resolve: Support imports of associated types and glob imports from traits

7 files changed, 51 insertions, 73 deletions

diff --git a/compiler/rustc_error_codes/src/error_codes/E0253.md b/compiler/rustc_error_codes/src/error_codes/E0253.md
index 705d1bfc53e..628f5e252fb 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0253.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0253.md
@@ -1,9 +1,13 @@
+#### Note: this error code is no longer emitted by the compiler.
+
 Attempt was made to import an unimportable type. This can happen when trying
 to import a type from a trait.
 
 Erroneous code example:
 
-```compile_fail,E0253
+```
+#![feature(import_trait_associated_functions)]
+
 mod foo {
     pub trait MyTrait {
         type SomeType;
diff --git a/compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs b/compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs
index fcb7382549f..5e79e932015 100644
--- a/compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs
+++ b/compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs
@@ -1770,7 +1770,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         span: Span,
         opt_self_ty: Option<Ty<'tcx>>,
         item_def_id: DefId,
-        trait_segment: &hir::PathSegment<'tcx>,
+        trait_segment: Option<&hir::PathSegment<'tcx>>,
         item_segment: &hir::PathSegment<'tcx>,
     ) -> Ty<'tcx> {
         match self.lower_qpath_shared(
@@ -1795,7 +1795,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         span: Span,
         opt_self_ty: Option<Ty<'tcx>>,
         item_def_id: DefId,
-        trait_segment: &hir::PathSegment<'tcx>,
+        trait_segment: Option<&hir::PathSegment<'tcx>>,
         item_segment: &hir::PathSegment<'tcx>,
     ) -> Const<'tcx> {
         match self.lower_qpath_shared(
@@ -1820,7 +1820,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         span: Span,
         opt_self_ty: Option<Ty<'tcx>>,
         item_def_id: DefId,
-        trait_segment: &hir::PathSegment<'tcx>,
+        trait_segment: Option<&hir::PathSegment<'tcx>>,
         item_segment: &hir::PathSegment<'tcx>,
         assoc_tag: ty::AssocTag,
     ) -> Result<(DefId, GenericArgsRef<'tcx>), ErrorGuaranteed> {
@@ -1840,7 +1840,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         debug!(?self_ty);
 
         let trait_ref =
-            self.lower_mono_trait_ref(span, trait_def_id, self_ty, trait_segment, false);
+            self.lower_mono_trait_ref(span, trait_def_id, self_ty, trait_segment.unwrap(), false);
         debug!(?trait_ref);
 
         let item_args =
@@ -2196,16 +2196,17 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 }
             }
             Res::Def(DefKind::AssocTy, def_id) => {
-                debug_assert!(path.segments.len() >= 2);
-                let _ = self.prohibit_generic_args(
-                    path.segments[..path.segments.len() - 2].iter(),
-                    GenericsArgsErrExtend::None,
-                );
+                let trait_segment = if let [modules @ .., trait_, _item] = path.segments {
+                    let _ = self.prohibit_generic_args(modules.iter(), GenericsArgsErrExtend::None);
+                    Some(trait_)
+                } else {
+                    None
+                };
                 self.lower_qpath_ty(
                     span,
                     opt_self_ty,
                     def_id,
-                    &path.segments[path.segments.len() - 2],
+                    trait_segment,
                     path.segments.last().unwrap(),
                 )
             }
@@ -2413,16 +2414,17 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 ty::Const::new_unevaluated(tcx, ty::UnevaluatedConst::new(did, args))
             }
             Res::Def(DefKind::AssocConst, did) => {
-                debug_assert!(path.segments.len() >= 2);
-                let _ = self.prohibit_generic_args(
-                    path.segments[..path.segments.len() - 2].iter(),
-                    GenericsArgsErrExtend::None,
-                );
+                let trait_segment = if let [modules @ .., trait_, _item] = path.segments {
+                    let _ = self.prohibit_generic_args(modules.iter(), GenericsArgsErrExtend::None);
+                    Some(trait_)
+                } else {
+                    None
+                };
                 self.lower_qpath_const(
                     span,
                     opt_self_ty,
                     did,
-                    &path.segments[path.segments.len() - 2],
+                    trait_segment,
                     path.segments.last().unwrap(),
                 )
             }
diff --git a/compiler/rustc_resolve/messages.ftl b/compiler/rustc_resolve/messages.ftl
index 558a01713dc..38cdfa72a14 100644
--- a/compiler/rustc_resolve/messages.ftl
+++ b/compiler/rustc_resolve/messages.ftl
@@ -218,10 +218,6 @@ resolve_invalid_asm_sym =
     .label = is a local variable
     .help = `sym` operands must refer to either a function or a static
 
