Auto merge of #49041 - nikomatsakis:issue-46541-impl-trait-hidden-lifetimes, r=cramertj

Detect illegal hidden lifetimes in `impl Trait`

This branch fixes #46541 -- however, it presently doesn't build because it also *breaks* a number of existing usages of impl Trait. I'm opening it as a WIP for now, just because we want to move on impl Trait, but I'll try to fix the problem in a bit.

~~(The problem is due to the fact that we apparently infer stricter lifetimes in closures that we need to; for example, if you capture a variable of type `&'a &'b u32`, we will put *precisely* those lifetimes into the closure, even if the closure would be happy with `&'a &'a u32`. This causes the present chance to affect things that are not invariant.)~~ fixed

r? @cramertj

src/test/mir-opt/end_region_8.rs

@@ -36,7 +36,7 @@ fn foo<F>(f: F) where F: FnOnce() -> i32 {
 //    let _2: &'21_1rs D;
 //    ...
 //    let mut _3: ();
-//    let mut _4: [closure@NodeId(22) r:&'21_1rs D];
+//    let mut _4: [closure@NodeId(22) r:&'19s D];
 //    let mut _5: &'21_1rs D;
 //    bb0: {
 //        StorageLive(_1);
@@ -54,6 +54,7 @@ fn foo<F>(f: F) where F: FnOnce() -> i32 {
 //        resume;
 //    }
 //    bb2: {
+//        EndRegion('19s);
 //        StorageDead(_4);
 //        _0 = ();
 //        EndRegion('21_1rs);
@@ -61,6 +62,7 @@ fn foo<F>(f: F) where F: FnOnce() -> i32 {
 //        drop(_1) -> [return: bb4, unwind: bb1];
 //    }
 //    bb3: {
+//        EndRegion('19s);
 //        EndRegion('21_1rs);
 //        drop(_1) -> bb1;
 //    }
@@ -72,7 +74,7 @@ fn foo<F>(f: F) where F: FnOnce() -> i32 {
 // END rustc.main.SimplifyCfg-qualify-consts.after.mir
 
 // START rustc.main-{{closure}}.SimplifyCfg-qualify-consts.after.mir
-// fn main::{{closure}}(_1: [closure@NodeId(22) r:&'21_1rs D]) -> i32 {
+// fn main::{{closure}}(_1: [closure@NodeId(22) r:&'19s D]) -> i32 {
 //     let mut _0: i32;
 //     let mut _2: i32;
 //

src/test/ui/generator/auto-trait-regions.stderr

@@ -1,15 +1,15 @@
-error[E0277]: the trait bound `No: Foo` is not satisfied in `[generator@$DIR/auto-trait-regions.rs:35:15: 39:6 x:&&'static OnlyFooIfStaticRef for<'r> {&'r OnlyFooIfStaticRef, ()}]`
+error[E0277]: the trait bound `No: Foo` is not satisfied in `[generator@$DIR/auto-trait-regions.rs:35:15: 39:6 x:&&OnlyFooIfStaticRef for<'r> {&'r OnlyFooIfStaticRef, ()}]`
   --> $DIR/auto-trait-regions.rs:40:5
    |
 LL |     assert_foo(gen); //~ ERROR the trait bound `No: Foo` is not satisfied
-   |     ^^^^^^^^^^ within `[generator@$DIR/auto-trait-regions.rs:35:15: 39:6 x:&&'static OnlyFooIfStaticRef for<'r> {&'r OnlyFooIfStaticRef, ()}]`, the trait `Foo` is not implemented for `No`
+   |     ^^^^^^^^^^ within `[generator@$DIR/auto-trait-regions.rs:35:15: 39:6 x:&&OnlyFooIfStaticRef for<'r> {&'r OnlyFooIfStaticRef, ()}]`, the trait `Foo` is not implemented for `No`
    |
    = help: the following implementations were found:
              <No as Foo>
    = note: required because it appears within the type `OnlyFooIfStaticRef`
    = note: required because it appears within the type `&OnlyFooIfStaticRef`
    = note: required because it appears within the type `for<'r> {&'r OnlyFooIfStaticRef, ()}`
-   = note: required because it appears within the type `[generator@$DIR/auto-trait-regions.rs:35:15: 39:6 x:&&'static OnlyFooIfStaticRef for<'r> {&'r OnlyFooIfStaticRef, ()}]`
+   = note: required because it appears within the type `[generator@$DIR/auto-trait-regions.rs:35:15: 39:6 x:&&OnlyFooIfStaticRef for<'r> {&'r OnlyFooIfStaticRef, ()}]`
 note: required by `assert_foo`
   --> $DIR/auto-trait-regions.rs:30:1
    |

