add warnings for unused lifetime parameters

Author: nikomatsakis

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

@@ -1358,7 +1358,25 @@ impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
                 Some(LifetimeUseSet::Many) => {
                     debug!("Not one use lifetime");
                 }
-                None => {}
+                None => {
+                    let node_id = self.tcx.hir.as_local_node_id(*def_id).unwrap();
+                    if let hir::map::NodeLifetime(hir_lifetime) = self.tcx.hir.get(node_id) {
+                        let span = hir_lifetime.span;
+                        let id = hir_lifetime.id;
+
+                        self.tcx
+                            .struct_span_lint_node(
+                                lint::builtin::UNUSED_LIFETIME,
+                                id,
+                                span,
+                                &format!(
+                                    "lifetime parameter `{}` never used",
+                                    hir_lifetime.name.name()
+                                ),
+                            )
+                            .emit();
+                    }
+                }
             }
         }
     }

src/test/ui/single-use-lifetime/zero-uses-in-fn.rs

@@ -0,0 +1,19 @@
+// 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.
+
+// Test that we DO warn when lifetime name is not used at all.
+
+#![deny(unused_lifetime)]
+#![allow(dead_code)]
+#![allow(unused_variables)]
+
+fn d<'a>() { } //~ ERROR `'a` never used
+
+fn main() { }

src/test/ui/single-use-lifetime/zero-uses-in-fn.stderr

@@ -0,0 +1,14 @@
+error: lifetime parameter `'a` never used
+  --> $DIR/zero-uses-in-fn.rs:17:6
+   |
+LL | fn d<'a>() { } //~ ERROR `'a` never used
+   |      ^^
+   |
+note: lint level defined here
+  --> $DIR/zero-uses-in-fn.rs:13:9
+   |
+LL | #![deny(unused_lifetime)]
+   |         ^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+

src/test/ui/single-use-lifetime/zero-uses-in-impl.rs

@@ -0,0 +1,21 @@
+// 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.
+
+// Test that we DO warn when lifetime name is not used at all.
+
+#![deny(unused_lifetime)]
+#![allow(dead_code)]
+#![allow(unused_variables)]
+
+struct Foo { }
+
+impl<'a> Foo { } //~ ERROR `'a` never used
+
+fn main() { }

src/test/ui/single-use-lifetime/zero-uses-in-impl.stderr

@@ -0,0 +1,14 @@
+error: lifetime parameter `'a` never used
+  --> $DIR/zero-uses-in-impl.rs:19:6
+   |
+LL | impl<'a> Foo { } //~ ERROR `'a` never used
+   |      ^^
+   |
+note: lint level defined here
+  --> $DIR/zero-uses-in-impl.rs:13:9
+   |
+LL | #![deny(unused_lifetime)]
+   |         ^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+