Tweak diagnostics

* Recover from invalid `'label: ` before block.
* Make suggestion to enclose statements in a block multipart.
* Point at `match`, `while`, `loop` and `unsafe` keywords when failing
  to parse their expression.
* Do not suggest `{ ; }`.
* Do not suggest `|` when very unlikely to be what was wanted (in `let`
  statements).

Author: estebank

compiler/rustc_expand/src/expand.rs

@@ -21,7 +21,7 @@ use rustc_data_structures::sync::Lrc;
 use rustc_errors::{Applicability, PResult};
 use rustc_feature::Features;
 use rustc_parse::parser::{
-    AttemptLocalParseRecovery, ForceCollect, Parser, RecoverColon, RecoverComma,
+    AttemptLocalParseRecovery, CommaRecoveryMode, ForceCollect, Parser, RecoverColon, RecoverComma,
 };
 use rustc_parse::validate_attr;
 use rustc_session::lint::builtin::{UNUSED_ATTRIBUTES, UNUSED_DOC_COMMENTS};
@@ -914,6 +914,7 @@ pub fn parse_ast_fragment<'a>(
             None,
             RecoverComma::No,
             RecoverColon::Yes,
+            CommaRecoveryMode::LikelyTuple,
         )?),
         AstFragmentKind::Crate => AstFragment::Crate(this.parse_crate_mod()?),
         AstFragmentKind::Arms

compiler/rustc_parse/src/lib.rs

@@ -3,6 +3,7 @@
 #![feature(array_windows)]
 #![feature(crate_visibility_modifier)]
 #![feature(if_let_guard)]
+#![feature(let_else)]
 #![feature(box_patterns)]
 #![recursion_limit = "256"]
 

compiler/rustc_parse/src/parser/diagnostics.rs

@@ -1,8 +1,8 @@
 use super::pat::Expected;
 use super::ty::AllowPlus;
 use super::{
-    BlockMode, Parser, PathStyle, RecoverColon, RecoverComma, Restrictions, SemiColonMode, SeqSep,
-    TokenExpectType, TokenType,
+    BlockMode, CommaRecoveryMode, Parser, PathStyle, RecoverColon, RecoverComma, Restrictions,
+    SemiColonMode, SeqSep, TokenExpectType, TokenType,
 };
 
 use rustc_ast as ast;
@@ -2200,12 +2200,32 @@ impl<'a> Parser<'a> {
         first_pat
     }
 
+    crate fn maybe_recover_unexpected_block_label(&mut self) -> bool {
+        let Some(label) = self.eat_label().filter(|_| {
+            self.eat(&token::Colon) && self.token.kind == token::OpenDelim(token::Brace)
+        }) else {
+            return false;
+        };
+        let span = label.ident.span.to(self.prev_token.span);
+        let mut err = self.struct_span_err(span, "block label not supported here");
+        err.span_label(span, "not supported here");
+        err.tool_only_span_suggestion(
+            label.ident.span.until(self.token.span),
+            "remove this block label",
+            String::new(),
+            Applicability::MachineApplicable,
+        );
+        err.emit();
+        true
+    }
+
     /// Some special error handling for the "top-level" patterns in a match arm,
     /// `for` loop, `let`, &c. (in contrast to subpatterns within such).
     crate fn maybe_recover_unexpected_comma(
         &mut self,
         lo: Span,
         rc: RecoverComma,
+        rt: CommaRecoveryMode,
     ) -> PResult<'a, ()> {
         if rc == RecoverComma::No || self.token != token::Comma {
             return Ok(());
@@ -2225,20 +2245,25 @@ impl<'a> Parser<'a> {
         let seq_span = lo.to(self.prev_token.span);
         let mut err = self.struct_span_err(comma_span, "unexpected `,` in pattern");
         if let Ok(seq_snippet) = self.span_to_snippet(seq_span) {
-            const MSG: &str = "try adding parentheses to match on a tuple...";
-
-            err.span_suggestion(
-                seq_span,
-                MSG,
-                format!("({})", seq_snippet),
-                Applicability::MachineApplicable,
-            );
-            err.span_suggestion(
-                seq_span,
-                "...or a vertical bar to match on multiple alternatives",
-                seq_snippet.replace(',', " |"),
+            err.multipart_suggestion(
+                &format!(
+                    "try adding parentheses to match on a tuple{}",
+                    if let CommaRecoveryMode::LikelyTuple = rt { "" } else { "..." },
+                ),
+                vec![
+                    (seq_span.shrink_to_lo(), "(".to_string()),
+                    (seq_span.shrink_to_hi(), ")".to_string()),
+                ],
                 Applicability::MachineApplicable,
             );
+            if let CommaRecoveryMode::EitherTupleOrPipe = rt {
+                err.span_suggestion(
+                    seq_span,
+                    "...or a vertical bar to match on multiple alternatives",
+                    seq_snippet.replace(',', " |"),
+                    Applicability::MachineApplicable,
+                );
+            }
         }
         Err(err)
     }

