Rollup merge of #65893 - jafern14:let-expr-stable-error-messaging, r=Centril

Output previous stable  error messaging when using stable build.

Fixes #65254

As I had mentioned previously there I have the logic running right now however I'm not getting the exact same syntax highlighting as there was originally for this error.

I'm currently getting the following:
```
error: expected expression, found statement (`let`)
 --> src/main.rs:2:14
  |
2 |     let x = (let y = 6);
  |              ^^^^^^^^^
  |
  = note: variable declaration using `let` is a statement
```

I'd like to get the following instead:

```
  |     let x = (let y = 6);
  |              ^^^
```

My current understanding is that the `span` being passed into `lower_expr_let` is coming from `lowering.rs`. I still don't know how the byte range is calculated for the erroneous syntax and need to look into it a bit more. In the meantime does anybody have any hints/tips regarding this??

Author: Centril

src/librustc/hir/lowering/expr.rs

@@ -235,11 +235,20 @@ impl LoweringContext<'_> {
     /// ```
     fn lower_expr_let(&mut self, span: Span, pat: &Pat, scrutinee: &Expr) -> hir::ExprKind {
         // If we got here, the `let` expression is not allowed.
-        self.sess
-            .struct_span_err(span, "`let` expressions are not supported here")
-            .note("only supported directly in conditions of `if`- and `while`-expressions")
-            .note("as well as when nested within `&&` and parenthesis in those conditions")
-            .emit();
+
+        if self.sess.opts.unstable_features.is_nightly_build() {
+            self.sess
+                .struct_span_err(span, "`let` expressions are not supported here")
+                .note("only supported directly in conditions of `if`- and `while`-expressions")
+                .note("as well as when nested within `&&` and parenthesis in those conditions")
+                .emit();
+        }
+        else {
+            self.sess
+                .struct_span_err(span, "expected expression, found statement (`let`)")
+                .note("variable declaration using `let` is a statement")
+                .emit();
+        }
 
         // For better recovery, we emit:
         // ```