 @@ impl<'a> Parser<'a> {
                                 .is_ok();
                             if pattern_follows && snapshot.check(&TokenKind::FatArrow) {
                                 err.cancel();
-                                this.sess.emit_err(MissingCommaAfterMatchArm {
+                                this.sess.emit_err(errors::MissingCommaAfterMatchArm {
                                     span: hi.shrink_to_hi(),
                                 });
                                 return Ok(true);
@@ -2809,7 +2880,7 @@ impl<'a> Parser<'a> {
     fn parse_try_block(&mut self, span_lo: Span) -> PResult<'a, P<Expr>> {
         let (attrs, body) = self.parse_inner_attrs_and_block()?;
         if self.eat_keyword(kw::Catch) {
-            Err(CatchAfterTry { span: self.prev_token.span }
+            Err(errors::CatchAfterTry { span: self.prev_token.span }
                 .into_diagnostic(&self.sess.span_diagnostic))
         } else {
             let span = span_lo.to(body.span);
@@ -2882,12 +2953,12 @@ impl<'a> Parser<'a> {
             if let Err(err) = self.expect(&token::OpenDelim(Delimiter::Brace)) {
                 return Some(Err(err));
             }
-            let expr = self.parse_struct_expr(qself.clone(), path.clone(), true);
+            let expr = self.parse_expr_struct(qself.clone(), path.clone(), true);
             if let (Ok(expr), false) = (&expr, struct_allowed) {
                 // This is a struct literal, but we don't can't accept them here.
-                self.sess.emit_err(StructLiteralNotAllowedHere {
+                self.sess.emit_err(errors::StructLiteralNotAllowedHere {
                     span: expr.span,
-                    sub: StructLiteralNotAllowedHereSugg {
+                    sub: errors::StructLiteralNotAllowedHereSugg {
                         left: path.span.shrink_to_lo(),
                         right: expr.span.shrink_to_hi(),
                     },
@@ -2903,15 +2974,15 @@ impl<'a> Parser<'a> {
         pth: ast::Path,
         recover: bool,
         close_delim: Delimiter,
-    ) -> PResult<'a, (Vec<ExprField>, ast::StructRest, bool)> {
-        let mut fields = Vec::new();
+    ) -> PResult<'a, (ThinVec<ExprField>, ast::StructRest, bool)> {
+        let mut fields = ThinVec::new();
         let mut base = ast::StructRest::None;
         let mut recover_async = false;
 
         let mut async_block_err = |e: &mut Diagnostic, span: Span| {
             recover_async = true;
-            e.span_label(span, "`async` blocks are only allowed in Rust 2018 or later");
-            e.help_use_latest_edition();
+            errors::AsyncBlockIn2015 { span }.add_to_diagnostic(e);
+            errors::HelpUseLatestEdition::new().add_to_diagnostic(e);
         };
 
         while self.token != token::CloseDelim(close_delim) {
@@ -3011,7 +3082,7 @@ impl<'a> Parser<'a> {
     }
 
     /// Precondition: already parsed the '{'.
-    pub(super) fn parse_struct_expr(
+    pub(super) fn parse_expr_struct(
         &mut self,
         qself: Option<P<ast::QSelf>>,
         pth: ast::Path,
@@ -3055,7 +3126,7 @@ impl<'a> Parser<'a> {
         if self.token != token::Comma {
             return;
         }
-        self.sess.emit_err(CommaAfterBaseStruct {
+        self.sess.emit_err(errors::CommaAfterBaseStruct {
             span: span.to(self.prev_token.span),
             comma: self.token.span,
         });
@@ -3068,7 +3139,7 @@ impl<'a> Parser<'a> {
         {
             // recover from typo of `...`, suggest `..`
             let span = self.prev_token.span;
-            self.sess.emit_err(MissingDotDot { token_span: span, sugg_span: span });
+            self.sess.emit_err(errors::MissingDotDot { token_span: span, sugg_span: span });
             return true;
         }
         false
@@ -3136,18 +3207,18 @@ impl<'a> Parser<'a> {
             return;
         }
 
-        self.sess.emit_err(EqFieldInit {
+        self.sess.emit_err(errors::EqFieldInit {
             span: self.token.span,
             eq: field_name.span.shrink_to_hi().to(self.token.span),
         });
     }
 
     fn err_dotdotdot_syntax(&self, span: Span) {
-        self.sess.emit_err(DotDotDot { span });
+        self.sess.emit_err(errors::DotDotDot { span });
     }
 
     fn err_larrow_operator(&self, span: Span) {
-        self.sess.emit_err(LeftArrowOperator { span });
+        self.sess.emit_err(errors::LeftArrowOperator { span });
     }
 
     fn mk_assign_op(&self, binop: BinOp, lhs: P<Expr>, rhs: P<Expr>) -> ExprKind {
@@ -3161,7 +3232,7 @@ impl<'a> Parser<'a> {
         limits: RangeLimits,
     ) -> ExprKind {
         if end.is_none() && limits == RangeLimits::Closed {
-            self.inclusive_range_with_incorrect_end(self.prev_token.span);
+            self.inclusive_range_with_incorrect_end();
             ExprKind::Err
         } else {
             ExprKind::Range(start, end, limits)
@@ -3180,7 +3251,7 @@ impl<'a> Parser<'a> {
         ExprKind::Index(expr, idx)
     }
 
-    fn mk_call(&self, f: P<Expr>, args: Vec<P<Expr>>) -> ExprKind {
+    fn mk_call(&self, f: P<Expr>, args: ThinVec<P<Expr>>) -> ExprKind {
         ExprKind::Call(f, args)
     }
 
diff --git a/compiler/rustc_parse/src/parser/generics.rs b/compiler/rustc_parse/src/parser/generics.rs
index 8ba811715..8d0f168e0 100644
--- a/compiler/rustc_parse/src/parser/generics.rs
+++ b/compiler/rustc_parse/src/parser/generics.rs
@@ -1,4 +1,8 @@
-use crate::errors::{WhereClauseBeforeTupleStructBody, WhereClauseBeforeTupleStructBodySugg};
+use crate::errors::{
+    MultipleWhereClauses, UnexpectedDefaultValueForLifetimeInGenericParameters,
+    UnexpectedSelfInGenericParameters, WhereClauseBeforeTupleStructBody,
+    WhereClauseBeforeTupleStructBodySugg,
+};
 
 use super::{ForceCollect, Parser, TrailingToken};
 
@@ -10,10 +14,11 @@ use rustc_ast::{
 use rustc_errors::{Applicability, PResult};
 use rustc_span::symbol::{kw, Ident};
 use rustc_span::Span;
+use thin_vec::ThinVec;
 
 enum PredicateOrStructBody {
     Predicate(ast::WherePredicate),
-    StructBody(Vec<ast::FieldDef>),
+    StructBody(ThinVec<ast::FieldDef>),
 }
 
 impl<'a> Parser<'a> {
@@ -117,8 +122,8 @@ impl<'a> Parser<'a> {
 
     /// Parses a (possibly empty) list of lifetime and type parameters, possibly including
     /// a trailing comma and erroneous trailing attributes.
-    pub(super) fn parse_generic_params(&mut self) -> PResult<'a, Vec<ast::GenericParam>> {
-        let mut params = Vec::new();
+    pub(super) fn parse_generic_params(&mut self) -> PResult<'a, ThinVec<ast::GenericParam>> {
+        let mut params = ThinVec::new();
         let mut done = false;
         while !done {
             let attrs = self.parse_outer_attributes()?;
@@ -127,12 +132,9 @@ impl<'a> Parser<'a> {
                     if this.eat_keyword_noexpect(kw::SelfUpper) {
                         // `Self` as a generic param is invalid. Here we emit the diagnostic and continue parsing
                         // as if `Self` never existed.
-                        this.struct_span_err(
-                            this.prev_token.span,
-                            "unexpected keyword `Self` in generic parameters",
-                        )
-                        .note("you cannot use `Self` as a generic parameter because it is reserved for associated items")
-                        .emit();
+                        this.sess.emit_err(UnexpectedSelfInGenericParameters {
+                            span: this.prev_token.span,
+                        });
 
                         this.eat(&token::Comma);
                     }
@@ -145,6 +147,20 @@ impl<'a> Parser<'a> {
                         } else {
                             (None, Vec::new())
                         };
+
+                        if this.check_noexpect(&token::Eq)
+                            && this.look_ahead(1, |t| t.is_lifetime())
+                        {
+                            let lo = this.token.span;
+                            // Parse `= 'lifetime`.
+                            this.bump(); // `=`
+                            this.bump(); // `'lifetime`
+                            let span = lo.to(this.prev_token.span);
+                            this.sess.emit_err(
+                                UnexpectedDefaultValueForLifetimeInGenericParameters { span },
+                            );
+                        }
+
                         Some(ast::GenericParam {
                             ident: lifetime.ident,
                             id: lifetime.id,
@@ -236,13 +252,13 @@ impl<'a> Parser<'a> {
             self.expect_gt()?;
             (params, span_lo.to(self.prev_token.span))
         } else {
-            (vec![], self.prev_token.span.shrink_to_hi())
+            (ThinVec::new(), self.prev_token.span.shrink_to_hi())
         };
         Ok(ast::Generics {
             params,
             where_clause: WhereClause {
                 has_where_token: false,
-                predicates: Vec::new(),
+                predicates: ThinVec::new(),
                 span: self.prev_token.span.shrink_to_hi(),
             },
             span,
@@ -262,17 +278,17 @@ impl<'a> Parser<'a> {
         &mut self,
         struct_name: Ident,
         body_insertion_point: Span,
-    ) -> PResult<'a, (WhereClause, Option<Vec<ast::FieldDef>>)> {
+    ) -> PResult<'a, (WhereClause, Option<ThinVec<ast::FieldDef>>)> {
         self.parse_where_clause_common(Some((struct_name, body_insertion_point)))
     }
 
     fn parse_where_clause_common(
         &mut self,
         struct_: Option<(Ident, Span)>,
-    ) -> PResult<'a, (WhereClause, Option<Vec<ast::FieldDef>>)> {
+    ) -> PResult<'a, (WhereClause, Option<ThinVec<ast::FieldDef>>)> {
         let mut where_clause = WhereClause {
             has_where_token: false,
-            predicates: Vec::new(),
+            predicates: ThinVec::new(),
             span: self.prev_token.span.shrink_to_hi(),
         };
         let mut tuple_struct_body = None;
@@ -329,16 +345,11 @@ impl<'a> Parser<'a> {
             let ate_comma = self.eat(&token::Comma);
 
             if self.eat_keyword_noexpect(kw::Where) {
-                let msg = "cannot define duplicate `where` clauses on an item";
-                let mut err = self.struct_span_err(self.token.span, msg);
-                err.span_label(pred_lo, "previous `where` clause starts here");
-                err.span_suggestion_verbose(
-                    prev_token.shrink_to_hi().to(self.prev_token.span),
-                    "consider joining the two `where` clauses into one",
-                    ",",
-                    Applicability::MaybeIncorrect,
-                );
-                err.emit();
+                self.sess.emit_err(MultipleWhereClauses {
+                    span: self.token.span,
+                    previous: pred_lo,
+                    between: prev_token.shrink_to_hi().to(self.prev_token.span),
+                });
             } else if !ate_comma {
                 break;
             }
diff --git a/compiler/rustc_parse/src/parser/item.rs b/compiler/rustc_parse/src/parser/item.rs
index 53680a82b..9d9ae154a 100644
--- a/compiler/rustc_parse/src/parser/item.rs
+++ b/compiler/rustc_parse/src/parser/item.rs
@@ -1,9 +1,8 @@
-use crate::errors::{DocCommentDoesNotDocumentAnything, UseEmptyBlockNotSemi};
+use crate::errors;
 
 use super::diagnostics::{dummy_arg, ConsumeClosingDelim};
 use super::ty::{AllowPlus, RecoverQPath, RecoverReturnSign};
 use super::{AttrWrapper, FollowedByType, ForceCollect, Parser, PathStyle, TrailingToken};
-use crate::errors::FnTypoWithImpl;
 use rustc_ast::ast::*;
 use rustc_ast::ptr::P;
 use rustc_ast::token::{self, Delimiter, TokenKind};
@@ -16,14 +15,18 @@ use rustc_ast::{EnumDef, FieldDef, Generics, TraitRef, Ty, TyKind, Variant, Vari
 use rustc_ast::{FnHeader, ForeignItem, Path, PathSegment, Visibility, VisibilityKind};
 use rustc_ast::{MacCall, MacDelimiter};
 use rustc_ast_pretty::pprust;
-use rustc_errors::{struct_span_err, Applicability, IntoDiagnostic, PResult, StashKey};
+use rustc_errors::{
+    struct_span_err, Applicability, DiagnosticBuilder, ErrorGuaranteed, IntoDiagnostic, PResult,
+    StashKey,
+};
+use rustc_span::edit_distance::edit_distance;
 use rustc_span::edition::Edition;
-use rustc_span::lev_distance::lev_distance;
 use rustc_span::source_map::{self, Span};
 use rustc_span::symbol::{kw, sym, Ident, Symbol};
 use rustc_span::DUMMY_SP;
+use std::fmt::Write;
 use std::mem;
-use thin_vec::ThinVec;
+use thin_vec::{thin_vec, ThinVec};
 
 impl<'a> Parser<'a> {
     /// Parses a source module as a crate. This is the main entry point for the parser.
@@ -53,12 +56,12 @@ impl<'a> Parser<'a> {
     pub fn parse_mod(
         &mut self,
         term: &TokenKind,
-    ) -> PResult<'a, (AttrVec, Vec<P<Item>>, ModSpans)> {
+    ) -> PResult<'a, (AttrVec, ThinVec<P<Item>>, ModSpans)> {
         let lo = self.token.span;
         let attrs = self.parse_inner_attributes()?;
 
         let post_attr_lo = self.token.span;
-        let mut items = vec![];
+        let mut items = ThinVec::new();
         while let Some(item) = self.parse_item(ForceCollect::No)? {
             items.push(item);
             self.maybe_consume_incorrect_semicolon(&items);
@@ -163,35 +166,18 @@ impl<'a> Parser<'a> {
         }
 
         // At this point, we have failed to parse an item.
-        self.error_on_unmatched_vis(&vis);
-        self.error_on_unmatched_defaultness(def);
-        if !attrs_allowed {
-            self.recover_attrs_no_item(&attrs)?;
+        if !matches!(vis.kind, VisibilityKind::Inherited) {
+            self.sess.emit_err(errors::VisibilityNotFollowedByItem { span: vis.span, vis });
         }
-        Ok(None)
-    }
 
-    /// Error in-case a non-inherited visibility was parsed but no item followed.
-    fn error_on_unmatched_vis(&self, vis: &Visibility) {
-        if let VisibilityKind::Inherited = vis.kind {
-            return;
+        if let Defaultness::Default(span) = def {
+            self.sess.emit_err(errors::DefaultNotFollowedByItem { span });
         }
-        let vs = pprust::vis_to_string(&vis);
-        let vs = vs.trim_end();
-        self.struct_span_err(vis.span, &format!("visibility `{vs}` is not followed by an item"))
-            .span_label(vis.span, "the visibility")
-            .help(&format!("you likely meant to define an item, e.g., `{vs} fn foo() {{}}`"))
-            .emit();
-    }
 
-    /// Error in-case a `default` was parsed but no item followed.
-    fn error_on_unmatched_defaultness(&self, def: Defaultness) {
-        if let Defaultness::Default(sp) = def {
-            self.struct_span_err(sp, "`default` is not followed by an item")
-                .span_label(sp, "the `default` qualifier")
-                .note("only `fn`, `const`, `type`, or `impl` items may be prefixed by `default`")
-                .emit();
+        if !attrs_allowed {
+            self.recover_attrs_no_item(&attrs)?;
         }
+        Ok(None)
     }
 
