Merging upstream version 1.69.0+dfsg1.

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

9 files changed, 856 insertions, 179 deletions

diff --git a/compiler/rustc_ast_lowering/Cargo.toml b/compiler/rustc_ast_lowering/Cargo.toml
index 6a59b9e61..eb2e82d79 100644
--- a/compiler/rustc_ast_lowering/Cargo.toml
+++ b/compiler/rustc_ast_lowering/Cargo.toml
@@ -7,7 +7,6 @@ edition = "2021"
 doctest = false
 
 [dependencies]
-rustc_arena = { path = "../rustc_arena" }
 rustc_ast = { path = "../rustc_ast" }
 rustc_ast_pretty = { path = "../rustc_ast_pretty" }
 rustc_data_structures = { path = "../rustc_data_structures" }
@@ -16,10 +15,9 @@ rustc_hir = { path = "../rustc_hir" }
 rustc_index = { path = "../rustc_index" }
 rustc_middle = { path = "../rustc_middle" }
 rustc_macros = { path = "../rustc_macros" }
-rustc_query_system = { path = "../rustc_query_system" }
 rustc_session = { path = "../rustc_session" }
 rustc_span = { path = "../rustc_span" }
 rustc_target = { path = "../rustc_target" }
 smallvec = { version = "1.8.1", features = ["union", "may_dangle"] }
-thin-vec = "0.2.9"
+thin-vec = "0.2.12"
 tracing = "0.1"
diff --git a/compiler/rustc_ast_lowering/locales/en-US.ftl b/compiler/rustc_ast_lowering/locales/en-US.ftl
new file mode 100644
index 000000000..3ccd84398
--- /dev/null
+++ b/compiler/rustc_ast_lowering/locales/en-US.ftl
@@ -0,0 +1,141 @@
+ast_lowering_generic_type_with_parentheses =
+    parenthesized type parameters may only be used with a `Fn` trait
+    .label = only `Fn` traits may use parentheses
+
+ast_lowering_use_angle_brackets = use angle brackets instead
+
+ast_lowering_invalid_abi =
+    invalid ABI: found `{$abi}`
+    .label = invalid ABI
+    .note = invoke `{$command}` for a full list of supported calling conventions.
+
+ast_lowering_invalid_abi_suggestion = did you mean
+
+ast_lowering_assoc_ty_parentheses =
+    parenthesized generic arguments cannot be used in associated type constraints
+
+ast_lowering_remove_parentheses = remove these parentheses
+
+ast_lowering_misplaced_impl_trait =
+    `impl Trait` only allowed in function and inherent method return types, not in {$position}
+
+ast_lowering_misplaced_assoc_ty_binding =
+    associated type bounds are only allowed in where clauses and function signatures, not in {$position}
+
+ast_lowering_underscore_expr_lhs_assign =
+    in expressions, `_` can only be used on the left-hand side of an assignment
+    .label = `_` not allowed here
+
+ast_lowering_base_expression_double_dot =
+    base expression required after `..`
+    .label = add a base expression here
+
+ast_lowering_await_only_in_async_fn_and_blocks =
+    `await` is only allowed inside `async` functions and blocks
+    .label = only allowed inside `async` functions and blocks
+
+ast_lowering_this_not_async = this is not `async`
+
+ast_lowering_generator_too_many_parameters =
+    too many parameters for a generator (expected 0 or 1 parameters)
+
+ast_lowering_closure_cannot_be_static = closures cannot be static
+
+ast_lowering_async_non_move_closure_not_supported =
+    `async` non-`move` closures with parameters are not currently supported
+    .help = consider using `let` statements to manually capture variables by reference before entering an `async move` closure
+
+ast_lowering_functional_record_update_destructuring_assignment =
+    functional record updates are not allowed in destructuring assignments
+    .suggestion = consider removing the trailing pattern
+
+ast_lowering_async_generators_not_supported =
+    `async` generators are not yet supported
+
+ast_lowering_inline_asm_unsupported_target =
+    inline assembly is unsupported on this target
+
+ast_lowering_att_syntax_only_x86 =
+    the `att_syntax` option is only supported on x86
+
+ast_lowering_abi_specified_multiple_times =
+    `{$prev_name}` ABI specified multiple times
+    .label = previously specified here
+    .note = these ABIs are equivalent on the current target
+
+ast_lowering_clobber_abi_not_supported =
+    `clobber_abi` is not supported on this target
+
+ast_lowering_invalid_abi_clobber_abi =
+    invalid ABI for `clobber_abi`
+    .note = the following ABIs are supported on this target: {$supported_abis}
+
+ast_lowering_invalid_register =
+    invalid register `{$reg}`: {$error}
+
+ast_lowering_invalid_register_class =
+    invalid register class `{$reg_class}`: {$error}
+
+ast_lowering_invalid_asm_template_modifier_reg_class =
+    invalid asm template modifier for this register class
+
+ast_lowering_argument = argument
+
+ast_lowering_template_modifier = template modifier
+
+ast_lowering_support_modifiers =
+    the `{$class_name}` register class supports the following template modifiers: {$modifiers}
+
+ast_lowering_does_not_support_modifiers =
+    the `{$class_name}` register class does not support template modifiers
+
+ast_lowering_invalid_asm_template_modifier_const =
+    asm template modifiers are not allowed for `const` arguments
+
+ast_lowering_invalid_asm_template_modifier_sym =
+    asm template modifiers are not allowed for `sym` arguments
+
+ast_lowering_register_class_only_clobber =
+    register class `{$reg_class_name}` can only be used as a clobber, not as an input or output
+
+ast_lowering_register_conflict =
+    register `{$reg1_name}` conflicts with register `{$reg2_name}`
+    .help = use `lateout` instead of `out` to avoid conflict
+
+ast_lowering_register1 = register `{$reg1_name}`
+
+ast_lowering_register2 = register `{$reg2_name}`
+
+ast_lowering_sub_tuple_binding =
+    `{$ident_name} @` is not allowed in a {$ctx}
+    .label = this is only allowed in slice patterns
+    .help = remove this and bind each tuple field independently
+
+ast_lowering_sub_tuple_binding_suggestion = if you don't need to use the contents of {$ident}, discard the tuple's remaining fields
+
+ast_lowering_extra_double_dot =
+    `..` can only be used once per {$ctx} pattern
+    .label = can only be used once per {$ctx} pattern
+
+ast_lowering_previously_used_here = previously used here
+
+ast_lowering_misplaced_double_dot =
+    `..` patterns are not allowed here
+    .note = only allowed in tuple, tuple struct, and slice patterns
+
+ast_lowering_misplaced_relax_trait_bound =
+    `?Trait` bounds are only permitted at the point where a type parameter is declared
+
+ast_lowering_not_supported_for_lifetime_binder_async_closure =
+    `for<...>` binders on `async` closures are not currently supported
+
+ast_lowering_arbitrary_expression_in_pattern =
+    arbitrary expressions aren't allowed in patterns
+
+ast_lowering_inclusive_range_with_no_end = inclusive range with no end
+
+ast_lowering_trait_fn_async =
+    functions in traits cannot be declared `async`
+    .label = `async` because of this
+    .note = `async` trait functions are not currently supported
+    .note2 = consider using the `async-trait` crate: https://crates.io/crates/async-trait
diff --git a/compiler/rustc_ast_lowering/src/errors.rs b/compiler/rustc_ast_lowering/src/errors.rs
index 21c6a2d26..5e6b6050b 100644
--- a/compiler/rustc_ast_lowering/src/errors.rs
+++ b/compiler/rustc_ast_lowering/src/errors.rs
@@ -79,11 +79,12 @@ pub struct MisplacedImplTrait<'a> {
     pub position: DiagnosticArgFromDisplay<'a>,
 }
 
-#[derive(Diagnostic, Clone, Copy)]
-#[diag(ast_lowering_rustc_box_attribute_error)]
-pub struct RustcBoxAttributeError {
+#[derive(Diagnostic)]
+#[diag(ast_lowering_misplaced_assoc_ty_binding)]
+pub struct MisplacedAssocTyBinding<'a> {
     #[primary_span]
     pub span: Span,
+    pub position: DiagnosticArgFromDisplay<'a>,
 }
 
