Merge remote-tracking branch 'origin/master' into frewsxcv-san

10 files changed, 271 insertions, 60 deletions

diff --git a/compiler/rustc_parse/src/parser/diagnostics.rs b/compiler/rustc_parse/src/parser/diagnostics.rs
index 350a372a684..98c7b9a63a5 100644
--- a/compiler/rustc_parse/src/parser/diagnostics.rs
+++ b/compiler/rustc_parse/src/parser/diagnostics.rs
@@ -1912,4 +1912,22 @@ impl<'a> Parser<'a> {
         *self = snapshot;
         Err(err)
     }
+
+    /// Get the diagnostics for the cases where `move async` is found.
+    ///
+    /// `move_async_span` starts at the 'm' of the move keyword and ends with the 'c' of the async keyword
+    pub(super) fn incorrect_move_async_order_found(
+        &self,
+        move_async_span: Span,
+    ) -> DiagnosticBuilder<'a> {
+        let mut err =
+            self.struct_span_err(move_async_span, "the order of `move` and `async` is incorrect");
+        err.span_suggestion_verbose(
+            move_async_span,
+            "try switching the order",
+            "async move".to_owned(),
+            Applicability::MaybeIncorrect,
+        );
+        err
+    }
 }
diff --git a/compiler/rustc_parse/src/parser/expr.rs b/compiler/rustc_parse/src/parser/expr.rs
index 93be478fc8c..b147f42fada 100644
--- a/compiler/rustc_parse/src/parser/expr.rs
+++ b/compiler/rustc_parse/src/parser/expr.rs
@@ -1,5 +1,5 @@
-use super::pat::{GateOr, PARAM_EXPECTED};
-use super::ty::{AllowPlus, RecoverQPath};
+use super::pat::{GateOr, RecoverComma, PARAM_EXPECTED};
+use super::ty::{AllowPlus, RecoverQPath, RecoverReturnSign};
 use super::{BlockMode, Parser, PathStyle, Restrictions, TokenType};
 use super::{SemiColonMode, SeqSep, TokenExpectType};
 use crate::maybe_recover_from_interpolated_ty_qpath;
@@ -1603,7 +1603,7 @@ impl<'a> Parser<'a> {
             self.sess.gated_spans.gate(sym::async_closure, span);
         }
 
-        let capture_clause = self.parse_capture_clause();
+        let capture_clause = self.parse_capture_clause()?;
         let decl = self.parse_fn_block_decl()?;
         let decl_hi = self.prev_token.span;
         let body = match decl.output {
@@ -1626,8 +1626,18 @@ impl<'a> Parser<'a> {
     }
 
     /// Parses an optional `move` prefix to a closure-like construct.
-    fn parse_capture_clause(&mut self) -> CaptureBy {
-        if self.eat_keyword(kw::Move) { CaptureBy::Value } else { CaptureBy::Ref }
+    fn parse_capture_clause(&mut self) -> PResult<'a, CaptureBy> {
+        if self.eat_keyword(kw::Move) {
+            // Check for `move async` and recover
+            if self.check_keyword(kw::Async) {
+                let move_async_span = self.token.span.with_lo(self.prev_token.span.data().lo);
+                Err(self.incorrect_move_async_order_found(move_async_span))
+            } else {
+                Ok(CaptureBy::Value)
+            }
+        } else {
+            Ok(CaptureBy::Ref)
+        }
     }
 
     /// Parses the `|arg, arg|` header of a closure.
@@ -1647,7 +1657,8 @@ impl<'a> Parser<'a> {
             self.expect_or()?;
             args
         };
-        let output = self.parse_ret_ty(AllowPlus::Yes, RecoverQPath::Yes)?;
+        let output =
+            self.parse_ret_ty(AllowPlus::Yes, RecoverQPath::Yes, RecoverReturnSign::Yes)?;
 
         Ok(P(FnDecl { inputs, output }))
     }
@@ -1728,7 +1739,7 @@ impl<'a> Parser<'a> {
     /// The `let` token has already been eaten.
     fn parse_let_expr(&mut self, attrs: AttrVec) -> PResult<'a, P<Expr>> {
         let lo = self.prev_token.span;
-        let pat = self.parse_top_pat(GateOr::No)?;
+        let pat = self.parse_top_pat(GateOr::No, RecoverComma::Yes)?;
         self.expect(&token::Eq)?;
         let expr = self.with_res(self.restrictions | Restrictions::NO_STRUCT_LITERAL, |this| {
             this.parse_assoc_expr_with(1 + prec_let_scrutinee_needs_par(), None.into())
@@ -1791,7 +1802,7 @@ impl<'a> Parser<'a> {
             _ => None,
         };
 