     /// Error in-case `default` was parsed in an in-appropriate context.
@@ -384,86 +370,74 @@ impl<'a> Parser<'a> {
         let sp = self.prev_token.span.between(self.token.span);
         let full_sp = self.prev_token.span.to(self.token.span);
         let ident_sp = self.token.span;
-        if self.look_ahead(1, |t| *t == token::OpenDelim(Delimiter::Brace)) {
+
+        let ident = if self.look_ahead(1, |t| {
+            [
+                token::Lt,
+                token::OpenDelim(Delimiter::Brace),
+                token::OpenDelim(Delimiter::Parenthesis),
+            ]
+            .contains(&t.kind)
+        }) {
+            self.parse_ident().unwrap()
+        } else {
+            return Ok(());
+        };
+
+        let mut found_generics = false;
+        if self.check(&token::Lt) {
+            found_generics = true;
+            self.eat_to_tokens(&[&token::Gt]);
+            self.bump(); // `>`
+        }
+
+        let err = if self.check(&token::OpenDelim(Delimiter::Brace)) {
             // possible public struct definition where `struct` was forgotten
-            let ident = self.parse_ident().unwrap();
-            let msg = format!("add `struct` here to parse `{ident}` as a public struct");
-            let mut err = self.struct_span_err(sp, "missing `struct` for struct definition");
-            err.span_suggestion_short(
-                sp,
-                &msg,
-                " struct ",
-                Applicability::MaybeIncorrect, // speculative
-            );
-            Err(err)
-        } else if self.look_ahead(1, |t| *t == token::OpenDelim(Delimiter::Parenthesis)) {
-            let ident = self.parse_ident().unwrap();
+            Some(errors::MissingKeywordForItemDefinition::Struct { span: sp, ident })
+        } else if self.check(&token::OpenDelim(Delimiter::Parenthesis)) {
+            // possible public function or tuple struct definition where `fn`/`struct` was
+            // forgotten
             self.bump(); // `(`
-            let kw_name = self.recover_first_param();
+            let is_method = self.recover_self_param();
+
             self.consume_block(Delimiter::Parenthesis, ConsumeClosingDelim::Yes);
-            let (kw, kw_name, ambiguous) = if self.check(&token::RArrow) {
-                self.eat_to_tokens(&[&token::OpenDelim(Delimiter::Brace)]);
-                self.bump(); // `{`
-                ("fn", kw_name, false)
-            } else if self.check(&token::OpenDelim(Delimiter::Brace)) {
-                self.bump(); // `{`
-                ("fn", kw_name, false)
-            } else if self.check(&token::Colon) {
-                let kw = "struct";
-                (kw, kw, false)
-            } else {
-                ("fn` or `struct", "function or struct", true)
-            };
 
-            let msg = format!("missing `{kw}` for {kw_name} definition");
-            let mut err = self.struct_span_err(sp, &msg);
-            if !ambiguous {
-                self.consume_block(Delimiter::Brace, ConsumeClosingDelim::Yes);
-                let suggestion =
-                    format!("add `{kw}` here to parse `{ident}` as a public {kw_name}");
-                err.span_suggestion_short(
-                    sp,
-                    &suggestion,
-                    format!(" {kw} "),
-                    Applicability::MachineApplicable,
-                );
-            } else if let Ok(snippet) = self.span_to_snippet(ident_sp) {
-                err.span_suggestion(
-                    full_sp,
-                    "if you meant to call a macro, try",
-                    format!("{}!", snippet),
-                    // this is the `ambiguous` conditional branch
-                    Applicability::MaybeIncorrect,
-                );
-            } else {
-                err.help(
-                    "if you meant to call a macro, remove the `pub` \
-                              and add a trailing `!` after the identifier",
-                );
-            }
-            Err(err)
-        } else if self.look_ahead(1, |t| *t == token::Lt) {
-            let ident = self.parse_ident().unwrap();
-            self.eat_to_tokens(&[&token::Gt]);
-            self.bump(); // `>`
-            let (kw, kw_name, ambiguous) = if self.eat(&token::OpenDelim(Delimiter::Parenthesis)) {
-                ("fn", self.recover_first_param(), false)
-            } else if self.check(&token::OpenDelim(Delimiter::Brace)) {
-                ("struct", "struct", false)
-            } else {
-                ("fn` or `struct", "function or struct", true)
-            };
-            let msg = format!("missing `{kw}` for {kw_name} definition");
-            let mut err = self.struct_span_err(sp, &msg);
-            if !ambiguous {
-                err.span_suggestion_short(
-                    sp,
-                    &format!("add `{kw}` here to parse `{ident}` as a public {kw_name}"),
-                    format!(" {} ", kw),
-                    Applicability::MachineApplicable,
-                );
-            }
-            Err(err)
+            let err =
+                if self.check(&token::RArrow) || self.check(&token::OpenDelim(Delimiter::Brace)) {
+                    self.eat_to_tokens(&[&token::OpenDelim(Delimiter::Brace)]);
+                    self.bump(); // `{`
+                    self.consume_block(Delimiter::Brace, ConsumeClosingDelim::Yes);
+                    if is_method {
+                        errors::MissingKeywordForItemDefinition::Method { span: sp, ident }
+                    } else {
+                        errors::MissingKeywordForItemDefinition::Function { span: sp, ident }
+                    }
+                } else if self.check(&token::Semi) {
+                    errors::MissingKeywordForItemDefinition::Struct { span: sp, ident }
+                } else {
+                    errors::MissingKeywordForItemDefinition::Ambiguous {
+                        span: sp,
+                        subdiag: if found_generics {
+                            None
+                        } else if let Ok(snippet) = self.span_to_snippet(ident_sp) {
+                            Some(errors::AmbiguousMissingKwForItemSub::SuggestMacro {
+                                span: full_sp,
+                                snippet,
+                            })
+                        } else {
+                            Some(errors::AmbiguousMissingKwForItemSub::HelpMacro)
+                        },
+                    }
+                };
+            Some(err)
+        } else if found_generics {
+            Some(errors::MissingKeywordForItemDefinition::Ambiguous { span: sp, subdiag: None })
+        } else {
+            None
+        };
+
+        if let Some(err) = err {
+            Err(err.into_diagnostic(&self.sess.span_diagnostic))
         } else {
             Ok(())
         }
@@ -485,7 +459,8 @@ impl<'a> Parser<'a> {
                 // Maybe the user misspelled `macro_rules` (issue #91227)
                 if self.token.is_ident()
                     && path.segments.len() == 1
-                    && lev_distance("macro_rules", &path.segments[0].ident.to_string(), 3).is_some()
+                    && edit_distance("macro_rules", &path.segments[0].ident.to_string(), 2)
+                        .is_some()
                 {
                     err.span_suggestion(
                         path.span,
@@ -512,16 +487,13 @@ impl<'a> Parser<'a> {
         let mut err = self.struct_span_err(end.span, msg);
         if end.is_doc_comment() {
             err.span_label(end.span, "this doc comment doesn't document anything");
-        }
-        if end.meta_kind().is_some() {
-            if self.token.kind == TokenKind::Semi {
-                err.span_suggestion_verbose(
-                    self.token.span,
-                    "consider removing this semicolon",
-                    "",
-                    Applicability::MaybeIncorrect,
-                );
-            }
+        } else if self.token.kind == TokenKind::Semi {
+            err.span_suggestion_verbose(
+                self.token.span,
+                "consider removing this semicolon",
+                "",
+                Applicability::MaybeIncorrect,
+            );
         }
         if let [.., penultimate, _] = attrs {
             err.span_label(start.span.to(penultimate.span), "other attributes here");
@@ -588,20 +560,11 @@ impl<'a> Parser<'a> {
         let ty_first = if self.token.is_keyword(kw::For) && self.look_ahead(1, |t| t != &token::Lt)
         {
             let span = self.prev_token.span.between(self.token.span);
-            self.struct_span_err(span, "missing trait in a trait impl")
-                .span_suggestion(
-                    span,
-                    "add a trait here",
-                    " Trait ",
-                    Applicability::HasPlaceholders,
-                )
-                .span_suggestion(
-                    span.to(self.token.span),
-                    "for an inherent impl, drop this `for`",
-                    "",
-                    Applicability::MaybeIncorrect,
-                )
-                .emit();
+            self.sess.emit_err(errors::MissingTraitInTraitImpl {
+                span,
+                for_span: span.to(self.token.span),
+            });
+
             P(Ty {
                 kind: TyKind::Path(None, err_path(span)),
                 span,
@@ -634,14 +597,7 @@ impl<'a> Parser<'a> {
             Some(ty_second) => {
                 // impl Trait for Type
                 if !has_for {
-                    self.struct_span_err(missing_for_span, "missing `for` in a trait impl")
-                        .span_suggestion_short(
-                            missing_for_span,
-                            "add `for` here",
-                            " for ",
-                            Applicability::MachineApplicable,
-                        )
-                        .emit();
+                    self.sess.emit_err(errors::MissingForInTraitImpl { span: missing_for_span });
                 }
 
                 let ty_first = ty_first.into_inner();
@@ -649,7 +605,9 @@ impl<'a> Parser<'a> {
                     // This notably includes paths passed through `ty` macro fragments (#46438).
                     TyKind::Path(None, path) => path,
                     _ => {
-                        self.struct_span_err(ty_first.span, "expected a trait, found type").emit();
+                        self.sess.emit_err(errors::ExpectedTraitInTraitImplFoundType {
+                            span: ty_first.span,
+                        });
                         err_path(ty_first.span)
                     }
                 };
@@ -688,20 +646,20 @@ impl<'a> Parser<'a> {
         &mut self,
         attrs: &mut AttrVec,
         mut parse_item: impl FnMut(&mut Parser<'a>) -> PResult<'a, Option<Option<T>>>,
-    ) -> PResult<'a, Vec<T>> {
+    ) -> PResult<'a, ThinVec<T>> {
         let open_brace_span = self.token.span;
 
         // Recover `impl Ty;` instead of `impl Ty {}`
         if self.token == TokenKind::Semi {
-            self.sess.emit_err(UseEmptyBlockNotSemi { span: self.token.span });
+            self.sess.emit_err(errors::UseEmptyBlockNotSemi { span: self.token.span });
             self.bump();
-            return Ok(vec![]);
+            return Ok(ThinVec::new());
         }
 
         self.expect(&token::OpenDelim(Delimiter::Brace))?;
         attrs.extend(self.parse_inner_attributes()?);
 
-        let mut items = Vec::new();
+        let mut items = ThinVec::new();
         while !self.eat(&token::CloseDelim(Delimiter::Brace)) {
             if self.recover_doc_comment_before_brace() {
                 continue;
@@ -783,6 +741,7 @@ impl<'a> Parser<'a> {
     fn recover_doc_comment_before_brace(&mut self) -> bool {
         if let token::DocComment(..) = self.token.kind {
             if self.look_ahead(1, |tok| tok == &token::CloseDelim(Delimiter::Brace)) {
+                // FIXME: merge with `DocCommentDoesNotDocumentAnything` (E0585)
                 struct_span_err!(
                     self.diagnostic(),
                     self.token.span,
@@ -849,7 +808,7 @@ impl<'a> Parser<'a> {
             // It's a trait alias.
             if had_colon {
                 let span = span_at_colon.to(span_before_eq);
-                self.struct_span_err(span, "bounds are not allowed on trait aliases").emit();
+                self.sess.emit_err(errors::BoundsNotAllowedOnTraitAliases { span });
             }
 
             let bounds = self.parse_generic_bounds(None)?;
@@ -858,12 +817,10 @@ impl<'a> Parser<'a> {
 
             let whole_span = lo.to(self.prev_token.span);
             if is_auto == IsAuto::Yes {
-                let msg = "trait aliases cannot be `auto`";
-                self.struct_span_err(whole_span, msg).span_label(whole_span, msg).emit();
+                self.sess.emit_err(errors::TraitAliasCannotBeAuto { span: whole_span });
             }
             if let Unsafe::Yes(_) = unsafety {
-                let msg = "trait aliases cannot be `unsafe`";
-                self.struct_span_err(whole_span, msg).span_label(whole_span, msg).emit();
+                self.sess.emit_err(errors::TraitAliasCannotBeUnsafe { span: whole_span });
             }
 
             self.sess.gated_spans.gate(sym::trait_alias, whole_span);
@@ -909,8 +866,7 @@ impl<'a> Parser<'a> {
                     Ok(kind) => kind,
                     Err(kind) => match kind {
                         ItemKind::Static(a, _, b) => {
-                            self.struct_span_err(span, "associated `static` items are not allowed")
-                                .emit();
+                            self.sess.emit_err(errors::AssociatedStaticItemNotAllowed { span });
                             AssocItemKind::Const(Defaultness::Final, a, b)
                         }
                         _ => return self.error_bad_item_kind(span, &kind, "`trait`s or `impl`s"),
@@ -1041,7 +997,7 @@ impl<'a> Parser<'a> {
     /// ```text
     /// USE_TREE_LIST = Ø | (USE_TREE `,`)* USE_TREE [`,`]
     /// ```
-    fn parse_use_tree_list(&mut self) -> PResult<'a, Vec<(UseTree, ast::NodeId)>> {
+    fn parse_use_tree_list(&mut self) -> PResult<'a, ThinVec<(UseTree, ast::NodeId)>> {
         self.parse_delim_comma_seq(Delimiter::Brace, |p| {
             p.recover_diff_marker();
             Ok((p.parse_use_tree()?, DUMMY_NODE_ID))
@@ -1084,41 +1040,37 @@ impl<'a> Parser<'a> {
     }
 
     fn parse_crate_name_with_dashes(&mut self) -> PResult<'a, Ident> {
-        let error_msg = "crate name using dashes are not valid in `extern crate` statements";
-        let suggestion_msg = "if the original crate name uses dashes you need to use underscores \
-                              in the code";
-        let mut ident = if self.token.is_keyword(kw::SelfLower) {
+        let ident = if self.token.is_keyword(kw::SelfLower) {
             self.parse_path_segment_ident()
         } else {
             self.parse_ident()
         }?;
-        let mut idents = vec![];
-        let mut replacement = vec![];
-        let mut fixed_crate_name = false;
-        // Accept `extern crate name-like-this` for better diagnostics.
+
         let dash = token::BinOp(token::BinOpToken::Minus);
-        if self.token == dash {
-            // Do not include `-` as part of the expected tokens list.
-            while self.eat(&dash) {
-                fixed_crate_name = true;
-                replacement.push((self.prev_token.span, "_".to_string()));
-                idents.push(self.parse_ident()?);
-            }
+        if self.token != dash {
+            return Ok(ident);
         }
-        if fixed_crate_name {
-            let fixed_name_sp = ident.span.to(idents.last().unwrap().span);
-            let mut fixed_name = ident.name.to_string();
-            for part in idents {
-                fixed_name.push_str(&format!("_{}", part.name));
-            }
-            ident = Ident::from_str_and_span(&fixed_name, fixed_name_sp);
 
-            self.struct_span_err(fixed_name_sp, error_msg)
-                .span_label(fixed_name_sp, "dash-separated idents are not valid")
-                .multipart_suggestion(suggestion_msg, replacement, Applicability::MachineApplicable)
-                .emit();
+        // Accept `extern crate name-like-this` for better diagnostics.
+        let mut dashes = vec![];
+        let mut idents = vec![];
+        while self.eat(&dash) {
+            dashes.push(self.prev_token.span);
+            idents.push(self.parse_ident()?);
         }
-        Ok(ident)
+
+        let fixed_name_sp = ident.span.to(idents.last().unwrap().span);
+        let mut fixed_name = ident.name.to_string();
+        for part in idents {
+            write!(fixed_name, "_{}", part.name).unwrap();
+        }
+
+        self.sess.emit_err(errors::ExternCrateNameWithDashes {
+            span: fixed_name_sp,
+            sugg: errors::ExternCrateNameWithDashesSugg { dashes },
+        });
+
+        Ok(Ident::from_str_and_span(&fixed_name, fixed_name_sp))
     }
 
     /// Parses `extern` for foreign ABIs modules.
@@ -1166,7 +1118,10 @@ impl<'a> Parser<'a> {
                     Ok(kind) => kind,
                     Err(kind) => match kind {
                         ItemKind::Const(_, a, b) => {
-                            self.error_on_foreign_const(span, ident);
+                            self.sess.emit_err(errors::ExternItemCannotBeConst {
+                                ident_span: ident.span,
+                                const_span: span.with_hi(ident.span.lo()),
+                            });
                             ForeignItemKind::Static(a, Mutability::Not, b)
                         }
                         _ => return self.error_bad_item_kind(span, &kind, "`extern` blocks"),
@@ -1178,6 +1133,7 @@ impl<'a> Parser<'a> {
     }
 
     fn error_bad_item_kind<T>(&self, span: Span, kind: &ItemKind, ctx: &str) -> Option<T> {
+        // FIXME(#100717): needs variant for each `ItemKind` (instead of using `ItemKind::descr()`)
         let span = self.sess.source_map().guess_head_span(span);
         let descr = kind.descr();
         self.struct_span_err(span, &format!("{descr} is not supported in {ctx}"))
@@ -1186,18 +1142,6 @@ impl<'a> Parser<'a> {
         None
     }
 
