only do the elided-lifetimes-in-paths lint on type paths

Instead of linting for invisible lifetimes during (a callee of)
`visit_path` (in the `LifetimeContext` visitor), we do it in the
`hir::TyPath` branch of `visit_ty`. This fixes a false positive where
we were suggesting `<'_>` on the struct name of a struct construction
expression. (Applying the suggestion would result in a parse error.)

Also, it seems better for the primary lint span (as distinguished from
the zero-width suggestion span where we propose the insertion of
anonymous lifetimes) to cover the entire path including any
angle-bracketed type parameters, rather than just the last path-name
segment. That is: we should highlight all of `Ref<T>` when we suggest
replacing it with `Ref<'_, T>`, rather than just highlighting the
`Ref`.

(Interestingly, this false positive didn't happen for tuple structs
like the one we already had in our UI test, probably because of
something something value vs. type namespace &c. that the present
author doesn't understand.)

This is in the matter of issue no. 52041.

src/librustc/middle/resolve_lifetime.rs

@@ -626,6 +626,10 @@ impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
                 self.with(scope, |_, this| this.visit_ty(&mt.ty));
             }
             hir::TyPath(hir::QPath::Resolved(None, ref path)) => {
+                let segment = &path.segments[path.segments.len()-1];
+                if let Some(ref args) = segment.args {
+                    self.lint_implicit_lifetimes_in_typath(path, args);
+                }
                 if let Def::Existential(exist_ty_did) = path.def {
                     assert!(exist_ty_did.is_local());
                     // Resolve the lifetimes that are applied to the existential type.
@@ -869,7 +873,7 @@ impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
         for (i, ref segment) in path.segments.iter().enumerate() {
             let depth = path.segments.len() - i - 1;
             if let Some(ref args) = segment.args {
-                self.visit_segment_args(path.def, depth, segment.ident, args);
+                self.visit_segment_args(path.def, depth, args);
             }
         }
     }
@@ -1667,7 +1671,6 @@ impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
         &mut self,
         def: Def,
         depth: usize,
-        segment_ident: ast::Ident,
         generic_args: &'tcx hir::GenericArgs,
     ) {
         if generic_args.parenthesized {
@@ -1690,9 +1693,6 @@ impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
             _ => None,
         }).collect::<Vec<_>>();
         if elide_lifetimes {
-            self.lint_implicit_lifetimes_in_segment(
-                segment_ident, generic_args, &lifetimes
-            );
             self.resolve_elided_lifetimes(lifetimes);
         } else {
             lifetimes.iter().for_each(|lt| self.visit_lifetime(lt));
@@ -2070,30 +2070,38 @@ impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
         }
     }
 
-    fn lint_implicit_lifetimes_in_segment(&mut self,
-                                          segment_ident: ast::Ident,
-                                          generic_args: &'tcx hir::GenericArgs,
-                                          lifetime_refs: &[&'tcx hir::Lifetime]) {
-        let num_implicit_lifetimes = lifetime_refs.iter()
-            .filter(|lt| lt.name.is_implicit()).count();
-        if num_implicit_lifetimes == 0 {
+    fn lint_implicit_lifetimes_in_typath(&mut self,
+                                         path: &hir::Path,
+                                         generic_args: &'tcx hir::GenericArgs) {
+        let implicit_lifetimes = generic_args.args.iter()
+            .filter_map(|arg| {
+                if let hir::GenericArg::Lifetime(lt) = arg {
+                    if lt.name.is_implicit() {
+                        return Some(lt)
+                    }
+                }
+                None
+            }).collect::<Vec<_>>();
+
+        if implicit_lifetimes.len() == 0 {
             return;
         }
 
         let mut err = self.tcx.struct_span_lint_node(
             lint::builtin::ELIDED_LIFETIMES_IN_PATHS,
-            lifetime_refs[0].id, // FIXME: HirIdify #50928
-            segment_ident.span,
+            implicit_lifetimes[0].id, // FIXME: HirIdify #50928
+            path.span,
             &format!("implicit lifetime parameters in types are deprecated"),
         );
 
