WIP

src/librustc/hir/lowering.rs

@@ -47,7 +47,8 @@ use hir::map::{DefKey, DefPathData, Definitions};
 use hir::def_id::{DefId, DefIndex, DefIndexAddressSpace, CRATE_DEF_INDEX};
 use hir::def::{Def, PathResolution, PerNS};
 use hir::GenericArg;
-use lint::builtin::{self, PARENTHESIZED_PARAMS_IN_TYPES_AND_MODULES};
+use lint::builtin::{self, PARENTHESIZED_PARAMS_IN_TYPES_AND_MODULES,
+                    ELIDED_LIFETIMES_IN_PATHS};
 use middle::cstore::CrateStore;
 use rustc_data_structures::indexed_vec::IndexVec;
 use session::Session;
@@ -1753,13 +1754,40 @@ impl<'a> LoweringContext<'a> {
             GenericArg::Lifetime(_) => true,
             _ => false,
         });
+        let first_generic_span = generic_args.args.iter().map(|a| a.span())
+            .chain(generic_args.bindings.iter().map(|b| b.span)).next();
         if !generic_args.parenthesized && !has_lifetimes {
             generic_args.args =
                 self.elided_path_lifetimes(path_span, expected_lifetimes)
                     .into_iter()
                     .map(|lt| GenericArg::Lifetime(lt))
                     .chain(generic_args.args.into_iter())
-                    .collect();
+                .collect();
+            if expected_lifetimes > 0 && param_mode == ParamMode::Explicit {
+                let anon_lt_suggestion = iter::repeat("'_").collect::<Vec<_>>().join(", ");
+                let no_ty_args = generic_args.args.len() == expected_lifetimes;
+                let no_bindings = generic_args.bindings.is_empty();
+                let (incl_angl_brckt, insertion_span, suggestion) = if no_ty_args && no_bindings {
+                    // If there are no (non-implicit) generic args or associated-type
+                    // bindings, our suggestion includes the angle brackets
+                    (true, path_span.shrink_to_hi(), format!("<{}>", anon_lt_suggestion))
+                } else {
+                    // Otherwise—sorry, this is kind of gross—we need to infer the
+                    // place to splice in the `'_, ` from the generics that do exist
+                    let first_generic_span = first_generic_span
+                        .expect("already checked that type args or bindings exist");
+                    (false, first_generic_span.shrink_to_lo(), anon_lt_suggestion)
+                };
+                self.sess.buffer_lint_with_diagnostic(
+                    ELIDED_LIFETIMES_IN_PATHS,
+                    CRATE_NODE_ID,
+                    path_span,
+                    "hidden lifetime parameters are deprecated",
+                    builtin::BuiltinLintDiagnostics::ElidedLifetimesInPaths(
+                        expected_lifetimes, path_span, incl_angl_brckt, insertion_span, suggestion
+                    )
+                );
+            }
         }
 
         hir::PathSegment::new(

src/librustc/lint/builtin.rs

@@ -400,6 +400,7 @@ pub enum BuiltinLintDiagnostics {
     AbsPathWithModule(Span),
     DuplicatedMacroExports(ast::Ident, Span, Span),
     ProcMacroDeriveResolutionFallback(Span),
+    ElidedLifetimesInPaths(usize, Span, bool, Span, String),
 }
 
 impl BuiltinLintDiagnostics {
@@ -440,6 +441,30 @@ impl BuiltinLintDiagnostics {
                 db.span_label(span, "names from parent modules are not \
                                      accessible without an explicit import");
             }
+            BuiltinLintDiagnostics::ElidedLifetimesInPaths(
+                n, path_span, incl_angl_brckt, insertion_span, anon_lts
+            ) => {
+                let (replace_span, suggestion) = if incl_angl_brckt {
+                    (insertion_span, anon_lts)
+                } else {
+                    // If our codemap can be trusted, construct the entire `Path<'_, T>` source
+                    // suggestion
+                    if let Ok(snippet) = sess.codemap().span_to_snippet(path_span) {
+                        let (before, after) = snippet.split_at(
+                            (insertion_span.lo().0 - path_span.lo().0) as usize
+                        );
+                        (path_span, format!("{}{}{}", before, anon_lts, after))
+                    } else {
+                        (insertion_span, anon_lts)
+                    }
+                };
+                db.span_suggestion_with_applicability(
+                    replace_span,
+                    &format!("indicate the anonymous lifetime{}", if n >= 2 { "s" } else { "" }),
+                    suggestion,
+                    Applicability::MachineApplicable
+                );
+            }
         }
     }
 }