-    fn error_on_foreign_const(&self, span: Span, ident: Ident) {
-        self.struct_span_err(ident.span, "extern items cannot be `const`")
-            .span_suggestion(
-                span.with_hi(ident.span.lo()),
-                "try using a static value",
-                "static ",
-                Applicability::MachineApplicable,
-            )
-            .note("for more information, visit https://doc.rust-lang.org/std/keyword.extern.html")
-            .emit();
-    }
-
     fn is_unsafe_foreign_mod(&self) -> bool {
         self.token.is_keyword(kw::Unsafe)
             && self.is_keyword_ahead(1, &[kw::Extern])
@@ -1225,25 +1169,10 @@ impl<'a> Parser<'a> {
     fn recover_const_mut(&mut self, const_span: Span) {
         if self.eat_keyword(kw::Mut) {
             let span = self.prev_token.span;
-            self.struct_span_err(span, "const globals cannot be mutable")
-                .span_label(span, "cannot be mutable")
-                .span_suggestion(
-                    const_span,
-                    "you might want to declare a static instead",
-                    "static",
-                    Applicability::MaybeIncorrect,
-                )
-                .emit();
+            self.sess.emit_err(errors::ConstGlobalCannotBeMutable { ident_span: span, const_span });
         } else if self.eat_keyword(kw::Let) {
             let span = self.prev_token.span;
-            self.struct_span_err(const_span.to(span), "`const` and `let` are mutually exclusive")
-                .span_suggestion(
-                    const_span.to(span),
-                    "remove `let`",
-                    "const",
-                    Applicability::MaybeIncorrect,
-                )
-                .emit();
+            self.sess.emit_err(errors::ConstLetMutuallyExclusive { span: const_span.to(span) });
         }
     }
 
@@ -1328,13 +1257,9 @@ impl<'a> Parser<'a> {
         };
 
         let span = self.prev_token.span.shrink_to_hi();
-        let mut err = self.struct_span_err(span, &format!("missing type for `{kind}` item"));
-        err.span_suggestion(
-            span,
-            "provide a type for the item",
-            format!("{colon} <type>"),
-            Applicability::HasPlaceholders,
-        );
+        let err: DiagnosticBuilder<'_, ErrorGuaranteed> =
+            errors::MissingConstType { span, colon, kind }
+                .into_diagnostic(&self.sess.span_diagnostic);
         err.stash(span, StashKey::ItemNoType);
 
         // The user intended that the type be inferred,
@@ -1346,18 +1271,12 @@ impl<'a> Parser<'a> {
     fn parse_item_enum(&mut self) -> PResult<'a, ItemInfo> {
         if self.token.is_keyword(kw::Struct) {
             let span = self.prev_token.span.to(self.token.span);
-            let mut err = self.struct_span_err(span, "`enum` and `struct` are mutually exclusive");
-            err.span_suggestion(
-                span,
-                "replace `enum struct` with",
-                "enum",
-                Applicability::MachineApplicable,
-            );
+            let err = errors::EnumStructMutuallyExclusive { span };
             if self.look_ahead(1, |t| t.is_ident()) {
                 self.bump();
-                err.emit();
+                self.sess.emit_err(err);
             } else {
-                return Err(err);
+                return Err(err.into_diagnostic(&self.sess.span_diagnostic));
             }
         }
 
@@ -1367,9 +1286,9 @@ impl<'a> Parser<'a> {
 
         // Possibly recover `enum Foo;` instead of `enum Foo {}`
         let (variants, _) = if self.token == TokenKind::Semi {
-            self.sess.emit_err(UseEmptyBlockNotSemi { span: self.token.span });
+            self.sess.emit_err(errors::UseEmptyBlockNotSemi { span: self.token.span });
             self.bump();
-            (vec![], false)
+            (thin_vec![], false)
         } else {
             self.parse_delim_comma_seq(Delimiter::Brace, |p| p.parse_enum_variant()).map_err(
                 |mut e| {
@@ -1412,7 +1331,7 @@ impl<'a> Parser<'a> {
                 };
 
                 let disr_expr =
-                    if this.eat(&token::Eq) { Some(this.parse_anon_const_expr()?) } else { None };
+                    if this.eat(&token::Eq) { Some(this.parse_expr_anon_const()?) } else { None };
 
                 let vr = ast::Variant {
                     ident,
@@ -1493,13 +1412,9 @@ impl<'a> Parser<'a> {
             self.expect_semi()?;
             body
         } else {
-            let token_str = super::token_descr(&self.token);
-            let msg = &format!(
-                "expected `where`, `{{`, `(`, or `;` after struct name, found {token_str}"
-            );
-            let mut err = self.struct_span_err(self.token.span, msg);
-            err.span_label(self.token.span, "expected `where`, `{`, `(`, or `;` after struct name");
-            return Err(err);
+            let err =
+                errors::UnexpectedTokenAfterStructName::new(self.token.span, self.token.clone());
+            return Err(err.into_diagnostic(&self.sess.span_diagnostic));
         };
 
         Ok((class_name, ItemKind::Struct(vdata, generics)))
@@ -1542,8 +1457,8 @@ impl<'a> Parser<'a> {
         adt_ty: &str,
         ident_span: Span,
         parsed_where: bool,
-    ) -> PResult<'a, (Vec<FieldDef>, /* recovered */ bool)> {
-        let mut fields = Vec::new();
+    ) -> PResult<'a, (ThinVec<FieldDef>, /* recovered */ bool)> {
+        let mut fields = ThinVec::new();
         let mut recovered = false;
         if self.eat(&token::OpenDelim(Delimiter::Brace)) {
             while self.token != token::CloseDelim(Delimiter::Brace) {
@@ -1583,7 +1498,7 @@ impl<'a> Parser<'a> {
         Ok((fields, recovered))
     }
 
-    pub(super) fn parse_tuple_struct_body(&mut self) -> PResult<'a, Vec<FieldDef>> {
+    pub(super) fn parse_tuple_struct_body(&mut self) -> PResult<'a, ThinVec<FieldDef>> {
         // This is the case where we find `struct Foo<T>(T) where T: Copy;`
         // Unit like structs are handled in parse_item_struct function
         self.parse_paren_comma_seq(|p| {
@@ -1676,7 +1591,7 @@ impl<'a> Parser<'a> {
             token::CloseDelim(Delimiter::Brace) => {}
             token::DocComment(..) => {
                 let previous_span = self.prev_token.span;
-                let mut err = DocCommentDoesNotDocumentAnything {
+                let mut err = errors::DocCommentDoesNotDocumentAnything {
                     span: self.token.span,
                     missing_comma: None,
                 };
@@ -1807,7 +1722,7 @@ impl<'a> Parser<'a> {
         }
         if self.token.kind == token::Eq {
             self.bump();
-            let const_expr = self.parse_anon_const_expr()?;
+            let const_expr = self.parse_expr_anon_const()?;
             let sp = ty.span.shrink_to_hi().to(const_expr.value.span);
             self.struct_span_err(sp, "default values on `struct` fields aren't supported")
                 .span_suggestion(
@@ -2186,7 +2101,7 @@ impl<'a> Parser<'a> {
                 // If we see `for Ty ...` then user probably meant `impl` item.
                 if self.token.is_keyword(kw::For) {
                     old_err.cancel();
-                    return Err(self.sess.create_err(FnTypoWithImpl { fn_span }));
+                    return Err(self.sess.create_err(errors::FnTypoWithImpl { fn_span }));
                 } else {
                     return Err(old_err);
                 }
@@ -2330,7 +2245,12 @@ impl<'a> Parser<'a> {
         let ext = self.parse_extern(case);
 
         if let Async::Yes { span, .. } = asyncness {
-            self.ban_async_in_2015(span);
+            if span.is_rust_2015() {
+                self.sess.emit_err(errors::AsyncFnIn2015 {
+                    span,
+                    help: errors::HelpUseLatestEdition::new(),
+                });
+            }
         }
 
         if !self.eat_keyword_case(kw::Fn, case) {
@@ -2440,17 +2360,6 @@ impl<'a> Parser<'a> {
         Ok(FnHeader { constness, unsafety, asyncness, ext })
     }
 
-    /// We are parsing `async fn`. If we are on Rust 2015, emit an error.
-    fn ban_async_in_2015(&self, span: Span) {
-        if span.rust_2015() {
-            let diag = self.diagnostic();
-            struct_span_err!(diag, span, E0670, "`async fn` is not permitted in Rust 2015")
-                .span_label(span, "to use `async fn`, switch to Rust 2018 or later")
-                .help_use_latest_edition()
-                .emit();
-        }
-    }
-
     /// Parses the parameter list and result type of a function declaration.
     pub(super) fn parse_fn_decl(
         &mut self,
@@ -2465,7 +2374,7 @@ impl<'a> Parser<'a> {
     }
 
     /// Parses the parameter list of a function, including the `(` and `)` delimiters.
-    pub(super) fn parse_fn_params(&mut self, req_name: ReqName) -> PResult<'a, Vec<Param>> {
+    pub(super) fn parse_fn_params(&mut self, req_name: ReqName) -> PResult<'a, ThinVec<Param>> {
         let mut first_param = true;
         // Parse the arguments, starting out with `self` being allowed...
         let (mut params, _) = self.parse_paren_comma_seq(|p| {
@@ -2593,9 +2502,7 @@ impl<'a> Parser<'a> {
         };
         // Recover for the grammar `*self`, `*const self`, and `*mut self`.
         let recover_self_ptr = |this: &mut Self| {
-            let msg = "cannot pass `self` by raw pointer";
-            let span = this.token.span;
-            this.struct_span_err(span, msg).span_label(span, msg).emit();
+            self.sess.emit_err(errors::SelfArgumentPointer { span: this.token.span });
 
             Ok((SelfKind::Value(Mutability::Not), expect_self_ident(this), this.prev_token.span))
         };
@@ -2676,14 +2583,14 @@ impl<'a> Parser<'a> {
             && self.look_ahead(offset + 1, |t| t == &token::Colon)
     }
 
-    fn recover_first_param(&mut self) -> &'static str {
+    fn recover_self_param(&mut self) -> bool {
         match self
             .parse_outer_attributes()
             .and_then(|_| self.parse_self_param())
             .map_err(|e| e.cancel())
         {
-            Ok(Some(_)) => "method",
-            _ => "function",
+            Ok(Some(_)) => true,
+            _ => false,
         }
     }
 }
diff --git a/compiler/rustc_parse/src/parser/mod.rs b/compiler/rustc_parse/src/parser/mod.rs
index ffb23b50a..da82e4724 100644
--- a/compiler/rustc_parse/src/parser/mod.rs
+++ b/compiler/rustc_parse/src/parser/mod.rs
@@ -10,7 +10,7 @@ mod path;
 mod stmt;
 mod ty;
 
-use crate::lexer::UnmatchedBrace;
+use crate::lexer::UnmatchedDelim;
 pub use attr_wrapper::AttrWrapper;
 pub use diagnostics::AttemptLocalParseRecovery;
 pub(crate) use item::FnParseMode;
@@ -19,9 +19,8 @@ pub use path::PathStyle;
 
 use rustc_ast::ptr::P;
 use rustc_ast::token::{self, Delimiter, Nonterminal, Token, TokenKind};
-use rustc_ast::tokenstream::AttributesData;
-use rustc_ast::tokenstream::{self, DelimSpan, Spacing};
-use rustc_ast::tokenstream::{TokenStream, TokenTree};
+use rustc_ast::tokenstream::{AttributesData, DelimSpan, Spacing};
+use rustc_ast::tokenstream::{TokenStream, TokenTree, TokenTreeCursor};
 use rustc_ast::util::case::Case;
 use rustc_ast::AttrId;
 use rustc_ast::DUMMY_NODE_ID;
@@ -37,9 +36,10 @@ use rustc_errors::{
 use rustc_session::parse::ParseSess;
 use rustc_span::source_map::{Span, DUMMY_SP};
 use rustc_span::symbol::{kw, sym, Ident, Symbol};
-
 use std::ops::Range;
 use std::{cmp, mem, slice};
+use thin_vec::ThinVec;
+use tracing::debug;
 
 use crate::errors::{
     DocCommentDoesNotDocumentAnything, IncorrectVisibilityRestriction, MismatchedClosingDelimiter,
@@ -149,7 +149,7 @@ pub struct Parser<'a> {
     /// A list of all unclosed delimiters found by the lexer. If an entry is used for error recovery
     /// it gets removed from here. Every entry left at the end gets emitted as an independent
     /// error.
-    pub(super) unclosed_delims: Vec<UnmatchedBrace>,
+    pub(super) unclosed_delims: Vec<UnmatchedDelim>,
     last_unexpected_token_span: Option<Span>,
     /// Span pointing at the `:` for the last type ascription the parser has seen, and whether it
     /// looked like it could have been a mistyped path or literal `Option:Some(42)`).
@@ -168,7 +168,7 @@ pub struct Parser<'a> {
 // This type is used a lot, e.g. it's cloned when matching many declarative macro rules with nonterminals. Make sure
 // it doesn't unintentionally get bigger.
 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
-rustc_data_structures::static_assert_size!(Parser<'_>, 336);
+rustc_data_structures::static_assert_size!(Parser<'_>, 312);
 
 /// Stores span information about a closure.
 #[derive(Clone)]
@@ -221,18 +221,27 @@ impl<'a> Drop for Parser<'a> {
     }
 }
 
+/// Iterator over a `TokenStream` that produces `Token`s. It's a bit odd that
+/// we (a) lex tokens into a nice tree structure (`TokenStream`), and then (b)
+/// use this type to emit them as a linear sequence. But a linear sequence is
+/// what the parser expects, for the most part.
 #[derive(Clone)]
 struct TokenCursor {
-    // The current (innermost) frame. `frame` and `stack` could be combined,
-    // but it's faster to have them separately to access `frame` directly
-    // rather than via something like `stack.last().unwrap()` or
-    // `stack[stack.len() - 1]`.
-    frame: TokenCursorFrame,
-    // Additional frames that enclose `frame`.
-    stack: Vec<TokenCursorFrame>,
+    // Cursor for the current (innermost) token stream. The delimiters for this
+    // token stream are found in `self.stack.last()`; when that is `None` then
+    // we are in the outermost token stream which never has delimiters.
+    tree_cursor: TokenTreeCursor,
+
+    // Token streams surrounding the current one. The delimiters for stack[n]'s
+    // tokens are in `stack[n-1]`. `stack[0]` (when present) has no delimiters
+    // because it's the outermost token stream which never has delimiters.
+    stack: Vec<(TokenTreeCursor, Delimiter, DelimSpan)>,
+
     desugar_doc_comments: bool,
+
     // Counts the number of calls to `{,inlined_}next`.
     num_next_calls: usize,
+
     // During parsing, we may sometimes need to 'unglue' a
     // glued token into two component tokens
     // (e.g. '>>' into '>' and '>), so that the parser
@@ -257,18 +266,6 @@ struct TokenCursor {
     break_last_token: bool,
 }
 
-#[derive(Clone)]
-struct TokenCursorFrame {
-    delim_sp: Option<(Delimiter, DelimSpan)>,
-    tree_cursor: tokenstream::Cursor,
-}
-
-impl TokenCursorFrame {
-    fn new(delim_sp: Option<(Delimiter, DelimSpan)>, tts: TokenStream) -> Self {
-        TokenCursorFrame { delim_sp, tree_cursor: tts.into_trees() }
-    }
-}
-
 impl TokenCursor {
     fn next(&mut self, desugar_doc_comments: bool) -> (Token, Spacing) {
         self.inlined_next(desugar_doc_comments)
@@ -281,38 +278,47 @@ impl TokenCursor {
             // FIXME: we currently don't return `Delimiter` open/close delims. To fix #67062 we will
             // need to, whereupon the `delim != Delimiter::Invisible` conditions below can be
             // removed.
-            if let Some(tree) = self.frame.tree_cursor.next_ref() {
+            if let Some(tree) = self.tree_cursor.next_ref() {
                 match tree {
                     &TokenTree::Token(ref token, spacing) => match (desugar_doc_comments, token) {
                         (true, &Token { kind: token::DocComment(_, attr_style, data), span }) => {
-                            return self.desugar(attr_style, data, span);
+                            let desugared = self.desugar(attr_style, data, span);
+                            self.tree_cursor.replace_prev_and_rewind(desugared);
+                            // Continue to get the first token of the desugared doc comment.
+                        }
+                        _ => {
+                            debug_assert!(!matches!(
+                                token.kind,
+                                token::OpenDelim(_) | token::CloseDelim(_)
+                            ));
+                            return (token.clone(), spacing);
                         }
-                        _ => return (token.clone(), spacing),
                     },
                     &TokenTree::Delimited(sp, delim, ref tts) => {
-                        // Set `open_delim` to true here because we deal with it immediately.
-                        let frame = TokenCursorFrame::new(Some((delim, sp)), tts.clone());
-                        self.stack.push(mem::replace(&mut self.frame, frame));
+                        let trees = tts.clone().into_trees();
+                        self.stack.push((mem::replace(&mut self.tree_cursor, trees), delim, sp));
                         if delim != Delimiter::Invisible {
                             return (Token::new(token::OpenDelim(delim), sp.open), Spacing::Alone);
                         }
                         // No open delimiter to return; continue on to the next iteration.
                     }
                 };
-            } else if let Some(frame) = self.stack.pop() {
-                if let Some((delim, span)) = self.frame.delim_sp && delim != Delimiter::Invisible {
-                    self.frame = frame;
+            } else if let Some((tree_cursor, delim, span)) = self.stack.pop() {
+                // We have exhausted this token stream. Move back to its parent token stream.
+                self.tree_cursor = tree_cursor;
+                if delim != Delimiter::Invisible {
                     return (Token::new(token::CloseDelim(delim), span.close), Spacing::Alone);
                 }
-                self.frame = frame;
                 // No close delimiter to return; continue on to the next iteration.
             } else {
+                // We have exhausted the outermost token stream.
                 return (Token::new(token::Eof, DUMMY_SP), Spacing::Alone);
             }
         }
     }
 
