Rollup merge of #127662 - estebank:gate-span, r=TaKO8Ki

When finding item gated behind a `cfg` flag, point at it

Previously we would only mention that the item was gated out, and opportunisitically mention the feature flag name when possible. We now point to the place where the item was gated, which can be behind layers of macro indirection, or in different modules.

```
error[E0433]: failed to resolve: could not find `doesnt_exist` in `inner`
  --> $DIR/diagnostics-cross-crate.rs:18:23
   |
LL |     cfged_out::inner::doesnt_exist::hello();
   |                       ^^^^^^^^^^^^ could not find `doesnt_exist` in `inner`
   |
note: found an item that was configured out
  --> $DIR/auxiliary/cfged_out.rs:6:13
   |
LL |     pub mod doesnt_exist {
   |             ^^^^^^^^^^^^
note: the item is gated here
  --> $DIR/auxiliary/cfged_out.rs:5:5
   |
LL |     #[cfg(FALSE)]
   |     ^^^^^^^^^^^^^
```

2 files changed, 13 insertions, 0 deletions

diff --git a/tests/ui/macros/builtin-std-paths-fail.stderr b/tests/ui/macros/builtin-std-paths-fail.stderr
index 331943843c0..49034c3987b 100644
--- a/tests/ui/macros/builtin-std-paths-fail.stderr
+++ b/tests/ui/macros/builtin-std-paths-fail.stderr
@@ -104,6 +104,8 @@ LL | #[std::test]
    |
 note: found an item that was configured out
   --> $SRC_DIR/std/src/lib.rs:LL:COL
+note: the item is gated here
+  --> $SRC_DIR/std/src/lib.rs:LL:COL
 
 error: aborting due to 16 previous errors
 
diff --git a/tests/ui/macros/macro-outer-attributes.stderr b/tests/ui/macros/macro-outer-attributes.stderr
index 87c0655a422..a8809f3fcff 100644
--- a/tests/ui/macros/macro-outer-attributes.stderr
+++ b/tests/ui/macros/macro-outer-attributes.stderr
@@ -9,6 +9,17 @@ note: found an item that was configured out
    |
 LL |       pub fn bar() { });
    |              ^^^
+note: the item is gated here
+  --> $DIR/macro-outer-attributes.rs:5:45
+   |
+LL |                        $i:item) => (mod $nm { #[$a] $i }); }
+   |                                               ^^^^^
+LL |
+LL | / test!(a,
+LL | |       #[cfg(FALSE)],
+LL | |       pub fn bar() { });
+   | |_______________________- in this macro invocation
+   = note: this error originates in the macro `test` (in Nightly builds, run with -Z macro-backtrace for more info)
 help: consider importing this function
    |
 LL + use b::bar;