Merging upstream version 1.70.0+dfsg1.

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

16 files changed, 312 insertions, 289 deletions

diff --git a/compiler/rustc_parse/locales/en-US.ftl b/compiler/rustc_parse/messages.ftl
index e76e91fc1..f11d0ed0f 100644
--- a/compiler/rustc_parse/locales/en-US.ftl
+++ b/compiler/rustc_parse/messages.ftl
@@ -336,7 +336,7 @@ parse_expected_identifier_found_reserved_keyword = expected identifier, found re
 parse_expected_identifier_found_doc_comment = expected identifier, found doc comment
 parse_expected_identifier = expected identifier
 
-parse_sugg_escape_to_use_as_identifier = escape `{$ident_name}` to use it as an identifier
+parse_sugg_escape_identifier = escape `{$ident_name}` to use it as an identifier
 
 parse_sugg_remove_comma = remove this comma
 
@@ -412,8 +412,7 @@ parse_fn_ptr_with_generics = function pointer types may not have generic paramet
         *[false] a
     } `for` parameter list
 
-parse_invalid_identifier_with_leading_number = expected identifier, found number literal
-    .label = identifiers cannot start with a number
+parse_invalid_identifier_with_leading_number = 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
@@ -710,7 +709,7 @@ parse_zero_chars = empty character literal
 parse_lone_slash = invalid trailing slash in literal
     .label = {parse_lone_slash}
 
-parse_unskipped_whitespace = non-ASCII whitespace symbol '{$ch}' is not skipped
+parse_unskipped_whitespace = whitespace symbol '{$ch}' is not skipped
     .label = {parse_unskipped_whitespace}
 
 parse_multiple_skipped_lines = multiple lines skipped by escaped newline
@@ -732,3 +731,14 @@ parse_unknown_start_of_token = unknown start of token: {$escaped}
         [one] once more
         *[other] {$repeats} more times
     }
+
+parse_box_syntax_removed = `box_syntax` has been removed
+    .suggestion = use `Box::new()` instead
+
+parse_bad_return_type_notation_output =
+    return type not allowed with return type notation
+    .suggestion = remove the return type
+
+parse_bad_return_type_notation_dotdot =
+    return type notation uses `()` instead of `(..)` for elided arguments
+    .suggestion = remove the `..`
diff --git a/compiler/rustc_parse/src/errors.rs b/compiler/rustc_parse/src/errors.rs
index 1662db36d..069217165 100644
--- a/compiler/rustc_parse/src/errors.rs
+++ b/compiler/rustc_parse/src/errors.rs
@@ -888,12 +888,12 @@ pub(crate) struct InvalidMetaItem {
 
 #[derive(Subdiagnostic)]
 #[suggestion(
-    parse_sugg_escape_to_use_as_identifier,
+    parse_sugg_escape_identifier,
     style = "verbose",
     applicability = "maybe-incorrect",
     code = "r#"
 )]
-pub(crate) struct SuggEscapeToUseAsIdentifier {
+pub(crate) struct SuggEscapeIdentifier {
     #[primary_span]
     pub span: Span,
     pub ident_name: String,
@@ -937,8 +937,9 @@ impl ExpectedIdentifierFound {
 pub(crate) struct ExpectedIdentifier {
     pub span: Span,
     pub token: Token,
-    pub suggest_raw: Option<SuggEscapeToUseAsIdentifier>,
+    pub suggest_raw: Option<SuggEscapeIdentifier>,
     pub suggest_remove_comma: Option<SuggRemoveComma>,
+    pub help_cannot_start_number: Option<HelpIdentifierStartsWithNumber>,
 }
 
 impl<'a, G: EmissionGuarantee> IntoDiagnostic<'a, G> for ExpectedIdentifier {
@@ -975,10 +976,21 @@ impl<'a, G: EmissionGuarantee> IntoDiagnostic<'a, G> for ExpectedIdentifier {
             sugg.add_to_diagnostic(&mut diag);
         }
 
+        if let Some(help) = self.help_cannot_start_number {
+            help.add_to_diagnostic(&mut diag);
+        }
+
         diag
     }
 }
 
+#[derive(Subdiagnostic)]
+#[help(parse_invalid_identifier_with_leading_number)]
+pub(crate) struct HelpIdentifierStartsWithNumber {
+    #[primary_span]
+    pub num_span: Span,
+}
+
 pub(crate) struct ExpectedSemi {
     pub span: Span,
     pub token: Token,
@@ -1208,14 +1220,6 @@ pub(crate) struct SelfParamNotFirst {
 }
 
 #[derive(Diagnostic)]
-#[diag(parse_invalid_identifier_with_leading_number)]
-pub(crate) struct InvalidIdentiferStartsWithNumber {
-    #[primary_span]
-    #[label]
-    pub span: Span,
-}
-
-#[derive(Diagnostic)]
 #[diag(parse_const_generic_without_braces)]
 pub(crate) struct ConstGenericWithoutBraces {
     #[primary_span]
@@ -2299,3 +2303,32 @@ impl HelpUseLatestEdition {
         }
     }
 }
+
+#[derive(Diagnostic)]
+#[diag(parse_box_syntax_removed)]
+pub struct BoxSyntaxRemoved<'a> {
+    #[primary_span]
+    #[suggestion(
+        code = "Box::new({code})",
+        applicability = "machine-applicable",
+        style = "verbose"
+    )]
+    pub span: Span,
+    pub code: &'a str,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_bad_return_type_notation_output)]
+pub(crate) struct BadReturnTypeNotationOutput {
+    #[primary_span]
+    #[suggestion(code = "", applicability = "maybe-incorrect")]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_bad_return_type_notation_dotdot)]
+pub(crate) struct BadReturnTypeNotationDotDot {
+    #[primary_span]
+    #[suggestion(code = "", applicability = "maybe-incorrect")]
+    pub span: Span,
+}
diff --git a/compiler/rustc_parse/src/lexer/diagnostics.rs b/compiler/rustc_parse/src/lexer/diagnostics.rs
index 27f4428d3..9e6d27bf0 100644
--- a/compiler/rustc_parse/src/lexer/diagnostics.rs
+++ b/compiler/rustc_parse/src/lexer/diagnostics.rs
@@ -21,7 +21,7 @@ pub struct TokenTreeDiagInfo {
     pub matching_block_spans: Vec<(Span, Span)>,
 }
 
-pub fn same_identation_level(sm: &SourceMap, open_sp: Span, close_sp: Span) -> bool {
+pub fn same_indentation_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,
@@ -67,13 +67,13 @@ pub fn report_suspicious_mismatch_block(
     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)))
+        .map(|&(open, close)| (open.with_hi(close.lo()), same_indentation_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()));
+    matched_spans.sort_by_key(|(span, _)| span.lo());
 