-resolve_is_not_directly_importable =
-    `{$target}` is not directly importable
-    .label = cannot be imported directly
-
 resolve_is_private =
     {$ident_descr} `{$ident}` is private
     .label = private {$ident_descr}
@@ -231,9 +227,6 @@ resolve_item_was_behind_feature =
 
 resolve_item_was_cfg_out = the item is gated here
 
-resolve_items_in_traits_are_not_importable =
-    items in traits are not importable
-
 resolve_label_with_similar_name_reachable =
     a label with a similar name is reachable
 
diff --git a/compiler/rustc_resolve/src/diagnostics.rs b/compiler/rustc_resolve/src/diagnostics.rs
index 363a75911ad..74daad08394 100644
--- a/compiler/rustc_resolve/src/diagnostics.rs
+++ b/compiler/rustc_resolve/src/diagnostics.rs
@@ -1181,11 +1181,9 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         } {
             let in_module_is_extern = !in_module.def_id().is_local();
             in_module.for_each_child(self, |this, ident, ns, name_binding| {
-                // avoid non-importable candidates
-                if !name_binding.is_importable()
-                    // FIXME(import_trait_associated_functions): remove this when `import_trait_associated_functions` is stable
-                    || name_binding.is_assoc_const_or_fn()
-                        && !this.tcx.features().import_trait_associated_functions()
+                // Avoid non-importable candidates.
+                if name_binding.is_assoc_item()
+                    && !this.tcx.features().import_trait_associated_functions()
                 {
                     return;
                 }
diff --git a/compiler/rustc_resolve/src/errors.rs b/compiler/rustc_resolve/src/errors.rs
index e26b300f13e..7fe74378b67 100644
--- a/compiler/rustc_resolve/src/errors.rs
+++ b/compiler/rustc_resolve/src/errors.rs
@@ -781,22 +781,6 @@ pub(crate) struct CannotGlobImportAllCrates {
     pub(crate) span: Span,
 }
 
-#[derive(Diagnostic)]
-#[diag(resolve_items_in_traits_are_not_importable)]
-pub(crate) struct ItemsInTraitsAreNotImportable {
-    #[primary_span]
-    pub(crate) span: Span,
-}
-
-#[derive(Diagnostic)]
-#[diag(resolve_is_not_directly_importable, code = E0253)]
-pub(crate) struct IsNotDirectlyImportable {
-    #[primary_span]
-    #[label]
-    pub(crate) span: Span,
-    pub(crate) target: Ident,
-}
-
 #[derive(Subdiagnostic)]
 #[suggestion(
     resolve_unexpected_res_change_ty_to_const_param_sugg,
diff --git a/compiler/rustc_resolve/src/imports.rs b/compiler/rustc_resolve/src/imports.rs
index 762e08b2be5..816efd0d5fa 100644
--- a/compiler/rustc_resolve/src/imports.rs
+++ b/compiler/rustc_resolve/src/imports.rs
@@ -30,8 +30,7 @@ use crate::diagnostics::{DiagMode, Suggestion, import_candidates};
 use crate::errors::{
     CannotBeReexportedCratePublic, CannotBeReexportedCratePublicNS, CannotBeReexportedPrivate,
     CannotBeReexportedPrivateNS, CannotDetermineImportResolution, CannotGlobImportAllCrates,
-    ConsiderAddingMacroExport, ConsiderMarkingAsPub, IsNotDirectlyImportable,
-    ItemsInTraitsAreNotImportable,
+    ConsiderAddingMacroExport, ConsiderMarkingAsPub,
 };
 use crate::{
     AmbiguityError, AmbiguityKind, BindingKey, Finalize, ImportSuggestion, Module,
@@ -835,11 +834,8 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
 
