Ensure let stmt compound assignment removal suggestion respect codepoint boundaries

compiler/rustc_parse/src/parser/stmt.rs

@@ -408,10 +408,14 @@ impl<'a> Parser<'a> {
     fn parse_initializer(&mut self, eq_optional: bool) -> PResult<'a, Option<P<Expr>>> {
         let eq_consumed = match self.token.kind {
             token::BinOpEq(..) => {
-                // Recover `let x <op>= 1` as `let x = 1`
+                // Recover `let x <op>= 1` as `let x = 1` We must not use `+ BytePos(1)` here
+                // because `<op>` can be a multi-byte lookalike that was recovered, e.g. `➖=` (the
+                // `➖` is a U+2796 Heavy Minus Sign Unicode Character) that was recovered as a
+                // `-=`.
+                let extra_op_span = self.psess.source_map().start_point(self.token.span);
                 self.dcx().emit_err(errors::CompoundAssignmentExpressionInLet {
                     span: self.token.span,
-                    suggestion: self.token.span.with_hi(self.token.span.lo() + BytePos(1)),
+                    suggestion: extra_op_span,
                 });
                 self.bump();
                 true

tests/ui/parser/suggest-remove-compount-assign-let-ice.rs

@@ -0,0 +1,16 @@
+//! Previously we would try to issue a suggestion for `let x <op>= 1`, i.e. a compound assignment
+//! within a `let` binding, to remove the `<op>`. The suggestion code unfortunately incorrectly
+//! assumed that the `<op>` is an exactly-1-byte ASCII character, but this assumption is incorrect
+//! because we also recover Unicode-confusables like `➖=` as `-=`. In this example, the suggestion
+//! code used a `+ BytePos(1)` to calculate the span of the `<op>` codepoint that looks like `-` but
+//! the mult-byte Unicode look-alike would cause the suggested removal span to be inside a
+//! multi-byte codepoint boundary, triggering a codepoint boundary assertion.
+//!
+//! issue: rust-lang/rust#128845
+
+fn main() {
+    // Adapted from #128845 but with irrelevant components removed and simplified.
+    let x ➖= 1;
+    //~^ ERROR unknown start of token: \u{2796}
+    //~| ERROR: can't reassign to an uninitialized variable
+}

tests/ui/parser/suggest-remove-compount-assign-let-ice.stderr

@@ -0,0 +1,26 @@
+error: unknown start of token: \u{2796}
+  --> $DIR/suggest-remove-compount-assign-let-ice.rs:13:11
+   |
+LL |     let x ➖= 1;
+   |           ^^
+   |
+help: Unicode character '➖' (Heavy Minus Sign) looks like '-' (Minus/Hyphen), but it is not
+   |
+LL |     let x -= 1;
+   |           ~
+
+error: can't reassign to an uninitialized variable
+  --> $DIR/suggest-remove-compount-assign-let-ice.rs:13:11
+   |
+LL |     let x ➖= 1;
+   |           ^^^
+   |
+   = help: if you meant to overwrite, remove the `let` binding
+help: initialize the variable
+   |
+LL -     let x ➖= 1;
+LL +     let x = 1;
+   |
+
+error: aborting due to 2 previous errors
+