Auto merge of #60683 - Centril:rollup-p05qh5d, r=Centril

Rollup of 8 pull requests

Successful merges:

 - #59348 (Clean up and add tests for slice drop shims)
 - #60188 (Identify when a stmt could have been parsed as an expr)
 - #60234 (std: Derive `Default` for `io::Cursor`)
 - #60618 (Comment ext::tt::transcribe)
 - #60648 (Skip codegen for one UI test with long file path)
 - #60671 (remove unneeded `extern crate`s from build tools)
 - #60675 (Remove the old await! macro)
 - #60676 (Fix async desugaring providing wrong input to procedural macros.)

Failed merges:

r? @ghost

9 files changed, 333 insertions, 86 deletions

diff --git a/src/libsyntax/ext/tt/macro_parser.rs b/src/libsyntax/ext/tt/macro_parser.rs
index ab5823eaca5..084a69f4cda 100644
--- a/src/libsyntax/ext/tt/macro_parser.rs
+++ b/src/libsyntax/ext/tt/macro_parser.rs
@@ -554,7 +554,10 @@ fn inner_parse_loop<'root, 'tt>(
             match item.top_elts.get_tt(idx) {
                 // Need to descend into a sequence
                 TokenTree::Sequence(sp, seq) => {
-                    // Examine the case where there are 0 matches of this sequence
+                    // Examine the case where there are 0 matches of this sequence. We are
+                    // implicitly disallowing OneOrMore from having 0 matches here. Thus, that will
+                    // result in a "no rules expected token" error by virtue of this matcher not
+                    // working.
                     if seq.op == quoted::KleeneOp::ZeroOrMore
                         || seq.op == quoted::KleeneOp::ZeroOrOne
                     {
diff --git a/src/libsyntax/ext/tt/macro_rules.rs b/src/libsyntax/ext/tt/macro_rules.rs
index b1b9d25b3d5..a53cc2fe661 100644
--- a/src/libsyntax/ext/tt/macro_rules.rs
+++ b/src/libsyntax/ext/tt/macro_rules.rs
@@ -151,7 +151,7 @@ fn generic_extension<'cx>(cx: &'cx mut ExtCtxt<'_>,
 
                 let rhs_spans = rhs.iter().map(|t| t.span()).collect::<Vec<_>>();
                 // rhs has holes ( `$id` and `$(...)` that need filled)
-                let mut tts = transcribe(cx, Some(named_matches), rhs);
+                let mut tts = transcribe(cx, &named_matches, rhs);
 
                 // Replace all the tokens for the corresponding positions in the macro, to maintain
                 // proper positions in error reporting, while maintaining the macro_backtrace.
diff --git a/src/libsyntax/ext/tt/quoted.rs b/src/libsyntax/ext/tt/quoted.rs
index b24edb57e52..ed8395f11ad 100644
--- a/src/libsyntax/ext/tt/quoted.rs
+++ b/src/libsyntax/ext/tt/quoted.rs
@@ -73,6 +73,7 @@ pub enum KleeneOp {
     ZeroOrMore,
     /// Kleene plus (`+`) for one or more repetitions
     OneOrMore,
+    /// Kleene optional (`?`) for zero or one reptitions
     ZeroOrOne,
 }
 
diff --git a/src/libsyntax/ext/tt/transcribe.rs b/src/libsyntax/ext/tt/transcribe.rs
index bd2adb5ac13..0cefcf1ce03 100644
--- a/src/libsyntax/ext/tt/transcribe.rs
+++ b/src/libsyntax/ext/tt/transcribe.rs
@@ -1,10 +1,10 @@
 use crate::ast::Ident;
 use crate::ext::base::ExtCtxt;
 use crate::ext::expand::Marker;
-use crate::ext::tt::macro_parser::{NamedMatch, MatchedSeq, MatchedNonterminal};
+use crate::ext::tt::macro_parser::{MatchedNonterminal, MatchedSeq, NamedMatch};
 use crate::ext::tt::quoted;
 use crate::mut_visit::noop_visit_tt;
-use crate::parse::token::{self, Token, NtTT};
+use crate::parse::token::{self, NtTT, Token};
 use crate::tokenstream::{DelimSpan, TokenStream, TokenTree, TreeAndJoint};
 
 use smallvec::{smallvec, SmallVec};
@@ -13,24 +13,16 @@ use syntax_pos::DUMMY_SP;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::sync::Lrc;
 use std::mem;
-use std::ops::Add;
 use std::rc::Rc;
 
