Suggest using anonymous lifetime in impl Trait return without hacks

Fallback to `static_impl_trait` for nice error message by peeking at the
return type and the lifetime type. Point at the return type instead of
the return expr/stmt in NLL mode.

Author: estebank

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

@@ -1,6 +1,7 @@
 //! Error Reporting for Anonymous Region Lifetime Errors
 //! where one region is named and the other is anonymous.
 use crate::infer::error_reporting::nice_region_error::NiceRegionError;
+use crate::hir::{FunctionRetTy, TyKind};
 use crate::ty;
 use errors::{Applicability, DiagnosticBuilder};
 
@@ -11,9 +12,10 @@ impl<'a, 'gcx, 'tcx> NiceRegionError<'a, 'gcx, 'tcx> {
         let (span, sub, sup) = self.get_regions();
 
         debug!(
-            "try_report_named_anon_conflict(sub={:?}, sup={:?})",
+            "try_report_named_anon_conflict(sub={:?}, sup={:?}, error={:?})",
             sub,
-            sup
+            sup,
+            self.error,
         );
 
         // Determine whether the sub and sup consist of one named region ('a)
@@ -84,6 +86,13 @@ impl<'a, 'gcx, 'tcx> NiceRegionError<'a, 'gcx, 'tcx> {
             {
                 return None;
             }
+            if let FunctionRetTy::Return(ty) = &fndecl.output {
+                if let (TyKind::Def(_, _), ty::ReStatic) = (&ty.node, sub) {
+                    // This is an impl Trait return that evaluates de need of 'static.
+                    // We handle this case better in `static_impl_trait`.
+                    return None;
+                }
+            }
         }
 
         let (error_var, span_label_var) = if let Some(simple_ident) = arg.pat.simple_ident() {
@@ -103,40 +112,10 @@ 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_label(span, format!("lifetime `{}` required", named));
         diag.span_suggestion(
             new_ty_span,
-            &format!("{}add explicit lifetime `{}` to {}",
-                if many {
-                    "otherwise, "
-                } else {
-                    ""
-                },
-                named,
-                span_label_var,
-            ),
+            &format!("add explicit lifetime `{}` to {}", named, span_label_var),
             new_ty.to_string(),
             Applicability::Unspecified,
         );

src/librustc_mir/borrow_check/nll/region_infer/error_reporting/mod.rs

@@ -132,6 +132,15 @@ impl<'tcx> RegionInferenceContext<'tcx> {
             }
         });
         if let Some(i) = best_choice {
+            if let Some(next) = categorized_path.get(i + 1) {
+                if categorized_path[i].0 == ConstraintCategory::Return
+                    && next.0 == ConstraintCategory::OpaqueType
+                {
+                    // The return expression is being influenced by the return type being
+                    // impl Trait, point at the return type and not the return expr.
+                    return *next;
+                }
+            }
             return categorized_path[i];
         }
 
@@ -240,6 +249,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
             self.provides_universal_region(r, fr, outlived_fr)
         });
 
+        debug!("report_error: category={:?} {:?}", category, span);
         // Check if we can use one of the "nice region errors".
         if let (Some(f), Some(o)) = (self.to_error_region(fr), self.to_error_region(outlived_fr)) {
             let tables = infcx.tcx.typeck_tables_of(mir_def_id);

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

@@ -1,7 +1,7 @@
 use std::fmt::Debug;
 
 fn elided(x: &i32) -> impl Copy { x }
-//~^ ERROR explicit lifetime required in the type of `x` [E0621]
+//~^ ERROR cannot infer an appropriate lifetime
 
 fn explicit<'a>(x: &'a i32) -> impl Copy { x }
 //~^ ERROR cannot infer an appropriate lifetime

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

@@ -1,16 +1,20 @@
-error[E0621]: explicit lifetime required in the type of `x`
-  --> $DIR/must_outlive_least_region_or_bound.rs:3:23
+error: cannot infer an appropriate lifetime
+  --> $DIR/must_outlive_least_region_or_bound.rs:3:35
    |
 LL | fn elided(x: &i32) -> impl Copy { x }
-   |                       ^^^^^^^^^ `impl Trait` types can only capture lifetimes that they reference
-help: add explicit unnamed lifetime `'_` to the return type to constrain it
+   |                       ---------   ^ ...but this borrow...
+   |                       |
+   |                       this return type evaluates to the `'static` lifetime...
+   |
+note: ...can't outlive the anonymous lifetime #1 defined on the function body at 3:1
+  --> $DIR/must_outlive_least_region_or_bound.rs:3:1
+   |
+LL | fn elided(x: &i32) -> impl Copy { x }
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+help: you can add a constraint to the return type to make it last less than `'static` and match the anonymous lifetime #1 defined on the function body at 3:1
    |
 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
@@ -73,5 +77,5 @@ LL | fn ty_param_wont_outlive_static<T:Debug>(x: T) -> impl Debug + 'static {
 
 error: aborting due to 5 previous errors
 
-Some errors occurred: E0310, E0621, E0623.
+Some errors occurred: E0310, E0623.
 For more information about an error, try `rustc --explain E0310`.

src/test/ui/nll/ty-outlives/impl-trait-captures.rs

@@ -8,8 +8,8 @@ trait Foo<'a> {
 impl<'a, T> Foo<'a> for T { }
 
 fn foo<'a, T>(x: &T) -> impl Foo<'a> {
+//~^ ERROR explicit lifetime required in the type of `x` [E0621]
     x
-        //~^ ERROR explicit lifetime required in the type of `x` [E0621]
 }
 
 fn main() {}

src/test/ui/nll/ty-outlives/impl-trait-captures.stderr

@@ -1,8 +1,8 @@
 error[E0621]: explicit lifetime required in the type of `x`
-  --> $DIR/impl-trait-captures.rs:11:5
+  --> $DIR/impl-trait-captures.rs:10:25
    |
-LL |     x
-   |     ^ lifetime `ReEarlyBound(0, 'a)` required
+LL | fn foo<'a, T>(x: &T) -> impl Foo<'a> {
+   |                         ^^^^^^^^^^^^ lifetime `ReEarlyBound(0, 'a)` required
 help: add explicit lifetime `ReEarlyBound(0, 'a)` to the type of `x`
    |
 LL | fn foo<'a, T>(x: &ReEarlyBound(0, 'a) T) -> impl Foo<'a> {