-        let pat = self.parse_top_pat(GateOr::Yes)?;
+        let pat = self.parse_top_pat(GateOr::Yes, RecoverComma::Yes)?;
         if !self.eat_keyword(kw::In) {
             self.error_missing_in_for_loop();
         }
@@ -1901,7 +1912,7 @@ impl<'a> Parser<'a> {
     pub(super) fn parse_arm(&mut self) -> PResult<'a, Arm> {
         let attrs = self.parse_outer_attributes()?;
         let lo = self.token.span;
-        let pat = self.parse_top_pat(GateOr::No)?;
+        let pat = self.parse_top_pat(GateOr::No, RecoverComma::Yes)?;
         let guard = if self.eat_keyword(kw::If) {
             let if_span = self.prev_token.span;
             let cond = self.parse_expr()?;
@@ -2018,7 +2029,7 @@ impl<'a> Parser<'a> {
     fn parse_async_block(&mut self, mut attrs: AttrVec) -> PResult<'a, P<Expr>> {
         let lo = self.token.span;
         self.expect_keyword(kw::Async)?;
-        let capture_clause = self.parse_capture_clause();
+        let capture_clause = self.parse_capture_clause()?;
         let (iattrs, body) = self.parse_inner_attrs_and_block()?;
         attrs.extend(iattrs);
         let kind = ExprKind::Async(capture_clause, DUMMY_NODE_ID, body);
diff --git a/compiler/rustc_parse/src/parser/generics.rs b/compiler/rustc_parse/src/parser/generics.rs
index dd99a7587dd..860e63020bb 100644
--- a/compiler/rustc_parse/src/parser/generics.rs
+++ b/compiler/rustc_parse/src/parser/generics.rs
@@ -5,7 +5,7 @@ use rustc_ast::{
     self as ast, Attribute, GenericBounds, GenericParam, GenericParamKind, WhereClause,
 };
 use rustc_errors::PResult;
-use rustc_span::symbol::{kw, sym};
+use rustc_span::symbol::kw;
 
 impl<'a> Parser<'a> {
     /// Parses bounds of a lifetime parameter `BOUND + BOUND + BOUND`, possibly with trailing `+`.
@@ -56,8 +56,6 @@ impl<'a> Parser<'a> {
         self.expect(&token::Colon)?;
         let ty = self.parse_ty()?;
 
-        self.sess.gated_spans.gate(sym::min_const_generics, const_span.to(self.prev_token.span));
-
         Ok(GenericParam {
             ident,
             id: ast::DUMMY_NODE_ID,
@@ -240,7 +238,7 @@ impl<'a> Parser<'a> {
 
         // Parse type with mandatory colon and (possibly empty) bounds,
         // or with mandatory equality sign and the second type.
-        let ty = self.parse_ty()?;
+        let ty = self.parse_ty_for_where_clause()?;
         if self.eat(&token::Colon) {
             let bounds = self.parse_generic_bounds(Some(self.prev_token.span))?;
             Ok(ast::WherePredicate::BoundPredicate(ast::WhereBoundPredicate {
diff --git a/compiler/rustc_parse/src/parser/item.rs b/compiler/rustc_parse/src/parser/item.rs
index 5954b370e6d..c6669f04682 100644
--- a/compiler/rustc_parse/src/parser/item.rs
+++ b/compiler/rustc_parse/src/parser/item.rs
@@ -1,5 +1,5 @@
 use super::diagnostics::{dummy_arg, ConsumeClosingDelim, Error};
-use super::ty::{AllowPlus, RecoverQPath};
+use super::ty::{AllowPlus, RecoverQPath, RecoverReturnSign};
 use super::{FollowedByType, Parser, PathStyle};
 
 use crate::maybe_whole;
@@ -247,9 +247,14 @@ impl<'a> Parser<'a> {
             (ident, ItemKind::Static(ty, m, expr))
         } else if let Const::Yes(const_span) = self.parse_constness() {
             // CONST ITEM
-            self.recover_const_mut(const_span);
-            let (ident, ty, expr) = self.parse_item_global(None)?;
-            (ident, ItemKind::Const(def(), ty, expr))
+            if self.token.is_keyword(kw::Impl) {
+                // recover from `const impl`, suggest `impl const`
+                self.recover_const_impl(const_span, attrs, def())?
+            } else {
+                self.recover_const_mut(const_span);
+                let (ident, ty, expr) = self.parse_item_global(None)?;
+                (ident, ItemKind::Const(def(), ty, expr))
+            }
         } else if self.check_keyword(kw::Trait) || self.check_auto_or_unsafe_trait_item() {
             // TRAIT ITEM
             self.parse_item_trait(attrs, lo)?
@@ -489,7 +494,7 @@ impl<'a> Parser<'a> {
         let polarity = self.parse_polarity();
 