-    // We use larger block whose identation is well to cover those inner mismatched blocks
+    // We use larger block whose indentation 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];
diff --git a/compiler/rustc_parse/src/lexer/mod.rs b/compiler/rustc_parse/src/lexer/mod.rs
index 59958a309..9e856c9f2 100644
--- a/compiler/rustc_parse/src/lexer/mod.rs
+++ b/compiler/rustc_parse/src/lexer/mod.rs
@@ -175,7 +175,7 @@ impl<'a> StringReader<'a> {
                     if !sym.can_be_raw() {
                         self.sess.emit_err(errors::CannotBeRawIdent { span, ident: sym });
                     }
-                    self.sess.raw_identifier_spans.borrow_mut().push(span);
+                    self.sess.raw_identifier_spans.push(span);
                     token::Ident(sym, true)
                 }
                 rustc_lexer::TokenKind::UnknownPrefix => {
@@ -553,8 +553,8 @@ impl<'a> StringReader<'a> {
         }
 
         if let Some(possible_offset) = possible_offset {
-            let lo = start + BytePos(possible_offset as u32);
-            let hi = lo + BytePos(found_terminators as u32);
+            let lo = start + BytePos(possible_offset);
+            let hi = lo + BytePos(found_terminators);
             let span = self.mk_sp(lo, hi);
             err.span_suggestion(
                 span,
diff --git a/compiler/rustc_parse/src/lexer/tokentrees.rs b/compiler/rustc_parse/src/lexer/tokentrees.rs
index 36fd1e37d..7c2c08951 100644
--- a/compiler/rustc_parse/src/lexer/tokentrees.rs
+++ b/compiler/rustc_parse/src/lexer/tokentrees.rs
@@ -1,5 +1,5 @@
 use super::diagnostics::report_suspicious_mismatch_block;
-use super::diagnostics::same_identation_level;
+use super::diagnostics::same_indentation_level;
 use super::diagnostics::TokenTreeDiagInfo;
 use super::{StringReader, UnmatchedDelim};
 use rustc_ast::token::{self, Delimiter, Token};
@@ -153,7 +153,7 @@ impl<'a> TokenTreesReader<'a> {
                         unclosed_delimiter = Some(sp);
                     };
                     for (brace, brace_span) in &self.diag_info.open_braces {
-                        if same_identation_level(&sm, self.token.span, *brace_span)
+                        if same_indentation_level(&sm, self.token.span, *brace_span)
                             && brace == &close_delim
                         {
                             // high likelihood of these two corresponding
diff --git a/compiler/rustc_parse/src/lexer/unicode_chars.rs b/compiler/rustc_parse/src/lexer/unicode_chars.rs
index d4f971d5b..1f027c08f 100644
--- a/compiler/rustc_parse/src/lexer/unicode_chars.rs
+++ b/compiler/rustc_parse/src/lexer/unicode_chars.rs
@@ -336,8 +336,8 @@ const ASCII_ARRAY: &[(&str, &str, Option<token::TokenKind>)] = &[
     ("\"", "Quotation Mark", None),
 ];
 
-pub(super) fn check_for_substitution<'a>(
-    reader: &StringReader<'a>,
+pub(super) fn check_for_substitution(
+    reader: &StringReader<'_>,
     pos: BytePos,
     ch: char,
     count: usize,
diff --git a/compiler/rustc_parse/src/lib.rs b/compiler/rustc_parse/src/lib.rs
index d1c3fd0cd..17466cd0e 100644
--- a/compiler/rustc_parse/src/lib.rs
+++ b/compiler/rustc_parse/src/lib.rs
@@ -36,7 +36,7 @@ pub mod validate_attr;
 
 mod errors;
 
-fluent_messages! { "../locales/en-US.ftl" }
+fluent_messages! { "../messages.ftl" }
 
 // A bunch of utility functions of the form `parse_<thing>_from_<source>`
 // where <thing> includes crate, expr, item, stmt, tts, and one that
diff --git a/compiler/rustc_parse/src/parser/diagnostics.rs b/compiler/rustc_parse/src/parser/diagnostics.rs
index a051dbe9f..e03ce5d71 100644
--- a/compiler/rustc_parse/src/parser/diagnostics.rs
+++ b/compiler/rustc_parse/src/parser/diagnostics.rs
@@ -6,20 +6,19 @@ use super::{
 use crate::errors::{
     AmbiguousPlus, AttributeOnParamType, BadQPathStage2, BadTypePlus, BadTypePlusSub,
     ComparisonOperatorsCannotBeChained, ComparisonOperatorsCannotBeChainedSugg,
-    ConstGenericWithoutBraces, ConstGenericWithoutBracesSugg, DocCommentOnParamType,
-    DoubleColonInBound, ExpectedIdentifier, ExpectedSemi, ExpectedSemiSugg,
-    GenericParamsWithoutAngleBrackets, GenericParamsWithoutAngleBracketsSugg, InInTypo,
-    IncorrectAwait, IncorrectSemicolon, IncorrectUseOfAwait, ParenthesesInForHead,
-    ParenthesesInForHeadSugg, PatternMethodParamWithoutBody, QuestionMarkInType,
-    QuestionMarkInTypeSugg, SelfParamNotFirst, StructLiteralBodyWithoutPath,
-    StructLiteralBodyWithoutPathSugg, StructLiteralNeedingParens, StructLiteralNeedingParensSugg,
-    SuggEscapeToUseAsIdentifier, SuggRemoveComma, UnexpectedConstInGenericParam,
-    UnexpectedConstParamDeclaration, UnexpectedConstParamDeclarationSugg, UnmatchedAngleBrackets,
-    UseEqInstead,
+    ConstGenericWithoutBraces, ConstGenericWithoutBracesSugg, DocCommentDoesNotDocumentAnything,
+    DocCommentOnParamType, DoubleColonInBound, ExpectedIdentifier, ExpectedSemi, ExpectedSemiSugg,
+    GenericParamsWithoutAngleBrackets, GenericParamsWithoutAngleBracketsSugg,
+    HelpIdentifierStartsWithNumber, InInTypo, IncorrectAwait, IncorrectSemicolon,
+    IncorrectUseOfAwait, ParenthesesInForHead, ParenthesesInForHeadSugg,
+    PatternMethodParamWithoutBody, QuestionMarkInType, QuestionMarkInTypeSugg, SelfParamNotFirst,
+    StructLiteralBodyWithoutPath, StructLiteralBodyWithoutPathSugg, StructLiteralNeedingParens,
+    StructLiteralNeedingParensSugg, SuggEscapeIdentifier, SuggRemoveComma,
+    UnexpectedConstInGenericParam, UnexpectedConstParamDeclaration,
+    UnexpectedConstParamDeclarationSugg, UnmatchedAngleBrackets, UseEqInstead,
 };
 