-// An iterator over the token trees in a delimited token tree (`{ ... }`) or a sequence (`$(...)`).
+/// An iterator over the token trees in a delimited token tree (`{ ... }`) or a sequence (`$(...)`).
 enum Frame {
-    Delimited {
-        forest: Lrc<quoted::Delimited>,
-        idx: usize,
-        span: DelimSpan,
-    },
-    Sequence {
-        forest: Lrc<quoted::SequenceRepetition>,
-        idx: usize,
-        sep: Option<Token>,
-    },
+    Delimited { forest: Lrc<quoted::Delimited>, idx: usize, span: DelimSpan },
+    Sequence { forest: Lrc<quoted::SequenceRepetition>, idx: usize, sep: Option<Token> },
 }
 
 impl Frame {
+    /// Construct a new frame around the delimited set of tokens.
     fn new(tts: Vec<quoted::TokenTree>) -> Frame {
         let forest = Lrc::new(quoted::Delimited { delim: token::NoDelim, tts: tts });
         Frame::Delimited { forest: forest, idx: 0, span: DelimSpan::dummy() }
@@ -54,84 +46,161 @@ impl Iterator for Frame {
     }
 }
 
-/// This can do Macro-By-Example transcription. On the other hand, if
-/// `src` contains no `TokenTree::{Sequence, MetaVar, MetaVarDecl}`s, `interp` can
-/// (and should) be None.
-pub fn transcribe(cx: &ExtCtxt<'_>,
-                  interp: Option<FxHashMap<Ident, Rc<NamedMatch>>>,
-                  src: Vec<quoted::TokenTree>)
-                  -> TokenStream {
+/// This can do Macro-By-Example transcription.
+/// - `interp` is a map of meta-variables to the tokens (non-terminals) they matched in the
+///   invocation. We are assuming we already know there is a match.
+/// - `src` is the RHS of the MBE, that is, the "example" we are filling in.
+///
+/// For example,
+///
+/// ```rust
+/// macro_rules! foo {
+///     ($id:ident) => { println!("{}", stringify!($id)); }
+/// }
+///
+/// foo!(bar);
+/// ```
+///
+/// `interp` would contain `$id => bar` and `src` would contain `println!("{}", stringify!($id));`.
+///
+/// `transcribe` would return a `TokenStream` containing `println!("{}", stringify!(bar));`.
+///
+/// Along the way, we do some additional error checking.
+pub fn transcribe(
+    cx: &ExtCtxt<'_>,
+    interp: &FxHashMap<Ident, Rc<NamedMatch>>,
+    src: Vec<quoted::TokenTree>,
+) -> TokenStream {
+    // Nothing for us to transcribe...
+    if src.is_empty() {
+        return TokenStream::empty();
+    }
+
+    // We descend into the RHS (`src`), expanding things as we go. This stack contains the things
+    // we have yet to expand/are still expanding. We start the stack off with the whole RHS.
     let mut stack: SmallVec<[Frame; 1]> = smallvec![Frame::new(src)];
-    let interpolations = interp.unwrap_or_else(FxHashMap::default); /* just a convenience */
+
+    // As we descend in the RHS, we will need to be able to match nested sequences of matchers.
+    // `repeats` keeps track of where we are in matching at each level, with the last element being
+    // the most deeply nested sequence. This is used as a stack.
     let mut repeats = Vec::new();
+
+    // `result` contains resulting token stream from the TokenTree we just finished processing. At
+    // the end, this will contain the full result of transcription, but at arbitrary points during
+    // `transcribe`, `result` will contain subsets of the final result.
+    //
+    // Specifically, as we descend into each TokenTree, we will push the existing results onto the
+    // `result_stack` and clear `results`. We will then produce the results of transcribing the
+    // TokenTree into `results`. Then, as we unwind back out of the `TokenTree`, we will pop the
+    // `result_stack` and append `results` too it to produce the new `results` up to that point.
+    //
+    // Thus, if we try to pop the `result_stack` and it is empty, we have reached the top-level
+    // again, and we are done transcribing.
     let mut result: Vec<TreeAndJoint> = Vec::new();
     let mut result_stack = Vec::new();
 
     loop {
+        // Look at the last frame on the stack.
         let tree = if let Some(tree) = stack.last_mut().unwrap().next() {
+            // If it still has a TokenTree we have not looked at yet, use that tree.
             tree
-        } else {
+        }
+        // The else-case never produces a value for `tree` (it `continue`s or `return`s).
+        else {
+            // Otherwise, if we have just reached the end of a sequence and we can keep repeating,
+            // go back to the beginning of the sequence.
             if let Frame::Sequence { ref mut idx, ref sep, .. } = *stack.last_mut().unwrap() {
                 let (ref mut repeat_idx, repeat_len) = *repeats.last_mut().unwrap();
                 *repeat_idx += 1;
                 if *repeat_idx < repeat_len {
                     *idx = 0;
                     if let Some(sep) = sep.clone() {
-                        // repeat same span, I guess
                         let prev_span = match result.last() {
                             Some((tt, _)) => tt.span(),
                             None => DUMMY_SP,
                         };
                         result.push(TokenTree::Token(prev_span, sep).into());
                     }
-                    continue
+                    continue;
                 }
             }
 
