Handle anon lifetime arg being returned with named lifetime return type

When there's a lifetime mismatch between an argument with an anonymous
lifetime being returned in a method with a return type that has a named
lifetime, show specialized lifetime error pointing at argument with a
hint to give it an explicit lifetime matching the return type.

```
error[E0621]: explicit lifetime required in the type of `other`
  --> file2.rs:21:21
   |
17 |     fn bar(&self, other: Foo) -> Foo<'a> {
   |                   ----- consider changing the type of `other` to `Foo<'a>`
...
21 |                     other
   |                     ^^^^^ lifetime `'a` required
```

Follow up to rust-lang#44124 and rust-lang#42669.

src/librustc/infer/error_reporting/different_lifetimes.rs

@@ -21,25 +21,42 @@ use hir::intravisit::{self, Visitor, NestedVisitorMap};
 use infer::error_reporting::util::AnonymousArgInfo;
 
 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
-    // This method prints the error message for lifetime errors when both the concerned regions
-    // are anonymous.
-    // Consider a case where we have
-    // fn foo(x: &mut Vec<&u8>, y: &u8)
-    //    { x.push(y); }.
-    // The example gives
-    // fn foo(x: &mut Vec<&u8>, y: &u8) {
-    //                    ---      --- these references are declared with different lifetimes...
-    //            x.push(y);
-    //            ^ ...but data from `y` flows into `x` here
-    // It has been extended for the case of structs too.
-    // Consider the example
-    // struct Ref<'a> { x: &'a u32 }
-    // fn foo(mut x: Vec<Ref>, y: Ref) {
-    //                   ---      --- these structs are declared with different lifetimes...
-    //               x.push(y);
-    //               ^ ...but data from `y` flows into `x` here
-    // }
-    // It will later be extended to trait objects.
+    /// Print the error message for lifetime errors when both the concerned regions are anonymous.
+    ///
+    /// Consider a case where we have
+    ///
+    /// ```no_run
+    /// fn foo(x: &mut Vec<&u8>, y: &u8) {
+    ///     x.push(y);
+    /// }
+    /// ```
+    ///
+    /// The example gives
+    ///
+    /// ```text
+    /// fn foo(x: &mut Vec<&u8>, y: &u8) {
+    ///                    ---      --- these references are declared with different lifetimes...
+    ///     x.push(y);
+    ///     ^ ...but data from `y` flows into `x` here
+    /// ```
+    ///
+    /// It has been extended for the case of structs too.
+    ///
+    /// Consider the example
+    ///
+    /// ```no_run
+    /// struct Ref<'a> { x: &'a u32 }
+    /// ```
+    ///
+    /// ```text
+    /// fn foo(mut x: Vec<Ref>, y: Ref) {
+    ///                   ---      --- these structs are declared with different lifetimes...
+    ///     x.push(y);
+    ///     ^ ...but data from `y` flows into `x` here
+    /// }
+    /// ````
+    ///
+    /// It will later be extended to trait objects.
     pub fn try_report_anon_anon_conflict(&self, error: &RegionResolutionError<'tcx>) -> bool {
         let (span, sub, sup) = match *error {
             ConcreteFailure(ref origin, sub, sup) => (origin.span(), sub, sup),

src/librustc/infer/error_reporting/named_anon_conflict.rs

@@ -16,18 +16,15 @@ use infer::region_inference::RegionResolutionError;
 use ty;
 
 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
-    // This method generates the error message for the case when
-    // the function arguments consist of a named region and an anonymous
-    // region and corresponds to `ConcreteFailure(..)`
+    /// Generate an error message for when the function arguments consist of a named region and
+    /// an anonymous region and corresponds to `ConcreteFailure(..)`
     pub fn try_report_named_anon_conflict(&self, error: &RegionResolutionError<'tcx>) -> bool {
         let (span, sub, sup) = match *error {
             ConcreteFailure(ref origin, sub, sup) => (origin.span(), sub, sup),
             _ => return false, // inapplicable
         };
 
-        debug!("try_report_named_anon_conflict(sub={:?}, sup={:?})",
-               sub,
-               sup);
+        debug!("try_report_named_anon_conflict(sub={:?}, sup={:?})", sub, sup);
 
         // Determine whether the sub and sup consist of one named region ('a)
         // and one anonymous (elided) region. If so, find the parameter arg
@@ -53,10 +50,8 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
             };
 
         debug!("try_report_named_anon_conflict: named = {:?}", named);
-        debug!("try_report_named_anon_conflict: anon_arg_info = {:?}",
-               anon_arg_info);
-        debug!("try_report_named_anon_conflict: region_info = {:?}",
-               region_info);
+        debug!("try_report_named_anon_conflict: anon_arg_info = {:?}", anon_arg_info);
+        debug!("try_report_named_anon_conflict: region_info = {:?}", region_info);
 
         let (arg, new_ty, br, is_first, scope_def_id, is_impl_item) = (anon_arg_info.arg,
                                                                        anon_arg_info.arg_ty,
@@ -101,6 +96,5 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                 .span_label(span, format!("lifetime `{}` required", named))
                 .emit();
         return true;
-
     }
 }

src/librustc/infer/error_reporting/util.rs

@@ -221,6 +221,7 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                     _ => false,
                 }
             }
+            ty::ReEarlyBound(_) => true,
             _ => false,
         }
     }

src/test/compile-fail/regions-infer-at-fn-not-param.rs

@@ -21,7 +21,7 @@ struct not_parameterized2 {
 }
 
 fn take1<'a>(p: parameterized1) -> parameterized1<'a> { p }
-//~^ ERROR mismatched types
+//~^ ERROR explicit lifetime required in the type of `p`
 
 fn take3(p: not_parameterized1) -> not_parameterized1 { p }
 fn take4(p: not_parameterized2) -> not_parameterized2 { p }

src/test/ui/lifetime-errors/ex3-both-anon-regions-earlybound-regions.stderr

@@ -1,11 +1,11 @@
-error[E0623]: lifetime mismatch
+error[E0621]: explicit lifetime required in the type of `y`
   --> $DIR/ex3-both-anon-regions-earlybound-regions.rs:17:12
    |
 13 | fn baz<'a, 'b, T>(x: &mut Vec<&'a T>, y: &T)
-   |                               -----      -- these two types are declared with different lifetimes...
+   |                                       - consider changing the type of `y` to `&'a T`
 ...
 17 |     x.push(y);
-   |            ^ ...but data from `y` flows into `x` here
+   |            ^ lifetime `'a` required
 
 error: aborting due to previous error
 

src/test/ui/lifetime-errors/ex4-anon-named-regions.rs

@@ -0,0 +1,30 @@
+// 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.
+
+#[derive(Clone)]
+enum Foo<'a> {
+    Bar(&'a str),
+}
+
+impl<'a> Foo<'a> {
+    fn bar(&self, other: Foo) -> Foo<'a> {
+        match *self {
+            Foo::Bar(s) => {
+                if s == "test" {
+                    other
+                } else {
+                    self.clone()
+                }
+            }
+        }
+    }
+}
+
+fn main() { }

src/test/ui/lifetime-errors/ex4-anon-named-regions.stderr

@@ -0,0 +1,11 @@
+error[E0621]: explicit lifetime required in the type of `other`
+  --> $DIR/ex4-anon-named-regions.rs:21:21
+   |
+17 |     fn bar(&self, other: Foo) -> Foo<'a> {
+   |                   ----- consider changing the type of `other` to `Foo<'a>`
+...
+21 |                     other
+   |                     ^^^^^ lifetime `'a` required
+
+error: aborting due to previous error
+