-        if num_implicit_lifetimes == 1 {
+        if implicit_lifetimes.len() == 1 {
+            let segment_span = &path.segments[path.segments.len()-1].ident.span;
             let (replace_span,
-                 suggestion) = if generic_args.args.len() == num_implicit_lifetimes &&
+                 suggestion) = if generic_args.args.len() == implicit_lifetimes.len() &&
                 generic_args.bindings.is_empty() {
                     // If there are no (non-implicit) generic args or bindings, our
                     // suggestion includes the angle brackets
-                    (segment_ident.span.shrink_to_hi(), "<'_>")
+                    (segment_span.shrink_to_hi(), "<'_>")
                 } else {
                     // Otherwise—sorry, this is kind of gross—we need to infer the
                     // replacement point span from the generics that do exist

src/test/ui/in-band-lifetimes/elided-lifetimes.fixed

@@ -30,12 +30,21 @@ fn bar(x: &Foo<'_>) {}
 
 struct Wrapped<'a>(&'a str);
 
+struct WrappedWithBow<'a> {
+    gift: &'a str
+}
+
 fn wrap_gift(gift: &str) -> Wrapped<'_> {
     //~^ ERROR implicit lifetime parameters in types are deprecated
     //~| HELP indicate the anonymous lifetime
     Wrapped(gift)
 }
 
+fn wrap_gift_with_bow(gift: &str) -> WrappedWithBow<'_> {
+    //~^ ERROR implicit lifetime parameters in types are deprecated
+    //~| HELP indicate the anonymous lifetime
+    WrappedWithBow { gift }
+}
 
 fn main() {
     let honesty = RefCell::new((4, 'e'));

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

@@ -30,12 +30,21 @@ fn bar(x: &Foo<'_>) {}
 
 struct Wrapped<'a>(&'a str);
 
+struct WrappedWithBow<'a> {
+    gift: &'a str
+}
+
 fn wrap_gift(gift: &str) -> Wrapped {
     //~^ ERROR implicit lifetime parameters in types are deprecated
     //~| HELP indicate the anonymous lifetime
     Wrapped(gift)
 }
 
+fn wrap_gift_with_bow(gift: &str) -> WrappedWithBow {
+    //~^ ERROR implicit lifetime parameters in types are deprecated
+    //~| HELP indicate the anonymous lifetime
+    WrappedWithBow { gift }
+}
 
 fn main() {
     let honesty = RefCell::new((4, 'e'));

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

@@ -11,16 +11,24 @@ LL | #![deny(elided_lifetimes_in_paths)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: implicit lifetime parameters in types are deprecated
-  --> $DIR/elided-lifetimes.rs:33:29
+  --> $DIR/elided-lifetimes.rs:37:29
    |
 LL | fn wrap_gift(gift: &str) -> Wrapped {
    |                             ^^^^^^^- help: indicate the anonymous lifetime: `<'_>`
 
 error: implicit lifetime parameters in types are deprecated
-  --> $DIR/elided-lifetimes.rs:42:18
+  --> $DIR/elided-lifetimes.rs:43:38
+   |
+LL | fn wrap_gift_with_bow(gift: &str) -> WrappedWithBow {
+   |                                      ^^^^^^^^^^^^^^- help: indicate the anonymous lifetime: `<'_>`
+
+error: implicit lifetime parameters in types are deprecated
+  --> $DIR/elided-lifetimes.rs:51:18
    |
 LL |     let loyalty: Ref<(u32, char)> = honesty.borrow();
-   |                  ^^^ - help: indicate the anonymous lifetime: `'_,`
+   |                  ^^^^-^^^^^^^^^^^
+   |                      |
+   |                      help: indicate the anonymous lifetime: `'_,`
 
-error: aborting due to 3 previous errors
+error: aborting due to 4 previous errors