Suggest using anonymous lifetime in impl Trait return

Author: estebank

src/librustc/infer/error_reporting/nice_region_error/named_anon_conflict.rs

@@ -103,13 +103,43 @@ impl<'a, 'gcx, 'tcx> NiceRegionError<'a, 'gcx, 'tcx> {
             error_var
         );
 
+        let many = if let Ok(snippet) = self.tcx().sess.source_map().span_to_snippet(span) {
+            if "'static" == &named.to_string() && snippet.starts_with("impl ") {
+                diag.span_suggestion(
+                    span,
+                    "add explicit unnamed lifetime `'_` to the return type to constrain it",
+                    format!("{} + '_", snippet),
+                    Applicability::Unspecified,
+                );
+                true
+            } else {
+                false
+            }
+        } else {
+            false
+        };
+        if many {
+            diag.span_label(
+                span,
+                "`impl Trait` types can only capture lifetimes that they reference"
+            );
+        } else {
+            diag.span_label(span, format!("lifetime `{}` required", named));
+        }
         diag.span_suggestion(
-                new_ty_span,
-                &format!("add explicit lifetime `{}` to {}", named, span_label_var),
-                new_ty.to_string(),
-                Applicability::Unspecified,
-            )
-            .span_label(span, format!("lifetime `{}` required", named));
+            new_ty_span,
+            &format!("{}add explicit lifetime `{}` to {}",
+                if many {
+                    "otherwise, "
+                } else {
+                    ""
+                },
+                named,
+                span_label_var,
+            ),
+            new_ty.to_string(),
+            Applicability::Unspecified,
+        );
 
         Some(diag)
     }

src/test/ui/impl-trait/must_outlive_least_region_or_bound.stderr

@@ -2,9 +2,15 @@ error[E0621]: explicit lifetime required in the type of `x`
   --> $DIR/must_outlive_least_region_or_bound.rs:3:23
    |
 LL | fn elided(x: &i32) -> impl Copy { x }
-   |              ----     ^^^^^^^^^ lifetime `'static` required
-   |              |
-   |              help: add explicit lifetime `'static` to the type of `x`: `&'static i32`
+   |                       ^^^^^^^^^ `impl Trait` types can only capture lifetimes that they reference
+help: add explicit unnamed lifetime `'_` to the return type to constrain it
+   |
+LL | fn elided(x: &i32) -> impl Copy + '_ { x }
+   |                       ^^^^^^^^^^^^^^
+help: otherwise, add explicit lifetime `'static` to the type of `x`
+   |
+LL | fn elided(x: &'static i32) -> impl Copy { x }
+   |              ^^^^^^^^^^^^
 
 error: cannot infer an appropriate lifetime
   --> $DIR/must_outlive_least_region_or_bound.rs:6:44