 #[derive(Diagnostic, Clone, Copy)]
@@ -339,7 +340,7 @@ pub struct InclusiveRangeWithNoEnd {
 #[derive(Diagnostic, Clone, Copy)]
 #[diag(ast_lowering_trait_fn_async, code = "E0706")]
 #[note]
-#[note(note2)]
+#[note(ast_lowering_note2)]
 pub struct TraitFnAsync {
     #[primary_span]
     pub fn_span: Span,
diff --git a/compiler/rustc_ast_lowering/src/expr.rs b/compiler/rustc_ast_lowering/src/expr.rs
index c3611b2f5..ffb30b1b3 100644
--- a/compiler/rustc_ast_lowering/src/expr.rs
+++ b/compiler/rustc_ast_lowering/src/expr.rs
@@ -2,7 +2,7 @@ use super::errors::{
     AsyncGeneratorsNotSupported, AsyncNonMoveClosureNotSupported, AwaitOnlyInAsyncFnAndBlocks,
     BaseExpressionDoubleDot, ClosureCannotBeStatic, FunctionalRecordUpdateDestructuringAssignemnt,
     GeneratorTooManyParameters, InclusiveRangeWithNoEnd, NotSupportedForLifetimeBinderAsyncClosure,
-    RustcBoxAttributeError, UnderscoreExprLhsAssign,
+    UnderscoreExprLhsAssign,
 };
 use super::ResolverAstLoweringExt;
 use super::{ImplTraitContext, LoweringContext, ParamMode, ParenthesizedGenericArgs};
@@ -16,9 +16,9 @@ use rustc_hir::def::Res;
 use rustc_hir::definitions::DefPathData;
 use rustc_session::errors::report_lit_error;
 use rustc_span::source_map::{respan, DesugaringKind, Span, Spanned};
-use rustc_span::symbol::{sym, Ident};
+use rustc_span::symbol::{sym, Ident, Symbol};
 use rustc_span::DUMMY_SP;
-use thin_vec::thin_vec;
+use thin_vec::{thin_vec, ThinVec};
 
 impl<'hir> LoweringContext<'_, 'hir> {
     fn lower_exprs(&mut self, exprs: &[AstP<Expr>]) -> &'hir [hir::Expr<'hir>] {
@@ -83,15 +83,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 }
                 ExprKind::Tup(elts) => hir::ExprKind::Tup(self.lower_exprs(elts)),
                 ExprKind::Call(f, args) => {
-                    if e.attrs.get(0).map_or(false, |a| a.has_name(sym::rustc_box)) {
-                        if let [inner] = &args[..] && e.attrs.len() == 1 {
-                            let kind = hir::ExprKind::Box(self.lower_expr(&inner));
-                            return hir::Expr { hir_id, kind, span: self.lower_span(e.span) };
-                        } else {
-                            self.tcx.sess.emit_err(RustcBoxAttributeError { span: e.span });
-                            hir::ExprKind::Err
-                        }
-                    } else if let Some(legacy_args) = self.resolver.legacy_const_generic_args(f) {
+                    if let Some(legacy_args) = self.resolver.legacy_const_generic_args(f) {
                         self.lower_legacy_const_generics((**f).clone(), args.clone(), &legacy_args)
                     } else {
                         let f = self.lower_expr(f);
@@ -139,13 +131,13 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 ExprKind::Cast(expr, ty) => {
                     let expr = self.lower_expr(expr);
                     let ty =
-                        self.lower_ty(ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Type));
+                        self.lower_ty(ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Cast));
                     hir::ExprKind::Cast(expr, ty)
                 }
                 ExprKind::Type(expr, ty) => {
                     let expr = self.lower_expr(expr);
                     let ty =
-                        self.lower_ty(ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Type));
+                        self.lower_ty(ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Cast));
                     hir::ExprKind::Type(expr, ty)
                 }
                 ExprKind::AddrOf(k, m, ohs) => {
@@ -266,8 +258,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     self.lower_expr_range(e.span, e1.as_deref(), e2.as_deref(), *lims)
                 }
                 ExprKind::Underscore => {
-                    self.tcx.sess.emit_err(UnderscoreExprLhsAssign { span: e.span });
-                    hir::ExprKind::Err
+                    let guar = self.tcx.sess.emit_err(UnderscoreExprLhsAssign { span: e.span });
+                    hir::ExprKind::Err(guar)
                 }
                 ExprKind::Path(qself, path) => {
                     let qpath = self.lower_qpath(
@@ -294,12 +286,14 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 ExprKind::InlineAsm(asm) => {
                     hir::ExprKind::InlineAsm(self.lower_inline_asm(e.span, asm))
                 }
+                ExprKind::FormatArgs(fmt) => self.lower_format_args(e.span, fmt),
                 ExprKind::Struct(se) => {
                     let rest = match &se.rest {
                         StructRest::Base(e) => Some(self.lower_expr(e)),
                         StructRest::Rest(sp) => {
-                            self.tcx.sess.emit_err(BaseExpressionDoubleDot { span: *sp });
-                            Some(&*self.arena.alloc(self.expr_err(*sp)))
+                            let guar =
+                                self.tcx.sess.emit_err(BaseExpressionDoubleDot { span: *sp });
+                            Some(&*self.arena.alloc(self.expr_err(*sp, guar)))
                         }
                         StructRest::None => None,
                     };
@@ -317,7 +311,9 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     )
                 }
                 ExprKind::Yield(opt_expr) => self.lower_expr_yield(e.span, opt_expr.as_deref()),
-                ExprKind::Err => hir::ExprKind::Err,
+                ExprKind::Err => hir::ExprKind::Err(
+                    self.tcx.sess.delay_span_bug(e.span, "lowered ExprKind::Err"),
+                ),
                 ExprKind::Try(sub_expr) => self.lower_expr_try(e.span, sub_expr),
 
                 ExprKind::Paren(_) | ExprKind::ForLoop(..) => unreachable!("already handled"),
@@ -366,7 +362,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
     fn lower_legacy_const_generics(
         &mut self,
         mut f: Expr,
-        args: Vec<AstP<Expr>>,
+        args: ThinVec<AstP<Expr>>,
         legacy_args_idx: &[usize],
     ) -> hir::ExprKind<'hir> {
         let ExprKind::Path(None, path) = &mut f.kind else {
@@ -375,7 +371,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
 
         // Split the arguments into const generics and normal arguments
         let mut real_args = vec![];
-        let mut generic_args = vec![];
+        let mut generic_args = ThinVec::new();
         for (idx, arg) in args.into_iter().enumerate() {
             if legacy_args_idx.contains(&idx) {
                 let parent_def_id = self.current_hir_id_owner;
@@ -760,7 +756,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 self.expr_ident_mut(span, task_context_ident, task_context_hid)
             } else {
                 // Use of `await` outside of an async context, we cannot use `task_context` here.
-                self.expr_err(span)
+                self.expr_err(span, self.tcx.sess.delay_span_bug(span, "no task_context hir id"))
             };
             let new_unchecked = self.expr_call_lang_item_fn_mut(
                 span,
@@ -1735,7 +1731,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         self.expr(span, hir::ExprKind::DropTemps(expr))
     }
 
-    fn expr_match(
+    pub(super) fn expr_match(
         &mut self,
         span: Span,
         arg: &'hir hir::Expr<'hir>,
@@ -1763,7 +1759,44 @@ impl<'hir> LoweringContext<'_, 'hir> {
         self.arena.alloc(self.expr(sp, hir::ExprKind::Tup(&[])))
     }
 
-    fn expr_call_mut(
+    pub(super) fn expr_usize(&mut self, sp: Span, value: usize) -> hir::Expr<'hir> {
+        self.expr(
+            sp,
+            hir::ExprKind::Lit(hir::Lit {
+                span: sp,
+                node: ast::LitKind::Int(
+                    value as u128,
+                    ast::LitIntType::Unsigned(ast::UintTy::Usize),
+                ),
+            }),
+        )
+    }
+
+    pub(super) fn expr_u32(&mut self, sp: Span, value: u32) -> hir::Expr<'hir> {
+        self.expr(
+            sp,
+            hir::ExprKind::Lit(hir::Lit {
+                span: sp,
+                node: ast::LitKind::Int(value.into(), ast::LitIntType::Unsigned(ast::UintTy::U32)),
+            }),
+        )
+    }
+
+    pub(super) fn expr_char(&mut self, sp: Span, value: char) -> hir::Expr<'hir> {
+        self.expr(sp, hir::ExprKind::Lit(hir::Lit { span: sp, node: ast::LitKind::Char(value) }))
+    }
+
+    pub(super) fn expr_str(&mut self, sp: Span, value: Symbol) -> hir::Expr<'hir> {
+        self.expr(
+            sp,
+            hir::ExprKind::Lit(hir::Lit {
+                span: sp,
+                node: ast::LitKind::Str(value, ast::StrStyle::Cooked),
+            }),
+        )
+    }
+
+    pub(super) fn expr_call_mut(
         &mut self,
         span: Span,
         e: &'hir hir::Expr<'hir>,
@@ -1772,7 +1805,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         self.expr(span, hir::ExprKind::Call(e, args))
     }
 
-    fn expr_call(
+    pub(super) fn expr_call(
         &mut self,
         span: Span,
         e: &'hir hir::Expr<'hir>,
@@ -1814,6 +1847,27 @@ impl<'hir> LoweringContext<'_, 'hir> {
         )
     }
 
+    /// `<LangItem>::name`
+    pub(super) fn expr_lang_item_type_relative(
+        &mut self,
+        span: Span,
+        lang_item: hir::LangItem,
+        name: Symbol,
+    ) -> hir::Expr<'hir> {
+        let path = hir::ExprKind::Path(hir::QPath::TypeRelative(
+            self.arena.alloc(self.ty(
+                span,
+                hir::TyKind::Path(hir::QPath::LangItem(lang_item, self.lower_span(span), None)),
+            )),
+            self.arena.alloc(hir::PathSegment::new(
+                Ident::new(name, span),
+                self.next_id(),
+                Res::Err,
+            )),
+        ));
+        self.expr(span, path)
+    }
+
     pub(super) fn expr_ident(
         &mut self,
         sp: Span,
@@ -1872,12 +1926,25 @@ impl<'hir> LoweringContext<'_, 'hir> {
         self.expr(b.span, hir::ExprKind::Block(b, None))
     }
 
