Add double-equals homoglyph

compiler/rustc_parse/src/lexer/unicode_chars.rs

@@ -296,6 +296,7 @@
     ('〉', "Right Angle Bracket", ">"),
     ('》', "Right Double Angle Bracket", ">"),
     ('＞', "Fullwidth Greater-Than Sign", ">"),
+    ('⩵', "Two Consecutive Equals Signs", "==")
 ];
 
 // FIXME: the lexer could be used to turn the ASCII version of unicode homoglyphs, instead of
@@ -325,6 +326,7 @@
     ("+", "Plus Sign", Some(token::BinOp(token::Plus))),
     ("<", "Less-Than Sign", Some(token::Lt)),
     ("=", "Equals Sign", Some(token::Eq)),
+    ("==", "Double Equals Sign", Some(token::EqEq)),
     (">", "Greater-Than Sign", Some(token::Gt)),
     // FIXME: Literals are already lexed by this point, so we can't recover gracefully just by
     // spitting the correct token out.

tests/ui/parser/unicode-chars.rs

@@ -6,4 +6,7 @@ fn main() {
     //~^ ERROR unknown start of token: \u{a0}
     //~^^ NOTE character appears 3 more times
     //~^^^ HELP Unicode character ' ' (No-Break Space) looks like ' ' (Space), but it is not
+    let _ = 1 ⩵ 2;
+    //~^ ERROR unknown start of token
+    //~^^ HELP Unicode character '⩵' (Two Consecutive Equals Signs) looks like '==' (Double Equals Sign), but it is not
 }

tests/ui/parser/unicode-chars.stderr

@@ -21,5 +21,16 @@ help: Unicode character ' ' (No-Break Space) looks like ' ' (Space), but it is
 LL |         let x = 0;
    |     ++++
 
-error: aborting due to 2 previous errors
+error: unknown start of token: \u{2a75}
+  --> $DIR/unicode-chars.rs:9:15
+   |
+LL |     let _ = 1 ⩵ 2;
+   |               ^
+   |
+help: Unicode character '⩵' (Two Consecutive Equals Signs) looks like '==' (Double Equals Sign), but it is not
+   |
+LL |     let _ = 1 == 2;
+   |               ~~
+
+error: aborting due to 3 previous errors