         // Parse both types and traits as a type, then reinterpret if necessary.
-        let err_path = |span| ast::Path::from_ident(Ident::new(kw::Invalid, span));
+        let err_path = |span| ast::Path::from_ident(Ident::new(kw::Empty, span));
         let ty_first = if self.token.is_keyword(kw::For) && self.look_ahead(1, |t| t != &token::Lt)
         {
             let span = self.prev_token.span.between(self.token.span);
@@ -988,6 +993,36 @@ impl<'a> Parser<'a> {
         }
     }
 
+    /// Recover on `const impl` with `const` already eaten.
+    fn recover_const_impl(
+        &mut self,
+        const_span: Span,
+        attrs: &mut Vec<Attribute>,
+        defaultness: Defaultness,
+    ) -> PResult<'a, ItemInfo> {
+        let impl_span = self.token.span;
+        let mut err = self.expected_ident_found();
+        let mut impl_info = self.parse_item_impl(attrs, defaultness)?;
+        match impl_info.1 {
+            // only try to recover if this is implementing a trait for a type
+            ItemKind::Impl { of_trait: Some(ref trai), ref mut constness, .. } => {
+                *constness = Const::Yes(const_span);
+
+                let before_trait = trai.path.span.shrink_to_lo();
+                let const_up_to_impl = const_span.with_hi(impl_span.lo());
+                err.multipart_suggestion(
+                    "you might have meant to write a const trait impl",
+                    vec![(const_up_to_impl, "".to_owned()), (before_trait, "const ".to_owned())],
+                    Applicability::MaybeIncorrect,
+                )
+                .emit();
+            }
+            ItemKind::Impl { .. } => return Err(err),
+            _ => unreachable!(),
+        }
+        Ok(impl_info)
+    }
+
     /// Parse `["const" | ("static" "mut"?)] $ident ":" $ty (= $expr)?` with
     /// `["const" | ("static" "mut"?)]` already parsed and stored in `m`.
     ///
@@ -1514,7 +1549,7 @@ impl<'a> Parser<'a> {
         let header = self.parse_fn_front_matter()?; // `const ... fn`
         let ident = self.parse_ident()?; // `foo`
         let mut generics = self.parse_generics()?; // `<'a, T, ...>`
-        let decl = self.parse_fn_decl(req_name, AllowPlus::Yes)?; // `(p: u8, ...)`
+        let decl = self.parse_fn_decl(req_name, AllowPlus::Yes, RecoverReturnSign::Yes)?; // `(p: u8, ...)`
         generics.where_clause = self.parse_where_clause()?; // `where T: Ord`
 
         let mut sig_hi = self.prev_token.span;
@@ -1645,10 +1680,11 @@ impl<'a> Parser<'a> {
         &mut self,
         req_name: ReqName,
         ret_allow_plus: AllowPlus,
+        recover_return_sign: RecoverReturnSign,
     ) -> PResult<'a, P<FnDecl>> {
         Ok(P(FnDecl {
             inputs: self.parse_fn_params(req_name)?,
-            output: self.parse_ret_ty(ret_allow_plus, RecoverQPath::Yes)?,
+            output: self.parse_ret_ty(ret_allow_plus, RecoverQPath::Yes, recover_return_sign)?,
         }))
     }
 
@@ -1663,7 +1699,7 @@ impl<'a> Parser<'a> {
                 // Skip every token until next possible arg or end.
                 p.eat_to_tokens(&[&token::Comma, &token::CloseDelim(token::Paren)]);
                 // Create a placeholder argument for proper arg count (issue #34264).
-                Ok(dummy_arg(Ident::new(kw::Invalid, lo.to(p.prev_token.span))))
+                Ok(dummy_arg(Ident::new(kw::Empty, lo.to(p.prev_token.span))))
             });
             // ...now that we've parsed the first argument, `self` is no longer allowed.
             first_param = false;
@@ -1723,7 +1759,7 @@ impl<'a> Parser<'a> {
             }
             match ty {
                 Ok(ty) => {
-                    let ident = Ident::new(kw::Invalid, self.prev_token.span);
+                    let ident = Ident::new(kw::Empty, self.prev_token.span);
                     let bm = BindingMode::ByValue(Mutability::Not);
                     let pat = self.mk_pat_ident(ty.span, bm, ident);
                     (pat, ty)
diff --git a/compiler/rustc_parse/src/parser/mod.rs b/compiler/rustc_parse/src/parser/mod.rs
index df4695b18e7..073e62c41d3 100644
--- a/compiler/rustc_parse/src/parser/mod.rs
+++ b/compiler/rustc_parse/src/parser/mod.rs
@@ -17,7 +17,7 @@ pub use path::PathStyle;
 use rustc_ast::ptr::P;
 use rustc_ast::token::{self, DelimToken, Token, TokenKind};
 use rustc_ast::tokenstream::{self, DelimSpan, LazyTokenStream, Spacing};
-use rustc_ast::tokenstream::{CreateTokenStream, TokenStream, TokenTree};
+use rustc_ast::tokenstream::{CreateTokenStream, TokenStream, TokenTree, TreeAndSpacing};
 use rustc_ast::DUMMY_NODE_ID;
 use rustc_ast::{self as ast, AnonConst, AttrStyle, AttrVec, Const, CrateSugar, Extern, Unsafe};
 use rustc_ast::{Async, Expr, ExprKind, MacArgs, MacDelimiter, Mutability, StrLit};
@@ -132,6 +132,28 @@ struct TokenCursor {
     // Counts the number of calls to `next` or `next_desugared`,
     // depending on whether `desugar_doc_comments` is set.
     num_next_calls: usize,
+    // During parsing, we may sometimes need to 'unglue' a
+    // glued token into two component tokens
+    // (e.g. '>>' into '>' and '>), so that the parser
+    // can consume them one at a time. This process
+    // bypasses the normal capturing mechanism
+    // (e.g. `num_next_calls` will not be incremented),
+    // since the 'unglued' tokens due not exist in
+    // the original `TokenStream`.
+    //
+    // If we end up consuming both unglued tokens,
+    // then this is not an issue - we'll end up
+    // capturing the single 'glued' token.
+    //
+    // However, in certain circumstances, we may
+    // want to capture just the first 'unglued' token.
+    // For example, capturing the `Vec<u8>`
+    // in `Option<Vec<u8>>` requires us to unglue
+    // the trailing `>>` token. The `append_unglued_token`
+    // field is used to track this token - it gets
+    // appended to the captured stream when
+    // we evaluate a `LazyTokenStream`
+    append_unglued_token: Option<TreeAndSpacing>,
 }
 