+    pub(super) fn expr_array_ref(
+        &mut self,
+        span: Span,
+        elements: &'hir [hir::Expr<'hir>],
+    ) -> hir::Expr<'hir> {
+        let addrof = hir::ExprKind::AddrOf(
+            hir::BorrowKind::Ref,
+            hir::Mutability::Not,
+            self.arena.alloc(self.expr(span, hir::ExprKind::Array(elements))),
+        );
+        self.expr(span, addrof)
+    }
+
     pub(super) fn expr(&mut self, span: Span, kind: hir::ExprKind<'hir>) -> hir::Expr<'hir> {
         let hir_id = self.next_id();
         hir::Expr { hir_id, kind, span: self.lower_span(span) }
     }
 
-    fn expr_field(
+    pub(super) fn expr_field(
         &mut self,
         ident: Ident,
         expr: &'hir hir::Expr<'hir>,
@@ -1892,7 +1959,11 @@ impl<'hir> LoweringContext<'_, 'hir> {
         }
     }
 
-    fn arm(&mut self, pat: &'hir hir::Pat<'hir>, expr: &'hir hir::Expr<'hir>) -> hir::Arm<'hir> {
+    pub(super) fn arm(
+        &mut self,
+        pat: &'hir hir::Pat<'hir>,
+        expr: &'hir hir::Expr<'hir>,
+    ) -> hir::Arm<'hir> {
         hir::Arm {
             hir_id: self.next_id(),
             pat,
diff --git a/compiler/rustc_ast_lowering/src/format.rs b/compiler/rustc_ast_lowering/src/format.rs
new file mode 100644
index 000000000..4095e225a
--- /dev/null
+++ b/compiler/rustc_ast_lowering/src/format.rs
@@ -0,0 +1,411 @@
+use super::LoweringContext;
+use rustc_ast as ast;
+use rustc_ast::visit::{self, Visitor};
+use rustc_ast::*;
+use rustc_data_structures::fx::FxIndexSet;
+use rustc_hir as hir;
+use rustc_span::{
+    sym,
+    symbol::{kw, Ident},
+    Span,
+};
+
+impl<'hir> LoweringContext<'_, 'hir> {
+    pub(crate) fn lower_format_args(&mut self, sp: Span, fmt: &FormatArgs) -> hir::ExprKind<'hir> {
+        expand_format_args(self, sp, fmt)
+    }
+}
+
+#[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)]
+enum ArgumentType {
+    Format(FormatTrait),
+    Usize,
+}
+
+/// Generate a hir expression representing an argument to a format_args invocation.
+///
+/// Generates:
+///
+/// ```text
+///     <core::fmt::ArgumentV1>::new_…(arg)
+/// ```
+fn make_argument<'hir>(
+    ctx: &mut LoweringContext<'_, 'hir>,
+    sp: Span,
+    arg: &'hir hir::Expr<'hir>,
+    ty: ArgumentType,
+) -> hir::Expr<'hir> {
+    use ArgumentType::*;
+    use FormatTrait::*;
+    let new_fn = ctx.arena.alloc(ctx.expr_lang_item_type_relative(
+        sp,
+        hir::LangItem::FormatArgument,
+        match ty {
+            Format(Display) => sym::new_display,
+            Format(Debug) => sym::new_debug,
+            Format(LowerExp) => sym::new_lower_exp,
+            Format(UpperExp) => sym::new_upper_exp,
+            Format(Octal) => sym::new_octal,
+            Format(Pointer) => sym::new_pointer,
+            Format(Binary) => sym::new_binary,
+            Format(LowerHex) => sym::new_lower_hex,
+            Format(UpperHex) => sym::new_upper_hex,
+            Usize => sym::from_usize,
+        },
+    ));
+    ctx.expr_call_mut(sp, new_fn, std::slice::from_ref(arg))
+}
+
+/// Generate a hir expression for a format_args Count.
+///
+/// Generates:
+///
+/// ```text
+///     <core::fmt::rt::v1::Count>::Is(…)
+/// ```
+///
+/// or
+///
+/// ```text
+///     <core::fmt::rt::v1::Count>::Param(…)
+/// ```
+///
+/// or
+///
+/// ```text
+///     <core::fmt::rt::v1::Count>::Implied
+/// ```
+fn make_count<'hir>(
+    ctx: &mut LoweringContext<'_, 'hir>,
+    sp: Span,
+    count: &Option<FormatCount>,
+    argmap: &mut FxIndexSet<(usize, ArgumentType)>,
+) -> hir::Expr<'hir> {
+    match count {
+        Some(FormatCount::Literal(n)) => {
+            let count_is = ctx.arena.alloc(ctx.expr_lang_item_type_relative(
+                sp,
+                hir::LangItem::FormatCount,
+                sym::Is,
+            ));
+            let value = ctx.arena.alloc_from_iter([ctx.expr_usize(sp, *n)]);
+            ctx.expr_call_mut(sp, count_is, value)
+        }
+        Some(FormatCount::Argument(arg)) => {
+            if let Ok(arg_index) = arg.index {
+                let (i, _) = argmap.insert_full((arg_index, ArgumentType::Usize));
+                let count_param = ctx.arena.alloc(ctx.expr_lang_item_type_relative(
+                    sp,
+                    hir::LangItem::FormatCount,
+                    sym::Param,
+                ));
+                let value = ctx.arena.alloc_from_iter([ctx.expr_usize(sp, i)]);
+                ctx.expr_call_mut(sp, count_param, value)
+            } else {
+                ctx.expr(
+                    sp,
+                    hir::ExprKind::Err(
+                        ctx.tcx.sess.delay_span_bug(sp, "lowered bad format_args count"),
+                    ),
+                )
+            }
+        }
+        None => ctx.expr_lang_item_type_relative(sp, hir::LangItem::FormatCount, sym::Implied),
+    }
+}
+
+/// Generate a hir expression for a format_args placeholder specification.
+///
+/// Generates
+///
+/// ```text
+///     <core::fmt::rt::v1::Argument::new(
+///         …usize, // position
+///         '…', // fill
+///         <core::fmt::rt::v1::Alignment>::…, // alignment
+///         …u32, // flags
+///         <core::fmt::rt::v1::Count::…>, // width
+///         <core::fmt::rt::v1::Count::…>, // precision
+///     )
+/// ```
+fn make_format_spec<'hir>(
+    ctx: &mut LoweringContext<'_, 'hir>,
+    sp: Span,
+    placeholder: &FormatPlaceholder,
+    argmap: &mut FxIndexSet<(usize, ArgumentType)>,
+) -> hir::Expr<'hir> {
+    let position = match placeholder.argument.index {
+        Ok(arg_index) => {
+            let (i, _) =
+                argmap.insert_full((arg_index, ArgumentType::Format(placeholder.format_trait)));
+            ctx.expr_usize(sp, i)
+        }
+        Err(_) => ctx.expr(
+            sp,
+            hir::ExprKind::Err(ctx.tcx.sess.delay_span_bug(sp, "lowered bad format_args count")),
+        ),
+    };
+    let &FormatOptions {
+        ref width,
+        ref precision,
+        alignment,
+        fill,
+        sign,
+        alternate,
+        zero_pad,
+        debug_hex,
+    } = &placeholder.format_options;
+    let fill = ctx.expr_char(sp, fill.unwrap_or(' '));
+    let align = ctx.expr_lang_item_type_relative(
+        sp,
+        hir::LangItem::FormatAlignment,
+        match alignment {
+            Some(FormatAlignment::Left) => sym::Left,
+            Some(FormatAlignment::Right) => sym::Right,
+            Some(FormatAlignment::Center) => sym::Center,
+            None => sym::Unknown,
+        },
+    );
+    // This needs to match `FlagV1` in library/core/src/fmt/mod.rs.
+    let flags: u32 = ((sign == Some(FormatSign::Plus)) as u32)
+        | ((sign == Some(FormatSign::Minus)) as u32) << 1
+        | (alternate as u32) << 2
+        | (zero_pad as u32) << 3
+        | ((debug_hex == Some(FormatDebugHex::Lower)) as u32) << 4
+        | ((debug_hex == Some(FormatDebugHex::Upper)) as u32) << 5;
+    let flags = ctx.expr_u32(sp, flags);
+    let precision = make_count(ctx, sp, &precision, argmap);
+    let width = make_count(ctx, sp, &width, argmap);
+    let format_placeholder_new = ctx.arena.alloc(ctx.expr_lang_item_type_relative(
+        sp,
+        hir::LangItem::FormatPlaceholder,
+        sym::new,
+    ));
+    let args = ctx.arena.alloc_from_iter([position, fill, align, flags, precision, width]);
+    ctx.expr_call_mut(sp, format_placeholder_new, args)
+}
+
+fn expand_format_args<'hir>(
+    ctx: &mut LoweringContext<'_, 'hir>,
+    macsp: Span,
+    fmt: &FormatArgs,
+) -> hir::ExprKind<'hir> {
+    let lit_pieces =
+        ctx.arena.alloc_from_iter(fmt.template.iter().enumerate().filter_map(|(i, piece)| {
+            match piece {
+                &FormatArgsPiece::Literal(s) => Some(ctx.expr_str(fmt.span, s)),
+                &FormatArgsPiece::Placeholder(_) => {
+                    // Inject empty string before placeholders when not already preceded by a literal piece.
+                    if i == 0 || matches!(fmt.template[i - 1], FormatArgsPiece::Placeholder(_)) {
+                        Some(ctx.expr_str(fmt.span, kw::Empty))
+                    } else {
+                        None
+                    }
+                }
+            }
+        }));
+    let lit_pieces = ctx.expr_array_ref(fmt.span, lit_pieces);
+
+    // Whether we'll use the `Arguments::new_v1_formatted` form (true),
+    // or the `Arguments::new_v1` form (false).
