in which the elided-lifetimes-in-paths lint stops firing on '_

`is_elided` is true for both underscore (`'_`) and implicit (no
representation in source code) lifetimes, but we don't want to fire the
lint on the former, because the entire point of the lint is to suggest
changing the latter to the former (see the initial issue rust-lang#45992).

It seems unfortunate for there to be ambiguity on whether the word
"elided" includes underscore lifetimes or not—the mandate of the
elided-lifetimes-in-paths lint seems to suggest it doesn't, whereas
the `is_elided` method seems to suggest it does—but it's beyond us to
resolve that in this commit. For now, let the message say "implicit"
for definiteness.

This relates to rust-lang#52041.

src/librustc/hir/mod.rs

@@ -272,6 +272,10 @@ impl LifetimeName {
         }
     }
 
+    pub fn is_implicit(&self) -> bool {
+        self == &LifetimeName::Implicit
+    }
+
     fn is_static(&self) -> bool {
         self == &LifetimeName::Static
     }

src/librustc/lint/builtin.rs

@@ -252,7 +252,7 @@ declare_lint! {
 declare_lint! {
     pub ELIDED_LIFETIMES_IN_PATHS,
     Allow,
-    "hidden lifetime parameters are deprecated, try `Foo<'_>`"
+    "implicit lifetime parameters are deprecated"
 }
 
 declare_lint! {

src/librustc/middle/resolve_lifetime.rs

@@ -2068,24 +2068,24 @@ impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
 
     fn resolve_elided_lifetimes(&mut self,
                                 lifetime_refs: Vec<&'tcx hir::Lifetime>,
-                                deprecated: bool) {
+                                deprecate_implicit: bool) {
         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();
+        if deprecate_implicit && lifetime_refs[0].name.is_implicit() {
+            let mut err = self.tcx.struct_span_lint_node(
+                lint::builtin::ELIDED_LIFETIMES_IN_PATHS,
+                lifetime_refs[0].id, // FIXME: HirIdify #50928
+                span,
+                &format!("implicit lifetime parameters in types are deprecated"),
+            );
+            // FIXME: suggest `'_` (need to take into account whether angle-bracketed
+            // params already exist)
+            err.emit();
         }
         let error = loop {
             match *scope {

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

@@ -14,11 +14,9 @@
 struct Foo<'a> { x: &'a u32 }
 
 fn foo(x: &Foo) {
-    //~^ ERROR: hidden lifetime parameters are deprecated, try `Foo<'_>`
+    //~^ ERROR: implicit lifetime parameters in types are deprecated
 }
 
-fn bar(x: &Foo<'_>) {
-    //~^ ERROR: hidden lifetime parameters are deprecated, try `Foo<'_>`
-}
+fn bar(x: &Foo<'_>) {}
 
 fn main() {}

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

@@ -1,4 +1,4 @@
-error: hidden lifetime parameters are deprecated, try `Foo<'_>`
+error: implicit lifetime parameters in types are deprecated
   --> $DIR/elided-lifetimes.rs:16:12
    |
 LL | fn foo(x: &Foo) {
@@ -10,11 +10,5 @@ note: lint level defined here
 LL | #![deny(elided_lifetimes_in_paths)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: hidden lifetime parameters are deprecated, try `Foo<'_>`
-  --> $DIR/elided-lifetimes.rs:20:16
-   |
-LL | fn bar(x: &Foo<'_>) {
-   |                ^^
-
-error: aborting due to 2 previous errors
+error: aborting due to previous error