+            // We are done with the top of the stack. Pop it. Depending on what it was, we do
+            // different things. Note that the outermost item must be the delimited, wrapped RHS
+            // that was passed in originally to `transcribe`.
             match stack.pop().unwrap() {
+                // Done with a sequence. Pop from repeats.
                 Frame::Sequence { .. } => {
                     repeats.pop();
                 }
+
+                // We are done processing a Delimited. If this is the top-level delimited, we are
+                // done. Otherwise, we unwind the result_stack to append what we have produced to
+                // any previous results.
                 Frame::Delimited { forest, span, .. } => {
                     if result_stack.is_empty() {
+                        // No results left to compute! We are back at the top-level.
                         return TokenStream::new(result);
                     }
-                    let tree = TokenTree::Delimited(
-                        span,
-                        forest.delim,
-                        TokenStream::new(result).into(),
-                    );
+
+                    // Step back into the parent Delimited.
+                    let tree =
+                        TokenTree::Delimited(span, forest.delim, TokenStream::new(result).into());
                     result = result_stack.pop().unwrap();
                     result.push(tree.into());
                 }
             }
-            continue
+            continue;
         };
 
+        // At this point, we know we are in the middle of a TokenTree (the last one on `stack`).
+        // `tree` contains the next `TokenTree` to be processed.
         match tree {
-            quoted::TokenTree::Sequence(sp, seq) => {
-                // FIXME(pcwalton): Bad copy.
-                match lockstep_iter_size(&quoted::TokenTree::Sequence(sp, seq.clone()),
-                                         &interpolations,
-                                         &repeats) {
+            // We are descending into a sequence. We first make sure that the matchers in the RHS
+            // and the matches in `interp` have the same shape. Otherwise, either the caller or the
+            // macro writer has made a mistake.
+            seq @ quoted::TokenTree::Sequence(..) => {
+                match lockstep_iter_size(&seq, interp, &repeats) {
                     LockstepIterSize::Unconstrained => {
-                        cx.span_fatal(sp.entire(), /* blame macro writer */
-                            "attempted to repeat an expression \
-                             containing no syntax \
-                             variables matched as repeating at this depth");
+                        cx.span_fatal(
+                            seq.span(), /* blame macro writer */
+                            "attempted to repeat an expression containing no syntax variables \
+                             matched as repeating at this depth",
+                        );
                     }
+
                     LockstepIterSize::Contradiction(ref msg) => {
+                        // FIXME: this should be impossible. I (mark-i-m) believe it would
+                        // represent a bug in the macro_parser.
                         // FIXME #2887 blame macro invoker instead
-                        cx.span_fatal(sp.entire(), &msg[..]);
+                        cx.span_fatal(seq.span(), &msg[..]);
                     }
+
                     LockstepIterSize::Constraint(len, _) => {
+                        // We do this to avoid an extra clone above. We know that this is a
+                        // sequence already.
+                        let (sp, seq) = if let quoted::TokenTree::Sequence(sp, seq) = seq {
+                            (sp, seq)
+                        } else {
+                            unreachable!()
+                        };
+
+                        // Is the repetition empty?
                         if len == 0 {
                             if seq.op == quoted::KleeneOp::OneOrMore {
+                                // FIXME: this should be impossible because we check for this in
+                                // macro_parser.rs
                                 // FIXME #2887 blame invoker
                                 cx.span_fatal(sp.entire(), "this must repeat at least once");
                             }
                         } else {
+                            // 0 is the initial counter (we have done 0 repretitions so far). `len`
+                            //   is the total number of reptitions we should generate.
                             repeats.push((0, len));
+
+                            // The first time we encounter the sequence we push it to the stack. It
+                            // then gets reused (see the beginning of the loop) until we are done
