Provide more context on derived obligation error primary label

Expand the primary span of E0277 when the immediate unmet bound is not what the user wrote:

```
error[E0277]: the trait bound `i32: Bar` is not satisfied
 --> f100.rs:6:6
  |
6 |     <i32 as Foo>::foo();
  |      ^^^ the trait `Bar` is not implemented for `i32`, which is required by `i32: Foo`
  |
help: this trait has no implementations, consider adding one
 --> f100.rs:2:1
  |
2 | trait Bar {}
  | ^^^^^^^^^
note: required for `i32` to implement `Foo`
 --> f100.rs:3:14
  |
3 | impl<T: Bar> Foo for T {}
  |         ---  ^^^     ^
  |         |
  |         unsatisfied trait bound introduced here
```

Fix #40120.

Author: estebank

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -1432,20 +1432,18 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
     ) -> bool {
         let span = obligation.cause.span;
 
-        let code = if let ObligationCauseCode::FunctionArgumentObligation { parent_code, .. } =
-            obligation.cause.code()
-        {
-            parent_code
-        } else if let ObligationCauseCode::ItemObligation(_)
-        | ObligationCauseCode::ExprItemObligation(..) = obligation.cause.code()
-        {
-            obligation.cause.code()
-        } else if let ExpnKind::Desugaring(DesugaringKind::ForLoop) =
-            span.ctxt().outer_expn_data().kind
-        {
-            obligation.cause.code()
-        } else {
-            return false;
+        let code = match obligation.cause.code() {
+            ObligationCauseCode::FunctionArgumentObligation { parent_code, .. } => parent_code,
+            c @ ObligationCauseCode::ItemObligation(_)
+            | c @ ObligationCauseCode::ExprItemObligation(..) => c,
+            c if matches!(
+                span.ctxt().outer_expn_data().kind,
+                ExpnKind::Desugaring(DesugaringKind::ForLoop)
+            ) =>
+            {
+                c
+            }
+            _ => return false,
         };
 
         // List of traits for which it would be nonsensical to suggest borrowing.
@@ -4978,16 +4976,27 @@ pub(super) fn get_explanation_based_on_obligation<'tcx>(
             _ => None,
         };
 
+        let pred = obligation.predicate;
+        let (_, base) = obligation.cause.code().peel_derives_with_predicate();
+        let post = if let ty::PredicateKind::Clause(clause) = pred.kind().skip_binder()
+            && let ty::ClauseKind::Trait(pred) = clause
+            && let Some(base) = base
+            && base.skip_binder() != pred
+        {
+            format!(", which is required by `{base}`")
+        } else {
+            String::new()
+        };
         match ty_desc {
             Some(desc) => format!(
-                "{}the trait `{}` is not implemented for {} `{}`",
+                "{}the trait `{}` is not implemented for {} `{}`{post}",
                 pre_message,
                 trait_predicate.print_modifiers_and_trait_path(),
                 desc,
                 tcx.short_ty_string(trait_ref.skip_binder().self_ty(), &mut None),
             ),
             None => format!(
-                "{}the trait `{}` is not implemented for `{}`",
+                "{}the trait `{}` is not implemented for `{}`{post}",
                 pre_message,
                 trait_predicate.print_modifiers_and_trait_path(),
                 tcx.short_ty_string(trait_ref.skip_binder().self_ty(), &mut None),

tests/ui/associated-consts/issue-58022.stderr

@@ -13,7 +13,7 @@ error[E0277]: the size for values of type `[u8]` cannot be known at compilation
 LL |     fn new(slice: &[u8; Self::SIZE]) -> Self {
    |                                         ^^^^ doesn't have a size known at compile-time
    |
-   = help: within `Bar<[u8]>`, the trait `Sized` is not implemented for `[u8]`
+   = help: within `Bar<[u8]>`, the trait `Sized` is not implemented for `[u8]`, which is required by `Bar<[u8]>: Sized`
 note: required because it appears within the type `Bar<[u8]>`
   --> $DIR/issue-58022.rs:8:12
    |

tests/ui/associated-type-bounds/return-type-notation/basic.without.stderr

@@ -13,7 +13,7 @@ error: future cannot be sent between threads safely
 LL |     is_send(foo::<T>());
    |             ^^^^^^^^^^ future returned by `foo` is not `Send`
    |
-   = help: within `impl Future<Output = Result<(), ()>>`, the trait `Send` is not implemented for `impl Future<Output = Result<(), ()>> { <T as Foo>::method() }`
+   = help: within `impl Future<Output = Result<(), ()>>`, the trait `Send` is not implemented for `impl Future<Output = Result<(), ()>> { <T as Foo>::method() }`, which is required by `impl Future<Output = Result<(), ()>>: Send`
 note: future is not `Send` as it awaits another future which is not `Send`
   --> $DIR/basic.rs:13:5
    |