 use crate::fluent_generated as fluent;
-use crate::lexer::UnmatchedDelim;
 use crate::parser;
 use rustc_ast as ast;
 use rustc_ast::ptr::P;
@@ -39,7 +38,7 @@ use rustc_errors::{
 use rustc_session::errors::ExprParenthesesNeeded;
 use rustc_span::source_map::Spanned;
 use rustc_span::symbol::{kw, sym, Ident};
-use rustc_span::{Span, SpanSnippetError, DUMMY_SP};
+use rustc_span::{Span, SpanSnippetError, Symbol, DUMMY_SP};
 use std::mem::take;
 use std::ops::{Deref, DerefMut};
 use thin_vec::{thin_vec, ThinVec};
@@ -220,7 +219,6 @@ impl MultiSugg {
 /// is dropped.
 pub struct SnapshotParser<'a> {
     parser: Parser<'a>,
-    unclosed_delims: Vec<UnmatchedDelim>,
 }
 
 impl<'a> Deref for SnapshotParser<'a> {
@@ -255,34 +253,36 @@ impl<'a> Parser<'a> {
         &self.sess.span_diagnostic
     }
 
-    /// Replace `self` with `snapshot.parser` and extend `unclosed_delims` with `snapshot.unclosed_delims`.
-    /// This is to avoid losing unclosed delims errors `create_snapshot_for_diagnostic` clears.
+    /// Replace `self` with `snapshot.parser`.
     pub(super) fn restore_snapshot(&mut self, snapshot: SnapshotParser<'a>) {
         *self = snapshot.parser;
-        self.unclosed_delims.extend(snapshot.unclosed_delims);
-    }
-
-    pub fn unclosed_delims(&self) -> &[UnmatchedDelim] {
-        &self.unclosed_delims
     }
 
     /// Create a snapshot of the `Parser`.
     pub fn create_snapshot_for_diagnostic(&self) -> SnapshotParser<'a> {
-        let mut snapshot = self.clone();
-        let unclosed_delims = self.unclosed_delims.clone();
-        // Clear `unclosed_delims` in snapshot to avoid
-        // duplicate errors being emitted when the `Parser`
-        // is dropped (which may or may not happen, depending
-        // if the parsing the snapshot is created for is successful)
-        snapshot.unclosed_delims.clear();
-        SnapshotParser { parser: snapshot, unclosed_delims }
+        let snapshot = self.clone();
+        SnapshotParser { parser: snapshot }
     }
 
     pub(super) fn span_to_snippet(&self, span: Span) -> Result<String, SpanSnippetError> {
         self.sess.source_map().span_to_snippet(span)
     }
 
-    pub(super) fn expected_ident_found(&mut self) -> DiagnosticBuilder<'a, ErrorGuaranteed> {
+    /// Emits an error with suggestions if an identifier was expected but not found.
+    ///
+    /// Returns a possibly recovered identifier.
+    pub(super) fn expected_ident_found(
+        &mut self,
+        recover: bool,
+    ) -> PResult<'a, (Ident, /* is_raw */ bool)> {
+        if let TokenKind::DocComment(..) = self.prev_token.kind {
+            return Err(DocCommentDoesNotDocumentAnything {
+                span: self.prev_token.span,
+                missing_comma: None,
+            }
+            .into_diagnostic(&self.sess.span_diagnostic));
+        }
+
         let valid_follow = &[
             TokenKind::Eq,
             TokenKind::Colon,
@@ -294,38 +294,61 @@ impl<'a> Parser<'a> {
             TokenKind::CloseDelim(Delimiter::Brace),
             TokenKind::CloseDelim(Delimiter::Parenthesis),
         ];
-        let suggest_raw = match self.token.ident() {
-            Some((ident, false))
-                if ident.is_raw_guess()
-                    && self.look_ahead(1, |t| valid_follow.contains(&t.kind)) =>
-            {
-                Some(SuggEscapeToUseAsIdentifier {
-                    span: ident.span.shrink_to_lo(),
-                    // `Symbol::to_string()` is different from `Symbol::into_diagnostic_arg()`,
-                    // which uses `Symbol::to_ident_string()` and "helpfully" adds an implicit `r#`
-                    ident_name: ident.name.to_string(),
-                })
-            }
-            _ => None,
-        };
+
+        let mut recovered_ident = None;
+        // we take this here so that the correct original token is retained in
+        // the diagnostic, regardless of eager recovery.
+        let bad_token = self.token.clone();
+
+        // suggest prepending a keyword in identifier position with `r#`
+        let suggest_raw = if let Some((ident, false)) = self.token.ident()
+            && ident.is_raw_guess()
+            && self.look_ahead(1, |t| valid_follow.contains(&t.kind))
+        {
+            recovered_ident = Some((ident, true));
+
+            // `Symbol::to_string()` is different from `Symbol::into_diagnostic_arg()`,
+            // which uses `Symbol::to_ident_string()` and "helpfully" adds an implicit `r#`
+            let ident_name = ident.name.to_string();
+
+            Some(SuggEscapeIdentifier {
+                span: ident.span.shrink_to_lo(),
+                ident_name
+            })
+        } else { None };
 
         let suggest_remove_comma =
             if self.token == token::Comma && self.look_ahead(1, |t| t.is_ident()) {
-                Some(SuggRemoveComma { span: self.token.span })
+                if recover {
+                    self.bump();
+                    recovered_ident = self.ident_or_err(false).ok();
+                };
+
+                Some(SuggRemoveComma { span: bad_token.span })
             } else {
                 None
             };
 
+        let help_cannot_start_number = self.is_lit_bad_ident().map(|(len, valid_portion)| {
+            let (invalid, valid) = self.token.span.split_at(len as u32);
+
+            recovered_ident = Some((Ident::new(valid_portion, valid), false));
+
+            HelpIdentifierStartsWithNumber { num_span: invalid }
+        });
+
         let err = ExpectedIdentifier {
-            span: self.token.span,
-            token: self.token.clone(),
+            span: bad_token.span,
+            token: bad_token,
             suggest_raw,
             suggest_remove_comma,
+            help_cannot_start_number,
         };
         let mut err = err.into_diagnostic(&self.sess.span_diagnostic);
 
         // if the token we have is a `<`
         // it *might* be a misplaced generic
+        // FIXME: could we recover with this?
         if self.token == token::Lt {
             // all keywords that could have generic applied
             let valid_prev_keywords =
@@ -376,7 +399,38 @@ impl<'a> Parser<'a> {
             }
         }
 