+    let mut use_format_options = false;
+
+    // Create a list of all _unique_ (argument, format trait) combinations.
+    // E.g. "{0} {0:x} {0} {1}" -> [(0, Display), (0, LowerHex), (1, Display)]
+    let mut argmap = FxIndexSet::default();
+    for piece in &fmt.template {
+        let FormatArgsPiece::Placeholder(placeholder) = piece else { continue };
+        if placeholder.format_options != Default::default() {
+            // Can't use basic form if there's any formatting options.
+            use_format_options = true;
+        }
+        if let Ok(index) = placeholder.argument.index {
+            if !argmap.insert((index, ArgumentType::Format(placeholder.format_trait))) {
+                // Duplicate (argument, format trait) combination,
+                // which we'll only put once in the args array.
+                use_format_options = true;
+            }
+        }
+    }
+
+    let format_options = use_format_options.then(|| {
+        // Generate:
+        //     &[format_spec_0, format_spec_1, format_spec_2]
+        let elements: Vec<_> = fmt
+            .template
+            .iter()
+            .filter_map(|piece| {
+                let FormatArgsPiece::Placeholder(placeholder) = piece else { return None };
+                Some(make_format_spec(ctx, macsp, placeholder, &mut argmap))
+            })
+            .collect();
+        ctx.expr_array_ref(macsp, ctx.arena.alloc_from_iter(elements))
+    });
+
+    let arguments = fmt.arguments.all_args();
+
+    // If the args array contains exactly all the original arguments once,
+    // in order, we can use a simple array instead of a `match` construction.
+    // However, if there's a yield point in any argument except the first one,
+    // we don't do this, because an ArgumentV1 cannot be kept across yield points.
+    //
+    // This is an optimization, speeding up compilation about 1-2% in some cases.
+    // See https://github.com/rust-lang/rust/pull/106770#issuecomment-1380790609
+    let use_simple_array = argmap.len() == arguments.len()
+        && argmap.iter().enumerate().all(|(i, &(j, _))| i == j)
+        && arguments.iter().skip(1).all(|arg| !may_contain_yield_point(&arg.expr));
+
+    let args = if use_simple_array {
+        // Generate:
+        //     &[
+        //         <core::fmt::ArgumentV1>::new_display(&arg0),
+        //         <core::fmt::ArgumentV1>::new_lower_hex(&arg1),
+        //         <core::fmt::ArgumentV1>::new_debug(&arg2),
+        //         …
+        //     ]
+        let elements: Vec<_> = arguments
+            .iter()
+            .zip(argmap)
+            .map(|(arg, (_, ty))| {
+                let sp = arg.expr.span.with_ctxt(macsp.ctxt());
+                let arg = ctx.lower_expr(&arg.expr);
+                let ref_arg = ctx.arena.alloc(ctx.expr(
+                    sp,
+                    hir::ExprKind::AddrOf(hir::BorrowKind::Ref, hir::Mutability::Not, arg),
+                ));
+                make_argument(ctx, sp, ref_arg, ty)
+            })
+            .collect();
+        ctx.expr_array_ref(macsp, ctx.arena.alloc_from_iter(elements))
+    } else {
+        // Generate:
+        //     &match (&arg0, &arg1, &…) {
+        //         args => [
+        //             <core::fmt::ArgumentV1>::new_display(args.0),
+        //             <core::fmt::ArgumentV1>::new_lower_hex(args.1),
+        //             <core::fmt::ArgumentV1>::new_debug(args.0),
+        //             …
+        //         ]
+        //     }
+        let args_ident = Ident::new(sym::args, macsp);
+        let (args_pat, args_hir_id) = ctx.pat_ident(macsp, args_ident);
+        let args = ctx.arena.alloc_from_iter(argmap.iter().map(|&(arg_index, ty)| {
+            if let Some(arg) = arguments.get(arg_index) {
+                let sp = arg.expr.span.with_ctxt(macsp.ctxt());
+                let args_ident_expr = ctx.expr_ident(macsp, args_ident, args_hir_id);
+                let arg = ctx.arena.alloc(ctx.expr(
+                    sp,
+                    hir::ExprKind::Field(
+                        args_ident_expr,
+                        Ident::new(sym::integer(arg_index), macsp),
+                    ),
+                ));
+                make_argument(ctx, sp, arg, ty)
+            } else {
+                ctx.expr(
+                    macsp,
+                    hir::ExprKind::Err(
+                        ctx.tcx.sess.delay_span_bug(macsp, format!("no arg at {arg_index}")),
+                    ),
+                )
+            }
+        }));
+        let elements: Vec<_> = arguments
+            .iter()
+            .map(|arg| {
+                let arg_expr = ctx.lower_expr(&arg.expr);
+                ctx.expr(
+                    arg.expr.span.with_ctxt(macsp.ctxt()),
+                    hir::ExprKind::AddrOf(hir::BorrowKind::Ref, hir::Mutability::Not, arg_expr),
+                )
+            })
+            .collect();
+        let args_tuple = ctx
+            .arena
+            .alloc(ctx.expr(macsp, hir::ExprKind::Tup(ctx.arena.alloc_from_iter(elements))));
+        let array = ctx.arena.alloc(ctx.expr(macsp, hir::ExprKind::Array(args)));
+        let match_arms = ctx.arena.alloc_from_iter([ctx.arm(args_pat, array)]);
+        let match_expr = ctx.arena.alloc(ctx.expr_match(
+            macsp,
+            args_tuple,
+            match_arms,
+            hir::MatchSource::FormatArgs,
+        ));
+        ctx.expr(
+            macsp,
+            hir::ExprKind::AddrOf(hir::BorrowKind::Ref, hir::Mutability::Not, match_expr),
+        )
+    };
+
+    if let Some(format_options) = format_options {
+        // Generate:
+        //     <core::fmt::Arguments>::new_v1_formatted(
+        //         lit_pieces,
+        //         args,
+        //         format_options,
+        //         unsafe { ::core::fmt::UnsafeArg::new() }
+        //     )
+        let new_v1_formatted = ctx.arena.alloc(ctx.expr_lang_item_type_relative(
+            macsp,
+            hir::LangItem::FormatArguments,
+            sym::new_v1_formatted,
+        ));
+        let unsafe_arg_new = ctx.arena.alloc(ctx.expr_lang_item_type_relative(
+            macsp,
+            hir::LangItem::FormatUnsafeArg,
+            sym::new,
+        ));
+        let unsafe_arg_new_call = ctx.expr_call(macsp, unsafe_arg_new, &[]);
+        let hir_id = ctx.next_id();
+        let unsafe_arg = ctx.expr_block(ctx.arena.alloc(hir::Block {
+            stmts: &[],
+            expr: Some(unsafe_arg_new_call),
+            hir_id,
+            rules: hir::BlockCheckMode::UnsafeBlock(hir::UnsafeSource::CompilerGenerated),
+            span: macsp,
+            targeted_by_break: false,
+        }));
+        let args = ctx.arena.alloc_from_iter([lit_pieces, args, format_options, unsafe_arg]);
+        hir::ExprKind::Call(new_v1_formatted, args)
+    } else {
+        // Generate:
+        //     <core::fmt::Arguments>::new_v1(
+        //         lit_pieces,
+        //         args,
+        //     )
+        let new_v1 = ctx.arena.alloc(ctx.expr_lang_item_type_relative(
+            macsp,
+            hir::LangItem::FormatArguments,
+            sym::new_v1,
+        ));
+        let new_args = ctx.arena.alloc_from_iter([lit_pieces, args]);
+        hir::ExprKind::Call(new_v1, new_args)
+    }
+}
+
+fn may_contain_yield_point(e: &ast::Expr) -> bool {
+    struct MayContainYieldPoint(bool);
+
+    impl Visitor<'_> for MayContainYieldPoint {
+        fn visit_expr(&mut self, e: &ast::Expr) {
+            if let ast::ExprKind::Await(_) | ast::ExprKind::Yield(_) = e.kind {
+                self.0 = true;
+            } else {
+                visit::walk_expr(self, e);
+            }
+        }
+
+        fn visit_mac_call(&mut self, _: &ast::MacCall) {
+            // Macros should be expanded at this point.
+            unreachable!("unexpanded macro in ast lowering");
+        }
+
+        fn visit_item(&mut self, _: &ast::Item) {
+            // Do not recurse into nested items.
+        }
+    }
+
+    let mut visitor = MayContainYieldPoint(false);
+    visitor.visit_expr(e);
+    visitor.0
+}
diff --git a/compiler/rustc_ast_lowering/src/index.rs b/compiler/rustc_ast_lowering/src/index.rs
index 63033085b..f7fe0d771 100644
--- a/compiler/rustc_ast_lowering/src/index.rs
+++ b/compiler/rustc_ast_lowering/src/index.rs
@@ -275,19 +275,6 @@ impl<'a, 'hir> Visitor<'hir> for NodeCollector<'a, 'hir> {
         });
     }
 