+                            // repeating.
                             stack.push(Frame::Sequence {
                                 idx: 0,
                                 sep: seq.separator.clone(),
@@ -141,10 +210,16 @@ pub fn transcribe(cx: &ExtCtxt<'_>,
                     }
                 }
             }
-            // FIXME #2887: think about span stuff here
+
+            // Replace the meta-var with the matched token tree from the invocation.
             quoted::TokenTree::MetaVar(mut sp, ident) => {
-                if let Some(cur_matched) = lookup_cur_matched(ident, &interpolations, &repeats) {
+                // Find the matched nonterminal from the macro invocation, and use it to replace
+                // the meta-var.
+                if let Some(cur_matched) = lookup_cur_matched(ident, interp, &repeats) {
                     if let MatchedNonterminal(ref nt) = *cur_matched {
+                        // FIXME #2887: why do we apply a mark when matching a token tree meta-var
+                        // (e.g. `$x:tt`), but not when we are matching any other type of token
+                        // tree?
                         if let NtTT(ref tt) = **nt {
                             result.push(tt.clone().into());
                         } else {
@@ -153,10 +228,15 @@ pub fn transcribe(cx: &ExtCtxt<'_>,
                             result.push(token.into());
                         }
                     } else {
-                        cx.span_fatal(sp, /* blame the macro writer */
-                            &format!("variable '{}' is still repeating at this depth", ident));
+                        // We were unable to descend far enough. This is an error.
+                        cx.span_fatal(
+                            sp, /* blame the macro writer */
+                            &format!("variable '{}' is still repeating at this depth", ident),
+                        );
                     }
                 } else {
+                    // If we aren't able to match the meta-var, we push it back into the result but
+                    // with modified syntax context. (I believe this supports nested macros).
                     let ident =
                         Ident::new(ident.name, ident.span.apply_mark(cx.current_expansion.mark));
                     sp = sp.apply_mark(cx.current_expansion.mark);
@@ -164,26 +244,44 @@ pub fn transcribe(cx: &ExtCtxt<'_>,
                     result.push(TokenTree::Token(sp, token::Token::from_ast_ident(ident)).into());
                 }
             }
+
+            // If we are entering a new delimiter, we push its contents to the `stack` to be
+            // processed, and we push all of the currently produced results to the `result_stack`.
+            // We will produce all of the results of the inside of the `Delimited` and then we will
+            // jump back out of the Delimited, pop the result_stack and add the new results back to
+            // the previous results (from outside the Delimited).
             quoted::TokenTree::Delimited(mut span, delimited) => {
                 span = span.apply_mark(cx.current_expansion.mark);
                 stack.push(Frame::Delimited { forest: delimited, idx: 0, span: span });
                 result_stack.push(mem::replace(&mut result, Vec::new()));
             }
+
+            // Nothing much to do here. Just push the token to the result, being careful to
+            // preserve syntax context.
             quoted::TokenTree::Token(sp, tok) => {
                 let mut marker = Marker(cx.current_expansion.mark);
                 let mut tt = TokenTree::Token(sp, tok);
                 noop_visit_tt(&mut tt, &mut marker);
                 result.push(tt.into());
             }
+
+            // There should be no meta-var declarations in the invocation of a macro.
             quoted::TokenTree::MetaVarDecl(..) => panic!("unexpected `TokenTree::MetaVarDecl"),
         }
     }
 }
 
-fn lookup_cur_matched(ident: Ident,
-                      interpolations: &FxHashMap<Ident, Rc<NamedMatch>>,
-                      repeats: &[(usize, usize)])
-                      -> Option<Rc<NamedMatch>> {
+/// Lookup the meta-var named `ident` and return the matched token tree from the invocation using
+/// the set of matches `interpolations`.
+///
+/// See the definition of `repeats` in the `transcribe` function. `repeats` is used to descend
+/// into the right place in nested matchers. If we attempt to descend too far, the macro writer has
+/// made a mistake, and we return `None`.
+fn lookup_cur_matched(
+    ident: Ident,
+    interpolations: &FxHashMap<Ident, Rc<NamedMatch>>,
+    repeats: &[(usize, usize)],
+) -> Option<Rc<NamedMatch>> {
     interpolations.get(&ident).map(|matched| {
         let mut matched = matched.clone();
         for &(idx, _) in repeats {
@@ -198,17 +296,30 @@ fn lookup_cur_matched(ident: Ident,
     })
 }
 
