Auto merge of #46254 - Dylan-DPC:ellided-lifetime, r=nikomatsakis

elided lifetime

Closes #45992

Hey
Having a problem with my config so decided to make a WIP PR nevertheless. Will add some more tests.

src/librustc/lint/builtin.rs

@@ -246,6 +246,12 @@ declare_lint! {
     "raw pointer to an inference variable"
 }
 
+declare_lint! {
+    pub ELIDED_LIFETIME_IN_PATH,
+    Allow,
+    "hidden lifetime parameters are deprecated, try `Foo<'_>`"
+}
+
 /// Does nothing as a lint pass, but registers some `Lint`s
 /// which are used by other parts of the compiler.
 #[derive(Copy, Clone)]
@@ -291,7 +297,9 @@ impl LintPass for HardwiredLints {
             UNUSED_MUT,
             COERCE_NEVER,
             SINGLE_USE_LIFETIME,
-            TYVAR_BEHIND_RAW_POINTER
+            TYVAR_BEHIND_RAW_POINTER,
+            ELIDED_LIFETIME_IN_PATH
+
         )
     }
 }

src/librustc/middle/resolve_lifetime.rs

@@ -737,7 +737,7 @@ impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
 
     fn visit_lifetime(&mut self, lifetime_ref: &'tcx hir::Lifetime) {
         if lifetime_ref.is_elided() {
-            self.resolve_elided_lifetimes(slice::from_ref(lifetime_ref));
+            self.resolve_elided_lifetimes(slice::from_ref(lifetime_ref), false);
             return;
         }
         if lifetime_ref.is_static() {
@@ -1444,7 +1444,7 @@ impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
         }
 
         if params.lifetimes.iter().all(|l| l.is_elided()) {
-            self.resolve_elided_lifetimes(&params.lifetimes);
+            self.resolve_elided_lifetimes(&params.lifetimes, true);
         } else {
             for l in &params.lifetimes {
                 self.visit_lifetime(l);
@@ -1803,14 +1803,24 @@ impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
         }
     }
 
-    fn resolve_elided_lifetimes(&mut self, lifetime_refs: &'tcx [hir::Lifetime]) {
+    fn resolve_elided_lifetimes(&mut self, lifetime_refs: &'tcx [hir::Lifetime], deprecated: bool) {
         if lifetime_refs.is_empty() {
             return;
         }
 
         let span = lifetime_refs[0].span;
+        let id = lifetime_refs[0].id;
         let mut late_depth = 0;
         let mut scope = self.scope;
+        if deprecated {
+            self.tcx
+                .struct_span_lint_node(
+                    lint::builtin::ELIDED_LIFETIME_IN_PATH,
+                    id,
+                    span,
+                    &format!("hidden lifetime parameters are deprecated, try `Foo<'_>`"))
+                .emit();
+        }
         let error = loop {
             match *scope {
                 // Do not assign any resolution, it will be inferred.

src/test/ui/in-band-lifetimes/ellided-lifetimes.rs

@@ -0,0 +1,19 @@
+// 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.
+#![allow(warnings)]
+#![allow(unused_variables, dead_code, unused, bad_style)]
+#![deny(elided_lifetime_in_path)]
+
+struct Foo<'a> { x: &'a u32 }
+fn foo(x: &Foo) {
+    //~^ ERROR: hidden lifetime parameters are deprecated, try `Foo<'_>`
+}
+
+fn main() {}

src/test/ui/in-band-lifetimes/ellided-lifetimes.stderr

@@ -0,0 +1,14 @@
+error: hidden lifetime parameters are deprecated, try `Foo<'_>`
+  --> $DIR/ellided-lifetimes.rs:15:12
+   |
+15 | fn foo(x: &Foo) {
+   |            ^^^
+   |
+note: lint level defined here
+  --> $DIR/ellided-lifetimes.rs:12:9
+   |
+12 | #![deny(elided_lifetime_in_path)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+