-    fn visit_fn(
-        &mut self,
-        fk: intravisit::FnKind<'hir>,
-        fd: &'hir FnDecl<'hir>,
-        b: BodyId,
-        _: Span,
-        id: HirId,
-    ) {
-        assert_eq!(self.owner, id.owner);
-        assert_eq!(self.parent_node, id.local_id);
-        intravisit::walk_fn(self, fk, fd, b, id);
-    }
-
     fn visit_block(&mut self, block: &'hir Block<'hir>) {
         self.insert(block.span, block.hir_id, Node::Block(block));
         self.with_parent(block.hir_id, |this| {
diff --git a/compiler/rustc_ast_lowering/src/item.rs b/compiler/rustc_ast_lowering/src/item.rs
index 5d2589cb2..41295f2b7 100644
--- a/compiler/rustc_ast_lowering/src/item.rs
+++ b/compiler/rustc_ast_lowering/src/item.rs
@@ -7,13 +7,14 @@ use rustc_ast::ptr::P;
 use rustc_ast::visit::AssocCtxt;
 use rustc_ast::*;
 use rustc_data_structures::sorted_map::SortedMap;
+use rustc_errors::ErrorGuaranteed;
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::{LocalDefId, CRATE_DEF_ID};
 use rustc_hir::PredicateOrigin;
 use rustc_index::vec::{Idx, IndexVec};
 use rustc_middle::ty::{DefIdTree, ResolverAstLowering, TyCtxt};
-use rustc_span::lev_distance::find_best_match_for_name;
+use rustc_span::edit_distance::find_best_match_for_name;
 use rustc_span::source_map::DesugaringKind;
 use rustc_span::symbol::{kw, sym, Ident};
 use rustc_span::{Span, Symbol};
@@ -67,7 +68,6 @@ impl<'a, 'hir> ItemLowerer<'a, 'hir> {
             current_hir_id_owner: hir::CRATE_OWNER_ID,
             item_local_id_counter: hir::ItemLocalId::new(0),
             node_id_to_local_id: Default::default(),
-            local_id_to_def_id: SortedMap::new(),
             trait_map: Default::default(),
 
             // Lowering state.
@@ -285,7 +285,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     .alloc_from_iter(fm.items.iter().map(|x| self.lower_foreign_item_ref(x))),
             },
             ItemKind::GlobalAsm(asm) => hir::ItemKind::GlobalAsm(self.lower_inline_asm(span, asm)),
-            ItemKind::TyAlias(box TyAlias { generics, where_clauses, ty: Some(ty), .. }) => {
+            ItemKind::TyAlias(box TyAlias { generics, where_clauses, ty, .. }) => {
                 // We lower
                 //
                 // type Foo = impl Trait
@@ -300,18 +300,16 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     &generics,
                     id,
                     &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
-                    |this| this.lower_ty(ty, &ImplTraitContext::TypeAliasesOpaqueTy),
-                );
-                hir::ItemKind::TyAlias(ty, generics)
-            }
-            ItemKind::TyAlias(box TyAlias { generics, where_clauses, ty: None, .. }) => {
-                let mut generics = generics.clone();
-                add_ty_alias_where_clause(&mut generics, *where_clauses, true);
-                let (generics, ty) = self.lower_generics(
-                    &generics,
-                    id,
-                    &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
-                    |this| this.arena.alloc(this.ty(span, hir::TyKind::Err)),
+                    |this| match ty {
+                        None => {
+                            let guar = this.tcx.sess.delay_span_bug(
+                                span,
+                                "expected to lower type alias type, but it was missing",
+                            );
+                            this.arena.alloc(this.ty(span, hir::TyKind::Err(guar)))
+                        }
+                        Some(ty) => this.lower_ty(ty, &ImplTraitContext::TypeAliasesOpaqueTy),
+                    },
                 );
                 hir::ItemKind::TyAlias(ty, generics)
             }
@@ -379,8 +377,10 @@ impl<'hir> LoweringContext<'_, 'hir> {
                             )
                         });
 
-                        let lowered_ty = this
-                            .lower_ty(ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Type));
+                        let lowered_ty = this.lower_ty(
+                            ty,
+                            &ImplTraitContext::Disallowed(ImplTraitPosition::ImplSelf),
+                        );
 
                         (trait_ref, lowered_ty)
                     });
@@ -459,7 +459,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         span: Span,
         body: Option<&Expr>,
     ) -> (&'hir hir::Ty<'hir>, hir::BodyId) {
-        let ty = self.lower_ty(ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Type));
+        let ty = self.lower_ty(ty, &ImplTraitContext::Disallowed(ImplTraitPosition::ConstTy));
         (ty, self.lower_const_body(span, body))
     }
 
@@ -609,8 +609,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     hir::ForeignItemKind::Fn(fn_dec, fn_args, generics)
                 }
                 ForeignItemKind::Static(t, m, _) => {
-                    let ty =
-                        self.lower_ty(t, &ImplTraitContext::Disallowed(ImplTraitPosition::Type));
+                    let ty = self
+                        .lower_ty(t, &ImplTraitContext::Disallowed(ImplTraitPosition::StaticTy));
                     hir::ForeignItemKind::Static(ty, *m)
                 }
                 ForeignItemKind::TyAlias(..) => hir::ForeignItemKind::Type,
@@ -680,11 +680,11 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 qself,
                 path,
                 ParamMode::ExplicitNamed, // no `'_` in declarations (Issue #61124)
-                &ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                &ImplTraitContext::Disallowed(ImplTraitPosition::FieldTy),
             );
             self.arena.alloc(t)
         } else {
-            self.lower_ty(&f.ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Type))
+            self.lower_ty(&f.ty, &ImplTraitContext::Disallowed(ImplTraitPosition::FieldTy))
         };
         let hir_id = self.lower_node_id(f.id);
         self.lower_attrs(hir_id, &f.attrs);
@@ -709,7 +709,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
 
         let (generics, kind, has_default) = match &i.kind {
             AssocItemKind::Const(_, ty, default) => {
-                let ty = self.lower_ty(ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Type));
+                let ty =
+                    self.lower_ty(ty, &ImplTraitContext::Disallowed(ImplTraitPosition::ConstTy));
                 let body = default.as_ref().map(|x| self.lower_const_body(i.span, Some(x)));
                 (hir::Generics::empty(), hir::TraitItemKind::Const(ty, body), body.is_some())
             }
@@ -747,7 +748,10 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                     |this| {
                         let ty = ty.as_ref().map(|x| {
-                            this.lower_ty(x, &ImplTraitContext::Disallowed(ImplTraitPosition::Type))
+                            this.lower_ty(
+                                x,
+                                &ImplTraitContext::Disallowed(ImplTraitPosition::AssocTy),
+                            )
                         });
                         hir::TraitItemKind::Type(
                             this.lower_param_bounds(
@@ -793,8 +797,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
     }
 
     /// Construct `ExprKind::Err` for the given `span`.
-    pub(crate) fn expr_err(&mut self, span: Span) -> hir::Expr<'hir> {
-        self.expr(span, hir::ExprKind::Err)
+    pub(crate) fn expr_err(&mut self, span: Span, guar: ErrorGuaranteed) -> hir::Expr<'hir> {
+        self.expr(span, hir::ExprKind::Err(guar))
     }
 
     fn lower_impl_item(&mut self, i: &AssocItem) -> &'hir hir::ImplItem<'hir> {
@@ -806,7 +810,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
 
         let (generics, kind) = match &i.kind {
             AssocItemKind::Const(_, ty, expr) => {
-                let ty = self.lower_ty(ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Type));
+                let ty =
+                    self.lower_ty(ty, &ImplTraitContext::Disallowed(ImplTraitPosition::ConstTy));
                 (
                     hir::Generics::empty(),
                     hir::ImplItemKind::Const(ty, self.lower_const_body(i.span, expr.as_deref())),
@@ -841,7 +846,11 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     &ImplTraitContext::Disallowed(ImplTraitPosition::Generic),
                     |this| match ty {
                         None => {
-                            let ty = this.arena.alloc(this.ty(i.span, hir::TyKind::Err));
+                            let guar = this.tcx.sess.delay_span_bug(
+                                i.span,
+                                "expected to lower associated type, but it was missing",
+                            );
+                            let ty = this.arena.alloc(this.ty(i.span, hir::TyKind::Err(guar)));
                             hir::ImplItemKind::Type(ty)
                         }
                         Some(ty) => {
@@ -967,7 +976,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
     fn lower_block_expr_opt(&mut self, span: Span, block: Option<&Block>) -> hir::Expr<'hir> {
         match block {
             Some(block) => self.lower_block_expr(block),
-            None => self.expr_err(span),
+            None => self.expr_err(span, self.tcx.sess.delay_span_bug(span, "no block")),
         }
     }
 
