Detect more `cfg`d out items in resolution errors

Use a visitor to collect *all* items (including those nested) that were stripped behind a `cfg` condition.

```
error[E0425]: cannot find function `f` in this scope
  --> $DIR/nested-cfg-attrs.rs:4:13
   |
LL | fn main() { f() }
   |             ^ not found in this scope
   |
note: found an item that was configured out
  --> $DIR/nested-cfg-attrs.rs:2:4
   |
LL | fn f() {}
   |    ^
note: the item is gated here
  --> $DIR/nested-cfg-attrs.rs:1:35
   |
LL | #[cfg_attr(all(), cfg_attr(all(), cfg(FALSE)))]
   |                                   ^^^^^^^^^^
```

14 files changed, 228 insertions, 20 deletions

diff --git a/tests/ui/cfg/both-true-false.stderr b/tests/ui/cfg/both-true-false.stderr
index 1526cc2b707..8550444d179 100644
--- a/tests/ui/cfg/both-true-false.stderr
+++ b/tests/ui/cfg/both-true-false.stderr
@@ -3,6 +3,27 @@ error[E0425]: cannot find function `foo` in this scope
    |
 LL |     foo();
    |     ^^^ not found in this scope
+   |
+note: found an item that was configured out
+  --> $DIR/both-true-false.rs:6:4
+   |
+LL | fn foo() {}
+   |    ^^^
+note: the item is gated here
+  --> $DIR/both-true-false.rs:4:7
+   |
+LL | #[cfg(false)]
+   |       ^^^^^
+note: found an item that was configured out
+  --> $DIR/both-true-false.rs:10:4
+   |
+LL | fn foo() {}
+   |    ^^^
+note: the item is gated here
+  --> $DIR/both-true-false.rs:9:7
+   |
+LL | #[cfg(false)]
+   |       ^^^^^
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/cfg/cfg-version/syntax.stderr b/tests/ui/cfg/cfg-version/syntax.stderr
index 188f6d7aa5d..4ec3e2de797 100644
--- a/tests/ui/cfg/cfg-version/syntax.stderr
+++ b/tests/ui/cfg/cfg-version/syntax.stderr
@@ -128,60 +128,170 @@ error[E0425]: cannot find function `key_value_form` in this scope
    |
 LL |     key_value_form();
    |     ^^^^^^^^^^^^^^ not found in this scope
+   |
+note: found an item that was configured out
+  --> $DIR/syntax.rs:32:4
+   |
+LL | fn key_value_form() {}
+   |    ^^^^^^^^^^^^^^
+note: the item is gated behind the `1.43` feature
+  --> $DIR/syntax.rs:30:7
+   |
+LL | #[cfg(version = "1.43")]
+   |       ^^^^^^^^^^^^^^^^
 
 error[E0425]: cannot find function `not_numbers_or_periods` in this scope
   --> $DIR/syntax.rs:143:5
    |
 LL |     not_numbers_or_periods();
    |     ^^^^^^^^^^^^^^^^^^^^^^ not found in this scope
+   |
+note: found an item that was configured out
+  --> $DIR/syntax.rs:53:4
+   |
+LL | fn not_numbers_or_periods() {}
+   |    ^^^^^^^^^^^^^^^^^^^^^^
+note: the item is gated here
+  --> $DIR/syntax.rs:51:14
+   |
+LL | #[cfg(version("foo"))]
+   |              ^^^^^^^
 
 error[E0425]: cannot find function `complex_semver_with_metadata` in this scope
   --> $DIR/syntax.rs:144:5
    |
 LL |     complex_semver_with_metadata();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ not found in this scope
+   |
+note: found an item that was configured out
+  --> $DIR/syntax.rs:57:4
+   |
+LL | fn complex_semver_with_metadata() {}
+   |    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: the item is gated here
+  --> $DIR/syntax.rs:55:14
+   |
+LL | #[cfg(version("1.20.0-stable"))]
+   |              ^^^^^^^^^^^^^^^^^
 
