Deduplicate Spans in Uninitialized Check

Prevents reporting labels or diagnostics on spans that are produced
multiple times.

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -678,14 +678,15 @@ fn report_use_of_uninitialized(
         let inits = &self.move_data.init_path_map[mpi];
         let move_path = &self.move_data.move_paths[mpi];
         let decl_span = self.body.local_decls[move_path.place.local].source_info.span;
-        let mut spans = vec![];
+        let mut spans_set = FxIndexSet::default();
         for init_idx in inits {
             let init = &self.move_data.inits[*init_idx];
             let span = init.span(self.body);
             if !span.is_dummy() {
-                spans.push(span);
+                spans_set.insert(span);
             }
         }
+        let spans: Vec<_> = spans_set.into_iter().collect();
 
         let (name, desc) = match self.describe_place_with_options(
             moved_place,

tests/ui/duplicate-label-E0381-issue-129274.rs

@@ -0,0 +1,13 @@
+fn main() {
+    fn test() {
+        loop {
+            let blah: Option<String>;
+            if true {
+                blah = Some("".to_string());
+            }
+            if let Some(blah) = blah.as_ref() { //~ ERROR E0381
+            }
+        }
+    }
+    println!("{:?}", test())
+}

tests/ui/duplicate-label-E0381-issue-129274.stderr

@@ -0,0 +1,15 @@
+error[E0381]: used binding `blah` is possibly-uninitialized
+  --> $DIR/duplicate-label-E0381-issue-129274.rs:8:33
+   |
+LL |             let blah: Option<String>;
+   |                 ---- binding declared here but left uninitialized
+LL |             if true {
+LL |                 blah = Some("".to_string());
+   |                 ---- binding initialized here in some conditions
+LL |             }
+LL |             if let Some(blah) = blah.as_ref() {
+   |                                 ^^^^ `blah` used here but it is possibly-uninitialized
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0381`.