Implement use associated items of traits

Author: frank-king

compiler/rustc_feature/src/unstable.rs

@@ -519,6 +519,8 @@ declare_features! (
     (unstable, impl_trait_in_bindings, "1.64.0", Some(63065)),
     /// Allows `impl Trait` as output type in `Fn` traits in return position of functions.
     (unstable, impl_trait_in_fn_trait_return, "1.64.0", Some(99697)),
+    /// Allows `use` associated functions from traits.
+    (unstable, import_trait_associated_functions, "CURRENT_RUSTC_VERSION", Some(134691)),
     /// Allows associated types in inherent impls.
     (incomplete, inherent_associated_types, "1.52.0", Some(8995)),
     /// Allow anonymous constants from an inline `const` block in pattern position

compiler/rustc_resolve/messages.ftl

@@ -221,10 +221,6 @@ 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}

compiler/rustc_resolve/src/errors.rs

@@ -785,15 +785,6 @@ pub(crate) struct ItemsInTraitsAreNotImportable {
     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,

compiler/rustc_resolve/src/ident.rs

@@ -1549,6 +1549,26 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                     ignore_import,
                 )
                 .map_err(|(determinacy, _)| determinacy)
+                .or_else(|err| {
+                    // HACK: try to resolve as a trait or type if it's the first segment in a `use` item.
+                    if ns == TypeNS
+                        && ribs.is_none()
+                        && segment_idx == 1
+                        && let ModuleOrUniformRoot::Module(module) = module
+                        && let Ok(binding) = self.early_resolve_ident_in_lexical_scope(
+                            ident,
+                            ScopeSet::Late(ns, module, finalize.map(|f| f.node_id)),
+                            parent_scope,
+                            finalize,
+                            finalize.is_some(),
+                            ignore_binding,
+                            ignore_import,
+                        )
+                    {
+                        return Ok(binding);
+                    }
+                    Err(err)
+                })
             } else if let Some(ribs) = ribs
                 && let Some(TypeNS | ValueNS) = opt_ns
             {

compiler/rustc_resolve/src/imports.rs

@@ -17,9 +17,10 @@ use rustc_session::lint::builtin::{
     AMBIGUOUS_GLOB_REEXPORTS, HIDDEN_GLOB_REEXPORTS, PUB_USE_OF_PRIVATE_EXTERN_CRATE,
     REDUNDANT_IMPORTS, UNUSED_IMPORTS,
 };
+use rustc_session::parse::feature_err;
 use rustc_span::edit_distance::find_best_match_for_name;
 use rustc_span::hygiene::LocalExpnId;
-use rustc_span::{Ident, Span, Symbol, kw};
+use rustc_span::{Ident, Span, Symbol, kw, sym};
 use smallvec::SmallVec;
 use tracing::debug;
 
@@ -29,8 +30,7 @@ use crate::diagnostics::{DiagMode, Suggestion, import_candidates};
 use crate::errors::{
     CannotBeReexportedCratePublic, CannotBeReexportedCratePublicNS, CannotBeReexportedPrivate,
     CannotBeReexportedPrivateNS, CannotDetermineImportResolution, CannotGlobImportAllCrates,
-    ConsiderAddingMacroExport, ConsiderMarkingAsPub, IsNotDirectlyImportable,
-    ItemsInTraitsAreNotImportable,
+    ConsiderAddingMacroExport, ConsiderMarkingAsPub, ItemsInTraitsAreNotImportable,
 };
 use crate::{
     AmbiguityError, AmbiguityKind, BindingKey, Finalize, ImportSuggestion, Module,
@@ -828,16 +828,23 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                     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() => {
+                    Ok(binding)
+                        if binding.is_importable()
+                            || this.tcx.features().import_trait_associated_functions() =>
+                    {
                         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();
+                            feature_err(
+                                this.tcx.sess,
+                                sym::import_trait_associated_functions,
+                                import.span,
+                                "`use` associated items of traits is unstable",
+                            )
+                            .emit();
                         }
                         let key = BindingKey::new(target, ns);
                         this.update_resolution(parent, key, false, |_, resolution| {

compiler/rustc_span/src/symbol.rs

@@ -1092,6 +1092,7 @@ symbols! {
         import,
         import_name_type,
         import_shadowing,
+        import_trait_associated_functions,
         imported_main,
         in_band_lifetimes,
         include,

