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.

5 files changed, 19 insertions, 19 deletions

diff --git a/tests/ui/iterators/float_iterator_hint.stderr b/tests/ui/iterators/float_iterator_hint.stderr
index c3cb00c3c68..29319b9400f 100644
--- a/tests/ui/iterators/float_iterator_hint.stderr
+++ b/tests/ui/iterators/float_iterator_hint.stderr
@@ -4,7 +4,7 @@ error[E0277]: `{float}` is not an iterator
 LL |     for i in 0.2 {
    |              ^^^ `{float}` is not an iterator
    |
-   = help: the trait `Iterator` is not implemented for `{float}`
+   = help: the trait `Iterator` is not implemented for `{float}`, which is required by `{float}: IntoIterator`
    = note: if you want to iterate between `start` until a value `end`, use the exclusive range syntax `start..end` or the inclusive range syntax `start..=end`
    = note: required for `{float}` to implement `IntoIterator`
 
diff --git a/tests/ui/iterators/integral.stderr b/tests/ui/iterators/integral.stderr
index c142fec8da0..74bbe28d6b7 100644
--- a/tests/ui/iterators/integral.stderr
+++ b/tests/ui/iterators/integral.stderr
@@ -4,7 +4,7 @@ error[E0277]: `{integer}` is not an iterator
 LL |     for _ in 42 {}
    |              ^^ `{integer}` is not an iterator
    |
-   = help: the trait `Iterator` is not implemented for `{integer}`
+   = help: the trait `Iterator` is not implemented for `{integer}`, which is required by `{integer}: IntoIterator`
    = note: if you want to iterate between `start` until a value `end`, use the exclusive range syntax `start..end` or the inclusive range syntax `start..=end`
    = note: required for `{integer}` to implement `IntoIterator`
 
@@ -14,7 +14,7 @@ error[E0277]: `u8` is not an iterator
 LL |     for _ in 42 as u8 {}
    |              ^^^^^^^^ `u8` is not an iterator
    |
-   = help: the trait `Iterator` is not implemented for `u8`
+   = help: the trait `Iterator` is not implemented for `u8`, which is required by `u8: IntoIterator`
    = note: if you want to iterate between `start` until a value `end`, use the exclusive range syntax `start..end` or the inclusive range syntax `start..=end`
    = note: required for `u8` to implement `IntoIterator`
 
@@ -24,7 +24,7 @@ error[E0277]: `i8` is not an iterator
 LL |     for _ in 42 as i8 {}
    |              ^^^^^^^^ `i8` is not an iterator
    |
-   = help: the trait `Iterator` is not implemented for `i8`
+   = help: the trait `Iterator` is not implemented for `i8`, which is required by `i8: IntoIterator`
    = note: if you want to iterate between `start` until a value `end`, use the exclusive range syntax `start..end` or the inclusive range syntax `start..=end`
    = note: required for `i8` to implement `IntoIterator`
 
@@ -34,7 +34,7 @@ error[E0277]: `u16` is not an iterator
 LL |     for _ in 42 as u16 {}
    |              ^^^^^^^^^ `u16` is not an iterator
    |
-   = help: the trait `Iterator` is not implemented for `u16`
+   = help: the trait `Iterator` is not implemented for `u16`, which is required by `u16: IntoIterator`
    = note: if you want to iterate between `start` until a value `end`, use the exclusive range syntax `start..end` or the inclusive range syntax `start..=end`
    = note: required for `u16` to implement `IntoIterator`
 
@@ -44,7 +44,7 @@ error[E0277]: `i16` is not an iterator
 LL |     for _ in 42 as i16 {}
    |              ^^^^^^^^^ `i16` is not an iterator
    |
-   = help: the trait `Iterator` is not implemented for `i16`
+   = help: the trait `Iterator` is not implemented for `i16`, which is required by `i16: IntoIterator`
    = note: if you want to iterate between `start` until a value `end`, use the exclusive range syntax `start..end` or the inclusive range syntax `start..=end`
    = note: required for `i16` to implement `IntoIterator`
 
