Fix ICE in suggestion caused by ⩵ being recovered as ==

The second suggestion shown here would previously incorrectly assume that the span corresponding to `⩵` was 2 bytes wide composed by 2 1 byte wide chars, so a span pointing at `==` could point only at one of the `=` to remove it. Instead, we now replace the whole thing (as we should have the whole time):

```
error: unknown start of token: \u{2a75}
  --> $DIR/unicode-double-equals-recovery.rs:1:16
   |
LL | const A: usize ⩵ 2;
   |                ^
   |
help: Unicode character '⩵' (Two Consecutive Equals Signs) looks like '==' (Double Equals Sign), but it is not
   |
LL | const A: usize == 2;
   |                ~~

error: unexpected `==`
  --> $DIR/unicode-double-equals-recovery.rs:1:16
   |
LL | const A: usize ⩵ 2;
   |                ^
   |
help: try using `=` instead
   |
LL | const A: usize = 2;
   |                ~
```

Author: estebank

compiler/rustc_parse/src/errors.rs

@@ -660,9 +660,8 @@ pub(crate) struct RemoveLet {
 #[diag(parse_use_eq_instead)]
 pub(crate) struct UseEqInstead {
     #[primary_span]
+    #[suggestion(style = "verbose", applicability = "machine-applicable", code = "=")]
     pub span: Span,
-    #[suggestion(style = "verbose", applicability = "machine-applicable", code = "")]
-    pub suggestion: Span,
 }
 
 #[derive(Diagnostic)]

compiler/rustc_parse/src/parser/diagnostics.rs

@@ -566,10 +566,7 @@ impl<'a> Parser<'a> {
             && expected.iter().any(|tok| matches!(tok, TokenType::Token(TokenKind::Eq)))
         {
             // Likely typo: `=` → `==` in let expr or enum item
-            return Err(self.dcx().create_err(UseEqInstead {
-                span: self.token.span,
-                suggestion: self.token.span.with_lo(self.token.span.lo() + BytePos(1)),
-            }));
+            return Err(self.dcx().create_err(UseEqInstead { span: self.token.span }));
         }
 
         if self.token.is_keyword(kw::Move) && self.prev_token.is_keyword(kw::Async) {

tests/ui/parser/recover/unicode-double-equals-recovery.rs

@@ -0,0 +1,3 @@
+const A: usize ⩵ 2; 
+//~^ ERROR unknown start of token: \u{2a75}
+//~| ERROR unexpected `==`

tests/ui/parser/recover/unicode-double-equals-recovery.stderr

@@ -0,0 +1,24 @@
+error: unknown start of token: \u{2a75}
+  --> $DIR/unicode-double-equals-recovery.rs:1:16
+   |
+LL | const A: usize ⩵ 2; 
+   |                ^
+   |
+help: Unicode character '⩵' (Two Consecutive Equals Signs) looks like '==' (Double Equals Sign), but it is not
+   |
+LL | const A: usize == 2; 
+   |                ~~
+
+error: unexpected `==`
+  --> $DIR/unicode-double-equals-recovery.rs:1:16
+   |
+LL | const A: usize ⩵ 2; 
+   |                ^
+   |
+help: try using `=` instead
+   |
+LL | const A: usize = 2; 
+   |                ~
+
+error: aborting due to 2 previous errors
+