@@ -977,7 +986,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 &[],
                 match expr {
                     Some(expr) => this.lower_expr_mut(expr),
-                    None => this.expr_err(span),
+                    None => this.expr_err(span, this.tcx.sess.delay_span_bug(span, "no block")),
                 },
             )
         })
@@ -1330,13 +1339,19 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 .map(|predicate| self.lower_where_predicate(predicate)),
         );
 
-        let mut params: SmallVec<[hir::GenericParam<'hir>; 4]> =
-            self.lower_generic_params_mut(&generics.params).collect();
+        let mut params: SmallVec<[hir::GenericParam<'hir>; 4]> = self
+            .lower_generic_params_mut(&generics.params, hir::GenericParamSource::Generics)
+            .collect();
 
         // Introduce extra lifetimes if late resolution tells us to.
         let extra_lifetimes = self.resolver.take_extra_lifetime_params(parent_node_id);
         params.extend(extra_lifetimes.into_iter().filter_map(|(ident, node_id, res)| {
-            self.lifetime_res_to_generic_param(ident, node_id, res)
+            self.lifetime_res_to_generic_param(
+                ident,
+                node_id,
+                res,
+                hir::GenericParamSource::Generics,
+            )
         }));
 
         let has_where_clause_predicates = !generics.where_clause.predicates.is_empty();
@@ -1440,9 +1455,10 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 span,
             }) => hir::WherePredicate::BoundPredicate(hir::WhereBoundPredicate {
                 hir_id: self.next_id(),
-                bound_generic_params: self.lower_generic_params(bound_generic_params),
+                bound_generic_params: self
+                    .lower_generic_params(bound_generic_params, hir::GenericParamSource::Binder),
                 bounded_ty: self
-                    .lower_ty(bounded_ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Type)),
+                    .lower_ty(bounded_ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Bound)),
                 bounds: self.arena.alloc_from_iter(bounds.iter().map(|bound| {
                     self.lower_param_bound(
                         bound,
@@ -1466,9 +1482,9 @@ impl<'hir> LoweringContext<'_, 'hir> {
             WherePredicate::EqPredicate(WhereEqPredicate { lhs_ty, rhs_ty, span }) => {
                 hir::WherePredicate::EqPredicate(hir::WhereEqPredicate {
                     lhs_ty: self
-                        .lower_ty(lhs_ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Type)),
+                        .lower_ty(lhs_ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Bound)),
                     rhs_ty: self
-                        .lower_ty(rhs_ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Type)),
+                        .lower_ty(rhs_ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Bound)),
                     span: self.lower_span(*span),
                 })
             }
diff --git a/compiler/rustc_ast_lowering/src/lib.rs b/compiler/rustc_ast_lowering/src/lib.rs
index bc6d2cf12..b20157f2c 100644
--- a/compiler/rustc_ast_lowering/src/lib.rs
+++ b/compiler/rustc_ast_lowering/src/lib.rs
@@ -52,23 +52,28 @@ use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::sorted_map::SortedMap;
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
 use rustc_data_structures::sync::Lrc;
-use rustc_errors::{DiagnosticArgFromDisplay, Handler, StashKey};
+use rustc_errors::{
+    DiagnosticArgFromDisplay, DiagnosticMessage, Handler, StashKey, SubdiagnosticMessage,
+};
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, LifetimeRes, Namespace, PartialRes, PerNS, Res};
 use rustc_hir::def_id::{LocalDefId, CRATE_DEF_ID};
 use rustc_hir::definitions::DefPathData;
 use rustc_hir::{ConstArg, GenericArg, ItemLocalId, ParamName, TraitCandidate};
 use rustc_index::vec::{Idx, IndexVec};
-use rustc_middle::span_bug;
-use rustc_middle::ty::{ResolverAstLowering, TyCtxt};
+use rustc_macros::fluent_messages;
+use rustc_middle::{
+    span_bug,
+    ty::{ResolverAstLowering, TyCtxt},
+};
 use rustc_session::parse::feature_err;
 use rustc_span::hygiene::MacroKind;
 use rustc_span::source_map::DesugaringKind;
 use rustc_span::symbol::{kw, sym, Ident, Symbol};
 use rustc_span::{Span, DUMMY_SP};
-
 use smallvec::SmallVec;
 use std::collections::hash_map::Entry;
+use thin_vec::ThinVec;
 
 macro_rules! arena_vec {
     ($this:expr; $($x:expr),*) => (
@@ -80,12 +85,15 @@ mod asm;
 mod block;
 mod errors;
 mod expr;
+mod format;
 mod index;
 mod item;
 mod lifetime_collector;
 mod pat;
 mod path;
 
+fluent_messages! { "../locales/en-US.ftl" }
+
 struct LoweringContext<'a, 'hir> {
     tcx: TyCtxt<'hir>,
     resolver: &'a mut ResolverAstLowering,
@@ -118,7 +126,6 @@ struct LoweringContext<'a, 'hir> {
 
     current_hir_id_owner: hir::OwnerId,
     item_local_id_counter: hir::ItemLocalId,
-    local_id_to_def_id: SortedMap<ItemLocalId, LocalDefId>,
     trait_map: FxHashMap<ItemLocalId, Box<[TraitCandidate]>>,
 
     impl_trait_defs: Vec<hir::GenericParam<'hir>>,
@@ -253,7 +260,6 @@ enum ImplTraitContext {
 enum ImplTraitPosition {
     Path,
     Variable,
-    Type,
     Trait,
     AsyncBlock,
     Bound,
@@ -270,30 +276,43 @@ enum ImplTraitPosition {
     FnTraitReturn,
     TraitReturn,
     ImplReturn,
+    GenericDefault,
+    ConstTy,
+    StaticTy,
+    AssocTy,
+    FieldTy,
+    Cast,
+    ImplSelf,
 }
 
 impl std::fmt::Display for ImplTraitPosition {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         let name = match self {
-            ImplTraitPosition::Path => "path",
-            ImplTraitPosition::Variable => "variable binding",
-            ImplTraitPosition::Type => "type",
-            ImplTraitPosition::Trait => "trait",
-            ImplTraitPosition::AsyncBlock => "async block",
-            ImplTraitPosition::Bound => "bound",
-            ImplTraitPosition::Generic => "generic",
-            ImplTraitPosition::ExternFnParam => "`extern fn` param",
-            ImplTraitPosition::ClosureParam => "closure param",
-            ImplTraitPosition::PointerParam => "`fn` pointer param",
-            ImplTraitPosition::FnTraitParam => "`Fn` trait param",
-            ImplTraitPosition::TraitParam => "trait method param",
-            ImplTraitPosition::ImplParam => "`impl` method param",
-            ImplTraitPosition::ExternFnReturn => "`extern fn` return",
-            ImplTraitPosition::ClosureReturn => "closure return",
-            ImplTraitPosition::PointerReturn => "`fn` pointer return",
-            ImplTraitPosition::FnTraitReturn => "`Fn` trait return",
-            ImplTraitPosition::TraitReturn => "trait method return",
-            ImplTraitPosition::ImplReturn => "`impl` method return",
+            ImplTraitPosition::Path => "paths",
+            ImplTraitPosition::Variable => "variable bindings",
+            ImplTraitPosition::Trait => "traits",
+            ImplTraitPosition::AsyncBlock => "async blocks",
+            ImplTraitPosition::Bound => "bounds",
+            ImplTraitPosition::Generic => "generics",
+            ImplTraitPosition::ExternFnParam => "`extern fn` params",
+            ImplTraitPosition::ClosureParam => "closure params",
+            ImplTraitPosition::PointerParam => "`fn` pointer params",
+            ImplTraitPosition::FnTraitParam => "`Fn` trait params",
+            ImplTraitPosition::TraitParam => "trait method params",
+            ImplTraitPosition::ImplParam => "`impl` method params",
+            ImplTraitPosition::ExternFnReturn => "`extern fn` return types",
+            ImplTraitPosition::ClosureReturn => "closure return types",
+            ImplTraitPosition::PointerReturn => "`fn` pointer return types",
+            ImplTraitPosition::FnTraitReturn => "`Fn` trait return types",
+            ImplTraitPosition::TraitReturn => "trait method return types",
+            ImplTraitPosition::ImplReturn => "`impl` method return types",
+            ImplTraitPosition::GenericDefault => "generic parameter defaults",
+            ImplTraitPosition::ConstTy => "const types",
+            ImplTraitPosition::StaticTy => "static types",
+            ImplTraitPosition::AssocTy => "associated types",
+            ImplTraitPosition::FieldTy => "field types",
+            ImplTraitPosition::Cast => "cast types",
+            ImplTraitPosition::ImplSelf => "impl headers",
         };
 