 #[derive(Clone)]
@@ -336,6 +358,7 @@ impl<'a> Parser<'a> {
                 stack: Vec::new(),
                 num_next_calls: 0,
                 desugar_doc_comments,
+                append_unglued_token: None,
             },
             desugar_doc_comments,
             unmatched_angle_bracket_count: 0,
@@ -359,6 +382,10 @@ impl<'a> Parser<'a> {
             self.token_cursor.next()
         };
         self.token_cursor.num_next_calls += 1;
+        // We've retrieved an token from the underlying
+        // cursor, so we no longer need to worry about
+        // an unglued token. See `break_and_eat` for more details
+        self.token_cursor.append_unglued_token = None;
         if next.span.is_dummy() {
             // Tweak the location for better diagnostics, but keep syntactic context intact.
             next.span = fallback_span.with_ctxt(next.span.ctxt());
@@ -555,6 +582,14 @@ impl<'a> Parser<'a> {
                 let first_span = self.sess.source_map().start_point(self.token.span);
                 let second_span = self.token.span.with_lo(first_span.hi());
                 self.token = Token::new(first, first_span);
+                // Keep track of this token - if we end token capturing now,
+                // we'll want to append this token to the captured stream.
+                //
+                // If we consume any additional tokens, then this token
+                // is not needed (we'll capture the entire 'glued' token),
+                // and `next_tok` will set this field to `None`
+                self.token_cursor.append_unglued_token =
+                    Some((TokenTree::Token(self.token.clone()), Spacing::Alone));
                 // Use the spacing of the glued token as the spacing
                 // of the unglued second token.
                 self.bump_with((Token::new(second, second_span), self.token_spacing));
@@ -685,13 +720,9 @@ impl<'a> Parser<'a> {
                                 Ok(t) => {
                                     // Parsed successfully, therefore most probably the code only
                                     // misses a separator.
-                                    let mut exp_span = self.sess.source_map().next_point(sp);
-                                    if self.sess.source_map().is_multiline(exp_span) {
-                                        exp_span = sp;
-                                    }
                                     expect_err
                                         .span_suggestion_short(
-                                            exp_span,
+                                            sp,
                                             &format!("missing `{}`", token_str),
                                             token_str,
                                             Applicability::MaybeIncorrect,
@@ -1230,6 +1261,7 @@ impl<'a> Parser<'a> {
             num_calls: usize,
             desugar_doc_comments: bool,
             trailing_semi: bool,
+            append_unglued_token: Option<TreeAndSpacing>,
         }
         impl CreateTokenStream for LazyTokenStreamImpl {
             fn create_token_stream(&self) -> TokenStream {
@@ -1253,12 +1285,18 @@ impl<'a> Parser<'a> {
                     }))
                     .take(num_calls);
 
