rustc: Use tcx.used_crates(()) more

And explain when it should be used.

compiler/rustc_passes/src/diagnostic_items.rs

@@ -82,7 +82,7 @@ fn all_diagnostic_items(tcx: TyCtxt<'_>, (): ()) -> DiagnosticItems {
     let mut items = DiagnosticItems::default();
 
     // Collect diagnostic items in other crates.
-    for &cnum in tcx.crates(()).iter().chain(std::iter::once(&LOCAL_CRATE)) {
+    for &cnum in tcx.crates_including_speculative(()).iter().chain(std::iter::once(&LOCAL_CRATE)) {
         for (&name, &def_id) in &tcx.diagnostic_items(cnum).name_to_id {
             collect_item(tcx, &mut items, name, def_id);
         }

compiler/rustc_passes/src/stability.rs

@@ -1020,7 +1020,7 @@ pub fn check_unused_or_stable_features(tcx: TyCtxt<'_>) {
         // stabilization diagnostic, but it can be avoided when there are no
         // `remaining_lib_features`.
         let mut all_implications = remaining_implications.clone();
-        for &cnum in tcx.crates(()) {
+        for &cnum in tcx.used_crates(()) {
             all_implications
                 .extend_unord(tcx.stability_implications(cnum).items().map(|(k, v)| (*k, *v)));
         }
@@ -1033,7 +1033,7 @@ pub fn check_unused_or_stable_features(tcx: TyCtxt<'_>) {
             &all_implications,
         );
 
-        for &cnum in tcx.crates(()) {
+        for &cnum in tcx.used_crates(()) {
             if remaining_lib_features.is_empty() && remaining_implications.is_empty() {
                 break;
             }

compiler/rustc_passes/src/weak_lang_items.rs

@@ -68,7 +68,7 @@ fn verify(tcx: TyCtxt<'_>, items: &lang_items::LanguageItems) {
     }
 
     let mut missing = FxHashSet::default();
-    for &cnum in tcx.crates(()).iter() {
+    for &cnum in tcx.used_crates(()).iter() {
         for &item in tcx.missing_lang_items(cnum).iter() {
             missing.insert(item);
         }