tests/ui/associated-types/defaults-suitability.stderr

@@ -39,7 +39,7 @@ error[E0277]: the trait bound `T: Clone` is not satisfied
   --> $DIR/defaults-suitability.rs:28:23
    |
 LL |     type Bar: Clone = Vec<T>;
-   |                       ^^^^^^ the trait `Clone` is not implemented for `T`
+   |                       ^^^^^^ the trait `Clone` is not implemented for `T`, which is required by `Vec<T>: Clone`
    |
    = note: required for `Vec<T>` to implement `Clone`
 note: required by a bound in `Foo::Bar`
@@ -88,7 +88,7 @@ error[E0277]: the trait bound `<Self as Foo2<T>>::Baz: Clone` is not satisfied
   --> $DIR/defaults-suitability.rs:65:23
    |
 LL |     type Bar: Clone = Vec<Self::Baz>;
-   |                       ^^^^^^^^^^^^^^ the trait `Clone` is not implemented for `<Self as Foo2<T>>::Baz`
+   |                       ^^^^^^^^^^^^^^ the trait `Clone` is not implemented for `<Self as Foo2<T>>::Baz`, which is required by `Vec<<Self as Foo2<T>>::Baz>: Clone`
    |
    = note: required for `Vec<<Self as Foo2<T>>::Baz>` to implement `Clone`
 note: required by a bound in `Foo2::Bar`
@@ -105,7 +105,7 @@ error[E0277]: the trait bound `<Self as Foo25<T>>::Baz: Clone` is not satisfied
   --> $DIR/defaults-suitability.rs:74:23
    |
 LL |     type Bar: Clone = Vec<Self::Baz>;
-   |                       ^^^^^^^^^^^^^^ the trait `Clone` is not implemented for `<Self as Foo25<T>>::Baz`
+   |                       ^^^^^^^^^^^^^^ the trait `Clone` is not implemented for `<Self as Foo25<T>>::Baz`, which is required by `Vec<<Self as Foo25<T>>::Baz>: Clone`
    |
    = note: required for `Vec<<Self as Foo25<T>>::Baz>` to implement `Clone`
 note: required by a bound in `Foo25::Bar`

tests/ui/associated-types/hr-associated-type-bound-1.stderr

@@ -2,7 +2,7 @@ error[E0277]: the trait bound `str: Clone` is not satisfied
   --> $DIR/hr-associated-type-bound-1.rs:12:14
    |
 LL |     type U = str;
-   |              ^^^ the trait `Clone` is not implemented for `str`
+   |              ^^^ the trait `Clone` is not implemented for `str`, which is required by `for<'b> <i32 as X<'b>>::U: Clone`
    |
    = help: the trait `Clone` is implemented for `String`
 note: required by a bound in `X`

tests/ui/associated-types/hr-associated-type-bound-param-1.stderr

@@ -2,7 +2,7 @@ error[E0277]: the trait bound `str: Clone` is not satisfied
   --> $DIR/hr-associated-type-bound-param-1.rs:14:14
    |
 LL |     type V = str;
-   |              ^^^ the trait `Clone` is not implemented for `str`
+   |              ^^^ the trait `Clone` is not implemented for `str`, which is required by `for<'b> <u8 as Y<'b, u8>>::V: Clone`
    |
    = help: the trait `Clone` is implemented for `String`
 note: required by a bound in `Y`

tests/ui/associated-types/hr-associated-type-bound-param-2.stderr

@@ -18,7 +18,7 @@ error[E0277]: the trait bound `str: Clone` is not satisfied
   --> $DIR/hr-associated-type-bound-param-2.rs:15:14
    |
 LL |     type W = str;
-   |              ^^^ the trait `Clone` is not implemented for `str`
+   |              ^^^ the trait `Clone` is not implemented for `str`, which is required by `for<'b> <u16 as Z<'b, u16>>::W: Clone`
    |
    = help: the trait `Clone` is implemented for `String`
 note: required by a bound in `Z`

tests/ui/associated-types/hr-associated-type-bound-param-3.stderr

@@ -2,7 +2,7 @@ error[E0277]: the trait bound `str: Clone` is not satisfied
   --> $DIR/hr-associated-type-bound-param-3.rs:13:14
    |
 LL |     type U = str;
-   |              ^^^ the trait `Clone` is not implemented for `str`
+   |              ^^^ the trait `Clone` is not implemented for `str`, which is required by `for<'b> <(T,) as X<'b, (T,)>>::U: Clone`
    |
    = help: the trait `Clone` is implemented for `String`
 note: required by a bound in `X`

tests/ui/associated-types/hr-associated-type-bound-param-4.stderr

@@ -2,7 +2,7 @@ error[E0277]: the trait bound `str: Clone` is not satisfied
   --> $DIR/hr-associated-type-bound-param-4.rs:13:14
    |
 LL |     type U = str;
-   |              ^^^ the trait `Clone` is not implemented for `str`
+   |              ^^^ the trait `Clone` is not implemented for `str`, which is required by `for<'b> <(T,) as X<'b, T>>::U: Clone`
    |
    = help: the trait `Clone` is implemented for `String`
 note: required by a bound in `X`

tests/ui/associated-types/hr-associated-type-bound-param-5.stderr

@@ -2,7 +2,7 @@ error[E0277]: the trait bound `str: Clone` is not satisfied
   --> $DIR/hr-associated-type-bound-param-5.rs:26:14
    |
 LL |     type U = str;
-   |              ^^^ the trait `Clone` is not implemented for `str`
+   |              ^^^ the trait `Clone` is not implemented for `str`, which is required by `for<'b> <<Vec<T> as Cycle>::Next as X<'b, <Vec<T> as Cycle>::Next>>::U: Clone`
    |
    = help: the trait `Clone` is implemented for `String`
 note: required by a bound in `X`
@@ -18,7 +18,7 @@ error[E0277]: the trait bound `str: Clone` is not satisfied
   --> $DIR/hr-associated-type-bound-param-5.rs:31:14
    |
 LL |     type U = str;
-   |              ^^^ the trait `Clone` is not implemented for `str`
+   |              ^^^ the trait `Clone` is not implemented for `str`, which is required by `for<'b> <<Box<T> as Cycle>::Next as X<'b, <Box<T> as Cycle>::Next>>::U: Clone`
    |
    = help: the trait `Clone` is implemented for `String`
 note: required by a bound in `X`