Auto merge of #88719 - estebank:point-at-arg-for-obligation, r=nagisa

Point at argument instead of call for their obligations

When an obligation is introduced by a specific `fn` argument, point at
the argument instead of the `fn` call if the obligation fails to be
fulfilled.

Move the information about pointing at the call argument expression in
an unmet obligation span from the `FulfillmentError` to a new
`ObligationCauseCode`.

When giving an error about an obligation introduced by a function call
that an argument doesn't fulfill, and that argument is a block, add a
span_label pointing at the innermost tail expression.

Current output:

```
error[E0425]: cannot find value `x` in this scope
 --> f10.rs:4:14
  |
4 |         Some(x * 2)
  |              ^ not found in this scope

error[E0277]: expected a `FnOnce<({integer},)>` closure, found `Option<_>`
 --> f10.rs:2:31
  |
2 |       let p = Some(45).and_then({
  |  ______________________--------_^
  | |                      |
  | |                      required by a bound introduced by this call
3 | |         |x| println!("doubling {}", x);
4 | |         Some(x * 2)
  | |         -----------
5 | |     });
  | |_____^ expected an `FnOnce<({integer},)>` closure, found `Option<_>`
  |
  = help: the trait `FnOnce<({integer},)>` is not implemented for `Option<_>`
```

Previous output:

```
error[E0425]: cannot find value `x` in this scope
 --> f10.rs:4:14
  |
4 |         Some(x * 2)
  |              ^ not found in this scope

error[E0277]: expected a `FnOnce<({integer},)>` closure, found `Option<_>`
 --> f10.rs:2:22
  |
2 |     let p = Some(45).and_then({
  |                      ^^^^^^^^ expected an `FnOnce<({integer},)>` closure, found `Option<_>`
  |
  = help: the trait `FnOnce<({integer},)>` is not implemented for `Option<_>`
```

Partially address #27300. Will require rebasing on top of #88546.

compiler/rustc_infer/src/infer/error_reporting/nice_region_error/mismatched_static_lifetime.rs

@@ -29,7 +29,13 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
             SubregionOrigin::Subtype(box TypeTrace { ref cause, .. }) => cause,
             _ => return None,
         };
-        let (parent, impl_def_id) = match &cause.code {
+        // If we added a "points at argument expression" obligation, we remove it here, we care
+        // about the original obligation only.
+        let code = match &cause.code {
+            ObligationCauseCode::FunctionArgumentObligation { parent_code, .. } => &*parent_code,
+            _ => &cause.code,
+        };
+        let (parent, impl_def_id) = match code {
             ObligationCauseCode::MatchImpl(parent, impl_def_id) => (parent, impl_def_id),
             _ => return None,
         };

compiler/rustc_infer/src/traits/mod.rs

@@ -66,10 +66,6 @@ pub type Selection<'tcx> = ImplSource<'tcx, PredicateObligation<'tcx>>;
 pub struct FulfillmentError<'tcx> {
     pub obligation: PredicateObligation<'tcx>,
     pub code: FulfillmentErrorCode<'tcx>,
-    /// Diagnostics only: we opportunistically change the `code.span` when we encounter an
-    /// obligation error caused by a call argument. When this is the case, we also signal that in
-    /// this field to ensure accuracy of suggestions.
-    pub points_at_arg_span: bool,
     /// Diagnostics only: the 'root' obligation which resulted in
     /// the failure to process `obligation`. This is the obligation
     /// that was initially passed to `register_predicate_obligation`
@@ -128,7 +124,7 @@ impl<'tcx> FulfillmentError<'tcx> {
         code: FulfillmentErrorCode<'tcx>,
         root_obligation: PredicateObligation<'tcx>,
     ) -> FulfillmentError<'tcx> {
-        FulfillmentError { obligation, code, points_at_arg_span: false, root_obligation }
+        FulfillmentError { obligation, code, root_obligation }
     }
 }