-        err
+        if let Some(recovered_ident) = recovered_ident && recover {
+            err.emit();
+            Ok(recovered_ident)
+        } else {
+            Err(err)
+        }
+    }
+
+    pub(super) fn expected_ident_found_err(&mut self) -> DiagnosticBuilder<'a, ErrorGuaranteed> {
+        self.expected_ident_found(false).unwrap_err()
+    }
+
+    /// Checks if the current token is a integer or float literal and looks like
+    /// it could be a invalid identifier with digits at the start.
+    ///
+    /// Returns the number of characters (bytes) composing the invalid portion
+    /// of the identifier and the valid portion of the identifier.
+    pub(super) fn is_lit_bad_ident(&mut self) -> Option<(usize, Symbol)> {
+        // ensure that the integer literal is followed by a *invalid*
+        // suffix: this is how we know that it is a identifier with an
+        // invalid beginning.
+        if let token::Literal(Lit {
+            kind: token::LitKind::Integer | token::LitKind::Float,
+            symbol,
+            suffix: Some(suffix), // no suffix makes it a valid literal
+        }) = self.token.kind
+            && rustc_ast::MetaItemLit::from_token(&self.token).is_none()
+        {
+            Some((symbol.as_str().len(), suffix))
+        } else {
+            None
+        }
     }
 
     pub(super) fn expected_one_of_not_found(
@@ -579,21 +633,6 @@ impl<'a> Parser<'a> {
         } else {
             label_sp
         };
-        match self.recover_closing_delimiter(
-            &expected
-                .iter()
-                .filter_map(|tt| match tt {
-                    TokenType::Token(t) => Some(t.clone()),
-                    _ => None,
-                })
-                .collect::<Vec<_>>(),
-            err,
-        ) {
-            Err(e) => err = e,
-            Ok(recovered) => {
-                return Ok(recovered);
-            }
-        }
 
         if self.check_too_many_raw_str_terminators(&mut err) {
             if expected.contains(&TokenType::Token(token::Semi)) && self.eat(&token::Semi) {
@@ -950,8 +989,7 @@ impl<'a> Parser<'a> {
                     }
                     if self.token.kind == token::OpenDelim(Delimiter::Parenthesis) {
                         // Recover from bad turbofish: `foo.collect::Vec<_>()`.
-                        let args = AngleBracketedArgs { args, span }.into();
-                        segment.args = args;
+                        segment.args = Some(AngleBracketedArgs { args, span }.into());
 
                         self.sess.emit_err(GenericParamsWithoutAngleBrackets {
                             span,
@@ -1573,12 +1611,6 @@ impl<'a> Parser<'a> {
         );
         let mut err = self.struct_span_err(sp, &msg);
         let label_exp = format!("expected `{token_str}`");
-        match self.recover_closing_delimiter(&[t.clone()], err) {
-            Err(e) => err = e,
-            Ok(recovered) => {
-                return Ok(recovered);
-            }
-        }
         let sm = self.sess.source_map();
         if !sm.is_multiline(prev_sp.until(sp)) {
             // When the spans are in the same line, it means that the only content
@@ -1795,81 +1827,6 @@ impl<'a> Parser<'a> {
         }
     }
 
-    pub(super) fn recover_closing_delimiter(
-        &mut self,
-        tokens: &[TokenKind],
-        mut err: DiagnosticBuilder<'a, ErrorGuaranteed>,
-    ) -> PResult<'a, bool> {
-        let mut pos = None;
-        // We want to use the last closing delim that would apply.
-        for (i, unmatched) in self.unclosed_delims.iter().enumerate().rev() {
-            if tokens.contains(&token::CloseDelim(unmatched.expected_delim))
-                && Some(self.token.span) > unmatched.unclosed_span
-            {
-                pos = Some(i);
-            }
-        }
-        match pos {
-            Some(pos) => {
-                // Recover and assume that the detected unclosed delimiter was meant for
-                // this location. Emit the diagnostic and act as if the delimiter was
-                // present for the parser's sake.
-
-                // Don't attempt to recover from this unclosed delimiter more than once.
-                let unmatched = self.unclosed_delims.remove(pos);
-                let delim = TokenType::Token(token::CloseDelim(unmatched.expected_delim));
-                if unmatched.found_delim.is_none() {
-                    // We encountered `Eof`, set this fact here to avoid complaining about missing
-                    // `fn main()` when we found place to suggest the closing brace.
-                    *self.sess.reached_eof.borrow_mut() = true;
-                }
-
-                // We want to suggest the inclusion of the closing delimiter where it makes
-                // the most sense, which is immediately after the last token:
-                //
-                //  {foo(bar {}}
-                //      ^      ^
-                //      |      |
-                //      |      help: `)` may belong here
-                //      |
-                //      unclosed delimiter
-                if let Some(sp) = unmatched.unclosed_span {
-                    let mut primary_span: Vec<Span> =
-                        err.span.primary_spans().iter().cloned().collect();
-                    primary_span.push(sp);
-                    let mut primary_span: MultiSpan = primary_span.into();
-                    for span_label in err.span.span_labels() {
-                        if let Some(label) = span_label.label {
-                            primary_span.push_span_label(span_label.span, label);
-                        }
-                    }
-                    err.set_span(primary_span);
-                    err.span_label(sp, "unclosed delimiter");
-                }
-                // Backticks should be removed to apply suggestions.
-                let mut delim = delim.to_string();
-                delim.retain(|c| c != '`');
-                err.span_suggestion_short(
-                    self.prev_token.span.shrink_to_hi(),
-                    &format!("`{delim}` may belong here"),
-                    delim,
-                    Applicability::MaybeIncorrect,
-                );
-                if unmatched.found_delim.is_none() {
-                    // Encountered `Eof` when lexing blocks. Do not recover here to avoid knockdown
-                    // errors which would be emitted elsewhere in the parser and let other error
-                    // recovery consume the rest of the file.
-                    Err(err)
-                } else {
-                    err.emit();
-                    self.expected_tokens.clear(); // Reduce the number of errors.
-                    Ok(true)
-                }
-            }
-            _ => Err(err),
-        }
-    }
-
     /// Eats tokens until we can be relatively sure we reached the end of the
     /// statement. This is something of a best-effort heuristic.
     ///
diff --git a/compiler/rustc_parse/src/parser/expr.rs b/compiler/rustc_parse/src/parser/expr.rs
index 95a7ca80d..03c82fbd3 100644
--- a/compiler/rustc_parse/src/parser/expr.rs
+++ b/compiler/rustc_parse/src/parser/expr.rs
@@ -8,6 +8,7 @@ use super::{
 
 use crate::errors;
 use crate::maybe_recover_from_interpolated_ty_qpath;
+use ast::{Path, PathSegment};
 use core::mem;
 use rustc_ast::ptr::P;
 use rustc_ast::token::{self, Delimiter, Token, TokenKind};
@@ -29,6 +30,7 @@ use rustc_session::errors::{report_lit_error, ExprParenthesesNeeded};
 use rustc_session::lint::builtin::BREAK_WITH_LABEL_AND_LOOP;
 use rustc_session::lint::BuiltinLintDiagnostics;
 use rustc_span::source_map::{self, Span, Spanned};
+use rustc_span::symbol::kw::PathRoot;
 use rustc_span::symbol::{kw, sym, Ident, Symbol};
 use rustc_span::{BytePos, Pos};
 use thin_vec::{thin_vec, ThinVec};
@@ -636,11 +638,27 @@ impl<'a> Parser<'a> {
         self.parse_expr_unary(lo, UnOp::Not)
     }
 