-    fn desugar(&mut self, attr_style: AttrStyle, data: Symbol, span: Span) -> (Token, Spacing) {
+    // Desugar a doc comment into something like `#[doc = r"foo"]`.
+    fn desugar(&mut self, attr_style: AttrStyle, data: Symbol, span: Span) -> Vec<TokenTree> {
         // Searches for the occurrences of `"#*` and returns the minimum number of `#`s
         // required to wrap the text. E.g.
         // - `abc d` is wrapped as `r"abc d"` (num_of_hashes = 0)
@@ -329,7 +335,7 @@ impl TokenCursor {
             num_of_hashes = cmp::max(num_of_hashes, count);
         }
 
-        // `/// foo` becomes `doc = r"foo".
+        // `/// foo` becomes `doc = r"foo"`.
         let delim_span = DelimSpan::from_single(span);
         let body = TokenTree::Delimited(
             delim_span,
@@ -346,27 +352,15 @@ impl TokenCursor {
             .collect::<TokenStream>(),
         );
 
-        self.stack.push(mem::replace(
-            &mut self.frame,
-            TokenCursorFrame::new(
-                None,
-                if attr_style == AttrStyle::Inner {
-                    [
-                        TokenTree::token_alone(token::Pound, span),
-                        TokenTree::token_alone(token::Not, span),
-                        body,
-                    ]
-                    .into_iter()
-                    .collect::<TokenStream>()
-                } else {
-                    [TokenTree::token_alone(token::Pound, span), body]
-                        .into_iter()
-                        .collect::<TokenStream>()
-                },
-            ),
-        ));
-
-        self.next(/* desugar_doc_comments */ false)
+        if attr_style == AttrStyle::Inner {
+            vec![
+                TokenTree::token_alone(token::Pound, span),
+                TokenTree::token_alone(token::Not, span),
+                body,
+            ]
+        } else {
+            vec![TokenTree::token_alone(token::Pound, span), body]
+        }
     }
 }
 
@@ -475,7 +469,7 @@ impl<'a> Parser<'a> {
             restrictions: Restrictions::empty(),
             expected_tokens: Vec::new(),
             token_cursor: TokenCursor {
-                frame: TokenCursorFrame::new(None, tokens),
+                tree_cursor: tokens.into_trees(),
                 stack: Vec::new(),
                 num_next_calls: 0,
                 desugar_doc_comments,
@@ -739,9 +733,10 @@ impl<'a> Parser<'a> {
     fn check_const_closure(&self) -> bool {
         self.is_keyword_ahead(0, &[kw::Const])
             && self.look_ahead(1, |t| match &t.kind {
-                token::Ident(kw::Move | kw::Static | kw::Async, _)
-                | token::OrOr
-                | token::BinOp(token::Or) => true,
+                // async closures do not work with const closures, so we do not parse that here.
+                token::Ident(kw::Move | kw::Static, _) | token::OrOr | token::BinOp(token::Or) => {
+                    true
+                }
                 _ => false,
             })
     }
@@ -859,11 +854,11 @@ impl<'a> Parser<'a> {
         sep: SeqSep,
         expect: TokenExpectType,
         mut f: impl FnMut(&mut Parser<'a>) -> PResult<'a, T>,
-    ) -> PResult<'a, (Vec<T>, bool /* trailing */, bool /* recovered */)> {
+    ) -> PResult<'a, (ThinVec<T>, bool /* trailing */, bool /* recovered */)> {
         let mut first = true;
         let mut recovered = false;
         let mut trailing = false;
-        let mut v = vec![];
+        let mut v = ThinVec::new();
         let unclosed_delims = !self.unclosed_delims.is_empty();
 
         while !self.expect_any_with_type(kets, expect) {
@@ -987,7 +982,11 @@ impl<'a> Parser<'a> {
         let initial_semicolon = self.token.span;
 
         while self.eat(&TokenKind::Semi) {
-            let _ = self.parse_stmt(ForceCollect::Yes)?;
+            let _ =
+                self.parse_stmt_without_recovery(false, ForceCollect::Yes).unwrap_or_else(|e| {
+                    e.cancel();
+                    None
+                });
         }
 
         expect_err.set_primary_message(
@@ -1043,7 +1042,7 @@ impl<'a> Parser<'a> {
         ket: &TokenKind,
         sep: SeqSep,
         f: impl FnMut(&mut Parser<'a>) -> PResult<'a, T>,
-    ) -> PResult<'a, (Vec<T>, bool, bool)> {
+    ) -> PResult<'a, (ThinVec<T>, bool, bool)> {
         self.parse_seq_to_before_tokens(&[ket], sep, TokenExpectType::Expect, f)
     }
 
@@ -1055,7 +1054,7 @@ impl<'a> Parser<'a> {
         ket: &TokenKind,
         sep: SeqSep,
         f: impl FnMut(&mut Parser<'a>) -> PResult<'a, T>,
-    ) -> PResult<'a, (Vec<T>, bool /* trailing */)> {
+    ) -> PResult<'a, (ThinVec<T>, bool /* trailing */)> {
         let (val, trailing, recovered) = self.parse_seq_to_before_end(ket, sep, f)?;
         if !recovered {
             self.eat(ket);
@@ -1072,7 +1071,7 @@ impl<'a> Parser<'a> {
         ket: &TokenKind,
         sep: SeqSep,
         f: impl FnMut(&mut Parser<'a>) -> PResult<'a, T>,
-    ) -> PResult<'a, (Vec<T>, bool)> {
+    ) -> PResult<'a, (ThinVec<T>, bool)> {
         self.expect(bra)?;
         self.parse_seq_to_end(ket, sep, f)
     }
@@ -1081,7 +1080,7 @@ impl<'a> Parser<'a> {
         &mut self,
         delim: Delimiter,
         f: impl FnMut(&mut Parser<'a>) -> PResult<'a, T>,
-    ) -> PResult<'a, (Vec<T>, bool)> {
+    ) -> PResult<'a, (ThinVec<T>, bool)> {
         self.parse_unspanned_seq(
             &token::OpenDelim(delim),
             &token::CloseDelim(delim),
@@ -1093,7 +1092,7 @@ impl<'a> Parser<'a> {
     fn parse_paren_comma_seq<T>(
         &mut self,
         f: impl FnMut(&mut Parser<'a>) -> PResult<'a, T>,
-    ) -> PResult<'a, (Vec<T>, bool)> {
+    ) -> PResult<'a, (ThinVec<T>, bool)> {
         self.parse_delim_comma_seq(Delimiter::Parenthesis, f)
     }
 
@@ -1142,14 +1141,16 @@ impl<'a> Parser<'a> {
             return looker(&self.token);
         }
 
-        let frame = &self.token_cursor.frame;
-        if let Some((delim, span)) = frame.delim_sp && delim != Delimiter::Invisible {
+        let tree_cursor = &self.token_cursor.tree_cursor;
+        if let Some(&(_, delim, span)) = self.token_cursor.stack.last()
+            && delim != Delimiter::Invisible
+        {
             let all_normal = (0..dist).all(|i| {
-                let token = frame.tree_cursor.look_ahead(i);
+                let token = tree_cursor.look_ahead(i);
                 !matches!(token, Some(TokenTree::Delimited(_, Delimiter::Invisible, _)))
             });
             if all_normal {
-                return match frame.tree_cursor.look_ahead(dist - 1) {
+                return match tree_cursor.look_ahead(dist - 1) {
                     Some(tree) => match tree {
                         TokenTree::Token(token, _) => looker(token),
                         TokenTree::Delimited(dspan, delim, _) => {
@@ -1203,8 +1204,18 @@ impl<'a> Parser<'a> {
 
     /// Parses constness: `const` or nothing.
     fn parse_constness(&mut self, case: Case) -> Const {
-        // Avoid const blocks to be parsed as const items
-        if self.look_ahead(1, |t| t != &token::OpenDelim(Delimiter::Brace))
+        self.parse_constness_(case, false)
+    }
+
+    /// Parses constness for closures
+    fn parse_closure_constness(&mut self, case: Case) -> Const {
+        self.parse_constness_(case, true)
+    }
+
+    fn parse_constness_(&mut self, case: Case, is_closure: bool) -> Const {
+        // Avoid const blocks and const closures to be parsed as const items
+        if (self.check_const_closure() == is_closure)
+            && self.look_ahead(1, |t| t != &token::OpenDelim(Delimiter::Brace))
             && self.eat_keyword_case(kw::Const, case)
         {
             Const::Yes(self.prev_token.uninterpolated_span())
@@ -1277,22 +1288,16 @@ impl<'a> Parser<'a> {
     }
 
     fn parse_delim_args_inner(&mut self) -> Option<DelimArgs> {
-        if self.check(&token::OpenDelim(Delimiter::Parenthesis))
+        let delimited = self.check(&token::OpenDelim(Delimiter::Parenthesis))
             || self.check(&token::OpenDelim(Delimiter::Bracket))
-            || self.check(&token::OpenDelim(Delimiter::Brace))
-        {
-            match self.parse_token_tree() {
-                // We've confirmed above that there is a delimiter so unwrapping is OK.
-                TokenTree::Delimited(dspan, delim, tokens) => Some(DelimArgs {
-                    dspan,
-                    delim: MacDelimiter::from_token(delim).unwrap(),
-                    tokens,
-                }),
-                _ => unreachable!(),
-            }
-        } else {
-            None
-        }
+            || self.check(&token::OpenDelim(Delimiter::Brace));
+
+        delimited.then(|| {
+            // We've confirmed above that there is a delimiter so unwrapping is OK.
+            let TokenTree::Delimited(dspan, delim, tokens) = self.parse_token_tree() else { unreachable!() };
+
+            DelimArgs { dspan, delim: MacDelimiter::from_token(delim).unwrap(), tokens }
+        })
     }
 
     fn parse_or_use_outer_attributes(
@@ -1310,10 +1315,10 @@ impl<'a> Parser<'a> {
     pub(crate) fn parse_token_tree(&mut self) -> TokenTree {
         match self.token.kind {
             token::OpenDelim(..) => {
-                // Grab the tokens from this frame.
-                let frame = &self.token_cursor.frame;
-                let stream = frame.tree_cursor.stream.clone();
-                let (delim, span) = frame.delim_sp.unwrap();
+                // Grab the tokens within the delimiters.
+                let tree_cursor = &self.token_cursor.tree_cursor;
+                let stream = tree_cursor.stream.clone();
+                let (_, delim, span) = *self.token_cursor.stack.last().unwrap();
 
                 // Advance the token cursor through the entire delimited
                 // sequence. After getting the `OpenDelim` we are *within* the
@@ -1516,11 +1521,11 @@ impl<'a> Parser<'a> {
 }
 
 pub(crate) fn make_unclosed_delims_error(
-    unmatched: UnmatchedBrace,
+    unmatched: UnmatchedDelim,
     sess: &ParseSess,
 ) -> Option<DiagnosticBuilder<'_, ErrorGuaranteed>> {
     // `None` here means an `Eof` was found. We already emit those errors elsewhere, we add them to
-    // `unmatched_braces` only for error recovery in the `Parser`.
+    // `unmatched_delims` only for error recovery in the `Parser`.
     let found_delim = unmatched.found_delim?;
     let mut spans = vec![unmatched.found_span];
     if let Some(sp) = unmatched.unclosed_span {
@@ -1537,7 +1542,7 @@ pub(crate) fn make_unclosed_delims_error(
     Some(err)
 }
 
-pub fn emit_unclosed_delims(unclosed_delims: &mut Vec<UnmatchedBrace>, sess: &ParseSess) {
+pub fn emit_unclosed_delims(unclosed_delims: &mut Vec<UnmatchedDelim>, sess: &ParseSess) {
     *sess.reached_eof.borrow_mut() |=
         unclosed_delims.iter().any(|unmatched_delim| unmatched_delim.found_delim.is_none());
     for unmatched in unclosed_delims.drain(..) {
diff --git a/compiler/rustc_parse/src/parser/nonterminal.rs b/compiler/rustc_parse/src/parser/nonterminal.rs
index 239ed79ce..7a4d53ed8 100644
--- a/compiler/rustc_parse/src/parser/nonterminal.rs
+++ b/compiler/rustc_parse/src/parser/nonterminal.rs
@@ -2,9 +2,11 @@ use rustc_ast::ptr::P;
 use rustc_ast::token::{self, Delimiter, NonterminalKind, Token};
 use rustc_ast::HasTokens;
 use rustc_ast_pretty::pprust;
+use rustc_errors::IntoDiagnostic;
 use rustc_errors::PResult;
 use rustc_span::symbol::{kw, Ident};
 
+use crate::errors::UnexpectedNonterminal;
 use crate::parser::pat::{CommaRecoveryMode, RecoverColon, RecoverComma};
 use crate::parser::{FollowedByType, ForceCollect, NtOrTt, Parser, PathStyle};
 
@@ -113,7 +115,8 @@ impl<'a> Parser<'a> {
             NonterminalKind::Item => match self.parse_item(ForceCollect::Yes)? {
                 Some(item) => token::NtItem(item),
                 None => {
-                    return Err(self.struct_span_err(self.token.span, "expected an item keyword"));
+                    return Err(UnexpectedNonterminal::Item(self.token.span)
+                               .into_diagnostic(&self.sess.span_diagnostic));
                 }
             },
             NonterminalKind::Block => {
@@ -124,7 +127,8 @@ impl<'a> Parser<'a> {
             NonterminalKind::Stmt => match self.parse_stmt(ForceCollect::Yes)? {
                 Some(s) => token::NtStmt(P(s)),
                 None => {
-                    return Err(self.struct_span_err(self.token.span, "expected a statement"));
+                    return Err(UnexpectedNonterminal::Statement(self.token.span)
+                               .into_diagnostic(&self.sess.span_diagnostic));
                 }
             },
             NonterminalKind::PatParam { .. } | NonterminalKind::PatWithOr { .. } => {
@@ -160,9 +164,10 @@ impl<'a> Parser<'a> {
                 token::NtIdent(ident, is_raw)
             }
             NonterminalKind::Ident => {
-                let token_str = pprust::token_to_string(&self.token);
-                let msg = &format!("expected ident, found {}", &token_str);
-                return Err(self.struct_span_err(self.token.span, msg));
+                return Err(UnexpectedNonterminal::Ident {
+                    span: self.token.span,
+                    token: self.token.clone(),
+                }.into_diagnostic(&self.sess.span_diagnostic));
             }
             NonterminalKind::Path => token::NtPath(
                 P(self.collect_tokens_no_attrs(|this| this.parse_path(PathStyle::Type))?),
@@ -175,9 +180,10 @@ impl<'a> Parser<'a> {
                 if self.check_lifetime() {
                     token::NtLifetime(self.expect_lifetime().ident)
                 } else {
-                    let token_str = pprust::token_to_string(&self.token);
-                    let msg = &format!("expected a lifetime, found `{}`", &token_str);
-                    return Err(self.struct_span_err(self.token.span, msg));
+                    return Err(UnexpectedNonterminal::Lifetime {
+                        span: self.token.span,
+                        token: self.token.clone(),
+                    }.into_diagnostic(&self.sess.span_diagnostic));
                 }
             }
         };
diff --git a/compiler/rustc_parse/src/parser/pat.rs b/compiler/rustc_parse/src/parser/pat.rs
index e73a17ced..8e920f1c4 100644
--- a/compiler/rustc_parse/src/parser/pat.rs
+++ b/compiler/rustc_parse/src/parser/pat.rs
@@ -1,5 +1,14 @@
 use super::{ForceCollect, Parser, PathStyle, TrailingToken};
-use crate::errors::RemoveLet;
+use crate::errors::{
+    AmbiguousRangePattern, DotDotDotForRemainingFields, DotDotDotRangeToPatternNotAllowed,
+    DotDotDotRestPattern, EnumPatternInsteadOfIdentifier, ExpectedBindingLeftOfAt,
+    ExpectedCommaAfterPatternField, InclusiveRangeExtraEquals, InclusiveRangeMatchArrow,
+    InclusiveRangeNoEnd, InvalidMutInPattern, PatternOnWrongSideOfAt, RefMutOrderIncorrect,
+    RemoveLet, RepeatedMutInPattern, TopLevelOrPatternNotAllowed, TopLevelOrPatternNotAllowedSugg,
+    TrailingVertNotAllowed, UnexpectedLifetimeInPattern, UnexpectedVertVertBeforeFunctionParam,
+    UnexpectedVertVertInPattern,
+};
+use crate::fluent_generated as fluent;
 use crate::{maybe_recover_from_interpolated_ty_qpath, maybe_whole};
 use rustc_ast::mut_visit::{noop_visit_pat, MutVisitor};
 use rustc_ast::ptr::P;
@@ -9,15 +18,32 @@ use rustc_ast::{
     PatField, PatKind, Path, QSelf, RangeEnd, RangeSyntax,
 };
 use rustc_ast_pretty::pprust;
-use rustc_errors::{struct_span_err, Applicability, DiagnosticBuilder, ErrorGuaranteed, PResult};
+use rustc_errors::{Applicability, DiagnosticBuilder, ErrorGuaranteed, IntoDiagnostic, PResult};
 use rustc_session::errors::ExprParenthesesNeeded;
 use rustc_span::source_map::{respan, Span, Spanned};
 use rustc_span::symbol::{kw, sym, Ident};
+use thin_vec::{thin_vec, ThinVec};
 
