Auto merge of #41990 - qnighy:disallow-underscore-suffix-for-string-like-literals, r=nikomatsakis

Disallow underscore suffix for string-like literals.

This patch turns string/bytestring/char/byte literals followed by an underscore, like `"Foo"_`, to an error.

`scan_optional_raw_name` will parse `_` as a valid raw name, but it will be rejected by the parser. I also considered just stopping parsing when the suffix is `_`, but in that case `"Foo"_` will be lexed as two valid tokens.

Fixes the latter half of #41723.

src/libsyntax/parse/lexer/mod.rs

@@ -480,6 +480,15 @@ impl<'a> StringReader<'a> {
 
         self.with_str_from(start, |string| {
             if string == "_" {
+                self.sess.span_diagnostic
+                    .struct_span_warn(mk_sp(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("for more information, see issue #42326 \
+                          <https://github.com/rust-lang/rust/issues/42326>")
+                    .emit();
                 None
             } else {
                 Some(Symbol::intern(string))