Make underscore_literal_suffix a hard error.

compiler/rustc_lexer/src/lib.rs

@@ -88,7 +88,9 @@ pub enum TokenKind {
  /// tokens.
  UnknownPrefix,
 
- /// Examples: `"12_u8"`, `"1.0e-40"`, `b"123`.
+ /// Examples: `12u8`, `1.0e-40`, `b"123"`. Note that `_` is an invalid
+ /// suffix, but may be present here on string and float literals. Users of
+ /// this type will need to check for and reject that case.
  ///
  /// See [LiteralKind] for more details.
  Literal { kind: LiteralKind, suffix_start: u32 },
@@ -840,12 +842,13 @@ impl Cursor<'_> {
  self.eat_decimal_digits()
  }
 
- // Eats the suffix of the literal, e.g. "_u8".
+ // Eats the suffix of the literal, e.g. "u8".
  fn eat_literal_suffix(&mut self) {
  self.eat_identifier();
  }
 
- // Eats the identifier.
+ // Eats the identifier. Note: succeeds on `_`, which isn't a valid
+ // identifer.
  fn eat_identifier(&mut self) {
  if !is_id_start(self.first()) {
  return;

compiler/rustc_parse/src/lexer/mod.rs

@@ -175,20 +175,10 @@ impl<'a> StringReader<'a> {
  if string == "_" {
  self.sess
  .span_diagnostic
- .struct_span_warn(
+ .struct_span_err(
  self.mk_sp(suffix_start, self.pos),
  "underscore literal suffix is not allowed",
  )
- .warn(
- "this was previously accepted by the compiler but is \
- being phased out; it will become a hard error in \
- a future release!",
- )
- .note(
- "see issue #42326 \
- <https://github.com/rust-lang/rust/issues/42326> \
- for more information",
- )
  .emit();
  None
  } else {

src/test/ui/parser/underscore-suffix-for-string.rs

@@ -1,8 +1,17 @@
-// check-pass
+macro_rules! sink {
+ ($tt:tt) => {()}
+}
 
 fn main() {
  let _ = "Foo"_;
- //~^ WARNING underscore literal suffix is not allowed
- //~| WARNING this was previously accepted
- //~| NOTE issue #42326
+ //~^ ERROR underscore literal suffix is not allowed
+
+ // This is ok, because `__` is a valid identifier and the macro consumes it
+ // before proper parsing happens.
+ let _ = sink!("Foo"__);
+
+ // This is not ok, even as an input to a macro, because the `_` suffix is
+ // never allowed.
+ sink!("Foo"_);
+ //~^ ERROR underscore literal suffix is not allowed
 }

src/test/ui/parser/underscore-suffix-for-string.stderr

@@ -1,11 +1,14 @@
-warning: underscore literal suffix is not allowed
- --> $DIR/underscore-suffix-for-string.rs:4:18
+error: underscore literal suffix is not allowed
+ --> $DIR/underscore-suffix-for-string.rs:6:18
  |
 LL | let _ = "Foo"_;
  | ^
+
+error: underscore literal suffix is not allowed
+ --> $DIR/underscore-suffix-for-string.rs:15:16
  |
- = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
- = note: see issue #42326 <https://github.com/rust-lang/rust/issues/42326> for more information
+LL | sink!("Foo"_);
+ | ^
 
-warning: 1 warning emitted
+error: aborting due to 2 previous errors