-pub(super) type Expected = Option<&'static str>;
+#[derive(PartialEq, Copy, Clone)]
+pub enum Expected {
+    ParameterName,
+    ArgumentName,
+    Identifier,
+    BindingPattern,
+}
 
-/// `Expected` for function and lambda parameter patterns.
-pub(super) const PARAM_EXPECTED: Expected = Some("parameter name");
+impl Expected {
+    // FIXME(#100717): migrate users of this to proper localization
+    fn to_string_or_fallback(expected: Option<Expected>) -> &'static str {
+        match expected {
+            Some(Expected::ParameterName) => "parameter name",
+            Some(Expected::ArgumentName) => "argument name",
+            Some(Expected::Identifier) => "identifier",
+            Some(Expected::BindingPattern) => "binding pattern",
+            None => "pattern",
+        }
+    }
+}
 
 const WHILE_PARSING_OR_MSG: &str = "while parsing this or-pattern starting here";
 
@@ -54,13 +80,19 @@ enum EatOrResult {
     None,
 }
 
+/// The syntax location of a given pattern. Used for diagnostics.
+pub(super) enum PatternLocation {
+    LetBinding,
+    FunctionParameter,
+}
+
 impl<'a> Parser<'a> {
     /// Parses a pattern.
     ///
     /// Corresponds to `pat<no_top_alt>` in RFC 2535 and does not admit or-patterns
     /// at the top level. Used when parsing the parameters of lambda expressions,
     /// functions, function pointers, and `pat` macro fragments.
-    pub fn parse_pat_no_top_alt(&mut self, expected: Expected) -> PResult<'a, P<Pat>> {
+    pub fn parse_pat_no_top_alt(&mut self, expected: Option<Expected>) -> PResult<'a, P<Pat>> {
         self.parse_pat_with_range_pat(true, expected)
     }
 
@@ -74,7 +106,7 @@ impl<'a> Parser<'a> {
     /// simplify the grammar somewhat.
     pub fn parse_pat_allow_top_alt(
         &mut self,
-        expected: Expected,
+        expected: Option<Expected>,
         rc: RecoverComma,
         ra: RecoverColon,
         rt: CommaRecoveryMode,
@@ -86,7 +118,7 @@ impl<'a> Parser<'a> {
     /// recovered).
     fn parse_pat_allow_top_alt_inner(
         &mut self,
-        expected: Expected,
+        expected: Option<Expected>,
         rc: RecoverComma,
         ra: RecoverColon,
         rt: CommaRecoveryMode,
@@ -123,7 +155,7 @@ impl<'a> Parser<'a> {
                 // If there was a leading vert, treat this as an or-pattern. This improves
                 // diagnostics.
                 let span = leading_vert_span.to(self.prev_token.span);
-                return Ok((self.mk_pat(span, PatKind::Or(vec![first_pat])), trailing_vert));
+                return Ok((self.mk_pat(span, PatKind::Or(thin_vec![first_pat])), trailing_vert));
             }
 
             return Ok((first_pat, trailing_vert));
@@ -131,7 +163,7 @@ impl<'a> Parser<'a> {
 
         // Parse the patterns `p_1 | ... | p_n` where `n > 0`.
         let lo = leading_vert_span.unwrap_or(first_pat.span);
-        let mut pats = vec![first_pat];
+        let mut pats = thin_vec![first_pat];
         loop {
             match self.eat_or_separator(Some(lo)) {
                 EatOrResult::AteOr => {}
@@ -165,9 +197,9 @@ impl<'a> Parser<'a> {
     /// otherwise).
     pub(super) fn parse_pat_before_ty(
         &mut self,
-        expected: Expected,
+        expected: Option<Expected>,
         rc: RecoverComma,
-        syntax_loc: &str,
+        syntax_loc: PatternLocation,
     ) -> PResult<'a, (P<Pat>, bool)> {
         // We use `parse_pat_allow_top_alt` regardless of whether we actually want top-level
         // or-patterns so that we can detect when a user tries to use it. This allows us to print a
@@ -181,27 +213,41 @@ impl<'a> Parser<'a> {
         let colon = self.eat(&token::Colon);
 
         if let PatKind::Or(pats) = &pat.kind {
-            let msg = format!("top-level or-patterns are not allowed in {}", syntax_loc);
-            let (help, fix) = if pats.len() == 1 {
-                // If all we have is a leading vert, then print a special message. This is the case
-                // if `parse_pat_allow_top_alt` returns an or-pattern with one variant.
-                let msg = "remove the `|`";
-                let fix = pprust::pat_to_string(&pat);
-                (msg, fix)
-            } else {
-                let msg = "wrap the pattern in parentheses";
-                let fix = format!("({})", pprust::pat_to_string(&pat));
-                (msg, fix)
-            };
+            let span = pat.span;
 
             if trailing_vert {
                 // We already emitted an error and suggestion to remove the trailing vert. Don't
                 // emit again.
-                self.sess.span_diagnostic.delay_span_bug(pat.span, &msg);
+
+                // FIXME(#100717): pass `TopLevelOrPatternNotAllowed::* { sub: None }` to
+                // `delay_span_bug()` instead of fluent message
+                self.sess.span_diagnostic.delay_span_bug(
+                    span,
+                    match syntax_loc {
+                        PatternLocation::LetBinding => {
+                            fluent::parse_or_pattern_not_allowed_in_let_binding
+                        }
+                        PatternLocation::FunctionParameter => {
+                            fluent::parse_or_pattern_not_allowed_in_fn_parameters
+                        }
+                    },
+                );
             } else {
-                self.struct_span_err(pat.span, &msg)
-                    .span_suggestion(pat.span, help, fix, Applicability::MachineApplicable)
-                    .emit();
+                let pat = pprust::pat_to_string(&pat);
+                let sub = if pats.len() == 1 {
+                    Some(TopLevelOrPatternNotAllowedSugg::RemoveLeadingVert { span, pat })
+                } else {
+                    Some(TopLevelOrPatternNotAllowedSugg::WrapInParens { span, pat })
+                };
+
+                self.sess.emit_err(match syntax_loc {
+                    PatternLocation::LetBinding => {
+                        TopLevelOrPatternNotAllowed::LetBinding { span, sub }
+                    }
+                    PatternLocation::FunctionParameter => {
+                        TopLevelOrPatternNotAllowed::FunctionParameter { span, sub }
+                    }
+                });
             }
         }
 
@@ -218,15 +264,15 @@ impl<'a> Parser<'a> {
         // a leading `||` probably doesn't indicate an or-pattern attempt, so we handle that
         // separately.
         if let token::OrOr = self.token.kind {
-            let span = self.token.span;
-            let mut err = self.struct_span_err(span, "unexpected `||` before function parameter");
-            err.span_suggestion(span, "remove the `||`", "", Applicability::MachineApplicable);
-            err.note("alternatives in or-patterns are separated with `|`, not `||`");
-            err.emit();
+            self.sess.emit_err(UnexpectedVertVertBeforeFunctionParam { span: self.token.span });
             self.bump();
         }
 
-        self.parse_pat_before_ty(PARAM_EXPECTED, RecoverComma::No, "function parameters")
+        self.parse_pat_before_ty(
+            Some(Expected::ParameterName),
+            RecoverComma::No,
+            PatternLocation::FunctionParameter,
+        )
     }
 
     /// Eat the or-pattern `|` separator.
@@ -236,7 +282,7 @@ impl<'a> Parser<'a> {
             EatOrResult::TrailingVert
         } else if matches!(self.token.kind, token::OrOr) {
             // Found `||`; Recover and pretend we parsed `|`.
-            self.ban_unexpected_or_or(lo);
+            self.sess.emit_err(UnexpectedVertVertInPattern { span: self.token.span, start: lo });
             self.bump();
             EatOrResult::AteOr
         } else if self.eat(&token::BinOp(token::Or)) {
@@ -270,7 +316,13 @@ impl<'a> Parser<'a> {
         });
         match (is_end_ahead, &self.token.kind) {
             (true, token::BinOp(token::Or) | token::OrOr) => {
-                self.ban_illegal_vert(lo, "trailing", "not allowed in an or-pattern");
+                // A `|` or possibly `||` token shouldn't be here. Ban it.
+                self.sess.emit_err(TrailingVertNotAllowed {
+                    span: self.token.span,
+                    start: lo,
+                    token: self.token.clone(),
+                    note_double_vert: matches!(self.token.kind, token::OrOr).then_some(()),
+                });
                 self.bump();
                 true
             }
@@ -278,46 +330,12 @@ impl<'a> Parser<'a> {
         }
     }
 
-    /// We have parsed `||` instead of `|`. Error and suggest `|` instead.
-    fn ban_unexpected_or_or(&mut self, lo: Option<Span>) {
-        let mut err = self.struct_span_err(self.token.span, "unexpected token `||` in pattern");
-        err.span_suggestion(
-            self.token.span,
-            "use a single `|` to separate multiple alternative patterns",
-            "|",
-            Applicability::MachineApplicable,
-        );
-        if let Some(lo) = lo {
-            err.span_label(lo, WHILE_PARSING_OR_MSG);
-        }
-        err.emit();
-    }
-
-    /// A `|` or possibly `||` token shouldn't be here. Ban it.
-    fn ban_illegal_vert(&mut self, lo: Option<Span>, pos: &str, ctx: &str) {
-        let span = self.token.span;
-        let mut err = self.struct_span_err(span, &format!("a {} `|` is {}", pos, ctx));
-        err.span_suggestion(
-            span,
-            &format!("remove the `{}`", pprust::token_to_string(&self.token)),
-            "",
-            Applicability::MachineApplicable,
-        );
-        if let Some(lo) = lo {
-            err.span_label(lo, WHILE_PARSING_OR_MSG);
-        }
-        if let token::OrOr = self.token.kind {
-            err.note("alternatives in or-patterns are separated with `|`, not `||`");
-        }
-        err.emit();
-    }
-
     /// Parses a pattern, with a setting whether modern range patterns (e.g., `a..=b`, `a..b` are
     /// allowed).
     fn parse_pat_with_range_pat(
         &mut self,
         allow_range_pat: bool,
-        expected: Expected,
+        expected: Option<Expected>,
     ) -> PResult<'a, P<Pat>> {
         maybe_recover_from_interpolated_ty_qpath!(self, true);
         maybe_whole!(self, NtPat, |x| x);
@@ -413,7 +431,7 @@ impl<'a> Parser<'a> {
             let lt = self.expect_lifetime();
             let (lit, _) =
                 self.recover_unclosed_char(lt.ident, Parser::mk_token_lit_char, |self_| {
-                    let expected = expected.unwrap_or("pattern");
+                    let expected = Expected::to_string_or_fallback(expected);
                     let msg = format!(
                         "expected {}, found {}",
                         expected,
@@ -454,15 +472,7 @@ impl<'a> Parser<'a> {
         self.bump(); // `...`
 
         // The user probably mistook `...` for a rest pattern `..`.
-        self.struct_span_err(lo, "unexpected `...`")
-            .span_label(lo, "not a valid pattern")
-            .span_suggestion_short(
-                lo,
-                "for a rest pattern, use `..` instead of `...`",
-                "..",
-                Applicability::MachineApplicable,
-            )
-            .emit();
+        self.sess.emit_err(DotDotDotRestPattern { span: lo });
         PatKind::Rest
     }
 
@@ -487,7 +497,7 @@ impl<'a> Parser<'a> {
         // At this point we attempt to parse `@ $pat_rhs` and emit an error.
         self.bump(); // `@`
         let mut rhs = self.parse_pat_no_top_alt(None)?;
-        let sp = lhs.span.to(rhs.span);
+        let whole_span = lhs.span.to(rhs.span);
 
         if let PatKind::Ident(_, _, sub @ None) = &mut rhs.kind {
             // The user inverted the order, so help them fix that.
@@ -496,27 +506,23 @@ impl<'a> Parser<'a> {
             // The RHS is now the full pattern.
             *sub = Some(lhs);
 
-            self.struct_span_err(sp, "pattern on wrong side of `@`")
-                .span_label(lhs_span, "pattern on the left, should be on the right")
-                .span_label(rhs.span, "binding on the right, should be on the left")
-                .span_suggestion(
-                    sp,
-                    "switch the order",
-                    pprust::pat_to_string(&rhs),
-                    Applicability::MachineApplicable,
-                )
-                .emit();
+            self.sess.emit_err(PatternOnWrongSideOfAt {
+                whole_span,
+                whole_pat: pprust::pat_to_string(&rhs),
+                pattern: lhs_span,
+                binding: rhs.span,
+            });
         } else {
             // The special case above doesn't apply so we may have e.g. `A(x) @ B(y)`.
             rhs.kind = PatKind::Wild;
-            self.struct_span_err(sp, "left-hand side of `@` must be a binding")
-                .span_label(lhs.span, "interpreted as a pattern, not a binding")
-                .span_label(rhs.span, "also a pattern")
-                .note("bindings are `x`, `mut x`, `ref x`, and `ref mut x`")
-                .emit();
+            self.sess.emit_err(ExpectedBindingLeftOfAt {
+                whole_span,
+                lhs: lhs.span,
+                rhs: rhs.span,
+            });
         }
 
-        rhs.span = sp;
+        rhs.span = whole_span;
         Ok(rhs)
     }
 
@@ -531,35 +537,23 @@ impl<'a> Parser<'a> {
             _ => return,
         }
 
-        self.struct_span_err(pat.span, "the range pattern here has ambiguous interpretation")
-            .span_suggestion(
-                pat.span,
-                "add parentheses to clarify the precedence",
-                format!("({})", pprust::pat_to_string(&pat)),
-                // "ambiguous interpretation" implies that we have to be guessing
-                Applicability::MaybeIncorrect,
-            )
-            .emit();
+        self.sess
+            .emit_err(AmbiguousRangePattern { span: pat.span, pat: pprust::pat_to_string(&pat) });
     }
 
     /// Parse `&pat` / `&mut pat`.
-    fn parse_pat_deref(&mut self, expected: Expected) -> PResult<'a, PatKind> {
+    fn parse_pat_deref(&mut self, expected: Option<Expected>) -> PResult<'a, PatKind> {
         self.expect_and()?;
-        self.recover_lifetime_in_deref_pat();
-        let mutbl = self.parse_mutability();
-        let subpat = self.parse_pat_with_range_pat(false, expected)?;
-        Ok(PatKind::Ref(subpat, mutbl))
-    }
-
-    fn recover_lifetime_in_deref_pat(&mut self) {
         if let token::Lifetime(name) = self.token.kind {
             self.bump(); // `'a`
 
-            let span = self.prev_token.span;
-            self.struct_span_err(span, &format!("unexpected lifetime `{}` in pattern", name))
-                .span_suggestion(span, "remove the lifetime", "", Applicability::MachineApplicable)
-                .emit();
+            self.sess
+                .emit_err(UnexpectedLifetimeInPattern { span: self.prev_token.span, symbol: name });
         }
+
+        let mutbl = self.parse_mutability();
+        let subpat = self.parse_pat_with_range_pat(false, expected)?;
+        Ok(PatKind::Ref(subpat, mutbl))
     }
 
     /// Parse a tuple or parenthesis pattern.
@@ -587,7 +581,8 @@ impl<'a> Parser<'a> {
         let mut_span = self.prev_token.span;
 
         if self.eat_keyword(kw::Ref) {
-            return self.recover_mut_ref_ident(mut_span);
+            self.sess.emit_err(RefMutOrderIncorrect { span: mut_span.to(self.prev_token.span) });
+            return self.parse_pat_ident(BindingAnnotation::REF_MUT);
         }
 
         self.recover_additional_muts();
@@ -600,7 +595,7 @@ impl<'a> Parser<'a> {
         }
 
         // Parse the pattern we hope to be an identifier.
-        let mut pat = self.parse_pat_no_top_alt(Some("identifier"))?;
+        let mut pat = self.parse_pat_no_top_alt(Some(Expected::Identifier))?;
 