-                make_token_stream(tokens)
+                make_token_stream(tokens, self.append_unglued_token.clone())
             }
             fn add_trailing_semi(&self) -> Box<dyn CreateTokenStream> {
                 if self.trailing_semi {
                     panic!("Called `add_trailing_semi` twice!");
                 }
+                if self.append_unglued_token.is_some() {
+                    panic!(
+                        "Cannot call `add_trailing_semi` when we have an unglued token {:?}",
+                        self.append_unglued_token
+                    );
+                }
                 let mut new = self.clone();
                 new.trailing_semi = true;
                 Box::new(new)
@@ -1271,6 +1309,7 @@ impl<'a> Parser<'a> {
             cursor_snapshot,
             desugar_doc_comments: self.desugar_doc_comments,
             trailing_semi: false,
+            append_unglued_token: self.token_cursor.append_unglued_token.clone(),
         };
         Ok((ret, Some(LazyTokenStream::new(lazy_impl))))
     }
@@ -1325,7 +1364,10 @@ pub fn emit_unclosed_delims(unclosed_delims: &mut Vec<UnmatchedBrace>, sess: &Pa
 /// Converts a flattened iterator of tokens (including open and close delimiter tokens)
 /// into a `TokenStream`, creating a `TokenTree::Delimited` for each matching pair
 /// of open and close delims.
-fn make_token_stream(tokens: impl Iterator<Item = (Token, Spacing)>) -> TokenStream {
+fn make_token_stream(
+    tokens: impl Iterator<Item = (Token, Spacing)>,
+    append_unglued_token: Option<TreeAndSpacing>,
+) -> TokenStream {
     #[derive(Debug)]
     struct FrameData {
         open: Span,
@@ -1348,14 +1390,17 @@ fn make_token_stream(tokens: impl Iterator<Item = (Token, Spacing)>) -> TokenStr
                     .inner
                     .push((delimited, Spacing::Alone));
             }
-            token => stack
-                .last_mut()
-                .expect("Bottom token frame is missing!")
-                .inner
-                .push((TokenTree::Token(token), spacing)),
+            token => {
+                stack
+                    .last_mut()
+                    .expect("Bottom token frame is missing!")
+                    .inner
+                    .push((TokenTree::Token(token), spacing));
+            }
         }
     }
-    let final_buf = stack.pop().expect("Missing final buf!");
+    let mut final_buf = stack.pop().expect("Missing final buf!");
+    final_buf.inner.extend(append_unglued_token);
     assert!(stack.is_empty(), "Stack should be empty: final_buf={:?} stack={:?}", final_buf, stack);
     TokenStream::new(final_buf.inner)
 }
diff --git a/compiler/rustc_parse/src/parser/nonterminal.rs b/compiler/rustc_parse/src/parser/nonterminal.rs
index c007f96a798..eb5d7075f00 100644
--- a/compiler/rustc_parse/src/parser/nonterminal.rs
+++ b/compiler/rustc_parse/src/parser/nonterminal.rs
@@ -4,6 +4,7 @@ use rustc_ast_pretty::pprust;
 use rustc_errors::PResult;
 use rustc_span::symbol::{kw, Ident};
 
+use crate::parser::pat::{GateOr, RecoverComma};
 use crate::parser::{FollowedByType, Parser, PathStyle};
 
 impl<'a> Parser<'a> {
@@ -27,6 +28,8 @@ impl<'a> Parser<'a> {
                 token.can_begin_expr()
                 // This exception is here for backwards compatibility.
                 && !token.is_keyword(kw::Let)
+                // This exception is here for backwards compatibility.
+                && !token.is_keyword(kw::Const)
             }
             NonterminalKind::Ty => token.can_begin_type(),
             NonterminalKind::Ident => get_macro_ident(token).is_some(),
@@ -55,7 +58,7 @@ impl<'a> Parser<'a> {
                 },
                 _ => false,
             },
-            NonterminalKind::Pat => match token.kind {
+            NonterminalKind::Pat2018 { .. } | NonterminalKind::Pat2021 { .. } => match token.kind {
                 token::Ident(..) |                  // box, ref, mut, and other identifiers (can stricten)
                 token::OpenDelim(token::Paren) |    // tuple pattern
                 token::OpenDelim(token::Bracket) |  // slice pattern
@@ -68,6 +71,8 @@ impl<'a> Parser<'a> {
                 token::ModSep |                     // path
                 token::Lt |                         // path (UFCS constant)
                 token::BinOp(token::Shl) => true,   // path (double UFCS)
+                // leading vert `|` or-pattern
+                token::BinOp(token::Or) =>  matches!(kind, NonterminalKind::Pat2021 {..}),
                 token::Interpolated(ref nt) => may_be_ident(nt),
                 _ => false,
             },
@@ -84,6 +89,7 @@ impl<'a> Parser<'a> {
         }
     }
 
