Rollup merge of #76468 - SNCPlay42:lifetime-names, r=Mark-Simulacrum

Improve lifetime name annotations for closures & async functions

* Don't refer to async functions as "generators" in error output
* Where possible, emit annotations pointing exactly at the `&` in the return type of closures (when they have explicit return types) and async functions, like we do for arguments.
Addresses #74072, but I wouldn't call that *closed* until annotations are identical for async and non-async functions.
* Emit a better annotation when the lifetime doesn't appear in the full name type, which currently happens for opaque types like `impl Future`. Addresses #74497, but further improves could probably be made (why *doesn't* it appear in the type as `impl Future + '1`?)
This is included in the same PR because the changes to `give_name_if_anonymous_region_appears_in_output` would introduce ICE otherwise (it would return `None` in cases where it didn't previously, which then gets `unwrap`ped)

src/test/ui/async-await/issue-74072-lifetime-name-annotations.rs

@@ -0,0 +1,37 @@
+// edition:2018
+#![feature(async_closure)]
+use std::future::Future;
+
+// test the quality of annotations giving lifetimes names (`'1`) when async constructs are involved
+
+pub async fn async_fn(x: &mut i32) -> &i32 {
+    let y = &*x;
+    *x += 1; //~ ERROR cannot assign to `*x` because it is borrowed
+    y
+}
+
+pub fn async_closure(x: &mut i32) -> impl Future<Output=&i32> {
+    (async move || {
+        let y = &*x;
+        *x += 1; //~ ERROR cannot assign to `*x` because it is borrowed
+        y
+    })()
+}
+
+pub fn async_closure_explicit_return_type(x: &mut i32) -> impl Future<Output=&i32> {
+    (async move || -> &i32 {
+        let y = &*x;
+        *x += 1; //~ ERROR cannot assign to `*x` because it is borrowed
+        y
+    })()
+}
+
+pub fn async_block(x: &mut i32) -> impl Future<Output=&i32> {
+    async move {
+        let y = &*x;
+        *x += 1; //~ ERROR cannot assign to `*x` because it is borrowed
+        y
+    }
+}
+
+fn main() {}

src/test/ui/async-await/issue-74072-lifetime-name-annotations.stderr

@@ -0,0 +1,51 @@
+error[E0506]: cannot assign to `*x` because it is borrowed
+  --> $DIR/issue-74072-lifetime-name-annotations.rs:9:5
+   |
+LL | pub async fn async_fn(x: &mut i32) -> &i32 {
+   |                                       - let's call the lifetime of this reference `'1`
+LL |     let y = &*x;
+   |             --- borrow of `*x` occurs here
+LL |     *x += 1;
+   |     ^^^^^^^ assignment to borrowed `*x` occurs here
+LL |     y
+   |     - returning this value requires that `*x` is borrowed for `'1`
+
+error[E0506]: cannot assign to `*x` because it is borrowed
+  --> $DIR/issue-74072-lifetime-name-annotations.rs:16:9
+   |
+LL |         let y = &*x;
+   |                 --- borrow of `*x` occurs here
+LL |         *x += 1;
+   |         ^^^^^^^ assignment to borrowed `*x` occurs here
+LL |         y
+   |         - returning this value requires that `*x` is borrowed for `'1`
+LL |     })()
+   |     - return type of async closure is &'1 i32
+
+error[E0506]: cannot assign to `*x` because it is borrowed
+  --> $DIR/issue-74072-lifetime-name-annotations.rs:24:9
+   |
+LL |     (async move || -> &i32 {
+   |                       - let's call the lifetime of this reference `'1`
+LL |         let y = &*x;
+   |                 --- borrow of `*x` occurs here
+LL |         *x += 1;
+   |         ^^^^^^^ assignment to borrowed `*x` occurs here
+LL |         y
+   |         - returning this value requires that `*x` is borrowed for `'1`
+
+error[E0506]: cannot assign to `*x` because it is borrowed
+  --> $DIR/issue-74072-lifetime-name-annotations.rs:32:9
+   |
+LL |         let y = &*x;
+   |                 --- borrow of `*x` occurs here
+LL |         *x += 1;
+   |         ^^^^^^^ assignment to borrowed `*x` occurs here
+LL |         y
+   |         - returning this value requires that `*x` is borrowed for `'1`
+LL |     }
+   |     - return type of async block is &'1 i32
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0506`.

src/test/ui/async-await/issue-74497-lifetime-in-opaque.rs

@@ -0,0 +1,19 @@
+// edition:2018
+
+// test that names give to anonymous lifetimes in opaque types like `impl Future` are correctly
+// introduced in error messages
+
+use std::future::Future;
+
+pub async fn foo<F, T>(_: F)
+where
+    F: Fn(&u8) -> T,
+    T: Future<Output = ()>,
+{
+}
+
+pub async fn bar(_: &u8) {}
+
+fn main() {
+    let _ = foo(|x| bar(x)); //~ ERROR lifetime may not live long enough
+}

src/test/ui/async-await/issue-74497-lifetime-in-opaque.stderr

@@ -0,0 +1,11 @@
+error: lifetime may not live long enough
+  --> $DIR/issue-74497-lifetime-in-opaque.rs:18:21
+   |
+LL |     let _ = foo(|x| bar(x));
+   |                  -- ^^^^^^ returning this value requires that `'1` must outlive `'2`
+   |                  ||
+   |                  |return type of closure `impl Future` contains a lifetime `'2`
+   |                  has type `&'1 u8`
+
+error: aborting due to previous error
+

src/test/ui/borrowck/issue-53432-nested-closure-outlives-borrowed-value.stderr

@@ -4,7 +4,7 @@ error: lifetime may not live long enough
 LL |     let _action = move || {
    |                   -------
    |                   |     |
-   |                   |     return type of closure is [closure@$DIR/issue-53432-nested-closure-outlives-borrowed-value.rs:4:9: 4:15]
+   |                   |     return type of closure `[closure@$DIR/issue-53432-nested-closure-outlives-borrowed-value.rs:4:9: 4:15]` contains a lifetime `'2`
    |                   lifetime `'1` represents this closure's body
 LL |         || f() // The `nested` closure
    |         ^^^^^^ returning this value requires that `'1` must outlive `'2`

src/test/ui/nll/issue-58053.stderr

@@ -2,9 +2,9 @@ error: lifetime may not live long enough
   --> $DIR/issue-58053.rs:6:33
    |
 LL |     let f = |x: &i32| -> &i32 { x };
-   |                 -        ----   ^ returning this value requires that `'1` must outlive `'2`
+   |                 -        -      ^ returning this value requires that `'1` must outlive `'2`
    |                 |        |
-   |                 |        return type of closure is &'2 i32
+   |                 |        let's call the lifetime of this reference `'2`
    |                 let's call the lifetime of this reference `'1`
 
 error: lifetime may not live long enough