Account for confusable codepoints when recovering emoji identifiers

compiler/rustc_parse/src/lexer/mod.rs

@@ -1,3 +1,4 @@
+use crate::lexer::unicode_chars::UNICODE_ARRAY;
 use rustc_ast::ast::{self, AttrStyle};
 use rustc_ast::token::{self, CommentKind, Token, TokenKind};
 use rustc_ast::tokenstream::{Spacing, TokenStream};
@@ -222,7 +223,17 @@ impl<'a> StringReader<'a> {
                 }
                 token::Ident(sym, is_raw_ident)
             }
-            rustc_lexer::TokenKind::InvalidIdent => {
+            rustc_lexer::TokenKind::InvalidIdent
+                // Do not recover an identifier with emojis if the codepoint is a confusable
+                // with a recoverable substitution token, like `➖`.
+                if UNICODE_ARRAY
+                    .iter()
+                    .find(|&&(c, _, _)| {
+                        let sym = self.str_from(start);
+                        sym.chars().count() == 1 && c == sym.chars().next().unwrap()
+                    })
+                    .is_none() =>
+            {
                 let sym = nfc_normalize(self.str_from(start));
                 let span = self.mk_sp(start, self.pos);
                 self.sess.bad_unicode_identifiers.borrow_mut().entry(sym).or_default().push(span);
@@ -299,7 +310,7 @@ impl<'a> StringReader<'a> {
             rustc_lexer::TokenKind::Caret => token::BinOp(token::Caret),
             rustc_lexer::TokenKind::Percent => token::BinOp(token::Percent),
 
-            rustc_lexer::TokenKind::Unknown => {
+            rustc_lexer::TokenKind::Unknown | rustc_lexer::TokenKind::InvalidIdent => {
                 let c = self.str_from(start).chars().next().unwrap();
                 let mut err =
                     self.struct_fatal_span_char(start, self.pos, "unknown start of token", c);

compiler/rustc_parse/src/lexer/unicode_chars.rs

@@ -7,7 +7,7 @@ use rustc_errors::{Applicability, DiagnosticBuilder};
 use rustc_span::{symbol::kw, BytePos, Pos, Span};
 
 #[rustfmt::skip] // for line breaks
-const UNICODE_ARRAY: &[(char, &str, char)] = &[
+pub(crate) const UNICODE_ARRAY: &[(char, &str, char)] = &[
     ('
', "Line Separator", ' '),
     ('
', "Paragraph Separator", ' '),
     (' ', "Ogham Space mark", ' '),

src/test/ui/parser/emoji-identifiers.rs

@@ -10,6 +10,7 @@ fn i_like_to_😅_a_lot() -> 👀 { //~ ERROR identifiers cannot contain emojis
     //~^ ERROR identifiers cannot contain emojis
 }
 fn main() {
-    let _ = i_like_to_😄_a_lot(); //~ ERROR cannot find function `i_like_to_😄_a_lot` in this scope
+    let _ = i_like_to_😄_a_lot() ➖ 4; //~ ERROR cannot find function `i_like_to_😄_a_lot` in this scope
     //~^ ERROR identifiers cannot contain emojis
+    //~| ERROR unknown start of token: \u{2796}
 }

src/test/ui/parser/emoji-identifiers.stderr

@@ -1,16 +1,27 @@
+error: unknown start of token: \u{2796}
+  --> $DIR/emoji-identifiers.rs:13:33
+   |
+LL |     let _ = i_like_to_😄_a_lot() ➖ 4;
+   |                                  ^^
+   |
+help: Unicode character '➖' (Heavy Minus Sign) looks like '-' (Minus/Hyphen), but it is not
+   |
+LL |     let _ = i_like_to_😄_a_lot() - 4;
+   |                                  ~
+
 error[E0425]: cannot find function `i_like_to_😄_a_lot` in this scope
   --> $DIR/emoji-identifiers.rs:13:13
    |
 LL | fn i_like_to_😅_a_lot() -> 👀 {
    | ----------------------------- similarly named function `i_like_to_😅_a_lot` defined here
 ...
-LL |     let _ = i_like_to_😄_a_lot();
+LL |     let _ = i_like_to_😄_a_lot() ➖ 4;
    |             ^^^^^^^^^^^^^^^^^^ help: a function with a similar name exists: `i_like_to_😅_a_lot`
 
 error: identifiers cannot contain emojis: `i_like_to_😄_a_lot`
   --> $DIR/emoji-identifiers.rs:13:13
    |
-LL |     let _ = i_like_to_😄_a_lot();
+LL |     let _ = i_like_to_😄_a_lot() ➖ 4;
    |             ^^^^^^^^^^^^^^^^^^
 
 error: identifiers cannot contain emojis: `full_of_✨`
@@ -66,7 +77,7 @@ LL |     👀::full_of✨()
    |         function or associated item not found in `👀`
    |         help: there is an associated function with a similar name: `full_of_✨`
 
-error: aborting due to 8 previous errors
+error: aborting due to 9 previous errors
 
 Some errors have detailed explanations: E0425, E0599.
 For more information about an error, try `rustc --explain E0425`.