path-type examples for single-use lifetime in fn argument UI test

Niko Matsakis commented in October 2017
(#44752 (comment))
that these should lint. They do! Let's reify that in the tests now
(and then we'll see a nice diff when we add suggestions in a future
commit).

src/test/ui/single-use-lifetime/one-use-in-fn-argument.rs

@@ -9,4 +9,11 @@ fn a<'a>(x: &'a u32) { //~ ERROR `'a` only used once
     //~^ HELP elide the single-use lifetime
 }
 
+struct Single<'a> { x: &'a u32 }
+struct Double<'a, 'b> { f: &'a &'b u32 }
+
+fn center<'m>(_: Single<'m>) {} //~ ERROR `'m` only used once
+fn left<'x, 'y>(foo: Double<'x, 'y>) -> &'x u32 { foo.f } //~ ERROR `'y` only used once
+fn right<'x, 'y>(foo: Double<'x, 'y>) -> &'y u32 { foo.f } //~ ERROR `'x` only used once
+
 fn main() { }

src/test/ui/single-use-lifetime/one-use-in-fn-argument.stderr

@@ -16,5 +16,25 @@ help: elide the single-use lifetime
 LL | fn a(x: &u32) {
    |    --   --
 
-error: aborting due to previous error
+error: lifetime parameter `'m` only used once
+  --> $DIR/one-use-in-fn-argument.rs:15:11
+   |
+LL | fn center<'m>(_: Single<'m>) {}
+   |           ^^            -- ...is used only here
+   |           |
+   |           this lifetime...
+
+error: lifetime parameter `'y` only used once
+  --> $DIR/one-use-in-fn-argument.rs:16:13
+   |
+LL | fn left<'x, 'y>(foo: Double<'x, 'y>) -> &'x u32 { foo.f }
+   |             ^^ this lifetime... -- ...is used only here
+
+error: lifetime parameter `'x` only used once
+  --> $DIR/one-use-in-fn-argument.rs:17:10
+   |
+LL | fn right<'x, 'y>(foo: Double<'x, 'y>) -> &'y u32 { foo.f }
+   |          ^^ this lifetime... -- ...is used only here
+
+error: aborting due to 4 previous errors