 error[E0425]: cannot find function `invalid_major_only` in this scope
   --> $DIR/syntax.rs:145:5
    |
 LL |     invalid_major_only();
    |     ^^^^^^^^^^^^^^^^^^ not found in this scope
+   |
+note: found an item that was configured out
+  --> $DIR/syntax.rs:80:4
+   |
+LL | fn invalid_major_only() {}
+   |    ^^^^^^^^^^^^^^^^^^
+note: the item is gated here
+  --> $DIR/syntax.rs:78:14
+   |
+LL | #[cfg(version("1"))]
+   |              ^^^^^
 
 error[E0425]: cannot find function `invalid_major_only_zero` in this scope
   --> $DIR/syntax.rs:146:5
    |
 LL |     invalid_major_only_zero();
    |     ^^^^^^^^^^^^^^^^^^^^^^^ not found in this scope
+   |
+note: found an item that was configured out
+  --> $DIR/syntax.rs:84:4
+   |
+LL | fn invalid_major_only_zero() {}
+   |    ^^^^^^^^^^^^^^^^^^^^^^^
+note: the item is gated here
+  --> $DIR/syntax.rs:82:14
+   |
+LL | #[cfg(version("0"))]
+   |              ^^^^^
 
 error[E0425]: cannot find function `invalid_major_only_negative` in this scope
   --> $DIR/syntax.rs:147:5
    |
 LL |     invalid_major_only_negative();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^ not found in this scope
+   |
+note: found an item that was configured out
+  --> $DIR/syntax.rs:97:4
+   |
+LL | fn invalid_major_only_negative() {}
+   |    ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: the item is gated here
+  --> $DIR/syntax.rs:95:14
+   |
+LL | #[cfg(version("-1"))]
+   |              ^^^^^^
 
 error[E0425]: cannot find function `exceed_u16_major` in this scope
   --> $DIR/syntax.rs:148:5
    |
 LL |     exceed_u16_major();
    |     ^^^^^^^^^^^^^^^^ not found in this scope
+   |
+note: found an item that was configured out
+  --> $DIR/syntax.rs:103:4
+   |
+LL | fn exceed_u16_major() {}
+   |    ^^^^^^^^^^^^^^^^
+note: the item is gated here
+  --> $DIR/syntax.rs:101:14
+   |
+LL | #[cfg(version("65536"))]
+   |              ^^^^^^^^^
 
 error[E0425]: cannot find function `exceed_u16_minor` in this scope
   --> $DIR/syntax.rs:149:5
    |
 LL |     exceed_u16_minor();
    |     ^^^^^^^^^^^^^^^^ not found in this scope
+   |
+note: found an item that was configured out
+  --> $DIR/syntax.rs:107:4
+   |
+LL | fn exceed_u16_minor() {}
+   |    ^^^^^^^^^^^^^^^^
+note: the item is gated here
+  --> $DIR/syntax.rs:105:14
+   |
+LL | #[cfg(version("1.65536.0"))]
+   |              ^^^^^^^^^^^^^
 
 error[E0425]: cannot find function `exceed_u16_patch` in this scope
   --> $DIR/syntax.rs:150:5
    |
 LL |     exceed_u16_patch();
    |     ^^^^^^^^^^^^^^^^ not found in this scope
+   |
+note: found an item that was configured out
+  --> $DIR/syntax.rs:111:4
+   |
+LL | fn exceed_u16_patch() {}
+   |    ^^^^^^^^^^^^^^^^
+note: the item is gated here
+  --> $DIR/syntax.rs:109:14
+   |
+LL | #[cfg(version("1.0.65536"))]
+   |              ^^^^^^^^^^^^^
 
 error[E0425]: cannot find function `exceed_u16_mixed` in this scope
   --> $DIR/syntax.rs:151:5
    |
 LL |     exceed_u16_mixed();
    |     ^^^^^^^^^^^^^^^^ not found in this scope
+   |
+note: found an item that was configured out
+  --> $DIR/syntax.rs:115:4
+   |
+LL | fn exceed_u16_mixed() {}
+   |    ^^^^^^^^^^^^^^^^
+note: the item is gated here
+  --> $DIR/syntax.rs:113:14
+   |
+LL | #[cfg(version("65536.0.65536"))]
+   |              ^^^^^^^^^^^^^^^^^
 