+/// An accumulator over a TokenTree to be used with `fold`. During transcription, we need to make
+/// sure that the size of each sequence and all of its nested sequences are the same as the sizes
+/// of all the matched (nested) sequences in the macro invocation. If they don't match, somebody
+/// has made a mistake (either the macro writer or caller).
 #[derive(Clone)]
 enum LockstepIterSize {
+    /// No constraints on length of matcher. This is true for any TokenTree variants except a
+    /// `MetaVar` with an actual `MatchedSeq` (as opposed to a `MatchedNonterminal`).
     Unconstrained,
+
+    /// A `MetaVar` with an actual `MatchedSeq`. The length of the match and the name of the
+    /// meta-var are returned.
     Constraint(usize, Ident),
+
+    /// Two `Constraint`s on the same sequence had different lengths. This is an error.
     Contradiction(String),
 }
 
-impl Add for LockstepIterSize {
-    type Output = LockstepIterSize;
-
-    fn add(self, other: LockstepIterSize) -> LockstepIterSize {
+impl LockstepIterSize {
+    /// Find incompatibilities in matcher/invocation sizes.
+    /// - `Unconstrained` is compatible with everything.
+    /// - `Contradiction` is incompatible with everything.
+    /// - `Constraint(len)` is only compatible with other constraints of the same length.
+    fn with(self, other: LockstepIterSize) -> LockstepIterSize {
         match self {
             LockstepIterSize::Unconstrained => other,
             LockstepIterSize::Contradiction(_) => self,
@@ -217,9 +328,11 @@ impl Add for LockstepIterSize {
                 LockstepIterSize::Contradiction(_) => other,
                 LockstepIterSize::Constraint(r_len, _) if l_len == r_len => self,
                 LockstepIterSize::Constraint(r_len, r_id) => {
-                    let msg = format!("inconsistent lockstep iteration: \
-                                       '{}' has {} items, but '{}' has {}",
-                                      l_id, l_len, r_id, r_len);
+                    let msg = format!(
+                        "inconsistent lockstep iteration: \
+                         '{}' has {} items, but '{}' has {}",
+                        l_id, l_len, r_id, r_len
+                    );
                     LockstepIterSize::Contradiction(msg)
                 }
             },
@@ -227,30 +340,38 @@ impl Add for LockstepIterSize {
     }
 }
 
-fn lockstep_iter_size(tree: &quoted::TokenTree,
-                      interpolations: &FxHashMap<Ident, Rc<NamedMatch>>,
-                      repeats: &[(usize, usize)])
-                      -> LockstepIterSize {
+/// Given a `tree`, make sure that all sequences have the same length as the matches for the
+/// appropriate meta-vars in `interpolations`.
+///
+/// Note that if `repeats` does not match the exact correct depth of a meta-var,
+/// `lookup_cur_matched` will return `None`, which is why this still works even in the presnece of
+/// multiple nested matcher sequences.
+fn lockstep_iter_size(
+    tree: &quoted::TokenTree,
+    interpolations: &FxHashMap<Ident, Rc<NamedMatch>>,
+    repeats: &[(usize, usize)],
+) -> LockstepIterSize {
     use quoted::TokenTree;
     match *tree {
         TokenTree::Delimited(_, ref delimed) => {
             delimed.tts.iter().fold(LockstepIterSize::Unconstrained, |size, tt| {
-                size + lockstep_iter_size(tt, interpolations, repeats)
+                size.with(lockstep_iter_size(tt, interpolations, repeats))
             })
-        },
+        }
         TokenTree::Sequence(_, ref seq) => {
             seq.tts.iter().fold(LockstepIterSize::Unconstrained, |size, tt| {
-                size + lockstep_iter_size(tt, interpolations, repeats)
+                size.with(lockstep_iter_size(tt, interpolations, repeats))
             })
-        },
-        TokenTree::MetaVar(_, name) | TokenTree::MetaVarDecl(_, name, _) =>
+        }
+        TokenTree::MetaVar(_, name) | TokenTree::MetaVarDecl(_, name, _) => {
             match lookup_cur_matched(name, interpolations, repeats) {
                 Some(matched) => match *matched {
                     MatchedNonterminal(_) => LockstepIterSize::Unconstrained,
                     MatchedSeq(ref ads, _) => LockstepIterSize::Constraint(ads.len(), name),
                 },
-                _ => LockstepIterSize::Unconstrained
-            },
+                _ => LockstepIterSize::Unconstrained,
+            }
+        }
         TokenTree::Token(..) => LockstepIterSize::Unconstrained,
     }
 }
diff --git a/src/libsyntax/parse/lexer/mod.rs b/src/libsyntax/parse/lexer/mod.rs
index 3c342c2ff26..2882acb0e78 100644
--- a/src/libsyntax/parse/lexer/mod.rs
+++ b/src/libsyntax/parse/lexer/mod.rs
@@ -1598,7 +1598,7 @@ mod tests {
     use std::io;
     use std::path::PathBuf;
     use syntax_pos::{BytePos, Span, NO_EXPANSION};
-    use rustc_data_structures::fx::FxHashSet;
+    use rustc_data_structures::fx::{FxHashSet, FxHashMap};
     use rustc_data_structures::sync::Lock;
 
