Rollup merge of #126302 - mu001999-contrib:ignore/default, r=michaelwoerister

Detect unused structs which derived Default

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Fixes #98871

tests/ui/deriving/deriving-default-enum.rs

@@ -22,6 +22,6 @@ enum MyOption<T> {
 }
 
 fn main() {
-    assert_eq!(Foo::default(), Foo::Alpha);
+    assert!(matches!(Foo::default(), Foo::Alpha));
     assert!(matches!(MyOption::<NotDefault>::default(), MyOption::None));
 }

tests/ui/issues/issue-68696-catch-during-unwind.rs

@@ -7,6 +7,7 @@
 
 use std::panic::catch_unwind;
 
+#[allow(dead_code)]
 #[derive(Default)]
 struct Guard;
 

tests/ui/lint/dead-code/unused-struct-derive-default.rs

@@ -0,0 +1,25 @@
+#![deny(dead_code)]
+
+#[derive(Default)]
+struct T; //~ ERROR struct `T` is never constructed
+
+#[derive(Default)]
+struct Used;
+
+#[derive(Default)]
+enum E {
+    #[default]
+    A,
+    B, //~ ERROR variant `B` is never constructed
+}
+
+// external crate can call T2::default() to construct T2,
+// so that no warnings for pub adts
+#[derive(Default)]
+pub struct T2 {
+    _unread: i32,
+}
+
+fn main() {
+    let _x: Used = Default::default();
+}

tests/ui/lint/dead-code/unused-struct-derive-default.stderr

@@ -0,0 +1,24 @@
+error: struct `T` is never constructed
+  --> $DIR/unused-struct-derive-default.rs:4:8
+   |
+LL | struct T;
+   |        ^
+   |
+   = note: `T` has a derived impl for the trait `Default`, but this is intentionally ignored during dead code analysis
+note: the lint level is defined here
+  --> $DIR/unused-struct-derive-default.rs:1:9
+   |
+LL | #![deny(dead_code)]
+   |         ^^^^^^^^^
+
+error: variant `B` is never constructed
+  --> $DIR/unused-struct-derive-default.rs:13:5
+   |
+LL | enum E {
+   |      - variant in this enum
+...
+LL |     B,
+   |     ^
+
+error: aborting due to 2 previous errors
+