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Fix error reporting for multibyte characters in byte string literal #139362

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Fix error reporting for multibyte characters in byte string literal #139362

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0031807
Improve unescape error reporting for multibyte characters in byte lit…
tapanprakasht Apr 4, 2025
7896e94
Fix failing test case
tapanprakasht Apr 4, 2025
8aa9676
Fix test case
tapanprakasht Apr 4, 2025
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11 changes: 1 addition & 10 deletions compiler/rustc_parse/src/lexer/unescape_error_reporting.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -199,16 +199,7 @@ pub(crate) fn emit_unescape_error(
err.span_label(span, format!("must be ASCII{postfix}"));
// Note: the \\xHH suggestions are not given for raw byte string
// literals, because they are araw and so cannot use any escapes.
if (c as u32) <= 0xFF && mode != Mode::RawByteStr {
err.span_suggestion(
span,
format!(
"if you meant to use the unicode code point for {c:?}, use a \\xHH escape"
),
format!("\\x{:X}", c as u32),
Applicability::MaybeIncorrect,
);
} else if mode == Mode::Byte {
if mode == Mode::Byte {
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I think there may also need to be consideration of the situation in the origin code, where it's possible that users may want to use single bytes, so we can explicitly state that the suggested value is a single byte value, not a UTF-8 encoding.
So, we could add this suggestion above if mode == Mode::Byte { such as below.

          if (c as u32) <= 0xFF && mode != Mode::RawByteStr {
              err.span_suggestion(
                  span,
                  format!(
                      "if you meant to use the byte with hex value {:#04X} (note: this is not the Unicode code point for {c:?})", 
                      c as u32
                  ),
                  format!("\\x{:02X}", c as u32),
                  Applicability::MaybeIncorrect,
              );
          }

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I believe that if a user explicitly specifies a character using a byte literal, their intention is likely to use the actual byte encoding. Otherwise, they would have directly provided the correct hex value.
Additionally, when dealing with a single byte value, it suggests a Unicode code point in hex even though that's not the full encoding, while for a multi-byte value, it throws a different error. That's more of an inconsistent behaviour.

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@Nadrieril Nadrieril Apr 6, 2025 •
edited
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I don't know enough about typical uses of byte chars. We should at the very least explicitly mention that what we're suggesting is not an utf8 encoding? For reference, this behavior was introduced in #87659 to fix #87397.

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@Nadrieril Nadrieril Apr 6, 2025 •
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Is it true that we can only reach this branch for multibyte characters? My limited knowledge of utf8 would suggest we do the obviously-correct thing:

if mode != Mode::RawByteStr {
	let mut utf8 = String::new();
	utf8.push(c);
	if mode == Mode::Byte && utf8.as_bytes().count() > 1 {
	    // error "this is multibyte"
	} else {
	    // suggest \xHH
	}
}

err.span_label(span, "this multibyte character does not fit into a single byte");
} else if mode != Mode::RawByteStr {
let mut utf8 = String::new();
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11 changes: 4 additions & 7 deletions tests/ui/parser/byte-literals.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -46,13 +46,10 @@ error: non-ASCII character in byte literal
--> $DIR/byte-literals.rs:10:7
|
LL | b'é';
| ^ must be ASCII
|
help: if you meant to use the unicode code point for 'é', use a \xHH escape
|
LL - b'é';
LL + b'\xE9';
|
| ^
| |
| must be ASCII
| this multibyte character does not fit into a single byte

error[E0763]: unterminated byte constant
--> $DIR/byte-literals.rs:11:6
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4 changes: 2 additions & 2 deletions tests/ui/parser/byte-string-literals.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -26,10 +26,10 @@ error: non-ASCII character in byte string literal
LL | b"é";
| ^ must be ASCII
|
help: if you meant to use the unicode code point for 'é', use a \xHH escape
help: if you meant to use the UTF-8 encoding of 'é', use \xHH escapes
|
LL - b"é";
LL + b"\xE9";
LL + b"\xC3\xA9";
|

error: non-ASCII character in raw byte string literal
Expand Down
12 changes: 11 additions & 1 deletion tests/ui/suggestions/multibyte-escapes.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -3,7 +3,7 @@
fn main() {
b'µ';
//~^ ERROR: non-ASCII character in byte literal
//~| HELP: if you meant to use the unicode code point for 'µ', use a \xHH escape
//~| NOTE: this multibyte character does not fit into a single byte
//~| NOTE: must be ASCII

b'字';
Expand All @@ -15,4 +15,14 @@ fn main() {
//~^ ERROR: non-ASCII character in byte string literal
//~| HELP: if you meant to use the UTF-8 encoding of '字', use \xHH escapes
//~| NOTE: must be ASCII

b"µ";
//~^ ERROR: non-ASCII character in byte string literal
//~| HELP: if you meant to use the UTF-8 encoding of 'µ', use \xHH escapes
//~| NOTE: must be ASCII

b"ñ";
//~^ ERROR: non-ASCII character in byte string literal
//~| HELP: if you meant to use the UTF-8 encoding of 'ñ', use \xHH escapes
//~| NOTE: must be ASCII
}
37 changes: 29 additions & 8 deletions tests/ui/suggestions/multibyte-escapes.stderr
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Original file line number Diff line number Diff line change
Expand Up @@ -2,13 +2,10 @@ error: non-ASCII character in byte literal
--> $DIR/multibyte-escapes.rs:4:7
|
LL | b'µ';
| ^ must be ASCII
|
help: if you meant to use the unicode code point for 'µ', use a \xHH escape
|
LL - b'µ';
LL + b'\xB5';
|
| ^
| |
| must be ASCII
| this multibyte character does not fit into a single byte

error: non-ASCII character in byte literal
--> $DIR/multibyte-escapes.rs:9:7
Expand All @@ -31,5 +28,29 @@ LL - b"字";
LL + b"\xE5\xAD\x97";
|

error: aborting due to 3 previous errors
error: non-ASCII character in byte string literal
--> $DIR/multibyte-escapes.rs:19:7
|
LL | b"µ";
| ^ must be ASCII
|
help: if you meant to use the UTF-8 encoding of 'µ', use \xHH escapes
|
LL - b"µ";
LL + b"\xC2\xB5";
|

error: non-ASCII character in byte string literal
--> $DIR/multibyte-escapes.rs:24:7
|
LL | b"ñ";
| ^ must be ASCII
|
help: if you meant to use the UTF-8 encoding of 'ñ', use \xHH escapes
|
LL - b"ñ";
LL + b"\xC3\xB1";
|

error: aborting due to 5 previous errors

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