@@ -54,7 +54,7 @@ error[E0277]: `u32` is not an iterator
 LL |     for _ in 42 as u32 {}
    |              ^^^^^^^^^ `u32` is not an iterator
    |
-   = help: the trait `Iterator` is not implemented for `u32`
+   = help: the trait `Iterator` is not implemented for `u32`, which is required by `u32: IntoIterator`
    = note: if you want to iterate between `start` until a value `end`, use the exclusive range syntax `start..end` or the inclusive range syntax `start..=end`
    = note: required for `u32` to implement `IntoIterator`
 
@@ -64,7 +64,7 @@ error[E0277]: `i32` is not an iterator
 LL |     for _ in 42 as i32 {}
    |              ^^^^^^^^^ `i32` is not an iterator
    |
-   = help: the trait `Iterator` is not implemented for `i32`
+   = help: the trait `Iterator` is not implemented for `i32`, which is required by `i32: IntoIterator`
    = note: if you want to iterate between `start` until a value `end`, use the exclusive range syntax `start..end` or the inclusive range syntax `start..=end`
    = note: required for `i32` to implement `IntoIterator`
 
@@ -74,7 +74,7 @@ error[E0277]: `u64` is not an iterator
 LL |     for _ in 42 as u64 {}
    |              ^^^^^^^^^ `u64` is not an iterator
    |
-   = help: the trait `Iterator` is not implemented for `u64`
+   = help: the trait `Iterator` is not implemented for `u64`, which is required by `u64: IntoIterator`
    = note: if you want to iterate between `start` until a value `end`, use the exclusive range syntax `start..end` or the inclusive range syntax `start..=end`
    = note: required for `u64` to implement `IntoIterator`
 
@@ -84,7 +84,7 @@ error[E0277]: `i64` is not an iterator
 LL |     for _ in 42 as i64 {}
    |              ^^^^^^^^^ `i64` is not an iterator
    |
-   = help: the trait `Iterator` is not implemented for `i64`
+   = help: the trait `Iterator` is not implemented for `i64`, which is required by `i64: IntoIterator`
    = note: if you want to iterate between `start` until a value `end`, use the exclusive range syntax `start..end` or the inclusive range syntax `start..=end`
    = note: required for `i64` to implement `IntoIterator`
 
@@ -94,7 +94,7 @@ error[E0277]: `usize` is not an iterator
 LL |     for _ in 42 as usize {}
    |              ^^^^^^^^^^^ `usize` is not an iterator
    |
-   = help: the trait `Iterator` is not implemented for `usize`
+   = help: the trait `Iterator` is not implemented for `usize`, which is required by `usize: IntoIterator`
    = note: if you want to iterate between `start` until a value `end`, use the exclusive range syntax `start..end` or the inclusive range syntax `start..=end`
    = note: required for `usize` to implement `IntoIterator`
 
@@ -104,7 +104,7 @@ error[E0277]: `isize` is not an iterator
 LL |     for _ in 42 as isize {}
    |              ^^^^^^^^^^^ `isize` is not an iterator
    |
-   = help: the trait `Iterator` is not implemented for `isize`
+   = help: the trait `Iterator` is not implemented for `isize`, which is required by `isize: IntoIterator`
    = note: if you want to iterate between `start` until a value `end`, use the exclusive range syntax `start..end` or the inclusive range syntax `start..=end`
    = note: required for `isize` to implement `IntoIterator`
 
@@ -114,7 +114,7 @@ error[E0277]: `{float}` is not an iterator
 LL |     for _ in 42.0 {}
    |              ^^^^ `{float}` is not an iterator
    |
-   = help: the trait `Iterator` is not implemented for `{float}`
+   = help: the trait `Iterator` is not implemented for `{float}`, which is required by `{float}: IntoIterator`
    = note: if you want to iterate between `start` until a value `end`, use the exclusive range syntax `start..end` or the inclusive range syntax `start..=end`
    = note: required for `{float}` to implement `IntoIterator`
 