     fn mk_sess(sm: Lrc<SourceMap>) -> ParseSess {
@@ -1617,6 +1617,7 @@ mod tests {
             raw_identifier_spans: Lock::new(Vec::new()),
             registered_diagnostics: Lock::new(ErrorMap::new()),
             buffered_lints: Lock::new(vec![]),
+            ambiguous_block_expr_parse: Lock::new(FxHashMap::default()),
         }
     }
 
diff --git a/src/libsyntax/parse/mod.rs b/src/libsyntax/parse/mod.rs
index 8c2fab9ada7..be44b964ba5 100644
--- a/src/libsyntax/parse/mod.rs
+++ b/src/libsyntax/parse/mod.rs
@@ -11,12 +11,12 @@ use crate::tokenstream::{TokenStream, TokenTree};
 use crate::diagnostics::plugin::ErrorMap;
 use crate::print::pprust::token_to_string;
 
-use errors::{FatalError, Level, Handler, ColorConfig, Diagnostic, DiagnosticBuilder};
+use errors::{Applicability, FatalError, Level, Handler, ColorConfig, Diagnostic, DiagnosticBuilder};
 use rustc_data_structures::sync::{Lrc, Lock};
 use syntax_pos::{Span, SourceFile, FileName, MultiSpan};
 use log::debug;
 
-use rustc_data_structures::fx::FxHashSet;
+use rustc_data_structures::fx::{FxHashSet, FxHashMap};
 use std::borrow::Cow;
 use std::path::{Path, PathBuf};
 use std::str;
@@ -52,6 +52,10 @@ pub struct ParseSess {
     included_mod_stack: Lock<Vec<PathBuf>>,
     source_map: Lrc<SourceMap>,
     pub buffered_lints: Lock<Vec<BufferedEarlyLint>>,
+    /// Contains the spans of block expressions that could have been incomplete based on the
+    /// operation token that followed it, but that the parser cannot identify without further
+    /// analysis.
+    pub ambiguous_block_expr_parse: Lock<FxHashMap<Span, Span>>,
 }
 
 impl ParseSess {
@@ -75,6 +79,7 @@ impl ParseSess {
             included_mod_stack: Lock::new(vec![]),
             source_map,
             buffered_lints: Lock::new(vec![]),
+            ambiguous_block_expr_parse: Lock::new(FxHashMap::default()),
         }
     }
 
@@ -98,6 +103,24 @@ impl ParseSess {
             });
         });
     }
+
+    /// Extend an error with a suggestion to wrap an expression with parentheses to allow the
+    /// parser to continue parsing the following operation as part of the same expression.
+    pub fn expr_parentheses_needed(
+        &self,
+        err: &mut DiagnosticBuilder<'_>,
+        span: Span,
+        alt_snippet: Option<String>,
+    ) {
+        if let Some(snippet) = self.source_map().span_to_snippet(span).ok().or(alt_snippet) {
+            err.span_suggestion(
+                span,
+                "parentheses are required to parse this as an expression",
+                format!("({})", snippet),
+                Applicability::MachineApplicable,
+            );
+        }
+    }
 }
 
 #[derive(Clone)]
diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index f3eac71ee77..d97d1e2f0f4 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -50,7 +50,10 @@ use crate::symbol::{Symbol, keywords};
 
 use errors::{Applicability, DiagnosticBuilder, DiagnosticId, FatalError};
 use rustc_target::spec::abi::{self, Abi};
-use syntax_pos::{Span, MultiSpan, BytePos, FileName};
+use syntax_pos::{
+    Span, MultiSpan, BytePos, FileName,
+    hygiene::CompilerDesugaringKind,
+};
 use log::{debug, trace};
 
 use std::borrow::Cow;
@@ -186,6 +189,7 @@ enum PrevTokenKind {
     Interpolated,
     Eof,
     Ident,
+    BitOr,
     Other,
 }
 
