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_metadata/src/rmeta/decoder.rs

@@ -995,6 +995,15 @@ impl<'a, 'tcx> CrateMetadataRef<'a> {
         )
     }
 
+    fn get_stripped_cfg_items(self, cnum: CrateNum, tcx: TyCtxt<'tcx>) -> &'tcx [StrippedCfgItem] {
+        let item_names = self
+            .root
+            .stripped_cfg_items
+            .decode((self, tcx))
+            .map(|item| item.map_mod_id(|index| DefId { krate: cnum, index }));
+        tcx.arena.alloc_from_iter(item_names)
+    }
+
     /// Iterates over the diagnostic items in the given crate.
     fn get_diagnostic_items(self) -> DiagnosticItems {
         let mut id_to_name = FxHashMap::default();

compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs

@@ -345,6 +345,7 @@ provide! { tcx, def_id, other, cdata,
     stability_implications => {
         cdata.get_stability_implications(tcx).iter().copied().collect()
     }
+    stripped_cfg_items => { cdata.get_stripped_cfg_items(cdata.cnum, tcx) }
     is_intrinsic => { cdata.get_is_intrinsic(def_id.index) }
     defined_lang_items => { cdata.get_lang_items(tcx) }
     diagnostic_items => { cdata.get_diagnostic_items() }

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -3,6 +3,7 @@ use crate::rmeta::def_path_hash_map::DefPathHashMapRef;
 use crate::rmeta::table::TableBuilder;
 use crate::rmeta::*;
 
+use rustc_ast::expand::StrippedCfgItem;
 use rustc_ast::Attribute;
 use rustc_data_structures::fingerprint::Fingerprint;
 use rustc_data_structures::fx::{FxHashMap, FxIndexSet};
@@ -584,6 +585,8 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
             (self.encode_lang_items(), self.encode_lang_items_missing())
         });
 
+        let stripped_cfg_items = stat!("stripped-cfg-items", || self.encode_stripped_cfg_items());
+
         let diagnostic_items = stat!("diagnostic-items", || self.encode_diagnostic_items());
 
         let native_libraries = stat!("native-libs", || self.encode_native_libraries());
@@ -694,6 +697,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
                 lang_items,
                 diagnostic_items,
                 lang_items_missing,
+                stripped_cfg_items,
                 native_libraries,
                 foreign_modules,
                 source_map,
@@ -1939,6 +1943,15 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
         self.lazy_array(&tcx.lang_items().missing)
     }
 
+    fn encode_stripped_cfg_items(&mut self) -> LazyArray<StrippedCfgItem<DefIndex>> {
+        self.lazy_array(
+            self.tcx
+                .stripped_cfg_items(LOCAL_CRATE)
+                .into_iter()
+                .map(|item| item.clone().map_mod_id(|def_id| def_id.index)),
+        )
+    }
+
     fn encode_traits(&mut self) -> LazyArray<DefIndex> {
         empty_proc_macro!(self);
         self.lazy_array(self.tcx.traits(LOCAL_CRATE).iter().map(|def_id| def_id.index))

compiler/rustc_metadata/src/rmeta/mod.rs

@@ -6,6 +6,7 @@ use rustc_middle::middle::debugger_visualizer::DebuggerVisualizerFile;
 use table::TableBuilder;
 
 use rustc_ast as ast;
+use rustc_ast::expand::StrippedCfgItem;
 use rustc_attr as attr;
 use rustc_data_structures::svh::Svh;
 use rustc_hir as hir;
@@ -256,6 +257,7 @@ pub(crate) struct CrateRoot {
     stability_implications: LazyArray<(Symbol, Symbol)>,
     lang_items: LazyArray<(DefIndex, LangItem)>,
     lang_items_missing: LazyArray<LangItem>,
+    stripped_cfg_items: LazyArray<StrippedCfgItem<DefIndex>>,
     diagnostic_items: LazyArray<(Symbol, DefIndex)>,
     native_libraries: LazyArray<NativeLib>,
     foreign_modules: LazyArray<ForeignModule>,