Auto merge of rust-lang#50440 - nikomatsakis:single-use-lifetimes, r=cramertj

Improve single-use and zero-use lifetime lints

The code now correctly identifies *when* to lint -- or more correctly, anyhow -- but it doesn't yet offer suggestions for how to fix.

(I just remembered when writing this I had meant to go back over some of these cases around e.g. impl Trait and double check that everything is right...)

cc rust-lang#44752

r? @cramertj

Author: bors

src/librustc/lint/builtin.rs

@@ -233,7 +233,13 @@ declare_lint! {
 declare_lint! {
     pub SINGLE_USE_LIFETIME,
     Allow,
-   "detects single use lifetimes"
+    "detects lifetime parameters that are only used once"
+}
+
+declare_lint! {
+    pub UNUSED_LIFETIME,
+    Allow,
+    "detects lifetime parameters that are never used"
 }
 
 declare_lint! {
@@ -318,6 +324,7 @@ impl LintPass for HardwiredLints {
             UNUSED_UNSAFE,
             UNUSED_MUT,
             SINGLE_USE_LIFETIME,
+            UNUSED_LIFETIME,
             TYVAR_BEHIND_RAW_POINTER,
             ELIDED_LIFETIME_IN_PATH,
             BARE_TRAIT_OBJECT,

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() { }