src/librustc/middle/resolve_lifetime.rs

@@ -609,7 +609,7 @@ impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
                         // resolved the same as the `'_` in `&'_ Foo`.
                         //
                         // cc #48468
-                        self.resolve_elided_lifetimes(vec![lifetime], false)
+                        self.resolve_elided_lifetimes(vec![lifetime])
                     }
                     LifetimeName::Param(_) | LifetimeName::Static => {
                         // If the user wrote an explicit name, use that.
@@ -857,7 +857,7 @@ 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(vec![lifetime_ref], false);
+            self.resolve_elided_lifetimes(vec![lifetime_ref]);
             return;
         }
         if lifetime_ref.is_static() {
@@ -1691,7 +1691,7 @@ impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
             _ => None,
         }).collect();
         if elide_lifetimes {
-            self.resolve_elided_lifetimes(lifetimes, true);
+            self.resolve_elided_lifetimes(lifetimes);
         } else {
             lifetimes.iter().for_each(|lt| self.visit_lifetime(lt));
         }
@@ -2069,26 +2069,14 @@ impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
     }
 
     fn resolve_elided_lifetimes(&mut self,
-                                lifetime_refs: Vec<&'tcx hir::Lifetime>,
-                                deprecated: bool) {
+                                lifetime_refs: Vec<&'tcx hir::Lifetime>) {
         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_LIFETIMES_IN_PATHS,
-                    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/elided-lifetimes.rs

@@ -0,0 +1,86 @@
+// 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.
+
+// run-rustfix
+// compile-flags: --edition 2018
+
+#![allow(unused)]
+#![deny(elided_lifetimes_in_paths)]
+//~^ NOTE lint level defined here
+
+use std::cell::{RefCell, Ref};
+
+
+struct Foo<'a> { x: &'a u32 }
+
+fn foo(x: &Foo) {
+    //~^ ERROR hidden lifetime parameters in types are deprecated
+    //~| HELP indicate the anonymous lifetime
+}
+
+fn bar(x: &Foo<'_>) {}
+
+
+struct Wrapped<'a>(&'a str);
+
+struct WrappedWithBow<'a> {
+    gift: &'a str
+}
+
+fn wrap_gift(gift: &str) -> Wrapped {
+    //~^ ERROR hidden lifetime parameters in types are deprecated
+    //~| HELP indicate the anonymous lifetime
+    Wrapped(gift)
+}
+
+fn wrap_gift_with_bow(gift: &str) -> WrappedWithBow {
+    //~^ ERROR hidden lifetime parameters in types are deprecated
+    //~| HELP indicate the anonymous lifetime
+    WrappedWithBow { gift }
+}
+
+macro_rules! autowrapper {
+    ($type_name:ident, $fn_name:ident, $lt:lifetime) => {
+        struct $type_name<$lt> {
+            gift: &$lt str
+        }
+
+        fn $fn_name(gift: &str) -> $type_name {
+            //~^ ERROR hidden lifetime parameters in types are deprecated
+            //~| HELP indicate the anonymous lifetime
+            $type_name { gift }
+        }
+    }
+}
+
+autowrapper!(Autowrapped, autowrap_gift, 'a);
+//~^ NOTE in this expansion of autowrapper!
+//~| NOTE in this expansion of autowrapper!
+
+macro_rules! anytuple_ref_ty {
+    ($($types:ty),*) => {
+        Ref<($($types),*)>
+        //~^ ERROR hidden lifetime parameters in types are deprecated
+        //~| HELP indicate the anonymous lifetime
+    }
+}
+
+fn main() {
+    let honesty = RefCell::new((4, 'e'));
+    let loyalty: Ref<(u32, char)> = honesty.borrow();
+    //~^ ERROR hidden lifetime parameters in types are deprecated
+    //~| HELP indicate the anonymous lifetime
+    let generosity = Ref::map(loyalty, |t| &t.0);
+
+    let laughter = RefCell::new((true, "magic"));
+    let yellow: anytuple_ref_ty!(bool, &str) = laughter.borrow();
+    //~^ NOTE in this expansion of anytuple_ref_ty!
+    //~| NOTE in this expansion of anytuple_ref_ty!
+}