 error: aborting due to 14 previous errors; 14 warnings emitted
 
diff --git a/tests/ui/cfg/cmdline-false.stderr b/tests/ui/cfg/cmdline-false.stderr
index 5f57c754c40..da5eb0c892a 100644
--- a/tests/ui/cfg/cmdline-false.stderr
+++ b/tests/ui/cfg/cmdline-false.stderr
@@ -3,6 +3,17 @@ error[E0425]: cannot find function `foo` in this scope
    |
 LL |     foo();
    |     ^^^ not found in this scope
+   |
+note: found an item that was configured out
+  --> $DIR/cmdline-false.rs:5:4
+   |
+LL | fn foo() {}
+   |    ^^^
+note: the item is gated here
+  --> $DIR/cmdline-false.rs:4:7
+   |
+LL | #[cfg(false)]
+   |       ^^^^^
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/cfg/diagnostics-reexport.rs b/tests/ui/cfg/diagnostics-reexport.rs
index 56fac562238..bdb57df2f25 100644
--- a/tests/ui/cfg/diagnostics-reexport.rs
+++ b/tests/ui/cfg/diagnostics-reexport.rs
@@ -1,7 +1,7 @@
 pub mod inner {
-    #[cfg(false)]
+    #[cfg(false)] //~ NOTE the item is gated here
     mod gone {
-        pub fn uwu() {}
+        pub fn uwu() {} //~ NOTE found an item that was configured out
     }
 
     #[cfg(false)] //~ NOTE the item is gated here
@@ -34,7 +34,7 @@ mod b {
 }
 
 fn main() {
-    // There is no uwu at this path - no diagnostic.
+    // There is no uwu at this path, but there's one in a cgfd out sub-module, so we mention it.
     inner::uwu(); //~ ERROR cannot find function
     //~^ NOTE not found in `inner`
 }
diff --git a/tests/ui/cfg/diagnostics-reexport.stderr b/tests/ui/cfg/diagnostics-reexport.stderr
index bf1bbcba7e4..82412ed49c5 100644
--- a/tests/ui/cfg/diagnostics-reexport.stderr
+++ b/tests/ui/cfg/diagnostics-reexport.stderr
@@ -51,6 +51,16 @@ LL |     inner::uwu();
    |            ^^^ not found in `inner`
    |
 note: found an item that was configured out
+  --> $DIR/diagnostics-reexport.rs:4:16
+   |
+LL |         pub fn uwu() {}
+   |                ^^^
+note: the item is gated here
+  --> $DIR/diagnostics-reexport.rs:2:11
+   |
+LL |     #[cfg(false)]
+   |           ^^^^^
+note: found an item that was configured out
   --> $DIR/diagnostics-reexport.rs:8:20
    |
 LL |     pub use super::uwu;
diff --git a/tests/ui/cfg/diagnostics-same-crate.rs b/tests/ui/cfg/diagnostics-same-crate.rs
index 9153f20b296..29209d5f3ea 100644
--- a/tests/ui/cfg/diagnostics-same-crate.rs
+++ b/tests/ui/cfg/diagnostics-same-crate.rs
@@ -37,8 +37,8 @@ mod placeholder {
     //~| NOTE could not find `doesnt_exist` in `inner`
 }
 
-#[cfg(i_dont_exist_and_you_can_do_nothing_about_it)]
-pub fn vanished() {}
+#[cfg(i_dont_exist_and_you_can_do_nothing_about_it)] //~ NOTE the item is gated here
+pub fn vanished() {} //~ NOTE found an item that was configured out
 
 fn main() {
     // There is no uwu at this path - no diagnostic.
@@ -49,8 +49,7 @@ fn main() {
     inner::uwu(); //~ ERROR cannot find function
     //~| NOTE not found in `inner`
 
-    // The module isn't found - we would like to get a diagnostic, but currently don't due to
-    // the awkward way the resolver diagnostics are currently implemented.
+    // The module isn't found - we get a diagnostic.
     inner::doesnt_exist::hello(); //~ ERROR failed to resolve
     //~| NOTE could not find `doesnt_exist` in `inner`
 
@@ -58,9 +57,8 @@ fn main() {
     inner::right::meow(); //~ ERROR cannot find function
     //~| NOTE not found in `inner::right
 
