fix single-use lint

Author: nikomatsakis

src/librustc/middle/resolve_lifetime.rs

@@ -0,0 +1,26 @@
+// Copyright 2016 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.
+
+#![deny(single_use_lifetime)]
+#![allow(dead_code)]
+#![allow(unused_variables)]
+
+// Test that we DO warn when lifetime name is used only
+// once in a fn argument.
+
+struct Foo {
+  a: for<'a> fn(&'a u32), //~ ERROR `'a` only used once
+  b: for<'a> fn(&'a u32, &'a u32), // OK, used twice.
+  c: for<'a> fn(&'a u32) -> &'a u32, // OK, used twice.
+  d: for<'a> fn() -> &'a u32, // OK, used only in return type.
+    //~^ ERROR return type references lifetime `'a`, which is not constrained by the fn input types
+}
+
+fn main() { }

src/test/ui/in-band-lifetimes/single_use_lifetimes-2.stderr

@@ -0,0 +1,21 @@
+error: lifetime parameter `'a` only used once
+  --> $DIR/fn-types.rs:19:10
+   |
+LL |   a: for<'a> fn(&'a u32), //~ ERROR `'a` only used once
+   |          ^^
+   |
+note: lint level defined here
+  --> $DIR/fn-types.rs:11:9
+   |
+LL | #![deny(single_use_lifetime)]
+   |         ^^^^^^^^^^^^^^^^^^^
+
+error[E0581]: return type references lifetime `'a`, which is not constrained by the fn input types
+  --> $DIR/fn-types.rs:22:22
+   |
+LL |   d: for<'a> fn() -> &'a u32, // OK, used only in return type.
+   |                      ^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0581`.

src/test/ui/in-band-lifetimes/single_use_lifetimes-3.stderr

@@ -0,0 +1,24 @@
+// Copyright 2016 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.
+
+#![feature(in_band_lifetimes)]
+#![deny(single_use_lifetime)]
+#![allow(dead_code)]
+#![allow(unused_variables)]
+
+// Test that we DO warn when lifetime name is used only
+// once in a fn argument, even with in band lifetimes.
+
+fn a(x: &'a u32, y: &'b u32) {
+    //~^ ERROR `'a` only used once
+    //~| ERROR `'b` only used once
+}
+
+fn main() { }