         write!(f, "{name}")
@@ -416,6 +435,7 @@ fn compute_hir_hash(
 
 pub fn lower_to_hir(tcx: TyCtxt<'_>, (): ()) -> hir::Crate<'_> {
     let sess = tcx.sess;
+    tcx.ensure().output_filenames(());
     let (mut resolver, krate) = tcx.resolver_for_lowering(()).steal();
 
     let ast_index = index_crate(&resolver.node_id_to_def_id, &krate);
@@ -565,7 +585,6 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         let current_attrs = std::mem::take(&mut self.attrs);
         let current_bodies = std::mem::take(&mut self.bodies);
         let current_node_ids = std::mem::take(&mut self.node_id_to_local_id);
-        let current_id_to_def_id = std::mem::take(&mut self.local_id_to_def_id);
         let current_trait_map = std::mem::take(&mut self.trait_map);
         let current_owner =
             std::mem::replace(&mut self.current_hir_id_owner, hir::OwnerId { def_id });
@@ -592,7 +611,6 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         self.attrs = current_attrs;
         self.bodies = current_bodies;
         self.node_id_to_local_id = current_node_ids;
-        self.local_id_to_def_id = current_id_to_def_id;
         self.trait_map = current_trait_map;
         self.current_hir_id_owner = current_owner;
         self.item_local_id_counter = current_local_counter;
@@ -627,7 +645,6 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     fn make_owner_info(&mut self, node: hir::OwnerNode<'hir>) -> &'hir hir::OwnerInfo<'hir> {
         let attrs = std::mem::take(&mut self.attrs);
         let mut bodies = std::mem::take(&mut self.bodies);
-        let local_id_to_def_id = std::mem::take(&mut self.local_id_to_def_id);
         let trait_map = std::mem::take(&mut self.trait_map);
 
         #[cfg(debug_assertions)]
@@ -643,13 +660,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         let (hash_including_bodies, hash_without_bodies) = self.hash_owner(node, &bodies);
         let (nodes, parenting) =
             index::index_hir(self.tcx.sess, &*self.tcx.definitions_untracked(), node, &bodies);
-        let nodes = hir::OwnerNodes {
-            hash_including_bodies,
-            hash_without_bodies,
-            nodes,
-            bodies,
-            local_id_to_def_id,
-        };
+        let nodes = hir::OwnerNodes { hash_including_bodies, hash_without_bodies, nodes, bodies };
         let attrs = {
             let hash = self.tcx.with_stable_hashing_context(|mut hcx| {
                 let mut stable_hasher = StableHasher::new();
@@ -708,7 +719,6 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 assert_ne!(local_id, hir::ItemLocalId::new(0));
                 if let Some(def_id) = self.opt_local_def_id(ast_node_id) {
                     self.children.push((def_id, hir::MaybeOwner::NonOwner(hir_id)));
-                    self.local_id_to_def_id.insert(local_id, def_id);
                 }
 
                 if let Some(traits) = self.resolver.trait_map.remove(&ast_node_id) {
@@ -794,6 +804,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         ident: Ident,
         node_id: NodeId,
         res: LifetimeRes,
+        source: hir::GenericParamSource,
     ) -> Option<hir::GenericParam<'hir>> {
         let (name, kind) = match res {
             LifetimeRes::Param { .. } => {
@@ -827,6 +838,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             pure_wrt_drop: false,
             kind: hir::GenericParamKind::Lifetime { kind },
             colon_span: None,
+            source,
         })
     }
 
@@ -842,11 +854,13 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         binder: NodeId,
         generic_params: &[GenericParam],
     ) -> &'hir [hir::GenericParam<'hir>] {
-        let mut generic_params: Vec<_> = self.lower_generic_params_mut(generic_params).collect();
+        let mut generic_params: Vec<_> = self
+            .lower_generic_params_mut(generic_params, hir::GenericParamSource::Binder)
+            .collect();
         let extra_lifetimes = self.resolver.take_extra_lifetime_params(binder);
         debug!(?extra_lifetimes);
         generic_params.extend(extra_lifetimes.into_iter().filter_map(|(ident, node_id, res)| {
-            self.lifetime_res_to_generic_param(ident, node_id, res)
+            self.lifetime_res_to_generic_param(ident, node_id, res, hir::GenericParamSource::Binder)
         }));
         let generic_params = self.arena.alloc_from_iter(generic_params);
         debug!(?generic_params);
@@ -992,8 +1006,6 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         } else {
             self.arena.alloc(hir::GenericArgs::none())
         };
-        let itctx_tait = &ImplTraitContext::TypeAliasesOpaqueTy;
-
         let kind = match &constraint.kind {
             AssocConstraintKind::Equality { term } => {
                 let term = match term {
@@ -1003,8 +1015,14 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 hir::TypeBindingKind::Equality { term }
             }
             AssocConstraintKind::Bound { bounds } => {
+                enum DesugarKind<'a> {
+                    ImplTrait,
+                    Error(&'a ImplTraitPosition),
+                    Bound,
+                }
+
                 // Piggy-back on the `impl Trait` context to figure out the correct behavior.
-                let (desugar_to_impl_trait, itctx) = match itctx {
+                let desugar_kind = match itctx {
                     // We are in the return position:
                     //
                     //     fn foo() -> impl Iterator<Item: Debug>
@@ -1013,7 +1031,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     //
                     //     fn foo() -> impl Iterator<Item = impl Debug>
                     ImplTraitContext::ReturnPositionOpaqueTy { .. }
-                    | ImplTraitContext::TypeAliasesOpaqueTy { .. } => (true, itctx),
+                    | ImplTraitContext::TypeAliasesOpaqueTy { .. } => DesugarKind::ImplTrait,
 
                     // We are in the argument position, but within a dyn type:
                     //
@@ -1022,15 +1040,11 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     // so desugar to
                     //
                     //     fn foo(x: dyn Iterator<Item = impl Debug>)
-                    ImplTraitContext::Universal if self.is_in_dyn_type => (true, itctx),
+                    ImplTraitContext::Universal if self.is_in_dyn_type => DesugarKind::ImplTrait,
 
-                    // In `type Foo = dyn Iterator<Item: Debug>` we desugar to
-                    // `type Foo = dyn Iterator<Item = impl Debug>` but we have to override the
-                    // "impl trait context" to permit `impl Debug` in this position (it desugars
-                    // then to an opaque type).
-                    //
-                    // FIXME: this is only needed until `impl Trait` is allowed in type aliases.
-                    ImplTraitContext::Disallowed(_) if self.is_in_dyn_type => (true, itctx_tait),
+                    ImplTraitContext::Disallowed(position) if self.is_in_dyn_type => {
+                        DesugarKind::Error(position)
+                    }
 
                     // We are in the parameter position, but not within a dyn type:
                     //
@@ -1039,35 +1053,47 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     // so we leave it as is and this gets expanded in astconv to a bound like
                     // `<T as Iterator>::Item: Debug` where `T` is the type parameter for the
                     // `impl Iterator`.
-                    _ => (false, itctx),
+                    _ => DesugarKind::Bound,
                 };
 
-                if desugar_to_impl_trait {
-                    // Desugar `AssocTy: Bounds` into `AssocTy = impl Bounds`. We do this by
-                    // constructing the HIR for `impl bounds...` and then lowering that.
-
-                    let impl_trait_node_id = self.next_node_id();
-
-                    self.with_dyn_type_scope(false, |this| {
-                        let node_id = this.next_node_id();
-                        let ty = this.lower_ty(
-                            &Ty {
-                                id: node_id,
-                                kind: TyKind::ImplTrait(impl_trait_node_id, bounds.clone()),
-                                span: this.lower_span(constraint.span),
-                                tokens: None,
-                            },
-                            itctx,
-                        );
+                match desugar_kind {
+                    DesugarKind::ImplTrait => {
+                        // Desugar `AssocTy: Bounds` into `AssocTy = impl Bounds`. We do this by
+                        // constructing the HIR for `impl bounds...` and then lowering that.
 