+    /// Parse a non-terminal (e.g. MBE `:pat` or `:ident`).
     pub fn parse_nonterminal(&mut self, kind: NonterminalKind) -> PResult<'a, Nonterminal> {
         // Any `Nonterminal` which stores its tokens (currently `NtItem` and `NtExpr`)
         // needs to have them force-captured here.
@@ -127,8 +133,14 @@ impl<'a> Parser<'a> {
                     }
                 }
             }
-            NonterminalKind::Pat => {
-                let (mut pat, tokens) = self.collect_tokens(|this| this.parse_pat(None))?;
+            NonterminalKind::Pat2018 { .. } | NonterminalKind::Pat2021 { .. } => {
+                let (mut pat, tokens) = self.collect_tokens(|this| match kind {
+                    NonterminalKind::Pat2018 { .. } => this.parse_pat(None),
+                    NonterminalKind::Pat2021 { .. } => {
+                        this.parse_top_pat(GateOr::Yes, RecoverComma::No)
+                    }
+                    _ => unreachable!(),
+                })?;
                 // We have have eaten an NtPat, which could already have tokens
                 if pat.tokens.is_none() {
                     pat.tokens = tokens;
diff --git a/compiler/rustc_parse/src/parser/pat.rs b/compiler/rustc_parse/src/parser/pat.rs
index b62c7373800..456e32680fe 100644
--- a/compiler/rustc_parse/src/parser/pat.rs
+++ b/compiler/rustc_parse/src/parser/pat.rs
@@ -26,7 +26,7 @@ pub(super) enum GateOr {
 
 /// Whether or not to recover a `,` when parsing or-patterns.
 #[derive(PartialEq, Copy, Clone)]
-enum RecoverComma {
+pub(super) enum RecoverComma {
     Yes,
     No,
 }
@@ -43,13 +43,17 @@ impl<'a> Parser<'a> {
 
     /// Entry point to the main pattern parser.
     /// Corresponds to `top_pat` in RFC 2535 and allows or-pattern at the top level.
-    pub(super) fn parse_top_pat(&mut self, gate_or: GateOr) -> PResult<'a, P<Pat>> {
+    pub(super) fn parse_top_pat(
+        &mut self,
+        gate_or: GateOr,
+        rc: RecoverComma,
+    ) -> PResult<'a, P<Pat>> {
         // Allow a '|' before the pats (RFCs 1925, 2530, and 2535).
         let gated_leading_vert = self.eat_or_separator(None) && gate_or == GateOr::Yes;
         let leading_vert_span = self.prev_token.span;
 
         // Parse the possibly-or-pattern.
-        let pat = self.parse_pat_with_or(None, gate_or, RecoverComma::Yes)?;
+        let pat = self.parse_pat_with_or(None, gate_or, rc)?;
 
         // If we parsed a leading `|` which should be gated,
         // and no other gated or-pattern has been parsed thus far,
diff --git a/compiler/rustc_parse/src/parser/path.rs b/compiler/rustc_parse/src/parser/path.rs
index 17e5bcf7605..60a47ca12b8 100644
--- a/compiler/rustc_parse/src/parser/path.rs
+++ b/compiler/rustc_parse/src/parser/path.rs
@@ -1,4 +1,4 @@
-use super::ty::{AllowPlus, RecoverQPath};
+use super::ty::{AllowPlus, RecoverQPath, RecoverReturnSign};
 use super::{Parser, TokenType};
 use crate::maybe_whole;
 use rustc_ast::ptr::P;
@@ -231,7 +231,8 @@ impl<'a> Parser<'a> {
                     // `(T, U) -> R`
                     let (inputs, _) = self.parse_paren_comma_seq(|p| p.parse_ty())?;
                     let span = ident.span.to(self.prev_token.span);
-                    let output = self.parse_ret_ty(AllowPlus::No, RecoverQPath::No)?;
+                    let output =
+                        self.parse_ret_ty(AllowPlus::No, RecoverQPath::No, RecoverReturnSign::No)?;
                     ParenthesizedArgs { inputs, output, span }.into()
                 };
 
@@ -500,10 +501,9 @@ impl<'a> Parser<'a> {
     pub(super) fn expr_is_valid_const_arg(&self, expr: &P<rustc_ast::Expr>) -> bool {
         match &expr.kind {
             ast::ExprKind::Block(_, _) | ast::ExprKind::Lit(_) => true,
-            ast::ExprKind::Unary(ast::UnOp::Neg, expr) => match &expr.kind {
-                ast::ExprKind::Lit(_) => true,
-                _ => false,
-            },
+            ast::ExprKind::Unary(ast::UnOp::Neg, expr) => {
+                matches!(expr.kind, ast::ExprKind::Lit(_))
+            }
             // We can only resolve single-segment paths at the moment, because multi-segment paths
             // require type-checking: see `visit_generic_arg` in `src/librustc_resolve/late.rs`.
             ast::ExprKind::Path(None, path)
diff --git a/compiler/rustc_parse/src/parser/stmt.rs b/compiler/rustc_parse/src/parser/stmt.rs
index e974556f43a..2942747991a 100644
--- a/compiler/rustc_parse/src/parser/stmt.rs
+++ b/compiler/rustc_parse/src/parser/stmt.rs
@@ -1,7 +1,7 @@
 use super::attr::DEFAULT_INNER_ATTR_FORBIDDEN;
 use super::diagnostics::{AttemptLocalParseRecovery, Error};
 use super::expr::LhsExpr;
-use super::pat::GateOr;
+use super::pat::{GateOr, RecoverComma};
 use super::path::PathStyle;
 use super::{BlockMode, Parser, Restrictions, SemiColonMode};
 use crate::maybe_whole;
@@ -185,7 +185,7 @@ impl<'a> Parser<'a> {
     /// Parses a local variable declaration.
     fn parse_local(&mut self, attrs: AttrVec) -> PResult<'a, P<Local>> {
         let lo = self.prev_token.span;
-        let pat = self.parse_top_pat(GateOr::Yes)?;
+        let pat = self.parse_top_pat(GateOr::Yes, RecoverComma::Yes)?;
 