compiler/rustc_parse/src/parser/expr.rs

@@ -1,4 +1,4 @@
-use super::pat::{RecoverColon, RecoverComma, PARAM_EXPECTED};
+use super::pat::{CommaRecoveryMode, RecoverColon, RecoverComma, PARAM_EXPECTED};
 use super::ty::{AllowPlus, RecoverQPath, RecoverReturnSign};
 use super::{
     AttrWrapper, BlockMode, ClosureSpans, ForceCollect, Parser, PathStyle, Restrictions, TokenType,
@@ -1269,18 +1269,27 @@ impl<'a> Parser<'a> {
         } else if let Some(label) = self.eat_label() {
             self.parse_labeled_expr(label, attrs, true)
         } else if self.eat_keyword(kw::Loop) {
-            self.parse_loop_expr(None, self.prev_token.span, attrs)
+            let sp = self.prev_token.span;
+            self.parse_loop_expr(None, self.prev_token.span, attrs).map_err(|mut err| {
+                err.span_label(sp, "while parsing this `loop` expression");
+                err
+            })
         } else if self.eat_keyword(kw::Continue) {
             let kind = ExprKind::Continue(self.eat_label());
             Ok(self.mk_expr(lo.to(self.prev_token.span), kind, attrs))
         } else if self.eat_keyword(kw::Match) {
             let match_sp = self.prev_token.span;
             self.parse_match_expr(attrs).map_err(|mut err| {
-                err.span_label(match_sp, "while parsing this match expression");
+                err.span_label(match_sp, "while parsing this `match` expression");
                 err
             })
         } else if self.eat_keyword(kw::Unsafe) {
+            let sp = self.prev_token.span;
             self.parse_block_expr(None, lo, BlockCheckMode::Unsafe(ast::UserProvided), attrs)
+                .map_err(|mut err| {
+                    err.span_label(sp, "while parsing this `unsafe` expression");
+                    err
+                })
         } else if self.check_inline_const(0) {
             self.parse_const_block(lo.to(self.token.span), false)
         } else if self.is_do_catch_block() {
@@ -2110,7 +2119,12 @@ 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_pat_allow_top_alt(None, RecoverComma::Yes, RecoverColon::Yes)?;
+        let pat = self.parse_pat_allow_top_alt(
+            None,
+            RecoverComma::Yes,
+            RecoverColon::Yes,
+            CommaRecoveryMode::LikelyTuple,
+        )?;
         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())
@@ -2173,7 +2187,12 @@ impl<'a> Parser<'a> {
             _ => None,
         };
 
-        let pat = self.parse_pat_allow_top_alt(None, RecoverComma::Yes, RecoverColon::Yes)?;
+        let pat = self.parse_pat_allow_top_alt(
+            None,
+            RecoverComma::Yes,
+            RecoverColon::Yes,
+            CommaRecoveryMode::LikelyTuple,
+        )?;
         if !self.eat_keyword(kw::In) {
             self.error_missing_in_for_loop();
         }
@@ -2216,8 +2235,15 @@ impl<'a> Parser<'a> {
         lo: Span,
         mut attrs: AttrVec,
     ) -> PResult<'a, P<Expr>> {
-        let cond = self.parse_cond_expr()?;
-        let (iattrs, body) = self.parse_inner_attrs_and_block()?;
+        let cond = self.parse_cond_expr().map_err(|mut err| {
+            err.span_label(lo, "while parsing the condition of this `while` expression");
+            err
+        })?;
+        let (iattrs, body) = self.parse_inner_attrs_and_block().map_err(|mut err| {
+            err.span_label(lo, "while parsing the body of this `while` expression");
+            err.span_label(cond.span, "this `while` condition successfully parsed");
+            err
+        })?;
         attrs.extend(iattrs);
         Ok(self.mk_expr(lo.to(self.prev_token.span), ExprKind::While(cond, body, opt_label), attrs))
     }
@@ -2234,7 +2260,7 @@ impl<'a> Parser<'a> {
         Ok(self.mk_expr(lo.to(self.prev_token.span), ExprKind::Loop(body, opt_label), attrs))
     }
 
