Rollup merge of rust-lang#40734 - adamransom:fix/40661, r=jseyfried

Add warning for use of lifetime parameter with 'static bound

Previously a `'static` lifetime bound would result in an `undeclared lifetime` error when compiling, even though it could be considered valid.

However, it is unnecessary to use it as a lifetime bound so we present the user with a warning instead and suggest using the `'static` lifetime directly, in place of the lifetime parameter. We can change this to an error (or warning with lint) if that's decided to be more appropriate.

Example output:
```
warning: unnecessary lifetime parameter `'a`
 --> ../static-lifetime-bound.rs:3:10
  |
3 | fn f<'a: 'static>(val: &'a i32) {
  |      ^^^^^^^^^^^
  |
  = help: you can use the `'static` lifetime directly, in place `'a`
```

Fixes rust-lang#40661

r? @jseyfried

src/librustc/hir/mod.rs

@@ -159,6 +159,10 @@ impl Lifetime {
     pub fn is_elided(&self) -> bool {
         self.name == keywords::Invalid.name()
     }
+
+    pub fn is_static(&self) -> bool {
+        self.name == keywords::StaticLifetime.name()
+    }
 }
 
 /// A lifetime definition, eg `'a: 'b+'c+'d`

src/librustc/middle/resolve_lifetime.rs

@@ -29,7 +29,7 @@ use syntax::ast;
 use syntax::attr;
 use syntax::ptr::P;
 use syntax::symbol::keywords;
-use syntax_pos::Span;
+use syntax_pos::{mk_sp, Span};
 use errors::DiagnosticBuilder;
 use util::nodemap::{NodeMap, NodeSet, FxHashSet, FxHashMap, DefIdMap};
 use rustc_back::slice;
@@ -434,7 +434,7 @@ impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
             self.resolve_elided_lifetimes(slice::ref_slice(lifetime_ref));
             return;
         }
-        if lifetime_ref.name == keywords::StaticLifetime.name() {
+        if lifetime_ref.is_static() {
             self.insert_lifetime(lifetime_ref, Region::Static);
             return;
         }
@@ -1434,7 +1434,7 @@ impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
             let lifetime_i = &lifetimes[i];
 
             for lifetime in lifetimes {
-                if lifetime.lifetime.name == keywords::StaticLifetime.name() {
+                if lifetime.lifetime.is_static() {
                     let lifetime = lifetime.lifetime;
                     let mut err = struct_span_err!(self.sess, lifetime.span, E0262,
                                   "invalid lifetime parameter name: `{}`", lifetime.name);
@@ -1464,7 +1464,17 @@ impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
             self.check_lifetime_def_for_shadowing(old_scope, &lifetime_i.lifetime);
 
             for bound in &lifetime_i.bounds {
-                self.resolve_lifetime_ref(bound);
+                if !bound.is_static() {
+                    self.resolve_lifetime_ref(bound);
+                } else {
+                    self.insert_lifetime(bound, Region::Static);
+                    let full_span = mk_sp(lifetime_i.lifetime.span.lo, bound.span.hi);
+                    self.sess.struct_span_warn(full_span,
+                        &format!("unnecessary lifetime parameter `{}`", lifetime_i.lifetime.name))
+                        .help(&format!("you can use the `'static` lifetime directly, in place \
+                                        of `{}`", lifetime_i.lifetime.name))
+                        .emit();
+                }
             }
         }
     }

src/test/compile-fail/static-lifetime-bound.rs

@@ -0,0 +1,16 @@
+// 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.
+
+fn f<'a: 'static>(_: &'a i32) {} //~WARN unnecessary lifetime parameter `'a`
+
+fn main() {
+    let x = 0;
+    f(&x); //~ERROR does not live long enough
+}