liveness: Test interaction with automatically_derived attribute

src/test/ui/liveness/liveness-derive.rs

@@ -0,0 +1,38 @@
+// Test for interaction between #[automatically_derived] attribute used by
+// built-in derives and lints generated by liveness pass.
+//
+// edition:2018
+// check-pass
+#![warn(unused)]
+
+pub trait T: Sized {
+    const N: usize;
+    fn t(&self) -> Self;
+}
+
+impl T for u32 {
+    const N: usize = {
+        let a = 0; // FIXME should warn about unused variable
+        4
+    };
+
+    fn t(&self) -> Self {
+        let b = 16; //~ WARN unused variable: `b`
+        0
+    }
+}
+
+#[automatically_derived]
+impl T for i32 {
+    const N: usize = {
+        let c = 0;
+        4
+    };
+
+    fn t(&self) -> Self {
+        let d = 17;
+        0
+    }
+}
+
+fn main() {}

src/test/ui/liveness/liveness-derive.stderr

@@ -0,0 +1,15 @@
+warning: unused variable: `b`
+  --> $DIR/liveness-derive.rs:20:13
+   |
+LL |         let b = 16;
+   |             ^ help: if this is intentional, prefix it with an underscore: `_b`
+   |
+note: the lint level is defined here
+  --> $DIR/liveness-derive.rs:6:9
+   |
+LL | #![warn(unused)]
+   |         ^^^^^^
+   = note: `#[warn(unused_variables)]` implied by `#[warn(unused)]`
+
+warning: 1 warning emitted
+