diff --git a/tests/ui/iterators/issue-28098.stderr b/tests/ui/iterators/issue-28098.stderr
index a724f03ad4a..3cb1b2f7270 100644
--- a/tests/ui/iterators/issue-28098.stderr
+++ b/tests/ui/iterators/issue-28098.stderr
@@ -14,7 +14,7 @@ error[E0277]: `bool` is not an iterator
 LL |     for _ in false {}
    |              ^^^^^ `bool` is not an iterator
    |
-   = help: the trait `Iterator` is not implemented for `bool`
+   = help: the trait `Iterator` is not implemented for `bool`, which is required by `bool: IntoIterator`
    = note: required for `bool` to implement `IntoIterator`
 
 error[E0277]: `()` is not an iterator
@@ -61,7 +61,7 @@ error[E0277]: `bool` is not an iterator
 LL |     for _ in false {}
    |              ^^^^^ `bool` is not an iterator
    |
-   = help: the trait `Iterator` is not implemented for `bool`
+   = help: the trait `Iterator` is not implemented for `bool`, which is required by `bool: IntoIterator`
    = note: required for `bool` to implement `IntoIterator`
 
 error[E0277]: `()` is not an iterator
diff --git a/tests/ui/iterators/ranges.stderr b/tests/ui/iterators/ranges.stderr
index b9fbcd5304b..a5d43ecbb63 100644
--- a/tests/ui/iterators/ranges.stderr
+++ b/tests/ui/iterators/ranges.stderr
@@ -4,7 +4,7 @@ error[E0277]: `RangeTo<{integer}>` is not an iterator
 LL |     for _ in ..10 {}
    |              ^^^^ if you meant to iterate until a value, add a starting value
    |
-   = help: the trait `Iterator` is not implemented for `RangeTo<{integer}>`
+   = help: the trait `Iterator` is not implemented for `RangeTo<{integer}>`, which is required by `RangeTo<{integer}>: IntoIterator`
    = note: `..end` is a `RangeTo`, which cannot be iterated on; you might have meant to have a bounded `Range`: `0..end`
    = note: required for `RangeTo<{integer}>` to implement `IntoIterator`
 
@@ -14,7 +14,7 @@ error[E0277]: `RangeToInclusive<{integer}>` is not an iterator
 LL |     for _ in ..=10 {}
    |              ^^^^^ if you meant to iterate until a value (including it), add a starting value
    |
-   = help: the trait `Iterator` is not implemented for `RangeToInclusive<{integer}>`
+   = help: the trait `Iterator` is not implemented for `RangeToInclusive<{integer}>`, which is required by `RangeToInclusive<{integer}>: IntoIterator`
    = note: `..=end` is a `RangeToInclusive`, which cannot be iterated on; you might have meant to have a bounded `RangeInclusive`: `0..=end`
    = note: required for `RangeToInclusive<{integer}>` to implement `IntoIterator`
 
diff --git a/tests/ui/iterators/string.stderr b/tests/ui/iterators/string.stderr
index ddfe0169b84..29f560677c0 100644
--- a/tests/ui/iterators/string.stderr
+++ b/tests/ui/iterators/string.stderr
@@ -4,7 +4,7 @@ error[E0277]: `String` is not an iterator
 LL |     for _ in "".to_owned() {}
    |              ^^^^^^^^^^^^^ `String` is not an iterator; try calling `.chars()` or `.bytes()`
    |
-   = help: the trait `Iterator` is not implemented for `String`
+   = help: the trait `Iterator` is not implemented for `String`, which is required by `String: IntoIterator`
    = note: required for `String` to implement `IntoIterator`
 
 error[E0277]: `&str` is not an iterator
@@ -13,7 +13,7 @@ error[E0277]: `&str` is not an iterator
 LL |     for _ in "" {}
    |              ^^ `&str` is not an iterator; try calling `.chars()` or `.bytes()`
    |
-   = help: the trait `Iterator` is not implemented for `&str`
+   = help: the trait `Iterator` is not implemented for `&str`, which is required by `&str: IntoIterator`
    = note: required for `&str` to implement `IntoIterator`
 
 error: aborting due to 2 previous errors