         let (err, ty) = if self.eat(&token::Colon) {
             // Save the state of the parser before parsing type normally, in case there is a `:`
diff --git a/compiler/rustc_parse/src/parser/ty.rs b/compiler/rustc_parse/src/parser/ty.rs
index 7a6ebca4e15..9553f5d09e8 100644
--- a/compiler/rustc_parse/src/parser/ty.rs
+++ b/compiler/rustc_parse/src/parser/ty.rs
@@ -43,6 +43,37 @@ pub(super) enum RecoverQPath {
     No,
 }
 
+/// Signals whether parsing a type should recover `->`.
+///
+/// More specifically, when parsing a function like:
+/// ```rust
+/// fn foo() => u8 { 0 }
+/// fn bar(): u8 { 0 }
+/// ```
+/// The compiler will try to recover interpreting `foo() => u8` as `foo() -> u8` when calling
+/// `parse_ty` with anything except `RecoverReturnSign::No`, and it will try to recover `bar(): u8`
+/// as `bar() -> u8` when passing `RecoverReturnSign::Yes` to `parse_ty`
+#[derive(Copy, Clone, PartialEq)]
+pub(super) enum RecoverReturnSign {
+    Yes,
+    OnlyFatArrow,
+    No,
+}
+
+impl RecoverReturnSign {
+    /// [RecoverReturnSign::Yes] allows for recovering `fn foo() => u8` and `fn foo(): u8`,
+    /// [RecoverReturnSign::OnlyFatArrow] allows for recovering only `fn foo() => u8` (recovering
+    /// colons can cause problems when parsing where clauses), and
+    /// [RecoverReturnSign::No] doesn't allow for any recovery of the return type arrow
+    fn can_recover(self, token: &TokenKind) -> bool {
+        match self {
+            Self::Yes => matches!(token, token::FatArrow | token::Colon),
+            Self::OnlyFatArrow => matches!(token, token::FatArrow),
+            Self::No => false,
+        }
+    }
+}
+
 // Is `...` (`CVarArgs`) legal at this level of type parsing?
 #[derive(PartialEq)]
 enum AllowCVariadic {
@@ -62,14 +93,24 @@ fn can_continue_type_after_non_fn_ident(t: &Token) -> bool {
 impl<'a> Parser<'a> {
     /// Parses a type.
     pub fn parse_ty(&mut self) -> PResult<'a, P<Ty>> {
-        self.parse_ty_common(AllowPlus::Yes, RecoverQPath::Yes, AllowCVariadic::No)
+        self.parse_ty_common(
+            AllowPlus::Yes,
+            AllowCVariadic::No,
+            RecoverQPath::Yes,
+            RecoverReturnSign::Yes,
+        )
     }
 
     /// Parse a type suitable for a function or function pointer parameter.
     /// The difference from `parse_ty` is that this version allows `...`
     /// (`CVarArgs`) at the top level of the type.
     pub(super) fn parse_ty_for_param(&mut self) -> PResult<'a, P<Ty>> {
-        self.parse_ty_common(AllowPlus::Yes, RecoverQPath::Yes, AllowCVariadic::Yes)
+        self.parse_ty_common(
+            AllowPlus::Yes,
+            AllowCVariadic::Yes,
+            RecoverQPath::Yes,
+            RecoverReturnSign::Yes,
+        )
     }
 
     /// Parses a type in restricted contexts where `+` is not permitted.
@@ -79,7 +120,22 @@ impl<'a> Parser<'a> {
     /// Example 2: `value1 as TYPE + value2`
     ///     `+` is prohibited to avoid interactions with expression grammar.
     pub(super) fn parse_ty_no_plus(&mut self) -> PResult<'a, P<Ty>> {
-        self.parse_ty_common(AllowPlus::No, RecoverQPath::Yes, AllowCVariadic::No)
+        self.parse_ty_common(
+            AllowPlus::No,
+            AllowCVariadic::No,
+            RecoverQPath::Yes,
+            RecoverReturnSign::Yes,
+        )
+    }
+
+    /// Parse a type without recovering `:` as `->` to avoid breaking code such as `where fn() : for<'a>`
+    pub(super) fn parse_ty_for_where_clause(&mut self) -> PResult<'a, P<Ty>> {
+        self.parse_ty_common(
+            AllowPlus::Yes,
+            AllowCVariadic::Yes,
+            RecoverQPath::Yes,
+            RecoverReturnSign::OnlyFatArrow,
+        )
     }
 