src/test/ui/impl-trait/region-escape-via-bound-contravariant-closure.rs

@@ -0,0 +1,31 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// In contrast to `region-escape-via-bound-invariant`, in this case we
+// *can* return a value of type `&'x u32`, even though `'x` does not
+// appear in the bounds. This is because `&` is contravariant, and so
+// we are *actually* returning a `&'y u32`.
+//
+// See https://github.com/rust-lang/rust/issues/46541 for more details.
+
+// run-pass
+
+#![allow(dead_code)]
+#![feature(conservative_impl_trait)]
+#![feature(in_band_lifetimes)]
+#![feature(nll)]
+
+fn foo(x: &'x u32) -> impl Fn() -> &'y u32
+where 'x: 'y
+{
+    move || x
+}
+
+fn main() { }

src/test/ui/impl-trait/region-escape-via-bound-contravariant.rs

@@ -0,0 +1,35 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// In contrast to `region-escape-via-bound-invariant`, in this case we
+// *can* return a value of type `&'x u32`, even though `'x` does not
+// appear in the bounds. This is because `&` is contravariant, and so
+// we are *actually* returning a `&'y u32`.
+//
+// See https://github.com/rust-lang/rust/issues/46541 for more details.
+
+// run-pass
+
+#![allow(dead_code)]
+#![feature(conservative_impl_trait)]
+#![feature(in_band_lifetimes)]
+#![feature(nll)]
+
+trait Trait<'a> { }
+
+impl Trait<'b> for &'a u32 { }
+
+fn foo(x: &'x u32) -> impl Trait<'y>
+where 'x: 'y
+{
+    x
+}
+
+fn main() { }

src/test/ui/impl-trait/region-escape-via-bound.rs

@@ -0,0 +1,34 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Test that we do not allow the region `'x` to escape in the impl
+// trait **even though** `'y` escapes, which outlives `'x`.
+//
+// See https://github.com/rust-lang/rust/issues/46541 for more details.
+
+#![allow(dead_code)]
+#![feature(conservative_impl_trait)]
+#![feature(in_band_lifetimes)]
+#![feature(nll)]
+
+use std::cell::Cell;
+
+trait Trait<'a> { }
+
+impl Trait<'b> for Cell<&'a u32> { }
+
+fn foo(x: Cell<&'x u32>) -> impl Trait<'y>
+    //~^ ERROR hidden type for `impl Trait` captures lifetime that does not appear in bounds [E0909]
+where 'x: 'y
+{
+    x
+}
+
+fn main() { }

src/test/ui/impl-trait/region-escape-via-bound.stderr

@@ -0,0 +1,20 @@
+error[E0909]: hidden type for `impl Trait` captures lifetime that does not appear in bounds
+  --> $DIR/region-escape-via-bound.rs:27:29
+   |
+LL | fn foo(x: Cell<&'x u32>) -> impl Trait<'y>
+   |                             ^^^^^^^^^^^^^^
+   |
+note: hidden type `std::cell::Cell<&'x u32>` captures the lifetime 'x as defined on the function body at 27:1
+  --> $DIR/region-escape-via-bound.rs:27:1
+   |
+LL | / fn foo(x: Cell<&'x u32>) -> impl Trait<'y>
+LL | |     //~^ ERROR hidden type for `impl Trait` captures lifetime that does not appear in bounds [E0909]
+LL | | where 'x: 'y
+LL | | {
+LL | |     x
+LL | | }
+   | |_^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0909`.