-    // Exists in the crate root - we would generally want a diagnostic,
-    // but currently don't have one.
-    // Not that it matters much though, this is highly unlikely to confuse anyone.
+    // Exists in the crate root - we show a diagnostic because we treat "no module DefId" as "crate
+    // root DefId".
     vanished(); //~ ERROR cannot find function
     //~^ NOTE not found in this scope
 }
diff --git a/tests/ui/cfg/diagnostics-same-crate.stderr b/tests/ui/cfg/diagnostics-same-crate.stderr
index 121f25ca090..f6a6fe388c5 100644
--- a/tests/ui/cfg/diagnostics-same-crate.stderr
+++ b/tests/ui/cfg/diagnostics-same-crate.stderr
@@ -33,7 +33,7 @@ LL |     #[cfg(false)]
    |           ^^^^^
 
 error[E0433]: failed to resolve: could not find `doesnt_exist` in `inner`
-  --> $DIR/diagnostics-same-crate.rs:54:12
+  --> $DIR/diagnostics-same-crate.rs:53:12
    |
 LL |     inner::doesnt_exist::hello();
    |            ^^^^^^^^^^^^ could not find `doesnt_exist` in `inner`
@@ -67,7 +67,7 @@ LL |     #[cfg(false)]
    |           ^^^^^
 
 error[E0425]: cannot find function `meow` in module `inner::right`
-  --> $DIR/diagnostics-same-crate.rs:58:19
+  --> $DIR/diagnostics-same-crate.rs:57:19
    |
 LL |     inner::right::meow();
    |                   ^^^^ not found in `inner::right`
@@ -90,10 +90,21 @@ LL |     uwu();
    |     ^^^ not found in this scope
 
 error[E0425]: cannot find function `vanished` in this scope
-  --> $DIR/diagnostics-same-crate.rs:64:5
+  --> $DIR/diagnostics-same-crate.rs:62:5
    |
 LL |     vanished();
    |     ^^^^^^^^ not found in this scope
+   |
+note: found an item that was configured out
+  --> $DIR/diagnostics-same-crate.rs:41:8
+   |
+LL | pub fn vanished() {}
+   |        ^^^^^^^^
+note: the item is gated here
+  --> $DIR/diagnostics-same-crate.rs:40:7
+   |
+LL | #[cfg(i_dont_exist_and_you_can_do_nothing_about_it)]
+   |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: aborting due to 7 previous errors
 
diff --git a/tests/ui/cfg/nested-cfg-attr-conditional-compilation.rs b/tests/ui/cfg/nested-cfg-attr-conditional-compilation.rs
index 7618e83a642..c5d86a27d52 100644
--- a/tests/ui/cfg/nested-cfg-attr-conditional-compilation.rs
+++ b/tests/ui/cfg/nested-cfg-attr-conditional-compilation.rs
@@ -10,9 +10,10 @@
 //!
 //! Added in <https://github.com/rust-lang/rust/pull/34216>.
 
-#[cfg_attr(all(), cfg_attr(all(), cfg(false)))]
-fn f() {}
+#[cfg_attr(all(), cfg_attr(all(), cfg(false)))] //~ NOTE the item is gated here
+fn f() {} //~ NOTE found an item that was configured out
 
 fn main() {
     f() //~ ERROR cannot find function `f` in this scope
+    //~^ NOTE not found in this scope
 }
diff --git a/tests/ui/cfg/nested-cfg-attr-conditional-compilation.stderr b/tests/ui/cfg/nested-cfg-attr-conditional-compilation.stderr
index ddb8ea1e13a..44a33adbc47 100644
--- a/tests/ui/cfg/nested-cfg-attr-conditional-compilation.stderr
+++ b/tests/ui/cfg/nested-cfg-attr-conditional-compilation.stderr
@@ -3,6 +3,17 @@ error[E0425]: cannot find function `f` in this scope
    |
 LL |     f()
    |     ^ not found in this scope
+   |
+note: found an item that was configured out
+  --> $DIR/nested-cfg-attr-conditional-compilation.rs:14:4
+   |
+LL | fn f() {}
+   |    ^
+note: the item is gated here
+  --> $DIR/nested-cfg-attr-conditional-compilation.rs:13:39
+   |
+LL | #[cfg_attr(all(), cfg_attr(all(), cfg(false)))]
+   |                                       ^^^^^
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/conditional-compilation/test-cfg.rs b/tests/ui/conditional-compilation/test-cfg.rs
index adbbc309be4..b3fff26a8fd 100644
--- a/tests/ui/conditional-compilation/test-cfg.rs
+++ b/tests/ui/conditional-compilation/test-cfg.rs
@@ -1,8 +1,10 @@
 //@ compile-flags: --cfg foo --check-cfg=cfg(foo,bar)
 
