Resolve the false positive dead_code lint when type is only used via destructuring

compiler/rustc_passes/src/dead.rs

@@ -239,6 +239,7 @@ impl<'tcx> MarkSymbolVisitor<'tcx> {
             let index = self.typeck_results().field_index(pat.hir_id);
             self.insert_def_id(variant.fields[index].did);
         }
+        self.check_def_id(variant.def_id);
     }
 
     fn handle_tuple_field_pattern_match(
@@ -265,6 +266,7 @@ impl<'tcx> MarkSymbolVisitor<'tcx> {
             }
             self.insert_def_id(variant.fields[FieldIdx::from_usize(idx)].did);
         }
+        self.check_def_id(variant.def_id);
     }
 
     fn handle_offset_of(&mut self, expr: &'tcx hir::Expr<'tcx>) {

tests/ui/lint/dead-code/lint-dead-code-4.rs

@@ -12,10 +12,10 @@ fn field_read(f: Foo) -> usize {
 }
 
 enum XYZ {
-    X, //~ ERROR variants `X` and `Y` are never constructed
+    X, //~ ERROR variant `X` is never constructed
     Y {
         a: String,
-        b: i32,
+        b: i32, //~ ERROR fields `b` and `c` are never read
         c: i32,
     },
     Z

tests/ui/lint/dead-code/lint-dead-code-4.stderr

@@ -13,15 +13,24 @@ note: the lint level is defined here
 LL | #![deny(dead_code)]
    |         ^^^^^^^^^
 
-error: variants `X` and `Y` are never constructed
+error: fields `b` and `c` are never read
+  --> $DIR/lint-dead-code-4.rs:18:9
+   |
+LL |     Y {
+   |     - fields in this variant
+LL |         a: String,
+LL |         b: i32,
+   |         ^
+LL |         c: i32,
+   |         ^
+
+error: variant `X` is never constructed
   --> $DIR/lint-dead-code-4.rs:15:5
    |
 LL | enum XYZ {
-   |      --- variants in this enum
+   |      --- variant in this enum
 LL |     X,
    |     ^
-LL |     Y {
-   |     ^
 
 error: enum `ABC` is never used
   --> $DIR/lint-dead-code-4.rs:24:6
@@ -62,5 +71,5 @@ LL |     b: bool,
 LL |     c: bool,
    |     ^
 
-error: aborting due to 6 previous errors
+error: aborting due to 7 previous errors
 

tests/ui/lint/dead-code/only-used-for-destructuring-struct.rs

@@ -0,0 +1,17 @@
+//@ check-pass
+
+// Make sure we don't have any false positives here.
+
+#![deny(dead_code)]
+
+struct Foo {
+    bar: usize,
+}
+
+fn get_thing<T>() -> T {
+    todo!()
+}
+
+pub fn main() {
+    let Foo { bar: _x } = get_thing();
+}

tests/ui/lint/dead-code/only-used-for-destructuring-tuple-struct.rs

@@ -0,0 +1,13 @@
+//@ check-pass
+
+// Make sure we don't have any false positives here.
+
+#![deny(dead_code)]
+
+struct Foo(usize);
+
+fn get_thing<T>() -> T { todo!() }
+
+pub fn main() {
+    let Foo(_x) = get_thing();
+}