-    /// Parse `box expr`.
+    /// Parse `box expr` - this syntax has been removed, but we still parse this
+    /// for now to provide an automated way to fix usages of it
     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)))
+        let code = self.sess.source_map().span_to_snippet(span.with_lo(lo.hi())).unwrap();
+        self.sess.emit_err(errors::BoxSyntaxRemoved { span, code: code.trim() });
+        // So typechecking works, parse `box <expr>` as `::std::boxed::Box::new(expr)`
+        let path = Path {
+            span,
+            segments: [
+                PathSegment::from_ident(Ident::with_dummy_span(PathRoot)),
+                PathSegment::from_ident(Ident::with_dummy_span(sym::std)),
+                PathSegment::from_ident(Ident::from_str("boxed")),
+                PathSegment::from_ident(Ident::from_str("Box")),
+                PathSegment::from_ident(Ident::with_dummy_span(sym::new)),
+            ]
+            .into(),
+            tokens: None,
+        };
+        let path = self.mk_expr(span, ExprKind::Path(None, path));
+        Ok((span, self.mk_call(path, ThinVec::from([expr]))))
     }
 
     fn is_mistaken_not_ident_negation(&self) -> bool {
@@ -1394,19 +1412,6 @@ impl<'a> Parser<'a> {
             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) {
-            // Don't complain about bare semicolons after unclosed braces
-            // recovery in order to keep the error count down. Fixing the
-            // delimiters will possibly also fix the bare semicolon found in
-            // expression context. For example, silence the following error:
-            //
-            //     error: expected expression, found `;`
-            //      --> file.rs:2:13
-            //       |
-            //     2 |     foo(bar(;
-            //       |             ^ expected expression
-            self.bump();
-            Ok(self.mk_expr_err(self.token.span))
         } else if self.token.uninterpolated_span().rust_2018() {
             // `Span::rust_2018()` is somewhat expensive; don't get it repeatedly.
             if self.check_keyword(kw::Async) {
@@ -1838,20 +1843,14 @@ impl<'a> Parser<'a> {
         &mut self,
         mk_lit_char: impl FnOnce(Symbol, Span) -> L,
     ) -> PResult<'a, L> {
-        if let token::Interpolated(inner) = &self.token.kind {
-            let expr = match inner.as_ref() {
-                token::NtExpr(expr) => Some(expr),
-                token::NtLiteral(expr) => Some(expr),
-                _ => None,
-            };
-            if let Some(expr) = expr {
-                if matches!(expr.kind, ExprKind::Err) {
-                    let mut err = errors::InvalidInterpolatedExpression { span: self.token.span }
-                        .into_diagnostic(&self.sess.span_diagnostic);
-                    err.downgrade_to_delayed_bug();
-                    return Err(err);
-                }
-            }
+        if let token::Interpolated(nt) = &self.token.kind
+            && let token::NtExpr(e) | token::NtLiteral(e) = &**nt
+            && matches!(e.kind, ExprKind::Err)
+        {
+            let mut err = errors::InvalidInterpolatedExpression { span: self.token.span }
+                .into_diagnostic(&self.sess.span_diagnostic);
+            err.downgrade_to_delayed_bug();
+            return Err(err);
         }
         let token = self.token.clone();
         let err = |self_: &Self| {
@@ -2118,7 +2117,7 @@ impl<'a> Parser<'a> {
             ClosureBinder::NotPresent
         };
 
-        let constness = self.parse_closure_constness(Case::Sensitive);
+        let constness = self.parse_closure_constness();
 
         let movability =
             if self.eat_keyword(kw::Static) { Movability::Static } else { Movability::Movable };
@@ -2768,7 +2767,7 @@ impl<'a> Parser<'a> {
                     (token::DotDotEq, token::Gt)
                 ) {
                     // `error_inclusive_range_match_arrow` handles cases like `0..=> {}`,
-                    // so we supress the error here
+                    // so we suppress the error here
                     err.delay_as_bug();
                     this.bump();
                 } else {
@@ -2912,7 +2911,7 @@ impl<'a> Parser<'a> {
         self.expect_keyword(kw::Async)?;
         let capture_clause = self.parse_capture_clause()?;
         let (attrs, body) = self.parse_inner_attrs_and_block()?;
-        let kind = ExprKind::Async(capture_clause, DUMMY_NODE_ID, body);
+        let kind = ExprKind::Async(capture_clause, body);
         Ok(self.mk_expr_with_attrs(lo.to(self.prev_token.span), kind, attrs))
     }
 
diff --git a/compiler/rustc_parse/src/parser/generics.rs b/compiler/rustc_parse/src/parser/generics.rs
index 8d0f168e0..f8ef1307c 100644
--- a/compiler/rustc_parse/src/parser/generics.rs
+++ b/compiler/rustc_parse/src/parser/generics.rs
@@ -53,7 +53,7 @@ impl<'a> Parser<'a> {
                 let snapshot = self.create_snapshot_for_diagnostic();
                 match self.parse_ty() {
                     Ok(p) => {
-                        if let TyKind::ImplTrait(_, bounds) = &(*p).kind {
+                        if let TyKind::ImplTrait(_, bounds) = &p.kind {
                             let span = impl_span.to(self.token.span.shrink_to_lo());
                             let mut err = self.struct_span_err(
                                 span,
diff --git a/compiler/rustc_parse/src/parser/item.rs b/compiler/rustc_parse/src/parser/item.rs
index 9d9ae154a..6422b8ac1 100644
--- a/compiler/rustc_parse/src/parser/item.rs
+++ b/compiler/rustc_parse/src/parser/item.rs
@@ -3,6 +3,7 @@ use crate::errors;
 use super::diagnostics::{dummy_arg, ConsumeClosingDelim};
 use super::ty::{AllowPlus, RecoverQPath, RecoverReturnSign};
 use super::{AttrWrapper, FollowedByType, ForceCollect, Parser, PathStyle, TrailingToken};
+use ast::StaticItem;
 use rustc_ast::ast::*;
 use rustc_ast::ptr::P;
 use rustc_ast::token::{self, Delimiter, TokenKind};
@@ -125,16 +126,13 @@ impl<'a> Parser<'a> {
             return Ok(Some(item.into_inner()));
         };
 
