Make the "extra if in let...else block" hint a suggestion

compiler/rustc_parse/src/errors.rs

@@ -351,7 +351,7 @@ pub(crate) enum IfExpressionMissingThenBlockSub {
 }
 
 #[derive(Subdiagnostic)]
-#[help(parse_extra_if_in_let_else)]
+#[suggestion(parse_extra_if_in_let_else, applicability = "maybe-incorrect", code = "")]
 pub(crate) struct IfExpressionLetSomeSub {
     #[primary_span]
     pub if_span: Span,

compiler/rustc_parse/src/parser/expr.rs

@@ -2281,7 +2281,7 @@ impl<'a> Parser<'a> {
                 block
             } else {
                 let let_else_sub = matches!(cond.kind, ExprKind::Let(..))
-                    .then(|| IfExpressionLetSomeSub { if_span: lo });
+                    .then(|| IfExpressionLetSomeSub { if_span: lo.until(cond_span) });
 
                 self.sess.emit_err(IfExpressionMissingThenBlock {
                     if_span: lo,

tests/ui/let-else/accidental-if.stderr

@@ -10,10 +10,10 @@ help: add a block here
 LL |     if let Some(y) = x else {
    |                       ^
 help: remove the `if` if you meant to write a `let...else` statement
-  --> $DIR/accidental-if.rs:3:5
    |
-LL |     if let Some(y) = x else {
-   |     ^^
+LL -     if let Some(y) = x else {
+LL +     let Some(y) = x else {
+   |
 
 error: aborting due to previous error
 

tests/ui/rfc-2497-if-let-chains/ensure-that-let-else-does-not-interact-with-let-chains.stderr

@@ -38,10 +38,10 @@ help: add a block here
 LL |     if let Some(n) = opt else {
    |                         ^
 help: remove the `if` if you meant to write a `let...else` statement
-  --> $DIR/ensure-that-let-else-does-not-interact-with-let-chains.rs:24:5
    |
-LL |     if let Some(n) = opt else {
-   |     ^^
+LL -     if let Some(n) = opt else {
+LL +     let Some(n) = opt else {
+   |
 
 error: this `if` expression is missing a block after the condition
   --> $DIR/ensure-that-let-else-does-not-interact-with-let-chains.rs:28:5