rustdoc: inline compiler-private items into compiler-private crates

Fixes #106421

Author: notriddle

Diff for: src/librustdoc/clean/inline.rs

@@ -13,6 +13,7 @@ use rustc_hir::def_id::DefId;
 use rustc_hir::Mutability;
 use rustc_metadata::creader::{CStore, LoadedMacro};
 use rustc_middle::ty::{self, TyCtxt};
+use rustc_span::def_id::LOCAL_CRATE;
 use rustc_span::hygiene::MacroKind;
 use rustc_span::symbol::{kw, sym, Symbol};
 
@@ -378,6 +379,14 @@ pub(crate) fn build_impl(
     let tcx = cx.tcx;
     let associated_trait = tcx.impl_trait_ref(did);
 
+    // Do not inline compiler-internal items unless we're a compiler-internal crate.
+    let document_compiler_internal =
+        if let Some(stab) = tcx.lookup_stability(LOCAL_CRATE.as_def_id()) {
+            stab.is_unstable() && stab.feature == sym::rustc_private
+        } else {
+            false
+        };
+
     // Only inline impl if the implemented trait is
     // reachable in rustdoc generated documentation
     if !did.is_local() {
@@ -387,10 +396,8 @@ pub(crate) fn build_impl(
                 return;
             }
 
-            if let Some(stab) = tcx.lookup_stability(did) {
-                if stab.is_unstable() && stab.feature == sym::rustc_private {
-                    return;
-                }
+            if !document_compiler_internal && let Some(stab) = tcx.lookup_stability(did) && stab.is_unstable() && stab.feature == sym::rustc_private {
+                return;
             }
         }
     }
@@ -416,10 +423,8 @@ pub(crate) fn build_impl(
                 return;
             }
 
-            if let Some(stab) = tcx.lookup_stability(did) {
-                if stab.is_unstable() && stab.feature == sym::rustc_private {
-                    return;
-                }
+            if !document_compiler_internal && let Some(stab) = tcx.lookup_stability(did) && stab.is_unstable() && stab.feature == sym::rustc_private {
+                return;
             }
         }
     }

Diff for: src/test/rustdoc/auxiliary/issue-106421-force-unstable.rs

@@ -0,0 +1,9 @@
+// compile-flags: -Zforce-unstable-if-unmarked
+#![crate_name="foo"]
+pub struct FatalError;
+
+impl FatalError {
+    pub fn raise(self) -> ! {
+        loop {}
+    }
+}

Diff for: src/test/rustdoc/issue-106421-not-internal.rs

@@ -0,0 +1,8 @@
+// aux-build:issue-106421-force-unstable.rs
+// ignore-cross-compile
+// This is the version where a non-compiler-internal crate inlines a compiler-internal one.
+// In this case, the item shouldn't be documented, because regular users can't get at it.
+extern crate foo;
+
+// @!has issue_106421_not_internal/struct.FatalError.html '//*[@id="method.raise"]' 'fn raise'
+pub use foo::FatalError;

Diff for: src/test/rustdoc/issue-106421.rs

@@ -0,0 +1,8 @@
+// aux-build:issue-106421-force-unstable.rs
+// ignore-cross-compile
+// compile-flags: -Zforce-unstable-if-unmarked
+
+extern crate foo;
+
+// @has issue_106421/struct.FatalError.html '//*[@id="method.raise"]' 'fn raise'
+pub use foo::FatalError;