-    fn eat_label(&mut self) -> Option<Label> {
+    crate fn eat_label(&mut self) -> Option<Label> {
         self.token.lifetime().map(|ident| {
             self.bump();
             Label { ident }
@@ -2255,7 +2281,12 @@ impl<'a> Parser<'a> {
                     Applicability::MaybeIncorrect, // speculative
                 );
             }
-            return Err(e);
+            if self.maybe_recover_unexpected_block_label() {
+                e.cancel();
+                self.bump();
+            } else {
+                return Err(e);
+            }
         }
         attrs.extend(self.parse_inner_attributes()?);
 
@@ -2380,7 +2411,12 @@ impl<'a> Parser<'a> {
         let attrs = self.parse_outer_attributes()?;
         self.collect_tokens_trailing_token(attrs, ForceCollect::No, |this, attrs| {
             let lo = this.token.span;
-            let pat = this.parse_pat_allow_top_alt(None, RecoverComma::Yes, RecoverColon::Yes)?;
+            let pat = this.parse_pat_allow_top_alt(
+                None,
+                RecoverComma::Yes,
+                RecoverColon::Yes,
+                CommaRecoveryMode::EitherTupleOrPipe,
+            )?;
             let guard = if this.eat_keyword(kw::If) {
                 let if_span = this.prev_token.span;
                 let cond = this.parse_expr()?;

compiler/rustc_parse/src/parser/mod.rs

@@ -15,7 +15,7 @@ pub use attr_wrapper::AttrWrapper;
 pub use diagnostics::AttemptLocalParseRecovery;
 use diagnostics::Error;
 pub(crate) use item::FnParseMode;
-pub use pat::{RecoverColon, RecoverComma};
+pub use pat::{CommaRecoveryMode, RecoverColon, RecoverComma};
 pub use path::PathStyle;
 
 use rustc_ast::ptr::P;

compiler/rustc_parse/src/parser/nonterminal.rs

@@ -5,7 +5,7 @@ use rustc_ast_pretty::pprust;
 use rustc_errors::PResult;
 use rustc_span::symbol::{kw, Ident};
 
-use crate::parser::pat::{RecoverColon, RecoverComma};
+use crate::parser::pat::{CommaRecoveryMode, RecoverColon, RecoverComma};
 use crate::parser::{FollowedByType, ForceCollect, Parser, PathStyle};
 
 impl<'a> Parser<'a> {
@@ -125,7 +125,7 @@ impl<'a> Parser<'a> {
                 token::NtPat(self.collect_tokens_no_attrs(|this| match kind {
                     NonterminalKind::PatParam { .. } => this.parse_pat_no_top_alt(None),
                     NonterminalKind::PatWithOr { .. } => {
-                        this.parse_pat_allow_top_alt(None, RecoverComma::No, RecoverColon::No)
+                        this.parse_pat_allow_top_alt(None, RecoverComma::No, RecoverColon::No, CommaRecoveryMode::EitherTupleOrPipe)
                     }
                     _ => unreachable!(),
                 })?)

compiler/rustc_parse/src/parser/pat.rs

@@ -33,6 +33,13 @@ pub enum RecoverColon {
     No,
 }
 
+/// Whether or not to recover a `a, b` when parsing patterns as `(a, b)` or that *and* `a | b`.
+#[derive(PartialEq, Copy, Clone)]
+pub enum CommaRecoveryMode {
+    LikelyTuple,
+    EitherTupleOrPipe,
+}
+
 /// The result of `eat_or_separator`. We want to distinguish which case we are in to avoid
 /// emitting duplicate diagnostics.
 #[derive(Debug, Clone, Copy)]
@@ -68,8 +75,9 @@ impl<'a> Parser<'a> {
         expected: Expected,
         rc: RecoverComma,
         ra: RecoverColon,
+        rt: CommaRecoveryMode,
     ) -> PResult<'a, P<Pat>> {
-        self.parse_pat_allow_top_alt_inner(expected, rc, ra).map(|(pat, _)| pat)
+        self.parse_pat_allow_top_alt_inner(expected, rc, ra, rt).map(|(pat, _)| pat)
     }
 
     /// Returns the pattern and a bool indicating whether we recovered from a trailing vert (true =
@@ -79,6 +87,7 @@ impl<'a> Parser<'a> {
         expected: Expected,
         rc: RecoverComma,
         ra: RecoverColon,
+        rt: CommaRecoveryMode,
     ) -> PResult<'a, (P<Pat>, bool)> {
         // Keep track of whether we recovered from a trailing vert so that we can avoid duplicated
         // suggestions (which bothers rustfix).
@@ -92,7 +101,7 @@ impl<'a> Parser<'a> {
 