@@ -1375,6 +1379,7 @@ impl<'a> Parser<'a> {
             token::DocComment(..) => PrevTokenKind::DocComment,
             token::Comma => PrevTokenKind::Comma,
             token::BinOp(token::Plus) => PrevTokenKind::Plus,
+            token::BinOp(token::Or) => PrevTokenKind::BitOr,
             token::Interpolated(..) => PrevTokenKind::Interpolated,
             token::Eof => PrevTokenKind::Eof,
             token::Ident(..) => PrevTokenKind::Ident,
@@ -2806,6 +2811,12 @@ impl<'a> Parser<'a> {
                             let msg = format!("expected expression, found {}",
                                               self.this_token_descr());
                             let mut err = self.fatal(&msg);
+                            let sp = self.sess.source_map().start_point(self.span);
+                            if let Some(sp) = self.sess.ambiguous_block_expr_parse.borrow()
+                                .get(&sp)
+                            {
+                                self.sess.expr_parentheses_needed(&mut err, *sp, None);
+                            }
                             err.span_label(self.span, "expected expression");
                             return Err(err);
                         }
@@ -2845,7 +2856,7 @@ impl<'a> Parser<'a> {
                     "struct literals are not allowed here",
                 );
                 err.multipart_suggestion(
-                    "surround the struct literal with parenthesis",
+                    "surround the struct literal with parentheses",
                     vec![
                         (lo.shrink_to_lo(), "(".to_string()),
                         (expr.span.shrink_to_hi(), ")".to_string()),
@@ -3506,9 +3517,42 @@ impl<'a> Parser<'a> {
             }
         };
 
-        if self.expr_is_complete(&lhs) {
-            // Semi-statement forms are odd. See https://github.com/rust-lang/rust/issues/29071
-            return Ok(lhs);
+        match (self.expr_is_complete(&lhs), AssocOp::from_token(&self.token)) {
+            (true, None) => {
+                // Semi-statement forms are odd. See https://github.com/rust-lang/rust/issues/29071
+                return Ok(lhs);
+            }
+            (false, _) => {} // continue parsing the expression
+            // An exhaustive check is done in the following block, but these are checked first
+            // because they *are* ambiguous but also reasonable looking incorrect syntax, so we
+            // want to keep their span info to improve diagnostics in these cases in a later stage.
+            (true, Some(AssocOp::Multiply)) | // `{ 42 } *foo = bar;` or `{ 42 } * 3`
+            (true, Some(AssocOp::Subtract)) | // `{ 42 } -5`
+            (true, Some(AssocOp::Add)) => { // `{ 42 } + 42
+                // These cases are ambiguous and can't be identified in the parser alone
+                let sp = self.sess.source_map().start_point(self.span);
+                self.sess.ambiguous_block_expr_parse.borrow_mut().insert(sp, lhs.span);
+                return Ok(lhs);
+            }
+            (true, Some(ref op)) if !op.can_continue_expr_unambiguously() => {
+                return Ok(lhs);
+            }
+            (true, Some(_)) => {
+                // We've found an expression that would be parsed as a statement, but the next
+                // token implies this should be parsed as an expression.
+                // For example: `if let Some(x) = x { x } else { 0 } / 2`
+                let mut err = self.sess.span_diagnostic.struct_span_err(self.span, &format!(
+                    "expected expression, found `{}`",
+                    pprust::token_to_string(&self.token),
+                ));
+                err.span_label(self.span, "expected expression");
+                self.sess.expr_parentheses_needed(
+                    &mut err,
+                    lhs.span,
+                    Some(pprust::expr_to_string(&lhs),
+                ));
+                err.emit();
+            }
         }
         self.expected_tokens.push(TokenType::Operator);
         while let Some(op) = AssocOp::from_token(&self.token) {
@@ -4819,6 +4863,10 @@ impl<'a> Parser<'a> {
                         );
                         let mut err = self.fatal(&msg);
                         err.span_label(self.span, format!("expected {}", expected));
+                        let sp = self.sess.source_map().start_point(self.span);
+                        if let Some(sp) = self.sess.ambiguous_block_expr_parse.borrow().get(&sp) {
+                            self.sess.expr_parentheses_needed(&mut err, *sp, None);
+                        }
                         return Err(err);
                     }
                 }
@@ -8727,6 +8775,10 @@ impl<'a> Parser<'a> {
     /// The arguments of the function are replaced in HIR lowering with the arguments created by
     /// this function and the statements created here are inserted at the top of the closure body.
     fn construct_async_arguments(&mut self, asyncness: &mut Spanned<IsAsync>, decl: &mut FnDecl) {
+        // FIXME(davidtwco): This function should really live in the HIR lowering but because
+        // the types constructed here need to be used in parts of resolve so that the correct
+        // locals are considered upvars, it is currently easier for it to live here in the parser,
+        // where it can be constructed once.
         if let IsAsync::Async { ref mut arguments, .. } = asyncness.node {
             for (index, input) in decl.inputs.iter_mut().enumerate() {
                 let id = ast::DUMMY_NODE_ID;
@@ -8741,6 +8793,15 @@ impl<'a> Parser<'a> {
                 // statement.
                 let (binding_mode, ident, is_simple_pattern) = match input.pat.node {
                     PatKind::Ident(binding_mode @ BindingMode::ByValue(_), ident, _) => {
+                        // Simple patterns like this don't have a generated argument, but they are
+                        // moved into the closure with a statement, so any `mut` bindings on the
+                        // argument will be unused. This binding mode can't be removed, because
+                        // this would affect the input to procedural macros, but they can have
+                        // their span marked as being the result of a compiler desugaring so
+                        // that they aren't linted against.
+                        input.pat.span = self.sess.source_map().mark_span_with_reason(
+                            CompilerDesugaringKind::Async, span, None);
+
                         (binding_mode, ident, true)
                     }
                     _ => (BindingMode::ByValue(Mutability::Mutable), ident, false),
@@ -8810,15 +8871,6 @@ impl<'a> Parser<'a> {
                     })
                 };
 
