test implied bounds + nested proj oblig

Author: lcnr

src/test/ui/implied-bounds/assoc-ty-wf-used-to-get-assoc-ty.rs

@@ -0,0 +1,27 @@
+// Test for a less than ideal interaction of implied bounds and normalization.
+trait Tr {
+    type Ty;
+}
+
+impl<T: 'static> Tr for T {
+    type Ty = &'static T;
+}
+
+// `<&'a u8 as Tr>::Ty` should cause an error because `&'a u8: Tr` doesn't hold for
+// all possible 'a. However, we consider normalized types for implied bounds.
+//
+// We normalize this projection to `&'static &'a u8` and add a nested `&'a u8: 'static`
+// bound. This bound is then proven using the implied bounds for `&'static &'a u8` which
+// we only get by normalizing in the first place.
+fn test<'a>(x: &'a u8, _wf: <&'a u8 as Tr>::Ty) -> &'static u8 { x }
+
+fn main() {
+    // This works as we have 'static references due to promotion.
+    let _: &'static u8 = test(&3, &&3);
+    // This causes an error because the projection requires 'a to be 'static.
+    // It would be unsound if this compiled.
+    let x: u8 = 3;
+    let _: &'static u8 = test(&x, &&3);
+    //~^ ERROR `x` does not live long enough
+
+}

src/test/ui/implied-bounds/assoc-ty-wf-used-to-get-assoc-ty.stderr

@@ -0,0 +1,15 @@
+error[E0597]: `x` does not live long enough
+  --> $DIR/assoc-ty-wf-used-to-get-assoc-ty.rs:24:31
+   |
+LL |     let _: &'static u8 = test(&x, &&3);
+   |                          -----^^------
+   |                          |    |
+   |                          |    borrowed value does not live long enough
+   |                          argument requires that `x` is borrowed for `'static`
+...
+LL | }
+   | - `x` dropped here while still borrowed
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0597`.