Save names of used extern crates

Tracks association between `self.sess.opts.externs` (aliases in `--extern alias=rlib`) and resolved `CrateNum`

Intended to allow Rustdoc match the aliases in `--extern-html-root-url` #76296

Force-injected extern crates aren't included, since they're meant for the linker only

Author: kornelski

compiler/rustc_metadata/src/creader.rs

@@ -12,6 +12,7 @@ use rustc_data_structures::fx::FxHashSet;
 use rustc_data_structures::owned_slice::OwnedSlice;
 use rustc_data_structures::svh::Svh;
 use rustc_data_structures::sync::{self, FreezeReadGuard, FreezeWriteGuard};
+use rustc_data_structures::unord::UnordMap;
 use rustc_errors::DiagCtxtHandle;
 use rustc_expand::base::SyntaxExtension;
 use rustc_fs_util::try_canonicalize;
@@ -68,6 +69,9 @@ pub struct CStore {
     /// This crate has a `#[alloc_error_handler]` item.
     has_alloc_error_handler: bool,
 
+    /// Names that were used to load the crates via `extern crate` or paths.
+    resolved_externs: UnordMap<Symbol, CrateNum>,
+
     /// Unused externs of the crate
     unused_externs: Vec<Symbol>,
 }
@@ -268,6 +272,14 @@ impl CStore {
         self.metas[cnum] = Some(Box::new(data));
     }
 
+    /// Save the name used to resolve the extern crate in the local crate
+    ///
+    /// The name isn't always the crate's own name, because `sess.opts.externs` can assign it another name.
+    /// It's also not always the same as the `DefId`'s symbol due to renames `extern crate resolved_name as defid_name`.
+    pub(crate) fn set_resolved_extern_crate_name(&mut self, name: Symbol, extern_crate: CrateNum) {
+        self.resolved_externs.insert(name, extern_crate);
+    }
+
     pub(crate) fn iter_crate_data(&self) -> impl Iterator<Item = (CrateNum, &CrateMetadata)> {
         self.metas
             .iter_enumerated()
@@ -494,6 +506,7 @@ impl CStore {
             alloc_error_handler_kind: None,
             has_global_allocator: false,
             has_alloc_error_handler: false,
+            resolved_externs: UnordMap::default(),
             unused_externs: Vec::new(),
         }
     }
@@ -1302,6 +1315,7 @@ impl<'a, 'tcx> CrateLoader<'a, 'tcx> {
                 let path_len = definitions.def_path(def_id).data.len();
                 self.cstore.update_extern_crate(
                     cnum,
+                    name,
                     ExternCrate {
                         src: ExternCrateSource::Extern(def_id.to_def_id()),
                         span: item.span,
@@ -1320,6 +1334,7 @@ impl<'a, 'tcx> CrateLoader<'a, 'tcx> {
 
         self.cstore.update_extern_crate(
             cnum,
+            name,
             ExternCrate {
                 src: ExternCrateSource::Path,
                 span,

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

@@ -631,15 +631,20 @@ impl CStore {
     }
 
     /// Track how an extern crate has been loaded. Called after resolving an import in the local crate.
+    ///
+    /// * the `name` is for [`Self::set_resolved_extern_crate_name`] saving `--extern name=`
+    /// * `extern_crate` is for diagnostics
     pub(crate) fn update_extern_crate(
         &mut self,
         cnum: CrateNum,
+        name: Symbol,
         extern_crate: ExternCrate,
     ) {
         debug_assert_eq!(
             extern_crate.dependency_of, LOCAL_CRATE,
             "this function should not be called on transitive dependencies"
         );
+        self.set_resolved_extern_crate_name(name, cnum);
         self.update_transitive_extern_crate_diagnostics(cnum, extern_crate);
     }