         // Parse the first pattern (`p_0`).
         let first_pat = self.parse_pat_no_top_alt(expected)?;
-        self.maybe_recover_unexpected_comma(first_pat.span, rc)?;
+        self.maybe_recover_unexpected_comma(first_pat.span, rc, rt)?;
 
         // If the next token is not a `|`,
         // this is not an or-pattern and we should exit here.
@@ -130,7 +139,7 @@ impl<'a> Parser<'a> {
                 err.span_label(lo, WHILE_PARSING_OR_MSG);
                 err
             })?;
-            self.maybe_recover_unexpected_comma(pat.span, rc)?;
+            self.maybe_recover_unexpected_comma(pat.span, rc, rt)?;
             pats.push(pat);
         }
         let or_pattern_span = lo.to(self.prev_token.span);
@@ -155,8 +164,12 @@ impl<'a> Parser<'a> {
         // We use `parse_pat_allow_top_alt` regardless of whether we actually want top-level
         // or-patterns so that we can detect when a user tries to use it. This allows us to print a
         // better error message.
-        let (pat, trailing_vert) =
-            self.parse_pat_allow_top_alt_inner(expected, rc, RecoverColon::No)?;
+        let (pat, trailing_vert) = self.parse_pat_allow_top_alt_inner(
+            expected,
+            rc,
+            RecoverColon::No,
+            CommaRecoveryMode::LikelyTuple,
+        )?;
         let colon = self.eat(&token::Colon);
 
         if let PatKind::Or(pats) = &pat.kind {
@@ -315,7 +328,12 @@ impl<'a> Parser<'a> {
         } else if self.check(&token::OpenDelim(token::Bracket)) {
             // Parse `[pat, pat,...]` as a slice pattern.
             let (pats, _) = self.parse_delim_comma_seq(token::Bracket, |p| {
-                p.parse_pat_allow_top_alt(None, RecoverComma::No, RecoverColon::No)
+                p.parse_pat_allow_top_alt(
+                    None,
+                    RecoverComma::No,
+                    RecoverColon::No,
+                    CommaRecoveryMode::EitherTupleOrPipe,
+                )
             })?;
             PatKind::Slice(pats)
         } else if self.check(&token::DotDot) && !self.is_pat_range_end_start(1) {
@@ -529,7 +547,12 @@ impl<'a> Parser<'a> {
     /// Parse a tuple or parenthesis pattern.
     fn parse_pat_tuple_or_parens(&mut self) -> PResult<'a, PatKind> {
         let (fields, trailing_comma) = self.parse_paren_comma_seq(|p| {
-            p.parse_pat_allow_top_alt(None, RecoverComma::No, RecoverColon::No)
+            p.parse_pat_allow_top_alt(
+                None,
+                RecoverComma::No,
+                RecoverColon::No,
+                CommaRecoveryMode::LikelyTuple,
+            )
         })?;
 
         // Here, `(pat,)` is a tuple pattern.
@@ -873,7 +896,12 @@ impl<'a> Parser<'a> {
     /// Parse tuple struct or tuple variant pattern (e.g. `Foo(...)` or `Foo::Bar(...)`).
     fn parse_pat_tuple_struct(&mut self, qself: Option<QSelf>, path: Path) -> PResult<'a, PatKind> {
         let (fields, _) = self.parse_paren_comma_seq(|p| {
-            p.parse_pat_allow_top_alt(None, RecoverComma::No, RecoverColon::No)
+            p.parse_pat_allow_top_alt(
+                None,
+                RecoverComma::No,
+                RecoverColon::No,
+                CommaRecoveryMode::EitherTupleOrPipe,
+            )
         })?;
         if qself.is_some() {
             self.sess.gated_spans.gate(sym::more_qualified_paths, path.span);
@@ -1033,7 +1061,12 @@ impl<'a> Parser<'a> {
             // Parsing a pattern of the form `fieldname: pat`.
             let fieldname = self.parse_field_name()?;
             self.bump();
-            let pat = self.parse_pat_allow_top_alt(None, RecoverComma::No, RecoverColon::No)?;
+            let pat = self.parse_pat_allow_top_alt(
+                None,
+                RecoverComma::No,
+                RecoverColon::No,
+                CommaRecoveryMode::EitherTupleOrPipe,
+            )?;
             hi = pat.span;
             (pat, fieldname, false)
         } else {