 #[cfg(all(foo, bar))] // foo AND bar
-fn foo() {}
+//~^ NOTE the item is gated here
+fn foo() {} //~ NOTE found an item that was configured out
 
 fn main() {
     foo(); //~ ERROR cannot find function `foo` in this scope
+    //~^ NOTE not found in this scope
 }
diff --git a/tests/ui/conditional-compilation/test-cfg.stderr b/tests/ui/conditional-compilation/test-cfg.stderr
index 9715f16acc2..1212074e417 100644
--- a/tests/ui/conditional-compilation/test-cfg.stderr
+++ b/tests/ui/conditional-compilation/test-cfg.stderr
@@ -1,8 +1,19 @@
 error[E0425]: cannot find function `foo` in this scope
-  --> $DIR/test-cfg.rs:7:5
+  --> $DIR/test-cfg.rs:8:5
    |
 LL |     foo();
    |     ^^^ not found in this scope
+   |
+note: found an item that was configured out
+  --> $DIR/test-cfg.rs:5:4
+   |
+LL | fn foo() {}
+   |    ^^^
+note: the item is gated here
+  --> $DIR/test-cfg.rs:3:16
+   |
+LL | #[cfg(all(foo, bar))] // foo AND bar
+   |                ^^^
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/macros/macro-inner-attributes.stderr b/tests/ui/macros/macro-inner-attributes.stderr
index d74b64db5ac..cda53497f39 100644
--- a/tests/ui/macros/macro-inner-attributes.stderr
+++ b/tests/ui/macros/macro-inner-attributes.stderr
@@ -4,6 +4,16 @@ error[E0433]: failed to resolve: use of unresolved module or unlinked crate `a`
 LL |     a::bar();
    |     ^ use of unresolved module or unlinked crate `a`
    |
+note: found an item that was configured out
+  --> $DIR/macro-inner-attributes.rs:7:7
+   |
+LL | test!(a,
+   |       ^
+note: the item is gated here
+  --> $DIR/macro-inner-attributes.rs:8:13
+   |
+LL |       #[cfg(false)],
+   |             ^^^^^
 help: there is a crate or module with a similar name
    |
 LL -     a::bar();
diff --git a/tests/ui/rustdoc/cfg-rustdoc.rs b/tests/ui/rustdoc/cfg-rustdoc.rs
index dd8e1ed97c4..11c48d07552 100644
--- a/tests/ui/rustdoc/cfg-rustdoc.rs
+++ b/tests/ui/rustdoc/cfg-rustdoc.rs
@@ -1,6 +1,7 @@
-#[cfg(doc)]
-pub struct Foo;
+#[cfg(doc)] //~ NOTE the item is gated here
+pub struct Foo; //~ NOTE found an item that was configured out
 
 fn main() {
-    let f = Foo; //~ ERROR
+    let f = Foo; //~ ERROR cannot find value `Foo` in this scope
+    //~^ NOTE not found in this scope
 }
diff --git a/tests/ui/rustdoc/cfg-rustdoc.stderr b/tests/ui/rustdoc/cfg-rustdoc.stderr
index 340a8e22482..fd0fcfd01aa 100644
--- a/tests/ui/rustdoc/cfg-rustdoc.stderr
+++ b/tests/ui/rustdoc/cfg-rustdoc.stderr
@@ -3,6 +3,17 @@ error[E0425]: cannot find value `Foo` in this scope
    |
 LL |     let f = Foo;
    |             ^^^ not found in this scope
+   |
+note: found an item that was configured out
+  --> $DIR/cfg-rustdoc.rs:2:12
+   |
+LL | pub struct Foo;
+   |            ^^^
+note: the item is gated here
+  --> $DIR/cfg-rustdoc.rs:1:7
+   |
+LL | #[cfg(doc)]
+   |       ^^^
 
 error: aborting due to 1 previous error