-                // Remove mutability from arguments. If this is not a simple pattern,
-                // those arguments are replaced by `__argN`, so there is no need to do this.
-                if let PatKind::Ident(BindingMode::ByValue(mutability @ Mutability::Mutable), ..) =
-                    &mut input.pat.node
-                {
-                    assert!(is_simple_pattern);
-                    *mutability = Mutability::Immutable;
-                }
-
                 let move_stmt = Stmt { id, node: StmtKind::Local(P(move_local)), span };
                 arguments.push(AsyncArgument { ident, arg, pat_stmt, move_stmt });
             }
diff --git a/src/libsyntax/source_map.rs b/src/libsyntax/source_map.rs
index 08abbf5e8a4..215618bd09c 100644
--- a/src/libsyntax/source_map.rs
+++ b/src/libsyntax/source_map.rs
@@ -930,6 +930,27 @@ impl SourceMap {
 
         None
     }
+
+    /// Reuses the span but adds information like the kind of the desugaring and features that are
+    /// allowed inside this span.
+    pub fn mark_span_with_reason(
+        &self,
+        reason: hygiene::CompilerDesugaringKind,
+        span: Span,
+        allow_internal_unstable: Option<Lrc<[symbol::Symbol]>>,
+    ) -> Span {
+        let mark = Mark::fresh(Mark::root());
+        mark.set_expn_info(ExpnInfo {
+            call_site: span,
+            def_site: Some(span),
+            format: CompilerDesugaring(reason),
+            allow_internal_unstable,
+            allow_internal_unsafe: false,
+            local_inner_macros: false,
+            edition: hygiene::default_edition(),
+        });
+        span.with_ctxt(SyntaxContext::empty().apply_mark(mark))
+    }
 }
 
 impl SourceMapper for SourceMap {
diff --git a/src/libsyntax/util/parser.rs b/src/libsyntax/util/parser.rs
index 80dabffaba9..86e89945afe 100644
--- a/src/libsyntax/util/parser.rs
+++ b/src/libsyntax/util/parser.rs
@@ -207,6 +207,31 @@ impl AssocOp {
             ObsoleteInPlace | Assign | AssignOp(_) | As | DotDot | DotDotEq | Colon => None
         }
     }
+
+    /// This operator could be used to follow a block unambiguously.
+    ///
+    /// This is used for error recovery at the moment, providing a suggestion to wrap blocks with
+    /// parentheses while having a high degree of confidence on the correctness of the suggestion.
+    pub fn can_continue_expr_unambiguously(&self) -> bool {
+        use AssocOp::*;
+        match self {
+            BitXor | // `{ 42 } ^ 3`
+            Assign | // `{ 42 } = { 42 }`
+            Divide | // `{ 42 } / 42`
+            Modulus | // `{ 42 } % 2`
+            ShiftRight | // `{ 42 } >> 2`
+            LessEqual | // `{ 42 } <= 3`
+            Greater | // `{ 42 } > 3`
+            GreaterEqual | // `{ 42 } >= 3`
+            AssignOp(_) | // `{ 42 } +=`
+            LAnd | // `{ 42 } &&foo`
+            As | // `{ 42 } as usize`
+            // Equal | // `{ 42 } == { 42 }`    Accepting these here would regress incorrect
+            // NotEqual | // `{ 42 } != { 42 }  struct literals parser recovery.
+            Colon => true, // `{ 42 }: usize`
+            _ => false,
+        }
+    }
 }
 
 pub const PREC_RESET: i8 = -100;