Introduce a query for visible crates

Add a query for crates that are visible and shoule appear in
diagnostics. This excludes stdlib-private crates unless the
`rustc_private` feature is set, in which case there is no difference
from the existing `.crates(())` query.

Author: tgross35

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

@@ -18,7 +18,7 @@ use rustc_middle::util::Providers;
 use rustc_session::cstore::{CrateStore, ExternCrate};
 use rustc_session::{Session, StableCrateId};
 use rustc_span::hygiene::ExpnId;
-use rustc_span::{Span, Symbol, kw};
+use rustc_span::{Span, Symbol, kw, sym};
 
 use super::{Decodable, DecodeContext, DecodeIterator};
 use crate::creader::{CStore, LoadedMacro};
@@ -561,6 +561,22 @@ pub(in crate::rmeta) fn provide(providers: &mut Providers) {
             tcx.untracked().cstore.freeze();
             tcx.arena.alloc_from_iter(CStore::from_tcx(tcx).iter_crate_data().map(|(cnum, _)| cnum))
         },
+        visible_crates: |tcx, ()| {
+            // The list of loaded crates is now frozen in query cache,
+            // so make sure cstore is not mutably accessed from here on.
+            tcx.untracked().cstore.freeze();
+
+            // if `#![feature(rustc_private)]` is enabled in the current crate, include
+            if tcx.features().enabled(sym::rustc_private) {
+                tcx.crates(())
+            } else {
+                tcx.arena.alloc_from_iter(
+                    CStore::from_tcx(tcx)
+                        .iter_crate_data()
+                        .filter_map(|(cnum, meta)| (!meta.stdlib_private_dep).then_some(cnum)),
+                )
+            }
+        },
         used_crates: |tcx, ()| {
             // The list of loaded crates is now frozen in query cache,
             // so make sure cstore is not mutably accessed from here on.

compiler/rustc_middle/src/query/mod.rs

@@ -2128,10 +2128,21 @@ rustc_queries! {
         eval_always
         desc { "calculating the stability index for the local crate" }
     }
+    /// All available crates in the graph. Use `.visible_crates(())` instead if results should only
+    /// include user-visible crates (such as for diagnostics).
     query crates(_: ()) -> &'tcx [CrateNum] {
         eval_always
         desc { "fetching all foreign CrateNum instances" }
     }
+    /// Crates that are meant to be accessible by the user. Use `.crates(())` if the entire crate
+    /// graph is required.
+    ///
+    /// This excludes e.g. private dependencies of the standard library, unless `rustc_private` is
+    /// enabled.
+    query visible_crates(_: ()) -> &'tcx [CrateNum] {
+        eval_always
+        desc { "fetching all user-reachable CrateNum instances" }
+    }
     // Crates that are loaded non-speculatively (not for diagnostics or doc links).
     // FIXME: This is currently only used for collecting lang items, but should be used instead of
     // `crates` in most other cases too.