-                        hir::TypeBindingKind::Equality { term: ty.into() }
-                    })
-                } else {
-                    // Desugar `AssocTy: Bounds` into a type binding where the
-                    // later desugars into a trait predicate.
-                    let bounds = self.lower_param_bounds(bounds, itctx);
+                        let impl_trait_node_id = self.next_node_id();
+
+                        self.with_dyn_type_scope(false, |this| {
+                            let node_id = this.next_node_id();
+                            let ty = this.lower_ty(
+                                &Ty {
+                                    id: node_id,
+                                    kind: TyKind::ImplTrait(impl_trait_node_id, bounds.clone()),
+                                    span: this.lower_span(constraint.span),
+                                    tokens: None,
+                                },
+                                itctx,
+                            );
+
+                            hir::TypeBindingKind::Equality { term: ty.into() }
+                        })
+                    }
+                    DesugarKind::Bound => {
+                        // Desugar `AssocTy: Bounds` into a type binding where the
+                        // later desugars into a trait predicate.
+                        let bounds = self.lower_param_bounds(bounds, itctx);
 
-                    hir::TypeBindingKind::Constraint { bounds }
+                        hir::TypeBindingKind::Constraint { bounds }
+                    }
+                    DesugarKind::Error(position) => {
+                        let guar = self.tcx.sess.emit_err(errors::MisplacedAssocTyBinding {
+                            span: constraint.span,
+                            position: DiagnosticArgFromDisplay(position),
+                        });
+                        let err_ty =
+                            &*self.arena.alloc(self.ty(constraint.span, hir::TyKind::Err(guar)));
+                        hir::TypeBindingKind::Equality { term: err_ty.into() }
+                    }
                 }
             }
         };
@@ -1204,7 +1230,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             let (bounds, lifetime_bound) = self.with_dyn_type_scope(true, |this| {
                 let bound = this.lower_poly_trait_ref(
                     &PolyTraitRef {
-                        bound_generic_params: vec![],
+                        bound_generic_params: ThinVec::new(),
                         trait_ref: TraitRef { path: path.clone(), ref_id: t.id },
                         span: t.span
                     },
@@ -1234,7 +1260,9 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     fn lower_ty_direct(&mut self, t: &Ty, itctx: &ImplTraitContext) -> hir::Ty<'hir> {
         let kind = match &t.kind {
             TyKind::Infer => hir::TyKind::Infer,
-            TyKind::Err => hir::TyKind::Err,
+            TyKind::Err => {
+                hir::TyKind::Err(self.tcx.sess.delay_span_bug(t.span, "TyKind::Err lowered"))
+            }
             TyKind::Slice(ty) => hir::TyKind::Slice(self.lower_ty(ty, itctx)),
             TyKind::Ptr(mt) => hir::TyKind::Ptr(self.lower_mt(mt, itctx)),
             TyKind::Ref(region, mt) => {
@@ -1351,8 +1379,12 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                             span,
                         );
                         let ident = Ident::from_str_and_span(&pprust::ty_to_string(t), span);
-                        let (param, bounds, path) =
-                            self.lower_generic_and_bounds(*def_node_id, span, ident, bounds);
+                        let (param, bounds, path) = self.lower_universal_param_and_bounds(
+                            *def_node_id,
+                            span,
+                            ident,
+                            bounds,
+                        );
                         self.impl_trait_defs.push(param);
                         if let Some(bounds) = bounds {
                             self.impl_trait_bounds.push(bounds);
@@ -1360,7 +1392,8 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                         path
                     }
                     ImplTraitContext::FeatureGated(position, feature) => {
-                        self.tcx
+                        let guar = self
+                            .tcx
                             .sess
                             .create_feature_err(
                                 MisplacedImplTrait {
@@ -1370,24 +1403,24 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                                 *feature,
                             )
                             .emit();
-                        hir::TyKind::Err
+                        hir::TyKind::Err(guar)
                     }
                     ImplTraitContext::Disallowed(position) => {
-                        self.tcx.sess.emit_err(MisplacedImplTrait {
+                        let guar = self.tcx.sess.emit_err(MisplacedImplTrait {
                             span: t.span,
                             position: DiagnosticArgFromDisplay(position),
                         });
-                        hir::TyKind::Err
+                        hir::TyKind::Err(guar)
                     }
                 }
             }
             TyKind::MacCall(_) => panic!("`TyKind::MacCall` should have been expanded by now"),
             TyKind::CVarArgs => {
-                self.tcx.sess.delay_span_bug(
+                let guar = self.tcx.sess.delay_span_bug(
                     t.span,
                     "`TyKind::CVarArgs` should have been handled elsewhere",
                 );
-                hir::TyKind::Err
+                hir::TyKind::Err(guar)
             }
         };
 
@@ -1505,6 +1538,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                             pure_wrt_drop: false,
                             kind: hir::GenericParamKind::Lifetime { kind },
                             colon_span: None,
+                            source: hir::GenericParamSource::Generics,
                         }
                     },
                 ));
@@ -1573,7 +1607,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         hir::OwnerNode::Item(self.arena.alloc(opaque_ty_item))
     }
 
-    /// Given a `parent_def_id`, a list of `lifetimes_in_bounds and a `remapping` hash to be
+    /// Given a `parent_def_id`, a list of `lifetimes_in_bounds` and a `remapping` hash to be
     /// filled, this function creates new definitions for `Param` and `Fresh` lifetimes, inserts the
     /// new definition, adds it to the remapping with the definition of the given lifetime and
     /// returns a list of lifetimes to be lowered afterwards.
@@ -1962,6 +1996,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                             pure_wrt_drop: false,
                             kind: hir::GenericParamKind::Lifetime { kind },
                             colon_span: None,
+                            source: hir::GenericParamSource::Generics,
                         }
                     },
                 ));
@@ -2127,16 +2162,25 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     fn lower_generic_params_mut<'s>(
         &'s mut self,
         params: &'s [GenericParam],
+        source: hir::GenericParamSource,
     ) -> impl Iterator<Item = hir::GenericParam<'hir>> + Captures<'a> + Captures<'s> {
-        params.iter().map(move |param| self.lower_generic_param(param))
+        params.iter().map(move |param| self.lower_generic_param(param, source))
     }
 
-    fn lower_generic_params(&mut self, params: &[GenericParam]) -> &'hir [hir::GenericParam<'hir>] {
-        self.arena.alloc_from_iter(self.lower_generic_params_mut(params))
+    fn lower_generic_params(
+        &mut self,
+        params: &[GenericParam],
+        source: hir::GenericParamSource,
+    ) -> &'hir [hir::GenericParam<'hir>] {
+        self.arena.alloc_from_iter(self.lower_generic_params_mut(params, source))
     }
 
     #[instrument(level = "trace", skip(self))]
-    fn lower_generic_param(&mut self, param: &GenericParam) -> hir::GenericParam<'hir> {
+    fn lower_generic_param(
+        &mut self,
+        param: &GenericParam,
+        source: hir::GenericParamSource,
+    ) -> hir::GenericParam<'hir> {
         let (name, kind) = self.lower_generic_param_kind(param);
 
         let hir_id = self.lower_node_id(param.id);
@@ -2149,6 +2193,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             pure_wrt_drop: self.tcx.sess.contains_name(&param.attrs, sym::may_dangle),
             kind,
             colon_span: param.colon_span.map(|s| self.lower_span(s)),
+            source,
         }
     }
 
@@ -2175,7 +2220,10 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             GenericParamKind::Type { default, .. } => {
                 let kind = hir::GenericParamKind::Type {
                     default: default.as_ref().map(|x| {
-                        self.lower_ty(x, &ImplTraitContext::Disallowed(ImplTraitPosition::Type))
+                        self.lower_ty(
+                            x,
+                            &ImplTraitContext::Disallowed(ImplTraitPosition::GenericDefault),
+                        )
                     }),
                     synthetic: false,
                 };
@@ -2183,7 +2231,10 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                 (hir::ParamName::Plain(self.lower_ident(param.ident)), kind)
             }
             GenericParamKind::Const { ty, kw_span: _, default } => {
-                let ty = self.lower_ty(&ty, &ImplTraitContext::Disallowed(ImplTraitPosition::Type));
+                let ty = self.lower_ty(
+                    &ty,
+                    &ImplTraitContext::Disallowed(ImplTraitPosition::GenericDefault),
+                );
                 let default = default.as_ref().map(|def| self.lower_anon_const(def));
                 (
                     hir::ParamName::Plain(self.lower_ident(param.ident)),
@@ -2235,7 +2286,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     }
 
     #[instrument(level = "debug", skip(self), ret)]
-    fn lower_generic_and_bounds(
+    fn lower_universal_param_and_bounds(
         &mut self,
         node_id: NodeId,
         span: Span,
@@ -2255,6 +2306,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             span,
             kind: hir::GenericParamKind::Type { default: None, synthetic: true },
             colon_span: None,
+            source: hir::GenericParamSource::Generics,
         };
 
         let preds = self.lower_generic_bound_predicate(
diff --git a/compiler/rustc_ast_lowering/src/pat.rs b/compiler/rustc_ast_lowering/src/pat.rs
index 06d885a45..2509b7056 100644
--- a/compiler/rustc_ast_lowering/src/pat.rs
+++ b/compiler/rustc_ast_lowering/src/pat.rs
@@ -330,8 +330,8 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             ExprKind::Path(..) if allow_paths => {}
             ExprKind::Unary(UnOp::Neg, inner) if matches!(inner.kind, ExprKind::Lit(_)) => {}
             _ => {
-                self.tcx.sess.emit_err(ArbitraryExpressionInPattern { span: expr.span });
-                return self.arena.alloc(self.expr_err(expr.span));
+                let guar = self.tcx.sess.emit_err(ArbitraryExpressionInPattern { span: expr.span });
+                return self.arena.alloc(self.expr_err(expr.span, guar));
             }
         }
         self.lower_expr(expr)