         // If we don't have `mut $ident (@ pat)?`, error.
         if let PatKind::Ident(BindingAnnotation(ByRef::No, m @ Mutability::Not), ..) = &mut pat.kind
@@ -617,22 +612,6 @@ impl<'a> Parser<'a> {
         Ok(pat.into_inner().kind)
     }
 
-    /// Recover on `mut ref? ident @ pat` and suggest
-    /// that the order of `mut` and `ref` is incorrect.
-    fn recover_mut_ref_ident(&mut self, lo: Span) -> PResult<'a, PatKind> {
-        let mutref_span = lo.to(self.prev_token.span);
-        self.struct_span_err(mutref_span, "the order of `mut` and `ref` is incorrect")
-            .span_suggestion(
-                mutref_span,
-                "try switching the order",
-                "ref mut",
-                Applicability::MachineApplicable,
-            )
-            .emit();
-
-        self.parse_pat_ident(BindingAnnotation::REF_MUT)
-    }
-
     /// Turn all by-value immutable bindings in a pattern into mutable bindings.
     /// Returns `true` if any change was made.
     fn make_all_value_bindings_mutable(pat: &mut P<Pat>) -> bool {
@@ -657,16 +636,13 @@ impl<'a> Parser<'a> {
     /// Error on `mut $pat` where `$pat` is not an ident.
     fn ban_mut_general_pat(&self, lo: Span, pat: &Pat, changed_any_binding: bool) {
         let span = lo.to(pat.span);
-        let fix = pprust::pat_to_string(&pat);
-        let (problem, suggestion) = if changed_any_binding {
-            ("`mut` must be attached to each individual binding", "add `mut` to each binding")
+        let pat = pprust::pat_to_string(&pat);
+
+        self.sess.emit_err(if changed_any_binding {
+            InvalidMutInPattern::NestedIdent { span, pat }
         } else {
-            ("`mut` must be followed by a named binding", "remove the `mut` prefix")
-        };
-        self.struct_span_err(span, problem)
-            .span_suggestion(span, suggestion, fix, Applicability::MachineApplicable)
-            .note("`mut` may be followed by `variable` and `variable @ pattern`")
-            .emit();
+            InvalidMutInPattern::NonIdent { span, pat }
+        });
     }
 
     /// Eat any extraneous `mut`s and error + recover if we ate any.
@@ -677,15 +653,7 @@ impl<'a> Parser<'a> {
             return;
         }
 
-        let span = lo.to(self.prev_token.span);
-        self.struct_span_err(span, "`mut` on a binding may not be repeated")
-            .span_suggestion(
-                span,
-                "remove the additional `mut`s",
-                "",
-                Applicability::MachineApplicable,
-            )
-            .emit();
+        self.sess.emit_err(RepeatedMutInPattern { span: lo.to(self.prev_token.span) });
     }
 
     /// Parse macro invocation
@@ -699,11 +667,11 @@ impl<'a> Parser<'a> {
     fn fatal_unexpected_non_pat(
         &mut self,
         err: DiagnosticBuilder<'a, ErrorGuaranteed>,
-        expected: Expected,
+        expected: Option<Expected>,
     ) -> PResult<'a, P<Pat>> {
         err.cancel();
 
-        let expected = expected.unwrap_or("pattern");
+        let expected = Expected::to_string_or_fallback(expected);
         let msg = format!("expected {}, found {}", expected, super::token_descr(&self.token));
 
         let mut err = self.struct_span_err(self.token.span, &msg);
@@ -745,49 +713,44 @@ impl<'a> Parser<'a> {
             // Parsing e.g. `X..`.
             if let RangeEnd::Included(_) = re.node {
                 // FIXME(Centril): Consider semantic errors instead in `ast_validation`.
-                self.inclusive_range_with_incorrect_end(re.span);
+                self.inclusive_range_with_incorrect_end();
             }
             None
         };
         Ok(PatKind::Range(Some(begin), end, re))
     }
 
-    pub(super) fn inclusive_range_with_incorrect_end(&mut self, span: Span) {
+    pub(super) fn inclusive_range_with_incorrect_end(&mut self) {
         let tok = &self.token;
-
+        let span = self.prev_token.span;
         // If the user typed "..==" instead of "..=", we want to give them
         // a specific error message telling them to use "..=".
+        // If they typed "..=>", suggest they use ".. =>".
         // Otherwise, we assume that they meant to type a half open exclusive
         // range and give them an error telling them to do that instead.
-        if matches!(tok.kind, token::Eq) && tok.span.lo() == span.hi() {
-            let span_with_eq = span.to(tok.span);
+        let no_space = tok.span.lo() == span.hi();
+        match tok.kind {
+            token::Eq if no_space => {
+                let span_with_eq = span.to(tok.span);
 
-            // Ensure the user doesn't receive unhelpful unexpected token errors
-            self.bump();
-            if self.is_pat_range_end_start(0) {
-                let _ = self.parse_pat_range_end().map_err(|e| e.cancel());
-            }
+                // Ensure the user doesn't receive unhelpful unexpected token errors
+                self.bump();
+                if self.is_pat_range_end_start(0) {
+                    let _ = self.parse_pat_range_end().map_err(|e| e.cancel());
+                }
 
-            self.error_inclusive_range_with_extra_equals(span_with_eq);
-        } else {
-            self.error_inclusive_range_with_no_end(span);
+                self.sess.emit_err(InclusiveRangeExtraEquals { span: span_with_eq });
+            }
+            token::Gt if no_space => {
+                let after_pat = span.with_hi(span.hi() - rustc_span::BytePos(1)).shrink_to_hi();
+                self.sess.emit_err(InclusiveRangeMatchArrow { span, arrow: tok.span, after_pat });
+            }
+            _ => {
+                self.sess.emit_err(InclusiveRangeNoEnd { span });
+            }
         }
     }
 
-    fn error_inclusive_range_with_extra_equals(&self, span: Span) {
-        self.struct_span_err(span, "unexpected `=` after inclusive range")
-            .span_suggestion_short(span, "use `..=` instead", "..=", Applicability::MaybeIncorrect)
-            .note("inclusive ranges end with a single equals sign (`..=`)")
-            .emit();
-    }
-
-    fn error_inclusive_range_with_no_end(&self, span: Span) {
-        struct_span_err!(self.sess.span_diagnostic, span, E0586, "inclusive range with no end")
-            .span_suggestion_short(span, "use `..` instead", "..", Applicability::MachineApplicable)
-            .note("inclusive ranges must be bounded at the end (`..=b` or `a..=b`)")
-            .emit();
-    }
-
     /// Parse a range-to pattern, `..X` or `..=X` where `X` remains to be parsed.
     ///
     /// The form `...X` is prohibited to reduce confusion with the potential
@@ -796,14 +759,7 @@ impl<'a> Parser<'a> {
         let end = self.parse_pat_range_end()?;
         if let RangeEnd::Included(syn @ RangeSyntax::DotDotDot) = &mut re.node {
             *syn = RangeSyntax::DotDotEq;
-            self.struct_span_err(re.span, "range-to patterns with `...` are not allowed")
-                .span_suggestion_short(
-                    re.span,
-                    "use `..=` instead",
-                    "..=",
-                    Applicability::MachineApplicable,
-                )
-                .emit();
+            self.sess.emit_err(DotDotDotRangeToPatternNotAllowed { span: re.span });
         }
         Ok(PatKind::Range(None, Some(end), re))
     }
@@ -868,7 +824,7 @@ impl<'a> Parser<'a> {
     fn parse_pat_ident(&mut self, binding_annotation: BindingAnnotation) -> PResult<'a, PatKind> {
         let ident = self.parse_ident()?;
         let sub = if self.eat(&token::At) {
-            Some(self.parse_pat_no_top_alt(Some("binding pattern"))?)
+            Some(self.parse_pat_no_top_alt(Some(Expected::BindingPattern))?)
         } else {
             None
         };
@@ -879,8 +835,8 @@ impl<'a> Parser<'a> {
         // binding mode then we do not end up here, because the lookahead
         // will direct us over to `parse_enum_variant()`.
         if self.token == token::OpenDelim(Delimiter::Parenthesis) {
-            return Err(self
-                .struct_span_err(self.prev_token.span, "expected identifier, found enum pattern"));
+            return Err(EnumPatternInsteadOfIdentifier { span: self.prev_token.span }
+                .into_diagnostic(&self.sess.span_diagnostic));
         }
 
         Ok(PatKind::Ident(binding_annotation, ident, sub))
@@ -897,7 +853,7 @@ impl<'a> Parser<'a> {
             e.span_label(path.span, "while parsing the fields for this pattern");
             e.emit();
             self.recover_stmt();
-            (vec![], true)
+            (ThinVec::new(), true)
         });
         self.bump();
         Ok(PatKind::Struct(qself, path, fields, etc))
@@ -962,7 +918,7 @@ impl<'a> Parser<'a> {
             // We cannot use `parse_pat_ident()` since it will complain `box`
             // is not an identifier.
             let sub = if self.eat(&token::At) {
-                Some(self.parse_pat_no_top_alt(Some("binding pattern"))?)
+                Some(self.parse_pat_no_top_alt(Some(Expected::BindingPattern))?)
             } else {
                 None
             };
@@ -976,8 +932,8 @@ impl<'a> Parser<'a> {
     }
 
     /// Parses the fields of a struct-like pattern.
-    fn parse_pat_fields(&mut self) -> PResult<'a, (Vec<PatField>, bool)> {
-        let mut fields = Vec::new();
+    fn parse_pat_fields(&mut self) -> PResult<'a, (ThinVec<PatField>, bool)> {
+        let mut fields = ThinVec::new();
         let mut etc = false;
         let mut ate_comma = true;
         let mut delayed_err: Option<DiagnosticBuilder<'a, ErrorGuaranteed>> = None;
@@ -997,7 +953,8 @@ impl<'a> Parser<'a> {
 
             // check that a comma comes after every field
             if !ate_comma {
-                let err = self.struct_span_err(self.token.span, "expected `,`");
+                let err = ExpectedCommaAfterPatternField { span: self.token.span }
+                    .into_diagnostic(&self.sess.span_diagnostic);
                 if let Some(mut delayed) = delayed_err {
                     delayed.emit();
                 }
@@ -1005,12 +962,15 @@ impl<'a> Parser<'a> {
             }
             ate_comma = false;
 
-            if self.check(&token::DotDot) || self.token == token::DotDotDot {
+            if self.check(&token::DotDot)
+                || self.check_noexpect(&token::DotDotDot)
+                || self.check_keyword(kw::Underscore)
+            {
                 etc = true;
                 let mut etc_sp = self.token.span;
 
-                self.recover_one_fewer_dotdot();
-                self.bump(); // `..` || `...`
+                self.recover_bad_dot_dot();
+                self.bump(); // `..` || `...` || `_`
 
                 if self.token == token::CloseDelim(Delimiter::Brace) {
                     etc_span = Some(etc_sp);
@@ -1103,21 +1063,15 @@ impl<'a> Parser<'a> {
         Ok((fields, etc))
     }
 
-    /// Recover on `...` as if it were `..` to avoid further errors.
+    /// Recover on `...` or `_` as if it were `..` to avoid further errors.
     /// See issue #46718.
-    fn recover_one_fewer_dotdot(&self) {
-        if self.token != token::DotDotDot {
+    fn recover_bad_dot_dot(&self) {
+        if self.token == token::DotDot {
             return;
         }
 
-        self.struct_span_err(self.token.span, "expected field pattern, found `...`")
-            .span_suggestion(
-                self.token.span,
-                "to omit remaining fields, use one fewer `.`",
-                "..",
-                Applicability::MachineApplicable,
-            )
-            .emit();
+        let token_str = pprust::token_to_string(&self.token);
+        self.sess.emit_err(DotDotDotForRemainingFields { span: self.token.span, token_str });
     }
 
     fn parse_pat_field(&mut self, lo: Span, attrs: AttrVec) -> PResult<'a, PatField> {
diff --git a/compiler/rustc_parse/src/parser/path.rs b/compiler/rustc_parse/src/parser/path.rs
index 5333d3b85..b50d2984a 100644
--- a/compiler/rustc_parse/src/parser/path.rs
+++ b/compiler/rustc_parse/src/parser/path.rs
@@ -332,7 +332,7 @@ impl<'a> Parser<'a> {
         style: PathStyle,
         lo: Span,
         ty_generics: Option<&Generics>,
-    ) -> PResult<'a, Vec<AngleBracketedArg>> {
+    ) -> PResult<'a, ThinVec<AngleBracketedArg>> {
         // We need to detect whether there are extra leading left angle brackets and produce an
         // appropriate error and suggestion. This cannot be implemented by looking ahead at
         // upcoming tokens for a matching `>` character - if there are unmatched `<` tokens
@@ -404,7 +404,7 @@ impl<'a> Parser<'a> {
 
         let is_first_invocation = style == PathStyle::Expr;
         // Take a snapshot before attempting to parse - we can restore this later.
-        let snapshot = if is_first_invocation { Some(self.clone()) } else { None };
+        let snapshot = is_first_invocation.then(|| self.clone());
 
         debug!("parse_generic_args_with_leading_angle_bracket_recovery: (snapshotting)");
         match self.parse_angle_args(ty_generics) {
@@ -472,8 +472,8 @@ impl<'a> Parser<'a> {
     pub(super) fn parse_angle_args(
         &mut self,
         ty_generics: Option<&Generics>,
-    ) -> PResult<'a, Vec<AngleBracketedArg>> {
-        let mut args = Vec::new();
+    ) -> PResult<'a, ThinVec<AngleBracketedArg>> {
+        let mut args = ThinVec::new();
         while let Some(arg) = self.parse_angle_arg(ty_generics)? {
             args.push(arg);
             if !self.eat(&token::Comma) {
@@ -653,7 +653,7 @@ impl<'a> Parser<'a> {
     pub(super) fn parse_const_arg(&mut self) -> PResult<'a, AnonConst> {
         // Parse const argument.
         let value = if let token::OpenDelim(Delimiter::Brace) = self.token.kind {
-            self.parse_block_expr(None, self.token.span, BlockCheckMode::Default)?
+            self.parse_expr_block(None, self.token.span, BlockCheckMode::Default)?
         } else {
             self.handle_unambiguous_unbraced_const_arg()?
         };
@@ -675,22 +675,42 @@ impl<'a> Parser<'a> {
             GenericArg::Const(self.parse_const_arg()?)
         } else if self.check_type() {
             // Parse type argument.
-            let is_const_fn =
-                self.look_ahead(1, |t| t.kind == token::OpenDelim(Delimiter::Parenthesis));
-            let mut snapshot = self.create_snapshot_for_diagnostic();
+
+            // Proactively create a parser snapshot enabling us to rewind and try to reparse the
+            // input as a const expression in case we fail to parse a type. If we successfully
+            // do so, we will report an error that it needs to be wrapped in braces.
+            let mut snapshot = None;
+            if self.may_recover() && self.token.can_begin_expr() {
+                snapshot = Some(self.create_snapshot_for_diagnostic());
+            }
+
             match self.parse_ty() {
-                Ok(ty) => GenericArg::Type(ty),
+                Ok(ty) => {
+                    // Since the type parser recovers from some malformed slice and array types and
+                    // successfully returns a type, we need to look for `TyKind::Err`s in the
+                    // type to determine if error recovery has occurred and if the input is not a
+                    // syntactically valid type after all.
+                    if let ast::TyKind::Slice(inner_ty) | ast::TyKind::Array(inner_ty, _) = &ty.kind
+                        && let ast::TyKind::Err = inner_ty.kind
+                        && let Some(snapshot) = snapshot
+                        && let Some(expr) = self.recover_unbraced_const_arg_that_can_begin_ty(snapshot)
+                    {
+                        return Ok(Some(self.dummy_const_arg_needs_braces(
+                            self.struct_span_err(expr.span, "invalid const generic expression"),
+                            expr.span,
+                        )));
+                    }
+
+                    GenericArg::Type(ty)
+                }
                 Err(err) => {
-                    if is_const_fn {
-                        match (*snapshot).parse_expr_res(Restrictions::CONST_EXPR, None) {
-                            Ok(expr) => {
-                                self.restore_snapshot(snapshot);
-                                return Ok(Some(self.dummy_const_arg_needs_braces(err, expr.span)));
-                            }
-                            Err(err) => {
-                                err.cancel();
-                            }
-                        }
+                    if let Some(snapshot) = snapshot
+                        && let Some(expr) = self.recover_unbraced_const_arg_that_can_begin_ty(snapshot)
+                    {
+                        return Ok(Some(self.dummy_const_arg_needs_braces(
+                            err,
+                            expr.span,
+                        )));
                     }
                     // Try to recover from possible `const` arg without braces.
                     return self.recover_const_arg(start, err).map(Some);
diff --git a/compiler/rustc_parse/src/parser/stmt.rs b/compiler/rustc_parse/src/parser/stmt.rs
index 4ff9927aa..92a22ffc2 100644
--- a/compiler/rustc_parse/src/parser/stmt.rs
+++ b/compiler/rustc_parse/src/parser/stmt.rs
@@ -1,18 +1,13 @@
 use super::attr::InnerAttrForbiddenReason;
 use super::diagnostics::AttemptLocalParseRecovery;
 use super::expr::LhsExpr;
-use super::pat::RecoverComma;
+use super::pat::{PatternLocation, RecoverComma};
 use super::path::PathStyle;
 use super::TrailingToken;
 use super::{
     AttrWrapper, BlockMode, FnParseMode, ForceCollect, Parser, Restrictions, SemiColonMode,
 };
-use crate::errors::{
-    AssignmentElseNotAllowed, CompoundAssignmentExpressionInLet, ConstLetMutuallyExclusive,
-    DocCommentDoesNotDocumentAnything, ExpectedStatementAfterOuterAttr, InvalidCurlyInLetElse,
-    InvalidExpressionInLetElse, InvalidIdentiferStartsWithNumber, InvalidVariableDeclaration,
-    InvalidVariableDeclarationSub, WrapExpressionInParentheses,
-};
+use crate::errors;
 use crate::maybe_whole;
 
 use rustc_ast as ast;
@@ -25,8 +20,8 @@ use rustc_ast::{StmtKind, DUMMY_NODE_ID};
 use rustc_errors::{Applicability, DiagnosticBuilder, ErrorGuaranteed, PResult};
 use rustc_span::source_map::{BytePos, Span};
 use rustc_span::symbol::{kw, sym};
-
 use std::mem;
+use thin_vec::{thin_vec, ThinVec};
 
 impl<'a> Parser<'a> {
     /// Parses a statement. This stops just before trailing semicolons on everything but items.
@@ -64,40 +59,45 @@ impl<'a> Parser<'a> {
         if self.token.is_keyword(kw::Mut) && self.is_keyword_ahead(1, &[kw::Let]) {
             self.bump();
             let mut_let_span = lo.to(self.token.span);
-            self.sess.emit_err(InvalidVariableDeclaration {
+            self.sess.emit_err(errors::InvalidVariableDeclaration {
                 span: mut_let_span,
-                sub: InvalidVariableDeclarationSub::SwitchMutLetOrder(mut_let_span),
+                sub: errors::InvalidVariableDeclarationSub::SwitchMutLetOrder(mut_let_span),
             });
         }
 
