Suppress let else suggestion for uninitialized refutable lets

Author: eggyal

compiler/rustc_mir_build/src/thir/pattern/check_match.rs

@@ -683,11 +683,11 @@ impl<'p, 'tcx> MatchVisitor<'p, 'tcx> {
             let end_span = semi_span.shrink_to_lo();
             let count = witnesses.len();
 
-            let_suggestion = Some(if bindings.is_empty() {
-                SuggestLet::If { start_span, semi_span, count }
-            } else {
-                SuggestLet::Else { end_span, count }
-            });
+            if bindings.is_empty() {
+                let_suggestion = Some(SuggestLet::If { start_span, semi_span, count });
+            } else if scrut.is_some() {
+                let_suggestion = Some(SuggestLet::Else { end_span, count });
+            }
         };
 
         let adt_defined_here = report_adt_defined_here(self.tcx, pattern_ty, &witnesses, false);

tests/ui/let-else/uninitialized-refutable-let-issue-123844.rs

@@ -0,0 +1,10 @@
+// https://github.com/rust-lang/rust/issues/123844
+// An uninitialized refutable let should not suggest `let else`, as it can't be used with deferred
+// initialization.
+//
+// @check-fail
+
+fn main() {
+    let Some(x); //~ ERROR refutable pattern in local binding
+    x = 1;
+}

tests/ui/let-else/uninitialized-refutable-let-issue-123844.stderr

@@ -0,0 +1,13 @@
+error[E0005]: refutable pattern in local binding
+  --> $DIR/uninitialized-refutable-let-issue-123844.rs:8:9
+   |
+LL |     let Some(x);
+   |         ^^^^^^^ pattern `None` not covered
+   |
+   = note: `let` bindings require an "irrefutable pattern", like a `struct` or an `enum` with only one variant
+   = note: for more information, visit https://doc.rust-lang.org/book/ch18-02-refutability.html
+   = note: the matched value is of type `Option<i32>`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0005`.