     /// Parses an optional return type `[ -> TY ]` in a function declaration.
@@ -87,10 +143,35 @@ impl<'a> Parser<'a> {
         &mut self,
         allow_plus: AllowPlus,
         recover_qpath: RecoverQPath,
+        recover_return_sign: RecoverReturnSign,
     ) -> PResult<'a, FnRetTy> {
         Ok(if self.eat(&token::RArrow) {
             // FIXME(Centril): Can we unconditionally `allow_plus`?
-            let ty = self.parse_ty_common(allow_plus, recover_qpath, AllowCVariadic::No)?;
+            let ty = self.parse_ty_common(
+                allow_plus,
+                AllowCVariadic::No,
+                recover_qpath,
+                recover_return_sign,
+            )?;
+            FnRetTy::Ty(ty)
+        } else if recover_return_sign.can_recover(&self.token.kind) {
+            // Don't `eat` to prevent `=>` from being added as an expected token which isn't
+            // actually expected and could only confuse users
+            self.bump();
+            self.struct_span_err(self.prev_token.span, "return types are denoted using `->`")
+                .span_suggestion_short(
+                    self.prev_token.span,
+                    "use `->` instead",
+                    "->".to_string(),
+                    Applicability::MachineApplicable,
+                )
+                .emit();
+            let ty = self.parse_ty_common(
+                allow_plus,
+                AllowCVariadic::No,
+                recover_qpath,
+                recover_return_sign,
+            )?;
             FnRetTy::Ty(ty)
         } else {
             FnRetTy::Default(self.token.span.shrink_to_lo())
@@ -100,8 +181,9 @@ impl<'a> Parser<'a> {
     fn parse_ty_common(
         &mut self,
         allow_plus: AllowPlus,
-        recover_qpath: RecoverQPath,
         allow_c_variadic: AllowCVariadic,
+        recover_qpath: RecoverQPath,
+        recover_return_sign: RecoverReturnSign,
     ) -> PResult<'a, P<Ty>> {
         let allow_qpath_recovery = recover_qpath == RecoverQPath::Yes;
         maybe_recover_from_interpolated_ty_qpath!(self, allow_qpath_recovery);
@@ -129,14 +211,14 @@ impl<'a> Parser<'a> {
             TyKind::Infer
         } else if self.check_fn_front_matter() {
             // Function pointer type
-            self.parse_ty_bare_fn(lo, Vec::new())?
+            self.parse_ty_bare_fn(lo, Vec::new(), recover_return_sign)?
         } else if self.check_keyword(kw::For) {
             // Function pointer type or bound list (trait object type) starting with a poly-trait.
             //   `for<'lt> [unsafe] [extern "ABI"] fn (&'lt S) -> T`
             //   `for<'lt> Trait1<'lt> + Trait2 + 'a`
             let lifetime_defs = self.parse_late_bound_lifetime_defs()?;
             if self.check_fn_front_matter() {
-                self.parse_ty_bare_fn(lo, lifetime_defs)?
+                self.parse_ty_bare_fn(lo, lifetime_defs, recover_return_sign)?
             } else {
                 let path = self.parse_path(PathStyle::Type)?;
                 let parse_plus = allow_plus == AllowPlus::Yes && self.check_plus();
@@ -338,9 +420,14 @@ impl<'a> Parser<'a> {
     /// Function Style    ABI  Parameter types
     /// ```
     /// We actually parse `FnHeader FnDecl`, but we error on `const` and `async` qualifiers.
-    fn parse_ty_bare_fn(&mut self, lo: Span, params: Vec<GenericParam>) -> PResult<'a, TyKind> {
+    fn parse_ty_bare_fn(
+        &mut self,
+        lo: Span,
+        params: Vec<GenericParam>,
+        recover_return_sign: RecoverReturnSign,
+    ) -> PResult<'a, TyKind> {
         let ast::FnHeader { ext, unsafety, constness, asyncness } = self.parse_fn_front_matter()?;
-        let decl = self.parse_fn_decl(|_| false, AllowPlus::No)?;
+        let decl = self.parse_fn_decl(|_| false, AllowPlus::No, recover_return_sign)?;
         let whole_span = lo.to(self.prev_token.span);
         if let ast::Const::Yes(span) = constness {
             self.error_fn_ptr_bad_qualifier(whole_span, span, "const");