Hint on unknown escape of Unicode quotation marks in string literal

compiler/rustc_parse/src/lexer/unescape_error_reporting.rs

@@ -158,6 +158,20 @@ pub(crate) fn emit_unescape_error(
                     "this is an isolated carriage return; consider checking your editor and \
                      version control settings",
                 );
+            } else if looks_like_quote(c) {
+                diag.span_suggestion(
+                    err_span,
+                    "if you meant to use a unicode quote; \
+                    consider using its escaped form for clarity",
+                    // lit.replace(c, &ec[1..]),
+                    &ec,
+                    Applicability::MaybeIncorrect,
+                );
+
+                diag.help(format!(
+                    "{ec} is not an ASCII quote, but may look like one in some fonts; \
+                     consider writing it in its escaped form for clarity."
+                ));
             } else {
                 if mode == Mode::Str || mode == Mode::Char {
                     diag.span_suggestion(
@@ -295,3 +309,17 @@ pub(crate) fn escaped_char(c: char) -> String {
         _ => c.escape_default().to_string(),
     }
 }
+
+/// Returns true if `c` may look identical to `"` in some fonts.
+fn looks_like_quote(c: char) -> bool {
+    // list of homoglyphs generated using the following wikidata query:
+    // SELECT ?u WHERE {
+    //   wd:Q87495536 wdt:P2444+ ?c.
+    //   ?c wdt:P4213 ?u.
+    // }
+    match c {
+        '\u{2033}' | '\u{02BA}' | '\u{02DD}' | '\u{030B}' | '\u{030E}' | '\u{05F4}'
+        | '\u{201C}' | '\u{201D}' => true,
+        _ => false,
+    }
+}

tests/ui/unicode-quote.rs

@@ -0,0 +1,20 @@
+// Provide a useful error message when attempting to escape a non-ascii quotation mark.
+// <https://github.com/rust-lang/rust/issues/128858>
+fn main() {
+    dbg!("\″"); //U+2033
+    //~^ERROR unknown character escape: `\u{2033}`
+    dbg!("\ʺ"); //U+02BA
+    //~^ERROR 6:12: 6:13: unknown character escape: `\u{2ba}`
+    dbg!("\˝"); //U+02DD
+    //~^ERROR 8:12: 8:13: unknown character escape: `\u{2dd}`
+    dbg!("\̋"); //U+030B
+    //~^ERROR 10:12: 10:13: unknown character escape: `\u{30b}`
+    dbg!("\̎"); //U+030E
+    //~^ERROR 12:12: 12:13: unknown character escape: `\u{30e}`
+    dbg!("\״"); //U+05F4
+    //~^ERROR 14:12: 14:13: unknown character escape: `\u{5f4}`
+    dbg!("\“"); //U+201C
+    //~^ERROR 16:12: 16:13: unknown character escape: `\u{201c}`
+    dbg!("\”"); //U+201D
+    //~^ERROR 18:12: 18:13: unknown character escape: `\u{201d}`
+}

tests/ui/unicode-quote.stderr

@@ -0,0 +1,106 @@
+error: unknown character escape: `\u{2033}`
+  --> $DIR/unicode-quote.rs:4:12
+   |
+LL |     dbg!("\″"); //U+2033
+   |            ^ unknown character escape
+   |
+   = help: \u{2033} is not an ascii quote, but may look like one in some fonts; consider writing it in its escaped form for clarity.
+help: if you meant to use a unicode quote; consider using its escaped form for clarity
+   |
+LL -     dbg!("\″"); //U+2033
+LL +     dbg!("\u{2033}"); //U+2033
+   |
+
+error: unknown character escape: `\u{2ba}`
+  --> $DIR/unicode-quote.rs:6:12
+   |
+LL |     dbg!("\ʺ"); //U+02BA
+   |            ^ unknown character escape
+   |
+   = help: \u{2ba} is not an ascii quote, but may look like one in some fonts; consider writing it in its escaped form for clarity.
+help: if you meant to use a unicode quote; consider using its escaped form for clarity
+   |
+LL -     dbg!("\ʺ"); //U+02BA
+LL +     dbg!("\u{2ba}"); //U+02BA
+   |
+
+error: unknown character escape: `\u{2dd}`
+  --> $DIR/unicode-quote.rs:8:12
+   |
+LL |     dbg!("\˝"); //U+02DD
+   |            ^ unknown character escape
+   |
+   = help: \u{2dd} is not an ascii quote, but may look like one in some fonts; consider writing it in its escaped form for clarity.
+help: if you meant to use a unicode quote; consider using its escaped form for clarity
+   |
+LL -     dbg!("\˝"); //U+02DD
+LL +     dbg!("\u{2dd}"); //U+02DD
+   |
+
+error: unknown character escape: `\u{30b}`
+  --> $DIR/unicode-quote.rs:10:12
+   |
+LL |     dbg!("\̋"); //U+030B
+   |            ^ unknown character escape
+   |
+   = help: \u{30b} is not an ascii quote, but may look like one in some fonts; consider writing it in its escaped form for clarity.
+help: if you meant to use a unicode quote; consider using its escaped form for clarity
+   |
+LL -     dbg!("\̋"); //U+030B
+LL +     dbg!("\u{30b}"); //U+030B
+   |
+
+error: unknown character escape: `\u{30e}`
+  --> $DIR/unicode-quote.rs:12:12
+   |
+LL |     dbg!("\̎"); //U+030E
+   |            ^ unknown character escape
+   |
+   = help: \u{30e} is not an ascii quote, but may look like one in some fonts; consider writing it in its escaped form for clarity.
+help: if you meant to use a unicode quote; consider using its escaped form for clarity
+   |
+LL -     dbg!("\̎"); //U+030E
+LL +     dbg!("\u{30e}"); //U+030E
+   |
+
+error: unknown character escape: `\u{5f4}`
+  --> $DIR/unicode-quote.rs:14:12
+   |
+LL |     dbg!("\״"); //U+05F4
+   |            ^ unknown character escape
+   |
+   = help: \u{5f4} is not an ascii quote, but may look like one in some fonts; consider writing it in its escaped form for clarity.
+help: if you meant to use a unicode quote; consider using its escaped form for clarity
+   |
+LL -     dbg!("\״"); //U+05F4
+LL +     dbg!("\u{5f4}"); //U+05F4
+   |
+
+error: unknown character escape: `\u{201c}`
+  --> $DIR/unicode-quote.rs:16:12
+   |
+LL |     dbg!("\“"); //U+201C
+   |            ^ unknown character escape
+   |
+   = help: \u{201c} is not an ascii quote, but may look like one in some fonts; consider writing it in its escaped form for clarity.
+help: if you meant to use a unicode quote; consider using its escaped form for clarity
+   |
+LL -     dbg!("\“"); //U+201C
+LL +     dbg!("\u{201c}"); //U+201C
+   |
+
+error: unknown character escape: `\u{201d}`
+  --> $DIR/unicode-quote.rs:18:12
+   |
+LL |     dbg!("\”"); //U+201D
+   |            ^ unknown character escape
+   |
+   = help: \u{201d} is not an ascii quote, but may look like one in some fonts; consider writing it in its escaped form for clarity.
+help: if you meant to use a unicode quote; consider using its escaped form for clarity
+   |
+LL -     dbg!("\”"); //U+201D
+LL +     dbg!("\u{201d}"); //U+201D
+   |
+
+error: aborting due to 8 previous errors
+