                 let parent = import.parent_scope.module;
                 match source_bindings[ns].get() {
-                    Err(Undetermined) => indeterminate_count += 1,
-                    // Don't update the resolution, because it was never added.
-                    Err(Determined) if target.name == kw::Underscore => {}
-                    Ok(binding) if binding.is_importable() => {
-                        if binding.is_assoc_const_or_fn()
+                    Ok(binding) => {
+                        if binding.is_assoc_item()
                             && !this.tcx.features().import_trait_associated_functions()
                         {
                             feature_err(
@@ -850,21 +846,21 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                             )
                             .emit();
                         }
+
                         let imported_binding = this.import(binding, import);
                         target_bindings[ns].set(Some(imported_binding));
                         this.define(parent, target, ns, imported_binding);
                     }
-                    source_binding @ (Ok(..) | Err(Determined)) => {
-                        if source_binding.is_ok() {
-                            this.dcx()
-                                .create_err(IsNotDirectlyImportable { span: import.span, target })
-                                .emit();
+                    Err(Determined) => {
+                        // Don't update the resolution for underscores, because it was never added.
+                        if target.name != kw::Underscore {
+                            let key = BindingKey::new(target, ns);
+                            this.update_resolution(parent, key, false, |_, resolution| {
+                                resolution.single_imports.swap_remove(&import);
+                            });
                         }
-                        let key = BindingKey::new(target, ns);
-                        this.update_resolution(parent, key, false, |_, resolution| {
-                            resolution.single_imports.swap_remove(&import);
-                        });
                     }
+                    Err(Undetermined) => indeterminate_count += 1,
                 }
             }
         });
@@ -1428,10 +1424,17 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
             return;
         };
 
-        if module.is_trait() {
-            self.dcx().emit_err(ItemsInTraitsAreNotImportable { span: import.span });
-            return;
-        } else if module == import.parent_scope.module {
+        if module.is_trait() && !self.tcx.features().import_trait_associated_functions() {
+            feature_err(
+                self.tcx.sess,
+                sym::import_trait_associated_functions,
+                import.span,
+                "`use` associated items of traits is unstable",
+            )
+            .emit();
+        }
+
+        if module == import.parent_scope.module {
             return;
         } else if is_prelude {
             self.prelude = Some(module);
diff --git a/compiler/rustc_resolve/src/lib.rs b/compiler/rustc_resolve/src/lib.rs
index d2da3ac7d86..d23e588e2e3 100644
--- a/compiler/rustc_resolve/src/lib.rs
+++ b/compiler/rustc_resolve/src/lib.rs
@@ -949,14 +949,8 @@ impl<'ra> NameBindingData<'ra> {
         }
     }
 
-    fn is_importable(&self) -> bool {
-        !matches!(self.res(), Res::Def(DefKind::AssocTy, _))
-    }
-
-    // FIXME(import_trait_associated_functions): associate `const` or `fn` are not importable unless
-    // the feature `import_trait_associated_functions` is enable
-    fn is_assoc_const_or_fn(&self) -> bool {
-        matches!(self.res(), Res::Def(DefKind::AssocConst | DefKind::AssocFn, _))
+    fn is_assoc_item(&self) -> bool {
+        matches!(self.res(), Res::Def(DefKind::AssocConst | DefKind::AssocFn | DefKind::AssocTy, _))
     }
 
     fn macro_kind(&self) -> Option<MacroKind> {