Auto merge of #75176 - jyn514:impl-link, r=GuillaumeGomez,petrochenkov

Fix intra-doc links for cross-crate re-exports of default trait methods

The original fix for this was very simple: https://github.com/rust-lang/rust/pull/58972 ignored `extern_traits` because before https://github.com/rust-lang/rust/issues/65983 was fixed, they would always fail to resolve, giving spurious warnings. So the first commit just undoes that change, so extern traits are now seen by the `collect_intra_doc_links` pass. There are also some minor changes in `librustdoc/fold.rs` to avoid borrowing the `extern_traits` RefCell more than once at a time.

However, that brought up a much more thorny problem. `rustc_resolve` started giving 'error: cannot find a built-in macro with name `cfg`' when documenting `libproc_macro` (I still haven't been able to reproduce on anything smaller than the full standard library). The chain of events looked like this (thanks @eddyb for the help debugging!):

0. `x.py build --stage 1` builds the standard library and creates a sysroot
1. `cargo doc` does something like `cargo check` to create `rmeta`s for all the crates (unrelated to what was built above)
2. the `cargo check`-like `libcore-*.rmeta` is loaded as a transitive dependency *and claims ownership* of builtin macros
3. `rustdoc` later tries to resolve some path in a doc link
4. suggestion logic fires and loads "extern prelude" crates by name
5. the sysroot `libcore-*.rlib` is loaded and *fails to claim ownership* of builtin macros

`rustc_resolve` gives the error after step 5. However, `rustdoc` doesn't need suggestions at all - `resolve_str_path_error` completely discards the `ResolutionError`! The fix implemented in this PR is to skip the suggestion logic for `resolve_ast_path`: pass `record_used: false` and skip `lookup_import_candidates` when `record_used` isn't set.

It's possible that if/when https://github.com/rust-lang/rust/issues/74207 is implemented this will need a more in-depth fix which returns a `ResolutionError` from `compile_macro`, to allow rustdoc to reuse the suggestions from rustc_resolve. However, that's a much larger change and there's no need for it yet, so I haven't implemented it here.

Fixes https://github.com/rust-lang/rust/issues/73829.

r? @GuillaumeGomez

src/librustc_metadata/creader.rs

@@ -222,6 +222,7 @@ impl<'a> CrateLoader<'a> {
         let mut ret = None;
         self.cstore.iter_crate_data(|cnum, data| {
             if data.name() != name {
+                tracing::trace!("{} did not match {}", data.name(), name);
                 return;
             }
 
@@ -230,7 +231,10 @@ impl<'a> CrateLoader<'a> {
                     ret = Some(cnum);
                     return;
                 }
-                Some(..) => return,
+                Some(hash) => {
+                    debug!("actual hash {} did not match expected {}", hash, data.hash());
+                    return;
+                }
                 None => {}
             }
 
@@ -273,6 +277,11 @@ impl<'a> CrateLoader<'a> {
                 .1;
             if kind.matches(prev_kind) {
                 ret = Some(cnum);
+            } else {
+                debug!(
+                    "failed to load existing crate {}; kind {:?} did not match prev_kind {:?}",
+                    name, kind, prev_kind
+                );
             }
         });
         ret

src/librustc_resolve/lib.rs

@@ -2385,8 +2385,12 @@ impl<'a> Resolver<'a> {
                             Res::Def(DefKind::Mod, _) => true,
                             _ => false,
                         };
-                        let mut candidates =
+                        // Don't look up import candidates if this is a speculative resolve
-                            self.lookup_import_candidates(ident, TypeNS, parent_scope, is_mod);
+                        let mut candidates = if record_used {
+                            self.lookup_import_candidates(ident, TypeNS, parent_scope, is_mod)
+                        } else {
+                            Vec::new()
+                        };
                         candidates.sort_by_cached_key(|c| {
                             (c.path.segments.len(), pprust::path_to_string(&c.path))
                         });
@@ -3200,7 +3204,7 @@ impl<'a> Resolver<'a> {
             &Segment::from_path(path),
             Some(ns),
             parent_scope,
-            true,
+            false,
             path.span,
             CrateLint::No,
         ) {

src/librustdoc/clean/inline.rs

@@ -628,7 +628,9 @@ pub fn record_extern_trait(cx: &DocContext<'_>, did: DefId) {
         }
     }
 
+    {
         cx.active_extern_traits.borrow_mut().insert(did);
+    }
 
     debug!("record_extern_trait: {:?}", did);
     let trait_ = build_external_trait(cx, did);

src/librustdoc/core.rs

@@ -439,6 +439,7 @@ pub fn run_core(
                 resolver.borrow_mut().access(|resolver| {
                     sess.time("load_extern_crates", || {
                         for extern_name in &extern_names {
+                            debug!("loading extern crate {}", extern_name);
                             resolver
                                 .resolve_str_path_error(
                                     DUMMY_SP,

src/librustdoc/fold.rs

@@ -93,15 +93,11 @@ pub trait DocFolder: Sized {
         c.module = c.module.take().and_then(|module| self.fold_item(module));
 
         {
-            let mut guard = c.external_traits.borrow_mut();
+            let external_traits = { std::mem::take(&mut *c.external_traits.borrow_mut()) };
-            let external_traits = std::mem::replace(&mut *guard, Default::default());
+            for (k, mut v) in external_traits {
-            *guard = external_traits
-                .into_iter()
-                .map(|(k, mut v)| {
                 v.items = v.items.into_iter().filter_map(|i| self.fold_item(i)).collect();
-                    (k, v)
+                c.external_traits.borrow_mut().insert(k, v);
-                })
+            }
-                .collect();
         }
         c
     }

src/librustdoc/passes/collect_intra_doc_links.rs

@@ -161,6 +161,7 @@ impl<'a, 'tcx> LinkCollector<'a, 'tcx> {
                 return Some(res.map_id(|_| panic!("unexpected id")));
             }
             if let Some(module_id) = parent_id {
+                debug!("resolving {} as a macro in the module {:?}", path_str, module_id);
                 if let Ok((_, res)) =
                     resolver.resolve_str_path_error(DUMMY_SP, path_str, MacroNS, module_id)
                 {
@@ -972,15 +973,6 @@ impl<'a, 'tcx> DocFolder for LinkCollector<'a, 'tcx> {
             self.fold_item_recur(item)
         }
     }
-
-    // FIXME: if we can resolve intra-doc links from other crates, we can use the stock
-    // `fold_crate`, but until then we should avoid scanning `krate.external_traits` since those
-    // will never resolve properly
-    fn fold_crate(&mut self, mut c: Crate) -> Crate {
-        c.module = c.module.take().and_then(|module| self.fold_item(module));
-
-        c
-    }
 }
 
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]

src/test/rustdoc/intra-doc-crate/traits.rs

@@ -1,5 +1,3 @@
-// ignore-test
-// ^ this is https://github.com/rust-lang/rust/issues/73829
 // aux-build:traits.rs
 // build-aux-docs
 // ignore-tidy-line-length