Rollup merge of #120460 - nnethercote:fix-120397, r=compiler-errors

Be more careful about interpreting a label/lifetime as a mistyped char literal.

Currently the parser interprets any label/lifetime in certain positions as a mistyped char literal, on the assumption that the trailing single quote was accidentally omitted. In such cases it gives an error with a suggestion to add the trailing single quote, and then puts the appropriate char literal into the AST. This behaviour was introduced in #101293.

This is reasonable for a case like this:
```
let c = 'a;
```
because `'a'` is a valid char literal. It's less reasonable for a case like this:
```
let c = 'abc;
```
because `'abc'` is not a valid char literal.

Prior to #120329 this could result in some sub-optimal suggestions in error messages, but nothing else. But #120329 changed `LitKind::from_token_lit` to assume that the char/byte/string literals it receives are valid, and to assert if not. This is reasonable because the lexer does not produce invalid char/byte/string literals in general. But in this "interpret label/lifetime as unclosed char literal" case the parser can produce an invalid char literal with contents such as `abc`, which triggers an assertion failure.

This PR changes the parser so it's more cautious about interpreting labels/lifetimes as unclosed char literals.

Fixes #120397.

r? `@compiler-errors`

1 files changed, 22 insertions, 9 deletions

diff --git a/compiler/rustc_parse/src/parser/expr.rs b/compiler/rustc_parse/src/parser/expr.rs
index c395decab12..9f748e2a3fe 100644
--- a/compiler/rustc_parse/src/parser/expr.rs
+++ b/compiler/rustc_parse/src/parser/expr.rs
@@ -28,6 +28,7 @@ use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_errors::{
     AddToDiagnostic, Applicability, Diagnostic, DiagnosticBuilder, PResult, StashKey,
 };
+use rustc_lexer::unescape::unescape_char;
 use rustc_macros::Subdiagnostic;
 use rustc_session::errors::{report_lit_error, ExprParenthesesNeeded};
 use rustc_session::lint::builtin::BREAK_WITH_LABEL_AND_LOOP;
@@ -1665,6 +1666,7 @@ impl<'a> Parser<'a> {
             && self.may_recover()
             && (matches!(self.token.kind, token::CloseDelim(_) | token::Comma)
                 || self.token.is_punct())
+            && could_be_unclosed_char_literal(label_.ident)
         {
             let (lit, _) =
                 self.recover_unclosed_char(label_.ident, Parser::mk_token_lit_char, |self_| {
@@ -1750,16 +1752,17 @@ impl<'a> Parser<'a> {
         Ok(expr)
     }
 
-    /// Emit an error when a char is parsed as a lifetime because of a missing quote.
+    /// Emit an error when a char is parsed as a lifetime or label because of a missing quote.
     pub(super) fn recover_unclosed_char<L>(
         &self,
-        lifetime: Ident,
+        ident: Ident,
         mk_lit_char: impl FnOnce(Symbol, Span) -> L,
         err: impl FnOnce(&Self) -> DiagnosticBuilder<'a>,
     ) -> L {
-        if let Some(diag) = self.dcx().steal_diagnostic(lifetime.span, StashKey::LifetimeIsChar) {
+        assert!(could_be_unclosed_char_literal(ident));
+        if let Some(diag) = self.dcx().steal_diagnostic(ident.span, StashKey::LifetimeIsChar) {
             diag.with_span_suggestion_verbose(
-                lifetime.span.shrink_to_hi(),
+                ident.span.shrink_to_hi(),
                 "add `'` to close the char literal",
                 "'",
                 Applicability::MaybeIncorrect,
@@ -1768,15 +1771,15 @@ impl<'a> Parser<'a> {
         } else {
             err(self)
                 .with_span_suggestion_verbose(
-                    lifetime.span.shrink_to_hi(),
+                    ident.span.shrink_to_hi(),
                     "add `'` to close the char literal",
                     "'",
                     Applicability::MaybeIncorrect,
                 )
                 .emit();
         }
-        let name = lifetime.without_first_quote().name;
-        mk_lit_char(name, lifetime.span)
+        let name = ident.without_first_quote().name;
+        mk_lit_char(name, ident.span)
     }
 
     /// Recover on the syntax `do catch { ... }` suggesting `try { ... }` instead.
@@ -2047,8 +2050,11 @@ impl<'a> Parser<'a> {
             let msg = format!("unexpected token: {}", super::token_descr(&token));
             self_.dcx().struct_span_err(token.span, msg)
         };
-        // On an error path, eagerly consider a lifetime to be an unclosed character lit
-        if self.token.is_lifetime() {
+        // On an error path, eagerly consider a lifetime to be an unclosed character lit, if that
+        // makes sense.
+        if let Some(ident) = self.token.lifetime()
+            && could_be_unclosed_char_literal(ident)
+        {
             let lt = self.expect_lifetime();
             Ok(self.recover_unclosed_char(lt.ident, mk_lit_char, err))
         } else {
@@ -3776,6 +3782,13 @@ impl<'a> Parser<'a> {
     }
 }
 
+/// Could this lifetime/label be an unclosed char literal? For example, `'a`
+/// could be, but `'abc` could not.
+pub(crate) fn could_be_unclosed_char_literal(ident: Ident) -> bool {
+    ident.name.as_str().starts_with('\'')
+        && unescape_char(ident.without_first_quote().name.as_str()).is_ok()
+}
+
 /// Used to forbid `let` expressions in certain syntactic locations.
 #[derive(Clone, Copy, Subdiagnostic)]
 pub(crate) enum ForbiddenLetReason {