-        let mut unclosed_delims = vec![];
         let item =
             self.collect_tokens_trailing_token(attrs, force_collect, |this: &mut Self, attrs| {
                 let item =
                     this.parse_item_common_(attrs, mac_allowed, attrs_allowed, fn_parse_mode);
-                unclosed_delims.append(&mut this.unclosed_delims);
                 Ok((item?, TrailingToken::None))
             })?;
 
-        self.unclosed_delims.append(&mut unclosed_delims);
         Ok(item)
     }
 
@@ -230,7 +228,7 @@ impl<'a> Parser<'a> {
             self.bump(); // `static`
             let m = self.parse_mutability();
             let (ident, ty, expr) = self.parse_item_global(Some(m))?;
-            (ident, ItemKind::Static(ty, m, expr))
+            (ident, ItemKind::Static(Box::new(StaticItem { ty, mutability: m, expr })))
         } else if let Const::Yes(const_span) = self.parse_constness(Case::Sensitive) {
             // CONST ITEM
             if self.token.is_keyword(kw::Impl) {
@@ -239,7 +237,7 @@ impl<'a> Parser<'a> {
             } else {
                 self.recover_const_mut(const_span);
                 let (ident, ty, expr) = self.parse_item_global(None)?;
-                (ident, ItemKind::Const(def_(), ty, expr))
+                (ident, ItemKind::Const(Box::new(ConstItem { defaultness: def_(), ty, expr })))
             }
         } else if self.check_keyword(kw::Trait) || self.check_auto_or_unsafe_trait_item() {
             // TRAIT ITEM
@@ -865,9 +863,13 @@ impl<'a> Parser<'a> {
                 let kind = match AssocItemKind::try_from(kind) {
                     Ok(kind) => kind,
                     Err(kind) => match kind {
-                        ItemKind::Static(a, _, b) => {
+                        ItemKind::Static(box StaticItem { ty, mutability: _, expr }) => {
                             self.sess.emit_err(errors::AssociatedStaticItemNotAllowed { span });
-                            AssocItemKind::Const(Defaultness::Final, a, b)
+                            AssocItemKind::Const(Box::new(ConstItem {
+                                defaultness: Defaultness::Final,
+                                ty,
+                                expr,
+                            }))
                         }
                         _ => return self.error_bad_item_kind(span, &kind, "`trait`s or `impl`s"),
                     },
@@ -1117,12 +1119,12 @@ impl<'a> Parser<'a> {
                 let kind = match ForeignItemKind::try_from(kind) {
                     Ok(kind) => kind,
                     Err(kind) => match kind {
-                        ItemKind::Const(_, a, b) => {
+                        ItemKind::Const(box ConstItem { ty, expr, .. }) => {
                             self.sess.emit_err(errors::ExternItemCannotBeConst {
                                 ident_span: ident.span,
                                 const_span: span.with_hi(ident.span.lo()),
                             });
-                            ForeignItemKind::Static(a, Mutability::Not, b)
+                            ForeignItemKind::Static(ty, Mutability::Not, expr)
                         }
                         _ => return self.error_bad_item_kind(span, &kind, "`extern` blocks"),
                     },
@@ -1184,7 +1186,7 @@ impl<'a> Parser<'a> {
         defaultness: Defaultness,
     ) -> PResult<'a, ItemInfo> {
         let impl_span = self.token.span;
-        let mut err = self.expected_ident_found();
+        let mut err = self.expected_ident_found_err();
 
         // Only try to recover if this is implementing a trait for a type
         let mut impl_info = match self.parse_item_impl(attrs, defaultness) {
@@ -1747,7 +1749,7 @@ impl<'a> Parser<'a> {
     /// Parses a field identifier. Specialized version of `parse_ident_common`
     /// for better diagnostics and suggestions.
     fn parse_field_ident(&mut self, adt_ty: &str, lo: Span) -> PResult<'a, Ident> {
-        let (ident, is_raw) = self.ident_or_err()?;
+        let (ident, is_raw) = self.ident_or_err(true)?;
         if !is_raw && ident.is_reserved() {
             let snapshot = self.create_snapshot_for_diagnostic();
             let err = if self.check_fn_front_matter(false, Case::Sensitive) {
@@ -1779,7 +1781,7 @@ impl<'a> Parser<'a> {
                     Err(err) => {
                         err.cancel();
                         self.restore_snapshot(snapshot);
-                        self.expected_ident_found()
+                        self.expected_ident_found_err()
                     }
                 }
             } else if self.eat_keyword(kw::Struct) {
@@ -1795,11 +1797,11 @@ impl<'a> Parser<'a> {
                     Err(err) => {
                         err.cancel();
                         self.restore_snapshot(snapshot);
-                        self.expected_ident_found()
+                        self.expected_ident_found_err()
                     }
                 }
             } else {
-                let mut err = self.expected_ident_found();
+                let mut err = self.expected_ident_found_err();
                 if self.eat_keyword_noexpect(kw::Let)
                     && let removal_span = self.prev_token.span.until(self.token.span)
                     && let Ok(ident) = self.parse_ident_common(false)
@@ -1960,21 +1962,12 @@ impl<'a> Parser<'a> {
         // FIXME: This will make us not emit the help even for declarative
         // macros within the same crate (that we can fix), which is sad.
         if !span.from_expansion() {
-            if self.unclosed_delims.is_empty() {
-                let DelimSpan { open, close } = args.dspan;
-                err.multipart_suggestion(
-                    "change the delimiters to curly braces",
-                    vec![(open, "{".to_string()), (close, '}'.to_string())],
-                    Applicability::MaybeIncorrect,
-                );
-            } else {
-                err.span_suggestion(
-                    span,
-                    "change the delimiters to curly braces",
-                    " { /* items */ }",
-                    Applicability::HasPlaceholders,
-                );
-            }
+            let DelimSpan { open, close } = args.dspan;
+            err.multipart_suggestion(
+                "change the delimiters to curly braces",
+                vec![(open, "{".to_string()), (close, '}'.to_string())],
+                Applicability::MaybeIncorrect,
+            );
             err.span_suggestion(
                 span.shrink_to_hi(),
                 "add a semicolon",
diff --git a/compiler/rustc_parse/src/parser/mod.rs b/compiler/rustc_parse/src/parser/mod.rs
index da82e4724..aa57b8047 100644
--- a/compiler/rustc_parse/src/parser/mod.rs
+++ b/compiler/rustc_parse/src/parser/mod.rs
@@ -29,6 +29,7 @@ use rustc_ast::{Async, AttrArgs, AttrArgsEq, Expr, ExprKind, MacDelimiter, Mutab
 use rustc_ast::{HasAttrs, HasTokens, Unsafe, Visibility, VisibilityKind};
 use rustc_ast_pretty::pprust;
 use rustc_data_structures::fx::FxHashMap;
+use rustc_data_structures::sync::Ordering;
 use rustc_errors::PResult;
 use rustc_errors::{
     Applicability, DiagnosticBuilder, ErrorGuaranteed, FatalError, IntoDiagnostic, MultiSpan,
@@ -42,8 +43,7 @@ use thin_vec::ThinVec;
 use tracing::debug;
 