         Ok(Some(if self.token.is_keyword(kw::Let) {
             self.parse_local_mk(lo, attrs, capture_semi, force_collect)?
         } else if self.is_kw_followed_by_ident(kw::Mut) && self.may_recover() {
-            self.recover_stmt_local_after_let(lo, attrs, InvalidVariableDeclarationSub::MissingLet)?
+            self.recover_stmt_local_after_let(
+                lo,
+                attrs,
+                errors::InvalidVariableDeclarationSub::MissingLet,
+            )?
         } else if self.is_kw_followed_by_ident(kw::Auto) && self.may_recover() {
             self.bump(); // `auto`
             self.recover_stmt_local_after_let(
                 lo,
                 attrs,
-                InvalidVariableDeclarationSub::UseLetNotAuto,
+                errors::InvalidVariableDeclarationSub::UseLetNotAuto,
             )?
         } else if self.is_kw_followed_by_ident(sym::var) && self.may_recover() {
             self.bump(); // `var`
             self.recover_stmt_local_after_let(
                 lo,
                 attrs,
-                InvalidVariableDeclarationSub::UseLetNotVar,
+                errors::InvalidVariableDeclarationSub::UseLetNotVar,
             )?
         } else if self.check_path() && !self.token.is_qpath_start() && !self.is_path_start_item() {
             // We have avoided contextual keywords like `union`, items with `crate` visibility,
             // or `auto trait` items. We aim to parse an arbitrary path `a::b` but not something
             // that starts like a path (1 token), but it fact not a path.
             // Also, we avoid stealing syntax from `parse_item_`.
-            if force_collect == ForceCollect::Yes {
-                self.collect_tokens_no_attrs(|this| this.parse_stmt_path_start(lo, attrs))
-            } else {
-                self.parse_stmt_path_start(lo, attrs)
-            }?
+            match force_collect {
+                ForceCollect::Yes => {
+                    self.collect_tokens_no_attrs(|this| this.parse_stmt_path_start(lo, attrs))?
+                }
+                ForceCollect::No => self.parse_stmt_path_start(lo, attrs)?,
+            }
         } else if let Some(item) = self.parse_item_common(
             attrs.clone(),
             false,
@@ -113,18 +113,17 @@ impl<'a> Parser<'a> {
             self.mk_stmt(lo, StmtKind::Empty)
         } else if self.token != token::CloseDelim(Delimiter::Brace) {
             // Remainder are line-expr stmts.
-            let e = if force_collect == ForceCollect::Yes {
-                self.collect_tokens_no_attrs(|this| {
+            let e = match force_collect {
+                ForceCollect::Yes => self.collect_tokens_no_attrs(|this| {
                     this.parse_expr_res(Restrictions::STMT_EXPR, Some(attrs))
-                })
-            } else {
-                self.parse_expr_res(Restrictions::STMT_EXPR, Some(attrs))
-            }?;
+                })?,
+                ForceCollect::No => self.parse_expr_res(Restrictions::STMT_EXPR, Some(attrs))?,
+            };
             if matches!(e.kind, ExprKind::Assign(..)) && self.eat_keyword(kw::Else) {
                 let bl = self.parse_block()?;
                 // Destructuring assignment ... else.
                 // This is not allowed, but point it out in a nice way.
-                self.sess.emit_err(AssignmentElseNotAllowed { span: e.span.to(bl.span) });
+                self.sess.emit_err(errors::AssignmentElseNotAllowed { span: e.span.to(bl.span) });
             }
             self.mk_stmt(lo.to(e.span), StmtKind::Expr(e))
         } else {
@@ -147,14 +146,14 @@ impl<'a> Parser<'a> {
             }
 
             let expr = if this.eat(&token::OpenDelim(Delimiter::Brace)) {
-                this.parse_struct_expr(None, path, true)?
+                this.parse_expr_struct(None, path, true)?
             } else {
                 let hi = this.prev_token.span;
                 this.mk_expr(lo.to(hi), ExprKind::Path(None, path))
             };
 
             let expr = this.with_res(Restrictions::STMT_EXPR, |this| {
-                this.parse_dot_or_call_expr_with(expr, lo, attrs)
+                this.parse_expr_dot_or_call_with(expr, lo, attrs)
             })?;
             // `DUMMY_SP` will get overwritten later in this function
             Ok((this.mk_stmt(rustc_span::DUMMY_SP, StmtKind::Expr(expr)), TrailingToken::None))
@@ -164,7 +163,7 @@ impl<'a> Parser<'a> {
             // Perform this outside of the `collect_tokens_trailing_token` closure,
             // since our outer attributes do not apply to this part of the expression
             let expr = self.with_res(Restrictions::STMT_EXPR, |this| {
-                this.parse_assoc_expr_with(
+                this.parse_expr_assoc_with(
                     0,
                     LhsExpr::AlreadyParsed { expr, starts_statement: true },
                 )
@@ -200,8 +199,8 @@ impl<'a> Parser<'a> {
             // Since none of the above applied, this is an expression statement macro.
             let e = self.mk_expr(lo.to(hi), ExprKind::MacCall(mac));
             let e = self.maybe_recover_from_bad_qpath(e)?;
-            let e = self.parse_dot_or_call_expr_with(e, lo, attrs)?;
-            let e = self.parse_assoc_expr_with(
+            let e = self.parse_expr_dot_or_call_with(e, lo, attrs)?;
+            let e = self.parse_expr_assoc_with(
                 0,
                 LhsExpr::AlreadyParsed { expr: e, starts_statement: false },
             )?;
@@ -217,12 +216,12 @@ impl<'a> Parser<'a> {
         && let attrs = attrs.take_for_recovery(self.sess)
         && let attrs @ [.., last] = &*attrs {
             if last.is_doc_comment() {
-                self.sess.emit_err(DocCommentDoesNotDocumentAnything {
+                self.sess.emit_err(errors::DocCommentDoesNotDocumentAnything {
                     span: last.span,
                     missing_comma: None,
                 });
             } else if attrs.iter().any(|a| a.style == AttrStyle::Outer) {
-                self.sess.emit_err(ExpectedStatementAfterOuterAttr { span: last.span });
+                self.sess.emit_err(errors::ExpectedStatementAfterOuterAttr { span: last.span });
             }
         }
     }
@@ -231,7 +230,7 @@ impl<'a> Parser<'a> {
         &mut self,
         lo: Span,
         attrs: AttrWrapper,
-        subdiagnostic: fn(Span) -> InvalidVariableDeclarationSub,
+        subdiagnostic: fn(Span) -> errors::InvalidVariableDeclarationSub,
     ) -> PResult<'a, Stmt> {
         let stmt =
             self.collect_tokens_trailing_token(attrs, ForceCollect::Yes, |this, attrs| {
@@ -242,7 +241,7 @@ impl<'a> Parser<'a> {
                     TrailingToken::None,
                 ))
             })?;
-        self.sess.emit_err(InvalidVariableDeclaration { span: lo, sub: subdiagnostic(lo) });
+        self.sess.emit_err(errors::InvalidVariableDeclaration { span: lo, sub: subdiagnostic(lo) });
         Ok(stmt)
     }
 
@@ -270,12 +269,13 @@ impl<'a> Parser<'a> {
         let lo = self.prev_token.span;
 
         if self.token.is_keyword(kw::Const) && self.look_ahead(1, |t| t.is_ident()) {
-            self.sess.emit_err(ConstLetMutuallyExclusive { span: lo.to(self.token.span) });
+            self.sess.emit_err(errors::ConstLetMutuallyExclusive { span: lo.to(self.token.span) });
             self.bump();
         }
 
         self.report_invalid_identifier_error()?;
-        let (pat, colon) = self.parse_pat_before_ty(None, RecoverComma::Yes, "`let` bindings")?;
+        let (pat, colon) =
+            self.parse_pat_before_ty(None, RecoverComma::Yes, PatternLocation::LetBinding)?;
 
         let (err, ty) = if colon {
             // Save the state of the parser before parsing type normally, in case there is a `:`
@@ -372,7 +372,7 @@ impl<'a> Parser<'a> {
             rustc_ast::MetaItemLit::from_token(&self.token).is_none() &&
             (lit.kind == token::LitKind::Integer || lit.kind == token::LitKind::Float) &&
             self.look_ahead(1, |t| matches!(t.kind, token::Eq) || matches!(t.kind, token::Colon ) ) {
-                return Err(self.sess.create_err(InvalidIdentiferStartsWithNumber { span: self.token.span }));
+                return Err(self.sess.create_err(errors::InvalidIdentiferStartsWithNumber { span: self.token.span }));
         }
         Ok(())
     }
@@ -380,10 +380,10 @@ impl<'a> Parser<'a> {
     fn check_let_else_init_bool_expr(&self, init: &ast::Expr) {
         if let ast::ExprKind::Binary(op, ..) = init.kind {
             if op.node.lazy() {
-                self.sess.emit_err(InvalidExpressionInLetElse {
+                self.sess.emit_err(errors::InvalidExpressionInLetElse {
                     span: init.span,
                     operator: op.node.to_string(),
-                    sugg: WrapExpressionInParentheses {
+                    sugg: errors::WrapExpressionInParentheses {
                         left: init.span.shrink_to_lo(),
                         right: init.span.shrink_to_hi(),
                     },
@@ -394,9 +394,9 @@ impl<'a> Parser<'a> {
 
     fn check_let_else_init_trailing_brace(&self, init: &ast::Expr) {
         if let Some(trailing) = classify::expr_trailing_brace(init) {
-            self.sess.emit_err(InvalidCurlyInLetElse {
+            self.sess.emit_err(errors::InvalidCurlyInLetElse {
                 span: trailing.span.with_lo(trailing.span.hi() - BytePos(1)),
-                sugg: WrapExpressionInParentheses {
+                sugg: errors::WrapExpressionInParentheses {
                     left: trailing.span.shrink_to_lo(),
                     right: trailing.span.shrink_to_hi(),
                 },
@@ -409,7 +409,8 @@ impl<'a> Parser<'a> {
         let eq_consumed = match self.token.kind {
             token::BinOpEq(..) => {
                 // Recover `let x <op>= 1` as `let x = 1`
-                self.sess.emit_err(CompoundAssignmentExpressionInLet { span: self.token.span });
+                self.sess
+                    .emit_err(errors::CompoundAssignmentExpressionInLet { span: self.token.span });
                 self.bump();
                 true
             }
@@ -543,7 +544,7 @@ impl<'a> Parser<'a> {
         s: BlockCheckMode,
         recover: AttemptLocalParseRecovery,
     ) -> PResult<'a, P<Block>> {
-        let mut stmts = vec![];
+        let mut stmts = ThinVec::new();
         let mut snapshot = None;
         while !self.eat(&token::CloseDelim(Delimiter::Brace)) {
             if self.token == token::Eof {
@@ -661,7 +662,12 @@ impl<'a> Parser<'a> {
         Ok(Some(stmt))
     }
 
-    pub(super) fn mk_block(&self, stmts: Vec<Stmt>, rules: BlockCheckMode, span: Span) -> P<Block> {
+    pub(super) fn mk_block(
+        &self,
+        stmts: ThinVec<Stmt>,
+        rules: BlockCheckMode,
+        span: Span,
+    ) -> P<Block> {
         P(Block {
             stmts,
             id: DUMMY_NODE_ID,
@@ -681,6 +687,6 @@ impl<'a> Parser<'a> {
     }
 
     pub(super) fn mk_block_err(&self, span: Span) -> P<Block> {
-        self.mk_block(vec![self.mk_stmt_err(span)], BlockCheckMode::Default, span)
+        self.mk_block(thin_vec![self.mk_stmt_err(span)], BlockCheckMode::Default, span)
     }
 }
diff --git a/compiler/rustc_parse/src/parser/ty.rs b/compiler/rustc_parse/src/parser/ty.rs
index 867974672..6fe4da71f 100644
--- a/compiler/rustc_parse/src/parser/ty.rs
+++ b/compiler/rustc_parse/src/parser/ty.rs
@@ -1,6 +1,12 @@
 use super::{Parser, PathStyle, TokenType};
 
-use crate::errors::{ExpectedFnPathFoundFnKeyword, FnPtrWithGenerics, FnPtrWithGenericsSugg};
+use crate::errors::{
+    DynAfterMut, ExpectedFnPathFoundFnKeyword, ExpectedMutOrConstInRawPointerType,
+    FnPointerCannotBeAsync, FnPointerCannotBeConst, FnPtrWithGenerics, FnPtrWithGenericsSugg,
+    InvalidDynKeyword, LifetimeAfterMut, NeedPlusAfterTraitObjectLifetime,
+    NegativeBoundsNotSupported, NegativeBoundsNotSupportedSugg, NestedCVariadicType,
+    ReturnTypesUseThinArrow,
+};
 use crate::{maybe_recover_from_interpolated_ty_qpath, maybe_whole};
 
 use ast::DUMMY_NODE_ID;
@@ -11,11 +17,11 @@ use rustc_ast::{
     self as ast, BareFnTy, FnRetTy, GenericBound, GenericBounds, GenericParam, Generics, Lifetime,
     MacCall, MutTy, Mutability, PolyTraitRef, TraitBoundModifier, TraitObjectSyntax, Ty, TyKind,
 };
-use rustc_errors::{pluralize, struct_span_err, Applicability, PResult};
+use rustc_errors::{Applicability, PResult};
 use rustc_span::source_map::Span;
 use rustc_span::symbol::{kw, sym, Ident};
 use rustc_span::Symbol;
-use thin_vec::thin_vec;
+use thin_vec::{thin_vec, ThinVec};
 
