Rollup merge of rust-lang#107709 - tialaramex:master, r=compiler-errors

Fix problem noticed in PR106859 with char -> u8 suggestion

HN reader `@ayosec` noticed that my rust-lang#106859 a few weeks back, malfunctions if you have a Unicode escape, the code suggested b'\u{0}' if you tried to use '\u{0}' where a byte should be, when of course b'\u{0}' is not a byte literal, regardless of the codepoint you can't write Unicode escapes in a byte literal at all.

My proposed fix here just checks that the "character" you wrote is fewer than 5 bytes, thus allowing \x7F and similar escapes but conveniently forbidding even the smallest Unicode escape \u{0} before offering the suggestion as before.

I have provided an updated test which includes examples which do and don't work because of this additional rule.

compiler/rustc_infer/src/infer/error_reporting/mod.rs

@@ -1922,7 +1922,8 @@ impl<'tcx> TypeErrCtxt<'_, 'tcx> {
                         (ty::Uint(ty::UintTy::U8), ty::Char) => {
                             if let Ok(code) = self.tcx.sess().source_map().span_to_snippet(span)
                                 && let Some(code) = code.strip_prefix('\'').and_then(|s| s.strip_suffix('\''))
-                                && code.chars().next().map_or(false, |c| c.is_ascii())
+                                && !code.starts_with("\\u") // forbid all Unicode escapes
+                                && code.chars().next().map_or(false, |c| c.is_ascii()) // forbids literal Unicode characters beyond ASCII
                             {
                                 err.span_suggestion(
                                     span,

tests/ui/suggestions/type-mismatch-byte-literal.rs

@@ -12,7 +12,19 @@ fn main() {
     //~^ ERROR: mismatched types [E0308]
     //~| HELP: if you meant to write a byte literal, prefix with `b`
 
+    let _a: u8 = '\x20';
+    //~^ ERROR: mismatched types [E0308]
+    //~| HELP: if you meant to write a byte literal, prefix with `b`
+
+    // Do not issue the suggestion if the char literal is a Unicode escape
+    foo('\u{0080}');
+    //~^ ERROR: mismatched types [E0308]
+
     // Do not issue the suggestion if the char literal isn't ASCII
     let _t: u8 = '€';
     //~^ ERROR: mismatched types [E0308]
+
+    // Do not issue the suggestion if the char literal isn't ASCII
+    foo('\u{1f980}');
+    //~^ ERROR: mismatched types [E0308]
 }

tests/ui/suggestions/type-mismatch-byte-literal.stderr

@@ -30,13 +30,54 @@ LL |     foo(b'#');
    |         ~~~~
 
 error[E0308]: mismatched types
-  --> $DIR/type-mismatch-byte-literal.rs:16:18
+  --> $DIR/type-mismatch-byte-literal.rs:15:18
+   |
+LL |     let _a: u8 = '\x20';
+   |             --   ^^^^^^ expected `u8`, found `char`
+   |             |
+   |             expected due to this
+   |
+help: if you meant to write a byte literal, prefix with `b`
+   |
+LL |     let _a: u8 = b'\x20';
+   |                  ~~~~~~~
+
+error[E0308]: mismatched types
+  --> $DIR/type-mismatch-byte-literal.rs:20:9
+   |
+LL |     foo('\u{0080}');
+   |     --- ^^^^^^^^^^ expected `u8`, found `char`
+   |     |
+   |     arguments to this function are incorrect
+   |
+note: function defined here
+  --> $DIR/type-mismatch-byte-literal.rs:4:4
+   |
+LL | fn foo(_t: u8) {}
+   |    ^^^ ------
+
+error[E0308]: mismatched types
+  --> $DIR/type-mismatch-byte-literal.rs:24:18
    |
 LL |     let _t: u8 = '€';
    |             --   ^^^ expected `u8`, found `char`
    |             |
    |             expected due to this
 
-error: aborting due to 3 previous errors
+error[E0308]: mismatched types
+  --> $DIR/type-mismatch-byte-literal.rs:28:9
+   |
+LL |     foo('\u{1f980}');
+   |     --- ^^^^^^^^^^^ expected `u8`, found `char`
+   |     |
+   |     arguments to this function are incorrect
+   |
+note: function defined here
+  --> $DIR/type-mismatch-byte-literal.rs:4:4
+   |
+LL | fn foo(_t: u8) {}
+   |    ^^^ ------
+
+error: aborting due to 6 previous errors
 
 For more information about this error, try `rustc --explain E0308`.