Auto merge of #109005 - Nilstrieb:dont-forgor-too-much-from-cfg, r=petrochenkov

Remember names of `cfg`-ed out items to mention them in diagnostics

# Examples

## `serde::Deserialize` without the `derive` feature (a classic beginner mistake)

I had to slightly modify serde so that it uses explicit re-exports instead of a glob re-export. (Update: a serde PR was merged that adds the manual re-exports)

```
error[E0433]: failed to resolve: could not find `Serialize` in `serde`
   --> src/main.rs:1:17
    |
1   | #[derive(serde::Serialize)]
    |                 ^^^^^^^^^ could not find `Serialize` in `serde`
    |
note: crate `serde` has an item named `Serialize` but it is inactive because its cfg predicate evaluated to false
   --> /home/gh-Nilstrieb/.cargo/registry/src/index.crates.io-6f17d22bba15001f/serde-1.0.160/src/lib.rs:343:1
    |
343 | #[cfg(feature = "serde_derive")]
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
344 | pub use serde_derive::{Deserialize, Serialize};
    |                                     ^^^^^^^^^
    = note: the item is gated behind the `serde_derive` feature
    = note: see https://doc.rust-lang.org/cargo/reference/features.html for how to activate a crate's feature
```
(the suggestion is not ideal but that's serde's fault)

I already tested the metadata size impact locally by compiling the `windows` crate without any features. `800k`  -> `809k`

r? `@ghost`

compiler/rustc_middle/src/query/mod.rs

@@ -54,7 +54,7 @@ use crate::ty::{
 };
 use rustc_arena::TypedArena;
 use rustc_ast as ast;
-use rustc_ast::expand::allocator::AllocatorKind;
+use rustc_ast::expand::{allocator::AllocatorKind, StrippedCfgItem};
 use rustc_attr as attr;
 use rustc_data_structures::fingerprint::Fingerprint;
 use rustc_data_structures::fx::{FxHashMap, FxIndexMap, FxIndexSet};
@@ -2174,6 +2174,15 @@ rustc_queries! {
     query check_tys_might_be_eq(arg: Canonical<'tcx, (ty::ParamEnv<'tcx>, Ty<'tcx>, Ty<'tcx>)>) -> Result<(), NoSolution> {
         desc { "check whether two const param are definitely not equal to eachother"}
     }
+
+    /// Get all item paths that were stripped by a `#[cfg]` in a particular crate.
+    /// Should not be called for the local crate before the resolver outputs are created, as it
+    /// is only fed there.
+    query stripped_cfg_items(cnum: CrateNum) -> &'tcx [StrippedCfgItem] {
+        feedable
+        desc { "getting cfg-ed out item names" }
+        separate_provide_extern
+    }
 }
 
 rustc_query_append! { define_callbacks! }