 /// Any `?` or `~const` modifiers that appear at the start of a bound.
 struct BoundModifiers {
@@ -217,14 +223,7 @@ impl<'a> Parser<'a> {
             // Don't `eat` to prevent `=>` from being added as an expected token which isn't
             // actually expected and could only confuse users
             self.bump();
-            self.struct_span_err(self.prev_token.span, "return types are denoted using `->`")
-                .span_suggestion_short(
-                    self.prev_token.span,
-                    "use `->` instead",
-                    "->",
-                    Applicability::MachineApplicable,
-                )
-                .emit();
+            self.sess.emit_err(ReturnTypesUseThinArrow { span: self.prev_token.span });
             let ty = self.parse_ty_common(
                 allow_plus,
                 AllowCVariadic::No,
@@ -274,7 +273,7 @@ impl<'a> Parser<'a> {
             TyKind::Infer
         } else if self.check_fn_front_matter(false, Case::Sensitive) {
             // Function pointer type
-            self.parse_ty_bare_fn(lo, Vec::new(), None, recover_return_sign)?
+            self.parse_ty_bare_fn(lo, ThinVec::new(), None, recover_return_sign)?
         } else if self.check_keyword(kw::For) {
             // Function pointer type or bound list (trait object type) starting with a poly-trait.
             //   `for<'lt> [unsafe] [extern "ABI"] fn (&'lt S) -> T`
@@ -305,13 +304,14 @@ impl<'a> Parser<'a> {
         } else if self.can_begin_bound() {
             self.parse_bare_trait_object(lo, allow_plus)?
         } else if self.eat(&token::DotDotDot) {
-            if allow_c_variadic == AllowCVariadic::Yes {
-                TyKind::CVarArgs
-            } else {
-                // FIXME(Centril): Should we just allow `...` syntactically
-                // anywhere in a type and use semantic restrictions instead?
-                self.error_illegal_c_varadic_ty(lo);
-                TyKind::Err
+            match allow_c_variadic {
+                AllowCVariadic::Yes => TyKind::CVarArgs,
+                AllowCVariadic::No => {
+                    // FIXME(Centril): Should we just allow `...` syntactically
+                    // anywhere in a type and use semantic restrictions instead?
+                    self.sess.emit_err(NestedCVariadicType { span: lo.to(self.prev_token.span) });
+                    TyKind::Err
+                }
             }
         } else {
             let msg = format!("expected type, found {}", super::token_descr(&self.token));
@@ -325,10 +325,9 @@ impl<'a> Parser<'a> {
         let mut ty = self.mk_ty(span, kind);
 
         // Try to recover from use of `+` with incorrect priority.
-        if matches!(allow_plus, AllowPlus::Yes) {
-            self.maybe_recover_from_bad_type_plus(&ty)?;
-        } else {
-            self.maybe_report_ambiguous_plus(impl_dyn_multi, &ty);
+        match allow_plus {
+            AllowPlus::Yes => self.maybe_recover_from_bad_type_plus(&ty)?,
+            AllowPlus::No => self.maybe_report_ambiguous_plus(impl_dyn_multi, &ty),
         }
         if let RecoverQuestionMark::Yes = recover_question_mark {
             ty = self.maybe_recover_from_question_mark(ty);
@@ -353,7 +352,7 @@ impl<'a> Parser<'a> {
             match ty.kind {
                 // `(TY_BOUND_NOPAREN) + BOUND + ...`.
                 TyKind::Path(None, path) if maybe_bounds => {
-                    self.parse_remaining_bounds_path(Vec::new(), path, lo, true)
+                    self.parse_remaining_bounds_path(ThinVec::new(), path, lo, true)
                 }
                 TyKind::TraitObject(bounds, TraitObjectSyntax::None)
                     if maybe_bounds && bounds.len() == 1 && !trailing_plus =>
@@ -372,15 +371,14 @@ impl<'a> Parser<'a> {
         let lt_no_plus = self.check_lifetime() && !self.look_ahead(1, |t| t.is_like_plus());
         let bounds = self.parse_generic_bounds_common(allow_plus, None)?;
         if lt_no_plus {
-            self.struct_span_err(lo, "lifetime in trait object type must be followed by `+`")
-                .emit();
+            self.sess.emit_err(NeedPlusAfterTraitObjectLifetime { span: lo });
         }
         Ok(TyKind::TraitObject(bounds, TraitObjectSyntax::None))
     }
 
     fn parse_remaining_bounds_path(
         &mut self,
-        generic_params: Vec<GenericParam>,
+        generic_params: ThinVec<GenericParam>,
         path: ast::Path,
         lo: Span,
         parse_plus: bool,
@@ -407,14 +405,10 @@ impl<'a> Parser<'a> {
     fn parse_ty_ptr(&mut self) -> PResult<'a, TyKind> {
         let mutbl = self.parse_const_or_mut().unwrap_or_else(|| {
             let span = self.prev_token.span;
-            self.struct_span_err(span, "expected `mut` or `const` keyword in raw pointer type")
-                .span_suggestions(
-                    span.shrink_to_hi(),
-                    "add `mut` or `const` here",
-                    ["mut ".to_string(), "const ".to_string()],
-                    Applicability::HasPlaceholders,
-                )
-                .emit();
+            self.sess.emit_err(ExpectedMutOrConstInRawPointerType {
+                span,
+                after_asterisk: span.shrink_to_hi(),
+            });
             Mutability::Not
         });
         let ty = self.parse_ty_no_plus()?;
@@ -439,7 +433,7 @@ impl<'a> Parser<'a> {
         };
 
         let ty = if self.eat(&token::Semi) {
-            let mut length = self.parse_anon_const_expr()?;
+            let mut length = self.parse_expr_anon_const()?;
             if let Err(e) = self.expect(&token::CloseDelim(Delimiter::Bracket)) {
                 // Try to recover from `X<Y, ...>` when `X::<Y, ...>` works
                 self.check_mistyped_turbofish_with_multiple_type_params(e, &mut length.value)?;
@@ -456,8 +450,7 @@ impl<'a> Parser<'a> {
 
     fn parse_borrowed_pointee(&mut self) -> PResult<'a, TyKind> {
         let and_span = self.prev_token.span;
-        let mut opt_lifetime =
-            if self.check_lifetime() { Some(self.expect_lifetime()) } else { None };
+        let mut opt_lifetime = self.check_lifetime().then(|| self.expect_lifetime());
         let mut mutbl = self.parse_mutability();
         if self.token.is_lifetime() && mutbl == Mutability::Mut && opt_lifetime.is_none() {
             // A lifetime is invalid here: it would be part of a bare trait bound, which requires
@@ -469,16 +462,13 @@ impl<'a> Parser<'a> {
                 let lifetime_span = self.token.span;
                 let span = and_span.to(lifetime_span);
 
-                let mut err = self.struct_span_err(span, "lifetime must precede `mut`");
-                if let Ok(lifetime_src) = self.span_to_snippet(lifetime_span) {
-                    err.span_suggestion(
-                        span,
-                        "place the lifetime before `mut`",
-                        format!("&{} mut", lifetime_src),
-                        Applicability::MaybeIncorrect,
-                    );
-                }
-                err.emit();
+                let (suggest_lifetime, snippet) =
+                    if let Ok(lifetime_src) = self.span_to_snippet(lifetime_span) {
+                        (Some(span), lifetime_src)
+                    } else {
+                        (None, String::new())
+                    };
+                self.sess.emit_err(LifetimeAfterMut { span, suggest_lifetime, snippet });
 
                 opt_lifetime = Some(self.expect_lifetime());
             }
@@ -488,14 +478,7 @@ impl<'a> Parser<'a> {
         {
             // We have `&dyn mut ...`, which is invalid and should be `&mut dyn ...`.
             let span = and_span.to(self.look_ahead(1, |t| t.span));
-            let mut err = self.struct_span_err(span, "`mut` must precede `dyn`");
-            err.span_suggestion(
-                span,
-                "place `mut` before `dyn`",
-                "&mut dyn",
-                Applicability::MachineApplicable,
-            );
-            err.emit();
+            self.sess.emit_err(DynAfterMut { span });
 
             // Recovery
             mutbl = Mutability::Mut;
@@ -511,7 +494,7 @@ impl<'a> Parser<'a> {
     // To avoid ambiguity, the type is surrounded by parentheses.
     fn parse_typeof_ty(&mut self) -> PResult<'a, TyKind> {
         self.expect(&token::OpenDelim(Delimiter::Parenthesis))?;
-        let expr = self.parse_anon_const_expr()?;
+        let expr = self.parse_expr_anon_const()?;
         self.expect(&token::CloseDelim(Delimiter::Parenthesis))?;
         Ok(TyKind::Typeof(expr))
     }
@@ -528,7 +511,7 @@ impl<'a> Parser<'a> {
     fn parse_ty_bare_fn(
         &mut self,
         lo: Span,
-        mut params: Vec<GenericParam>,
+        mut params: ThinVec<GenericParam>,
         param_insertion_point: Option<Span>,
         recover_return_sign: RecoverReturnSign,
     ) -> PResult<'a, TyKind> {
@@ -549,10 +532,10 @@ impl<'a> Parser<'a> {
             // If we ever start to allow `const fn()`, then update
             // feature gating for `#![feature(const_extern_fn)]` to
             // cover it.
-            self.error_fn_ptr_bad_qualifier(whole_span, span, "const");
+            self.sess.emit_err(FnPointerCannotBeConst { span: whole_span, qualifier: span });
         }
         if let ast::Async::Yes { span, .. } = asyncness {
-            self.error_fn_ptr_bad_qualifier(whole_span, span, "async");
+            self.sess.emit_err(FnPointerCannotBeAsync { span: whole_span, qualifier: span });
         }
         let decl_span = span_start.to(self.token.span);
         Ok(TyKind::BareFn(P(BareFnTy { ext, unsafety, generic_params: params, decl, decl_span })))
@@ -562,13 +545,13 @@ impl<'a> Parser<'a> {
     fn recover_fn_ptr_with_generics(
         &mut self,
         lo: Span,
-        params: &mut Vec<GenericParam>,
+        params: &mut ThinVec<GenericParam>,
         param_insertion_point: Option<Span>,
     ) -> PResult<'a, ()> {
         let generics = self.parse_generics()?;
         let arity = generics.params.len();
 
-        let mut lifetimes: Vec<_> = generics
+        let mut lifetimes: ThinVec<_> = generics
             .params
             .into_iter()
             .filter(|param| matches!(param.kind, ast::GenericParamKind::Lifetime))
@@ -600,19 +583,6 @@ impl<'a> Parser<'a> {
         Ok(())
     }
 
-    /// Emit an error for the given bad function pointer qualifier.
-    fn error_fn_ptr_bad_qualifier(&self, span: Span, qual_span: Span, qual: &str) {
-        self.struct_span_err(span, &format!("an `fn` pointer type cannot be `{}`", qual))
-            .span_label(qual_span, format!("`{}` because of this", qual))
-            .span_suggestion_short(
-                qual_span,
-                &format!("remove the `{}` qualifier", qual),
-                "",
-                Applicability::MaybeIncorrect,
-            )
-            .emit();
-    }
-
     /// Parses an `impl B0 + ... + Bn` type.
     fn parse_impl_ty(&mut self, impl_dyn_multi: &mut bool) -> PResult<'a, TyKind> {
         // Always parse bounds greedily for better error recovery.
@@ -643,7 +613,7 @@ impl<'a> Parser<'a> {
     /// Is a `dyn B0 + ... + Bn` type allowed here?
     fn is_explicit_dyn_type(&mut self) -> bool {
         self.check_keyword(kw::Dyn)
-            && (!self.token.uninterpolated_span().rust_2015()
+            && (self.token.uninterpolated_span().rust_2018()
                 || self.look_ahead(1, |t| {
                     (t.can_begin_bound() || t.kind == TokenKind::BinOp(token::Star))
                         && !can_continue_type_after_non_fn_ident(t)
@@ -692,23 +662,13 @@ impl<'a> Parser<'a> {
             })))
         } else if allow_plus == AllowPlus::Yes && self.check_plus() {
             // `Trait1 + Trait2 + 'a`
-            self.parse_remaining_bounds_path(Vec::new(), path, lo, true)
+            self.parse_remaining_bounds_path(ThinVec::new(), path, lo, true)
         } else {
             // Just a type path.
             Ok(TyKind::Path(None, path))
         }
     }
 
-    fn error_illegal_c_varadic_ty(&self, lo: Span) {
-        struct_span_err!(
-            self.sess.span_diagnostic,
-            lo.to(self.prev_token.span),
-            E0743,
-            "C-variadic type `...` may not be nested inside another type",
-        )
-        .emit();
-    }
-
     pub(super) fn parse_generic_bounds(
         &mut self,
         colon_span: Option<Span>,
@@ -739,15 +699,7 @@ impl<'a> Parser<'a> {
         {
             if self.token.is_keyword(kw::Dyn) {
                 // Account for `&dyn Trait + dyn Other`.
-                self.struct_span_err(self.token.span, "invalid `dyn` keyword")
-                    .help("`dyn` is only needed at the start of a trait `+`-separated list")
-                    .span_suggestion(
-                        self.token.span,
-                        "remove this keyword",
-                        "",
-                        Applicability::MachineApplicable,
-                    )
-                    .emit();
+                self.sess.emit_err(InvalidDynKeyword { span: self.token.span });
                 self.bump();
             }
             match self.parse_generic_bound()? {
@@ -784,11 +736,7 @@ impl<'a> Parser<'a> {
         bounds: &[GenericBound],
         negative_bounds: Vec<Span>,
     ) {
-        let negative_bounds_len = negative_bounds.len();
-        let last_span = *negative_bounds.last().expect("no negative bounds, but still error?");
-        let mut err = self.struct_span_err(negative_bounds, "negative bounds are not supported");
-        err.span_label(last_span, "negative bounds are not supported");
-        if let Some(bound_list) = colon_span {
+        let sub = if let Some(bound_list) = colon_span {
             let bound_list = bound_list.to(self.prev_token.span);
             let mut new_bound_list = String::new();
             if !bounds.is_empty() {
@@ -799,14 +747,18 @@ impl<'a> Parser<'a> {
                 }
                 new_bound_list = new_bound_list.replacen(" +", ":", 1);
             }
-            err.tool_only_span_suggestion(
+
+            Some(NegativeBoundsNotSupportedSugg {
                 bound_list,
-                &format!("remove the bound{}", pluralize!(negative_bounds_len)),
-                new_bound_list,
-                Applicability::MachineApplicable,
-            );
-        }
-        err.emit();
+                num_bounds: negative_bounds.len(),
+                fixed: new_bound_list,
+            })
+        } else {
+            None
+        };
+
+        let last_span = *negative_bounds.last().expect("no negative bounds, but still error?");
+        self.sess.emit_err(NegativeBoundsNotSupported { negative_bounds, last_span, sub });
     }
 
     /// Parses a bound according to the grammar:
@@ -918,7 +870,7 @@ impl<'a> Parser<'a> {
             None
         };
 
-        let maybe = if self.eat(&token::Question) { Some(self.prev_token.span) } else { None };
+        let maybe = self.eat(&token::Question).then_some(self.prev_token.span);
 
         Ok(BoundModifiers { maybe, maybe_const })
     }
@@ -990,7 +942,7 @@ impl<'a> Parser<'a> {
                 self.parse_remaining_bounds(bounds, true)?;
                 self.expect(&token::CloseDelim(Delimiter::Parenthesis))?;
                 let sp = vec![lo, self.prev_token.span];
-                let sugg: Vec<_> = sp.iter().map(|sp| (*sp, String::new())).collect();
+                let sugg = vec![(lo, String::from(" ")), (self.prev_token.span, String::new())];
                 self.struct_span_err(sp, "incorrect braces around trait bounds")
                     .multipart_suggestion(
                         "remove the parentheses",
@@ -1041,7 +993,7 @@ impl<'a> Parser<'a> {
     }
 
     /// Optionally parses `for<$generic_params>`.
-    pub(super) fn parse_late_bound_lifetime_defs(&mut self) -> PResult<'a, Vec<GenericParam>> {
+    pub(super) fn parse_late_bound_lifetime_defs(&mut self) -> PResult<'a, ThinVec<GenericParam>> {
         if self.eat_keyword(kw::For) {
             self.expect_lt()?;
             let params = self.parse_generic_params()?;
@@ -1050,7 +1002,7 @@ impl<'a> Parser<'a> {
             // parameters, and the lifetime parameters must not have bounds.
             Ok(params)
         } else {
-            Ok(Vec::new())
+            Ok(ThinVec::new())
         }
     }
 
@@ -1060,7 +1012,7 @@ impl<'a> Parser<'a> {
     fn recover_fn_trait_with_lifetime_params(
         &mut self,
         fn_path: &mut ast::Path,
-        lifetime_defs: &mut Vec<GenericParam>,
+        lifetime_defs: &mut ThinVec<GenericParam>,
     ) -> PResult<'a, ()> {
         let fn_path_segment = fn_path.segments.last_mut().unwrap();
         let generic_args = if let Some(p_args) = &fn_path_segment.args {
@@ -1094,7 +1046,7 @@ impl<'a> Parser<'a> {
 
         // Parse `(T, U) -> R`.
         let inputs_lo = self.token.span;
-        let inputs: Vec<_> =
+        let inputs: ThinVec<_> =
             self.parse_fn_params(|_| false)?.into_iter().map(|input| input.ty).collect();
         let inputs_span = inputs_lo.to(self.prev_token.span);
         let output = self.parse_ret_ty(AllowPlus::No, RecoverQPath::No, RecoverReturnSign::No)?;
@@ -1120,7 +1072,7 @@ impl<'a> Parser<'a> {
                 kind: ast::GenericParamKind::Lifetime,
                 colon_span: None,
             })
-            .collect::<Vec<GenericParam>>();
+            .collect::<ThinVec<GenericParam>>();
         lifetime_defs.append(&mut generic_params);
 
         let generic_args_span = generic_args.span();