Auto merge of #54810 - 1aim:unused-impl-trait, r=oli-obk

Fix dead code lint for functions using impl Trait

Fixes https://github.com/rust-lang/rust/issues/54754

This is a minimal fix that doesn't add any new queries or touches unnecessary code. Please nominate for beta backport if wanted.

src/test/ui/impl-trait/existential-minimal.rs

@@ -0,0 +1,15 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// compile-pass
+
+fn main() {}
+
+fn foo() -> impl std::fmt::Debug { "cake" }

src/test/ui/impl-trait/issue-42479.rs

@@ -0,0 +1,27 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// compile-pass
+
+use std::iter::once;
+
+struct Foo {
+    x: i32,
+}
+
+impl Foo {
+    fn inside(&self) -> impl Iterator<Item = &i32> {
+        once(&self.x)
+    }
+}
+
+fn main() {
+    println!("hi");
+}

src/test/ui/impl-trait/issue-49376.rs

@@ -0,0 +1,31 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// compile-pass
+
+// Tests for nested self-reference which caused a stack overflow.
+
+use std::fmt::Debug;
+use std::ops::*;
+
+fn gen() -> impl PartialOrd + PartialEq + Debug { }
+
+struct Bar {}
+trait Foo<T = Self> {}
+impl Foo for Bar {}
+
+fn foo() -> impl Foo {
+    Bar {}
+}
+
+fn test_impl_ops() -> impl Add + Sub + Mul + Div { 1 }
+fn test_impl_assign_ops() -> impl AddAssign + SubAssign + MulAssign + DivAssign { 1 }
+
+fn main() {}

src/test/ui/issue-49556.rs

@@ -0,0 +1,23 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// compile-pass
+fn iter<'a>(data: &'a [usize]) -> impl Iterator<Item = usize> + 'a {
+    data.iter()
+        .map(
+            |x| x // fn(&'a usize) -> &'(ReScope) usize
+        )
+        .map(
+            |x| *x // fn(&'(ReScope) usize) -> usize
+        )
+}
+
+fn main() {
+}

src/test/ui/lint/lint-dead-code-1.rs

@@ -109,6 +109,10 @@ fn bar() { //~ ERROR: function is never used
     foo();
 }
 
+fn baz() -> impl Copy { //~ ERROR: function is never used
+    "I'm unused, too"
+}
+
 // Code with #[allow(dead_code)] should be marked live (and thus anything it
 // calls is marked live)
 #[allow(dead_code)]

src/test/ui/lint/lint-dead-code-1.stderr

@@ -58,5 +58,11 @@ error: function is never used: `bar`
 LL | fn bar() { //~ ERROR: function is never used
    | ^^^^^^^^
 
-error: aborting due to 9 previous errors
+error: function is never used: `baz`
+  --> $DIR/lint-dead-code-1.rs:112:1
+   |
+LL | fn baz() -> impl Copy { //~ ERROR: function is never used
+   | ^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 10 previous errors
 

src/test/ui/traits/conservative_impl_trait.rs

@@ -0,0 +1,18 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// compile-pass
+// #39665
+
+fn batches(n: &u32) -> impl Iterator<Item=&u32> {
+    std::iter::once(n)
+}
+
+fn main() {}