Rollup merge of #103876 - oli-obk:tait_implications, r=lcnr

type alias impl trait: add tests showing that hidden type only outlives lifetimes that occur in bounds

fixes #103642

https://github.com/rust-lang/rust/pull/102417 only made sure that hidden types cannot outlive lifetimes other than the ones mentioned on bounds, but didn't allow us to actually infer anything from that.

cc `@aliemjay`

compiler/rustc_borrowck/src/region_infer/mod.rs

@@ -1705,6 +1705,7 @@ impl<'tcx> RegionInferenceContext<'tcx> {
         });
     }
 
+    #[instrument(level = "debug", skip(self, infcx, errors_buffer))]
     fn check_member_constraints(
         &self,
         infcx: &InferCtxt<'tcx>,
@@ -1712,22 +1713,21 @@ impl<'tcx> RegionInferenceContext<'tcx> {
     ) {
         let member_constraints = self.member_constraints.clone();
         for m_c_i in member_constraints.all_indices() {
-            debug!("check_member_constraint(m_c_i={:?})", m_c_i);
+            debug!(?m_c_i);
             let m_c = &member_constraints[m_c_i];
             let member_region_vid = m_c.member_region_vid;
             debug!(
-                "check_member_constraint: member_region_vid={:?} with value {}",
-                member_region_vid,
-                self.region_value_str(member_region_vid),
+                ?member_region_vid,
+                value = ?self.region_value_str(member_region_vid),
             );
             let choice_regions = member_constraints.choice_regions(m_c_i);
-            debug!("check_member_constraint: choice_regions={:?}", choice_regions);
+            debug!(?choice_regions);
 
             // Did the member region wind up equal to any of the option regions?
             if let Some(o) =
                 choice_regions.iter().find(|&&o_r| self.eval_equal(o_r, m_c.member_region_vid))
             {
-                debug!("check_member_constraint: evaluated as equal to {:?}", o);
+                debug!("evaluated as equal to {:?}", o);
                 continue;
             }
 

src/test/ui/type-alias-impl-trait/imply_bounds_from_bounds.rs

@@ -0,0 +1,25 @@
+// check-pass
+
+#![feature(type_alias_impl_trait)]
+
+trait Callable {
+    type Output;
+    fn call() -> Self::Output;
+}
+
+impl<'a> Callable for &'a () {
+    type Output = impl Sized;
+    fn call() -> Self::Output {}
+}
+
+fn test<'a>() -> impl Sized {
+    <&'a () as Callable>::call()
+}
+
+fn want_static<T: 'static>(_: T) {}
+
+fn test2<'a>() {
+    want_static(<&'a () as Callable>::call());
+}
+
+fn main() {}

src/test/ui/type-alias-impl-trait/imply_bounds_from_bounds_param.rs

@@ -0,0 +1,38 @@
+#![feature(type_alias_impl_trait)]
+
+trait Callable {
+    type Output;
+    fn call(x: Self) -> Self::Output;
+}
+
+trait PlusOne {
+    fn plus_one(&mut self);
+}
+
+impl<'a> PlusOne for &'a mut i32 {
+    fn plus_one(&mut self) {
+        **self += 1;
+    }
+}
+
+impl<T: PlusOne> Callable for T {
+    type Output = impl PlusOne;
+    fn call(t: T) -> Self::Output { t }
+}
+
+fn test<'a>(y: &'a mut i32) -> impl PlusOne {
+    <&'a mut i32 as Callable>::call(y)
+    //~^ ERROR hidden type for `impl PlusOne` captures lifetime that does not appear in bounds
+}
+
+fn main() {
+    let mut z = 42;
+    let mut thing = test(&mut z);
+    let mut thing2 = test(&mut z);
+    thing.plus_one();
+    assert_eq!(z, 43);
+    thing2.plus_one();
+    assert_eq!(z, 44);
+    thing.plus_one();
+    assert_eq!(z, 45);
+}

src/test/ui/type-alias-impl-trait/imply_bounds_from_bounds_param.stderr

@@ -0,0 +1,16 @@
+error[E0700]: hidden type for `impl PlusOne` captures lifetime that does not appear in bounds
+  --> $DIR/imply_bounds_from_bounds_param.rs:24:5
+   |
+LL | fn test<'a>(y: &'a mut i32) -> impl PlusOne {
+   |         -- hidden type `<&'a mut i32 as Callable>::Output` captures the lifetime `'a` as defined here
+LL |     <&'a mut i32 as Callable>::call(y)
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: to declare that `impl PlusOne` captures `'a`, you can add an explicit `'a` lifetime bound
+   |
+LL | fn test<'a>(y: &'a mut i32) -> impl PlusOne + 'a {
+   |                                             ++++
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0700`.

src/test/ui/type-alias-impl-trait/self_implication.rs

@@ -0,0 +1,38 @@
+// check-pass
+
+#![feature(type_alias_impl_trait)]
+fn foo() {
+    struct Foo<'a> {
+        x: &'a mut u8,
+    }
+    impl<'a> Foo<'a> {
+        fn foo(&self) -> impl Sized {}
+    }
+    // use site
+    let mut x = 5;
+    let y = Foo { x: &mut x };
+    let z = y.foo();
+    let _a = &x; // invalidate the `&'a mut`in `y`
+    let _b = z; // this should *not* check that `'a` in the type `Foo<'a>::foo::opaque` is live
+}
+
+fn bar() {
+    struct Foo<'a> {
+        x: &'a mut u8,
+    }
+
+    // desugared
+    type FooX<'a> = impl Sized;
+    impl<'a> Foo<'a> {
+        fn foo(&self) -> FooX<'a> {}
+    }
+
+    // use site
+    let mut x = 5;
+    let y = Foo { x: &mut x };
+    let z = y.foo();
+    let _a = &x; // invalidate the `&'a mut`in `y`
+    let _b = z; // this should *not* check that `'a` in the type `Foo<'a>::foo::opaque` is live
+}
+
+fn main() {}