 use crate::errors::{
-    DocCommentDoesNotDocumentAnything, IncorrectVisibilityRestriction, MismatchedClosingDelimiter,
-    NonStringAbiLiteral,
+    IncorrectVisibilityRestriction, MismatchedClosingDelimiter, NonStringAbiLiteral,
 };
 
 bitflags::bitflags! {
@@ -146,10 +146,7 @@ pub struct Parser<'a> {
     /// See the comments in the `parse_path_segment` function for more details.
     unmatched_angle_bracket_count: u32,
     max_angle_bracket_count: u32,
-    /// 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<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 +165,7 @@ 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<'_>, 312);
+rustc_data_structures::static_assert_size!(Parser<'_>, 288);
 
 /// Stores span information about a closure.
 #[derive(Clone)]
@@ -215,12 +212,6 @@ struct CaptureState {
     inner_attr_ranges: FxHashMap<AttrId, ReplaceRange>,
 }
 
-impl<'a> Drop for Parser<'a> {
-    fn drop(&mut self) {
-        emit_unclosed_delims(&mut self.unclosed_delims, &self.sess);
-    }
-}
-
 /// 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
@@ -478,7 +469,6 @@ impl<'a> Parser<'a> {
             desugar_doc_comments,
             unmatched_angle_bracket_count: 0,
             max_angle_bracket_count: 0,
-            unclosed_delims: Vec::new(),
             last_unexpected_token_span: None,
             last_type_ascription: None,
             subparser_name,
@@ -562,21 +552,11 @@ impl<'a> Parser<'a> {
         self.parse_ident_common(true)
     }
 
-    fn ident_or_err(&mut self) -> PResult<'a, (Ident, /* is_raw */ bool)> {
-        self.token.ident().ok_or_else(|| match self.prev_token.kind {
-            TokenKind::DocComment(..) => DocCommentDoesNotDocumentAnything {
-                span: self.prev_token.span,
-                missing_comma: None,
-            }
-            .into_diagnostic(&self.sess.span_diagnostic),
-            _ => self.expected_ident_found(),
-        })
-    }
-
     fn parse_ident_common(&mut self, recover: bool) -> PResult<'a, Ident> {
-        let (ident, is_raw) = self.ident_or_err()?;
+        let (ident, is_raw) = self.ident_or_err(recover)?;
+
         if !is_raw && ident.is_reserved() {
-            let mut err = self.expected_ident_found();
+            let mut err = self.expected_ident_found_err();
             if recover {
                 err.emit();
             } else {
@@ -587,6 +567,21 @@ impl<'a> Parser<'a> {
         Ok(ident)
     }
 
+    fn ident_or_err(&mut self, recover: bool) -> PResult<'a, (Ident, /* is_raw */ bool)> {
+        let result = self.token.ident().ok_or_else(|| self.expected_ident_found(recover));
+
+        let (ident, is_raw) = match result {
+            Ok(ident) => ident,
+            Err(err) => match err {
+                // we recovered!
+                Ok(ident) => ident,
+                Err(err) => return Err(err),
+            },
+        };
+
+        Ok((ident, is_raw))
+    }
+
     /// Checks if the next token is `tok`, and returns `true` if so.
     ///
     /// This method will automatically add `tok` to `expected_tokens` if `tok` is not
@@ -859,7 +854,6 @@ impl<'a> Parser<'a> {
         let mut recovered = false;
         let mut trailing = false;
         let mut v = ThinVec::new();
-        let unclosed_delims = !self.unclosed_delims.is_empty();
 
         while !self.expect_any_with_type(kets, expect) {
             if let token::CloseDelim(..) | token::Eof = self.token.kind {
@@ -901,7 +895,7 @@ impl<'a> Parser<'a> {
                                 _ => {
                                     // Attempt to keep parsing if it was a similar separator.
                                     if let Some(tokens) = t.similar_tokens() {
-                                        if tokens.contains(&self.token.kind) && !unclosed_delims {
+                                        if tokens.contains(&self.token.kind) {
                                             self.bump();
                                         }
                                     }
@@ -1207,9 +1201,13 @@ impl<'a> Parser<'a> {
         self.parse_constness_(case, false)
     }
 
-    /// Parses constness for closures
-    fn parse_closure_constness(&mut self, case: Case) -> Const {
-        self.parse_constness_(case, true)
+    /// Parses constness for closures (case sensitive, feature-gated)
+    fn parse_closure_constness(&mut self) -> Const {
+        let constness = self.parse_constness_(Case::Sensitive, true);
+        if let Const::Yes(span) = constness {
+            self.sess.gated_spans.gate(sym::const_closures, span);
+        }
+        constness
     }
 
     fn parse_constness_(&mut self, case: Case, is_closure: bool) -> Const {
@@ -1543,8 +1541,10 @@ pub(crate) fn make_unclosed_delims_error(
 }
 
 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());
+    let _ = sess.reached_eof.fetch_or(
+        unclosed_delims.iter().any(|unmatched_delim| unmatched_delim.found_delim.is_none()),
+        Ordering::Relaxed,
+    );
     for unmatched in unclosed_delims.drain(..) {
         if let Some(mut e) = make_unclosed_delims_error(unmatched, sess) {
             e.emit();
diff --git a/compiler/rustc_parse/src/parser/pat.rs b/compiler/rustc_parse/src/parser/pat.rs
index 8e920f1c4..2246002f5 100644
--- a/compiler/rustc_parse/src/parser/pat.rs
+++ b/compiler/rustc_parse/src/parser/pat.rs
@@ -391,7 +391,13 @@ impl<'a> Parser<'a> {
             } else {
                 PatKind::Lit(const_expr)
             }
-        } else if self.can_be_ident_pat() {
+        // Don't eagerly error on semantically invalid tokens when matching
+        // declarative macros, as the input to those doesn't have to be
+        // semantically valid. For attribute/derive proc macros this is not the
+        // case, so doing the recovery for them is fine.
+        } else if self.can_be_ident_pat()
+            || (self.is_lit_bad_ident().is_some() && self.may_recover())
+        {
             // Parse `ident @ pat`
             // This can give false positives and parse nullary enums,
             // they are dealt with later in resolve.
@@ -590,7 +596,7 @@ impl<'a> Parser<'a> {
         // Make sure we don't allow e.g. `let mut $p;` where `$p:pat`.
         if let token::Interpolated(nt) = &self.token.kind {
             if let token::NtPat(_) = **nt {
-                self.expected_ident_found().emit();
+                self.expected_ident_found_err().emit();
             }
         }
 
