Remove silencing logic for unknown prefix

This logic is tricky to get right. At some point, revisit using the stash mechanism.

Author: estebank

compiler/rustc_parse/src/lexer/mod.rs

@@ -701,7 +701,6 @@ impl<'psess, 'src> StringReader<'psess, 'src> {
         let expn_data = prefix_span.ctxt().outer_expn_data();
 
         if expn_data.edition >= Edition::Edition2021 {
-            let mut silence = false;
             // In Rust 2021, this is a hard error.
             let sugg = if prefix == "rb" {
                 Some(errors::UnknownPrefixSugg::UseBr(prefix_span))
@@ -710,8 +709,9 @@ impl<'psess, 'src> StringReader<'psess, 'src> {
                     && let Some(start) = self.last_lifetime
                     && self.cursor.third() != '\''
                 {
-                    // An "unclosed `char`" error will be emitted already, silence redundant error.
-                    silence = true;
+                    // FIXME: An "unclosed `char`" error will be emitted already in some cases,
+                    // but it's hard to silence this error while not also silencing important cases
+                    // too. We should use the error stashing machinery instead.
                     Some(errors::UnknownPrefixSugg::MeantStr {
                         start,
                         end: self.mk_sp(self.pos, self.pos + BytePos(1)),
@@ -722,12 +722,7 @@ impl<'psess, 'src> StringReader<'psess, 'src> {
             } else {
                 None
             };
-            let err = errors::UnknownPrefix { span: prefix_span, prefix, sugg };
-            if silence {
-                self.dcx().create_err(err).delay_as_bug();
-            } else {
-                self.dcx().emit_err(err);
-            }
+            self.dcx().emit_err(errors::UnknownPrefix { span: prefix_span, prefix, sugg });
         } else {
             // Before Rust 2021, only emit a lint for migration.
             self.psess.buffer_lint_with_diagnostic(

tests/ui/lexer/lex-bad-str-literal-as-char-3.rs

@@ -4,4 +4,5 @@
 fn main() {
     println!('hello world');
     //~^ ERROR unterminated character literal
+    //[rust2021]~| ERROR prefix `world` is unknown
 }

tests/ui/lexer/lex-bad-str-literal-as-char-3.rust2021.stderr

@@ -1,3 +1,15 @@
+error: prefix `world` is unknown
+  --> $DIR/lex-bad-str-literal-as-char-3.rs:5:21
+   |
+LL |     println!('hello world');
+   |                     ^^^^^ unknown prefix
+   |
+   = note: prefixed identifiers and literals are reserved since Rust 2021
+help: if you meant to write a string literal, use double quotes
+   |
+LL |     println!("hello world");
+   |              ~           ~
+
 error[E0762]: unterminated character literal
   --> $DIR/lex-bad-str-literal-as-char-3.rs:5:26
    |
@@ -9,6 +21,6 @@ help: if you meant to write a string literal, use double quotes
 LL |     println!("hello world");
    |              ~           ~
 
-error: aborting due to 1 previous error
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0762`.

tests/ui/lexer/lex-bad-str-literal-as-char-4.rs

@@ -3,6 +3,7 @@ macro_rules! foo {
     () => {
         println!('hello world');
         //~^ ERROR unterminated character literal
+        //~| ERROR prefix `world` is unknown
     }
 }
 fn main() {}

tests/ui/lexer/lex-bad-str-literal-as-char-4.stderr

@@ -1,3 +1,15 @@
+error: prefix `world` is unknown
+  --> $DIR/lex-bad-str-literal-as-char-4.rs:4:25
+   |
+LL |         println!('hello world');
+   |                         ^^^^^ unknown prefix
+   |
+   = note: prefixed identifiers and literals are reserved since Rust 2021
+help: if you meant to write a string literal, use double quotes
+   |
+LL |         println!("hello world");
+   |                  ~           ~
+
 error[E0762]: unterminated character literal
   --> $DIR/lex-bad-str-literal-as-char-4.rs:4:30
    |
@@ -9,6 +21,6 @@ help: if you meant to write a string literal, use double quotes
 LL |         println!("hello world");
    |                  ~           ~
 
-error: aborting due to 1 previous error
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0762`.