tests/ui/feature-gates/feature-gate-import-trait-associated-functions.rs

@@ -0,0 +1,62 @@
+use std::collections::HashMap;
+
+use A::{DEFAULT, new};
+//~^ ERROR `use` associated items of traits is unstable [E0658]
+//~| ERROR `use` associated items of traits is unstable [E0658]
+use Default::default;
+//~^ ERROR `use` associated items of traits is unstable [E0658]
+
+struct S {
+    a: HashMap<i32, i32>,
+}
+
+impl S {
+    fn new() -> S {
+        S { a: default() }
+    }
+}
+
+trait A: Sized {
+    const DEFAULT: Option<Self> = None;
+    fn new() -> Self;
+    fn do_something(&self);
+}
+
+mod b {
+    use super::A::{self, DEFAULT, new};
+    //~^ ERROR `use` associated items of traits is unstable [E0658]
+    //~| ERROR `use` associated items of traits is unstable [E0658]
+
+    struct B();
+
+    impl A for B {
+        const DEFAULT: Option<Self> = Some(B());
+        fn new() -> Self {
+            B()
+        }
+
+        fn do_something(&self) {}
+    }
+
+    fn f() {
+        let b: B = new();
+        b.do_something();
+        let c: B = DEFAULT.unwrap();
+    }
+}
+
+impl A for S {
+    fn new() -> Self {
+        S::new()
+    }
+
+    fn do_something(&self) {}
+}
+
+fn f() {
+    let s: S = new();
+    s.do_something();
+    let t: Option<S> = DEFAULT;
+}
+
+fn main() {}

tests/ui/feature-gates/feature-gate-import-trait-associated-functions.stderr

@@ -0,0 +1,53 @@
+error[E0658]: `use` associated items of traits is unstable
+  --> $DIR/feature-gate-import-trait-associated-functions.rs:3:9
+   |
+LL | use A::{DEFAULT, new};
+   |         ^^^^^^^
+   |
+   = note: see issue #134691 <https://github.com/rust-lang/rust/issues/134691> for more information
+   = help: add `#![feature(import_trait_associated_functions)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0658]: `use` associated items of traits is unstable
+  --> $DIR/feature-gate-import-trait-associated-functions.rs:3:18
+   |
+LL | use A::{DEFAULT, new};
+   |                  ^^^
+   |
+   = note: see issue #134691 <https://github.com/rust-lang/rust/issues/134691> for more information
+   = help: add `#![feature(import_trait_associated_functions)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0658]: `use` associated items of traits is unstable
+  --> $DIR/feature-gate-import-trait-associated-functions.rs:6:5
+   |
+LL | use Default::default;
+   |     ^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #134691 <https://github.com/rust-lang/rust/issues/134691> for more information
+   = help: add `#![feature(import_trait_associated_functions)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0658]: `use` associated items of traits is unstable
+  --> $DIR/feature-gate-import-trait-associated-functions.rs:26:26
+   |
+LL |     use super::A::{self, DEFAULT, new};
+   |                          ^^^^^^^
+   |
+   = note: see issue #134691 <https://github.com/rust-lang/rust/issues/134691> for more information
+   = help: add `#![feature(import_trait_associated_functions)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0658]: `use` associated items of traits is unstable
+  --> $DIR/feature-gate-import-trait-associated-functions.rs:26:35
+   |
+LL |     use super::A::{self, DEFAULT, new};
+   |                                   ^^^
+   |
+   = note: see issue #134691 <https://github.com/rust-lang/rust/issues/134691> for more information
+   = help: add `#![feature(import_trait_associated_functions)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: aborting due to 5 previous errors
+
+For more information about this error, try `rustc --explain E0658`.

tests/ui/use/import_trait_associated_functions.rs

@@ -0,0 +1,60 @@
+//@ check-pass
+#![feature(import_trait_associated_functions)]
+
+use std::collections::HashMap;
+
+use A::{DEFAULT, new};
+use Default::default;
+
+struct S {
+    a: HashMap<i32, i32>,
+}
+
+impl S {
+    fn new() -> S {
+        S { a: default() }
+    }
+}
+
+trait A: Sized {
+    const DEFAULT: Option<Self> = None;
+    fn new() -> Self;
+    fn do_something(&self);
+}
+
+mod b {
+    use super::A::{self, DEFAULT, new};
+
+    struct B();
+
+    impl A for B {
+        const DEFAULT: Option<Self> = Some(B());
+        fn new() -> Self {
+            B()
+        }
+
+        fn do_something(&self) {}
+    }
+
+    fn f() {
+        let b: B = new();
+        b.do_something();
+        let c: B = DEFAULT.unwrap();
+    }
+}
+
+impl A for S {
+    fn new() -> Self {
+        S::new()
+    }
+
+    fn do_something(&self) {}
+}
+
+fn f() {
+    let s: S = new();
+    s.do_something();
+    let t: Option<S> = DEFAULT;
+}
+
+fn main() {}