diff --git a/compiler/rustc_parse/src/parser/path.rs b/compiler/rustc_parse/src/parser/path.rs
index b50d2984a..c25c23d84 100644
--- a/compiler/rustc_parse/src/parser/path.rs
+++ b/compiler/rustc_parse/src/parser/path.rs
@@ -1,6 +1,6 @@
 use super::ty::{AllowPlus, RecoverQPath, RecoverReturnSign};
 use super::{Parser, Restrictions, TokenType};
-use crate::maybe_whole;
+use crate::{errors, maybe_whole};
 use rustc_ast::ptr::P;
 use rustc_ast::token::{self, Delimiter, Token, TokenKind};
 use rustc_ast::{
@@ -290,6 +290,32 @@ impl<'a> Parser<'a> {
                     })?;
                     let span = lo.to(self.prev_token.span);
                     AngleBracketedArgs { args, span }.into()
+                } else if self.may_recover()
+                    && self.token.kind == token::OpenDelim(Delimiter::Parenthesis)
+                    // FIXME(return_type_notation): Could also recover `...` here.
+                    && self.look_ahead(1, |tok| tok.kind == token::DotDot)
+                {
+                    self.bump();
+                    self.sess
+                        .emit_err(errors::BadReturnTypeNotationDotDot { span: self.token.span });
+                    self.bump();
+                    self.expect(&token::CloseDelim(Delimiter::Parenthesis))?;
+                    let span = lo.to(self.prev_token.span);
+
+                    if self.eat_noexpect(&token::RArrow) {
+                        let lo = self.prev_token.span;
+                        let ty = self.parse_ty()?;
+                        self.sess
+                            .emit_err(errors::BadReturnTypeNotationOutput { span: lo.to(ty.span) });
+                    }
+
+                    ParenthesizedArgs {
+                        span,
+                        inputs: ThinVec::new(),
+                        inputs_span: span,
+                        output: ast::FnRetTy::Default(self.prev_token.span.shrink_to_hi()),
+                    }
+                    .into()
                 } else {
                     // `(T, U) -> R`
                     let (inputs, _) = self.parse_paren_comma_seq(|p| p.parse_ty())?;
@@ -300,7 +326,7 @@ impl<'a> Parser<'a> {
                     ParenthesizedArgs { span, inputs, inputs_span, output }.into()
                 };
 
-                PathSegment { ident, args, id: ast::DUMMY_NODE_ID }
+                PathSegment { ident, args: Some(args), id: ast::DUMMY_NODE_ID }
             } else {
                 // Generic arguments are not found.
                 PathSegment::from_ident(ident)
@@ -547,10 +573,16 @@ impl<'a> Parser<'a> {
                     };
 
                     let span = lo.to(self.prev_token.span);
-
                     // Gate associated type bounds, e.g., `Iterator<Item: Ord>`.
                     if let AssocConstraintKind::Bound { .. } = kind {
-                        self.sess.gated_spans.gate(sym::associated_type_bounds, span);
+                        if let Some(ast::GenericArgs::Parenthesized(args)) = &gen_args
+                            && args.inputs.is_empty()
+                            && matches!(args.output, ast::FnRetTy::Default(..))
+                        {
+                            self.sess.gated_spans.gate(sym::return_type_notation, span);
+                        } else {
+                            self.sess.gated_spans.gate(sym::associated_type_bounds, span);
+                        }
                     }
                     let constraint =
                         AssocConstraint { id: ast::DUMMY_NODE_ID, ident, gen_args, kind, span };
diff --git a/compiler/rustc_parse/src/parser/stmt.rs b/compiler/rustc_parse/src/parser/stmt.rs
index 92a22ffc2..fbe5b88c4 100644
--- a/compiler/rustc_parse/src/parser/stmt.rs
+++ b/compiler/rustc_parse/src/parser/stmt.rs
@@ -273,7 +273,6 @@ impl<'a> Parser<'a> {
             self.bump();
         }
 
-        self.report_invalid_identifier_error()?;
         let (pat, colon) =
             self.parse_pat_before_ty(None, RecoverComma::Yes, PatternLocation::LetBinding)?;
 
@@ -366,17 +365,6 @@ impl<'a> Parser<'a> {
         Ok(P(ast::Local { ty, pat, kind, id: DUMMY_NODE_ID, span: lo.to(hi), attrs, tokens: None }))
     }
 
-    /// report error for `let 1x = 123`
-    pub fn report_invalid_identifier_error(&mut self) -> PResult<'a, ()> {
-        if let token::Literal(lit) = self.token.uninterpolate().kind &&
-            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(errors::InvalidIdentiferStartsWithNumber { span: self.token.span }));
-        }
-        Ok(())
-    }
-
     fn check_let_else_init_bool_expr(&self, init: &ast::Expr) {
         if let ast::ExprKind::Binary(op, ..) = init.kind {
             if op.node.lazy() {
diff --git a/compiler/rustc_parse/src/parser/ty.rs b/compiler/rustc_parse/src/parser/ty.rs
index 6fe4da71f..400c8dbe9 100644
--- a/compiler/rustc_parse/src/parser/ty.rs
+++ b/compiler/rustc_parse/src/parser/ty.rs
@@ -624,10 +624,12 @@ impl<'a> Parser<'a> {
     ///
     /// Note that this does *not* parse bare trait objects.
     fn parse_dyn_ty(&mut self, impl_dyn_multi: &mut bool) -> PResult<'a, TyKind> {
+        let lo = self.token.span;
         self.bump(); // `dyn`
 
         // parse dyn* types
         let syntax = if self.eat(&TokenKind::BinOp(token::Star)) {
+            self.sess.gated_spans.gate(sym::dyn_star, lo.to(self.prev_token.span));
             TraitObjectSyntax::DynStar
         } else {
             TraitObjectSyntax::Dyn
@@ -1057,8 +1059,11 @@ impl<'a> Parser<'a> {
             output,
         }
         .into();
-        *fn_path_segment =
-            ast::PathSegment { ident: fn_path_segment.ident, args, id: ast::DUMMY_NODE_ID };
+        *fn_path_segment = ast::PathSegment {
+            ident: fn_path_segment.ident,
+            args: Some(args),
+            id: ast::DUMMY_NODE_ID,
+        };
 
         // Convert parsed `<'a>` in `Fn<'a>` into `for<'a>`.
         let mut generic_params = lifetimes