Rollup merge of #98259 - jyn514:improve-obligation-errors, r=estebank

Greatly improve error reporting for futures and generators in `note_obligation_cause_code`

Most futures don't go through this code path, because they're caught by
`maybe_note_obligation_cause_for_async_await`. But all generators do,
and `maybe_note` is imperfect and doesn't catch all futures. Improve the error message for those it misses.

At some point, we may want to consider unifying this with the code for `maybe_note_async_await`,
so that `async_await` notes all parent constraints, and `note_obligation` can point to yield points.
But both functions are quite complicated, and it's not clear to me how to combine them;
this seems like a good incremental improvement.

Helps with https://github.com/rust-lang/rust/issues/97332.

r? ``@estebank`` cc ``@eholk`` ``@compiler-errors``

2 files changed, 40 insertions, 10 deletions

diff --git a/src/test/ui/impl-trait/auto-trait-leak2.rs b/src/test/ui/impl-trait/auto-trait-leak2.rs
index a464f576dc7..09450089ada 100644
--- a/src/test/ui/impl-trait/auto-trait-leak2.rs
+++ b/src/test/ui/impl-trait/auto-trait-leak2.rs
@@ -3,23 +3,37 @@ use std::rc::Rc;
 
 // Fast path, main can see the concrete type returned.
 fn before() -> impl Fn(i32) {
+//~^ NOTE within this `impl Fn
+//~| NOTE within the type `impl Fn
+//~| NOTE expansion of desugaring
     let p = Rc::new(Cell::new(0));
-    move |x| p.set(x)
+    move |x| p.set(x) //~ NOTE used within this closure
 }
 
 fn send<T: Send>(_: T) {}
+//~^ NOTE required by a bound
+//~| NOTE required by a bound
+//~| NOTE required by this bound
+//~| NOTE required by this bound
 
 fn main() {
     send(before());
     //~^ ERROR `Rc<Cell<i32>>` cannot be sent between threads safely
+    //~| NOTE `Rc<Cell<i32>>` cannot be sent between threads safely
+    //~| NOTE required by a bound
 
     send(after());
     //~^ ERROR `Rc<Cell<i32>>` cannot be sent between threads safely
+    //~| NOTE `Rc<Cell<i32>>` cannot be sent between threads safely
+    //~| NOTE required by a bound
 }
 
 // Deferred path, main has to wait until typeck finishes,
 // to check if the return type of after is Send.
 fn after() -> impl Fn(i32) {
+//~^ NOTE within this `impl Fn(i32)`
+//~| NOTE in this expansion
+//~| NOTE appears within the type
     let p = Rc::new(Cell::new(0));
-    move |x| p.set(x)
+    move |x| p.set(x) //~ NOTE used within this closure
 }
diff --git a/src/test/ui/impl-trait/auto-trait-leak2.stderr b/src/test/ui/impl-trait/auto-trait-leak2.stderr
index 37ae3c68029..d825843492d 100644
--- a/src/test/ui/impl-trait/auto-trait-leak2.stderr
+++ b/src/test/ui/impl-trait/auto-trait-leak2.stderr
@@ -1,5 +1,5 @@
 error[E0277]: `Rc<Cell<i32>>` cannot be sent between threads safely
-  --> $DIR/auto-trait-leak2.rs:13:10
+  --> $DIR/auto-trait-leak2.rs:20:10
    |
 LL | fn before() -> impl Fn(i32) {
    |                ------------ within this `impl Fn(i32)`
@@ -10,16 +10,24 @@ LL |     send(before());
    |     required by a bound introduced by this call
    |
    = help: within `impl Fn(i32)`, the trait `Send` is not implemented for `Rc<Cell<i32>>`
-   = note: required because it appears within the type `[closure@$DIR/auto-trait-leak2.rs:7:5: 7:22]`
-   = note: required because it appears within the type `impl Fn(i32)`
+note: required because it's used within this closure
+  --> $DIR/auto-trait-leak2.rs:10:5
+   |
+LL |     move |x| p.set(x)
+   |     ^^^^^^^^^^^^^^^^^
+note: required because it appears within the type `impl Fn(i32)`
+  --> $DIR/auto-trait-leak2.rs:5:16
+   |
+LL | fn before() -> impl Fn(i32) {
+   |                ^^^^^^^^^^^^
 note: required by a bound in `send`
-  --> $DIR/auto-trait-leak2.rs:10:12
+  --> $DIR/auto-trait-leak2.rs:13:12
    |
 LL | fn send<T: Send>(_: T) {}
    |            ^^^^ required by this bound in `send`
 
 error[E0277]: `Rc<Cell<i32>>` cannot be sent between threads safely
-  --> $DIR/auto-trait-leak2.rs:16:10
+  --> $DIR/auto-trait-leak2.rs:25:10
    |
 LL |     send(after());
    |     ---- ^^^^^^^ `Rc<Cell<i32>>` cannot be sent between threads safely
@@ -30,10 +38,18 @@ LL | fn after() -> impl Fn(i32) {
    |               ------------ within this `impl Fn(i32)`
    |
    = help: within `impl Fn(i32)`, the trait `Send` is not implemented for `Rc<Cell<i32>>`
-   = note: required because it appears within the type `[closure@$DIR/auto-trait-leak2.rs:24:5: 24:22]`
-   = note: required because it appears within the type `impl Fn(i32)`
+note: required because it's used within this closure
+  --> $DIR/auto-trait-leak2.rs:38:5
+   |
+LL |     move |x| p.set(x)
+   |     ^^^^^^^^^^^^^^^^^
+note: required because it appears within the type `impl Fn(i32)`
+  --> $DIR/auto-trait-leak2.rs:33:15
+   |
+LL | fn after() -> impl Fn(i32) {
+   |               ^^^^^^^^^^^^
 note: required by a bound in `send`
-  --> $DIR/auto-trait-leak2.rs:10:12
+  --> $DIR/auto-trait-leak2.rs:13:12
    |
 LL | fn send<T: Send>(_: T) {}
    |            ^^^^ required by this bound in `send`