Implement use associated items of traits

compiler/rustc_error_codes/src/error_codes/E0253.md

@@ -1,19 +1,19 @@
-Attempt was made to import an unimportable value. This can happen when trying
-to import a method from a trait.
+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
 mod foo {
     pub trait MyTrait {
-        fn do_something();
+        type SomeType;
     }
 }
 
-use foo::MyTrait::do_something;
-// error: `do_something` is not directly importable
+use foo::MyTrait::SomeType;
+// error: `SomeType` is not directly importable
 
 fn main() {}
 ```
 
-It's invalid to directly import methods belonging to a trait or concrete type.
+It's invalid to directly import type belonging to a trait or concrete type.

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/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;
 
@@ -828,16 +829,33 @@ 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()
+                            || binding.is_assoc_const_or_fn()
+                                && 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 })
+                        if let Ok(binding) = source_binding {
+                            if binding.is_assoc_const_or_fn() {
+                                feature_err(
+                                    this.tcx.sess,
+                                    sym::import_trait_associated_functions,
+                                    import.span,
+                                    "`use` associated items of traits is unstable",
+                                )
                                 .emit();
+                            } else {
+                                this.dcx()
+                                    .create_err(IsNotDirectlyImportable {
+                                        span: import.span,
+                                        target,
+                                    })
+                                    .emit();
+                            }
                         }
                         let key = BindingKey::new(target, ns);
                         this.update_resolution(parent, key, false, |_, resolution| {

compiler/rustc_resolve/src/lib.rs

@@ -926,6 +926,10 @@ impl<'ra> NameBindingData<'ra> {
         )
     }
 
+    fn is_assoc_const_or_fn(&self) -> bool {
+        matches!(self.res(), Res::Def(DefKind::AssocConst | DefKind::AssocFn, _))
+    }
+
     fn macro_kind(&self) -> Option<MacroKind> {
         self.res().macro_kind()
     }

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/error-codes/E0253.rs

@@ -1,10 +1,10 @@
 mod foo {
     pub trait MyTrait {
-        fn do_something();
+        type SomeType;
     }
 }
 
-use foo::MyTrait::do_something;
+use foo::MyTrait::SomeType;
     //~^ ERROR E0253
 
 fn main() {}

tests/ui/error-codes/E0253.stderr

@@ -1,8 +1,8 @@
-error[E0253]: `do_something` is not directly importable
+error[E0253]: `SomeType` is not directly importable
   --> $DIR/E0253.rs:7:5
    |
-LL | use foo::MyTrait::do_something;
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot be imported directly
+LL | use foo::MyTrait::SomeType;
+   |     ^^^^^^^^^^^^^^^^^^^^^^ cannot be imported directly
 
 error: aborting due to 1 previous error
 

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

@@ -0,0 +1,63 @@
+//@ edition:2018
+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:4:9
+   |
+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:4:18
+   |
+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:7:5
+   |
+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:27:26
+   |
+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:27:35
+   |
+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/imports/import-trait-method.rs

@@ -2,6 +2,6 @@ trait Foo {
     fn foo();
 }
 
-use Foo::foo; //~ ERROR not directly importable
+use Foo::foo; //~ ERROR `use` associated items of traits is unstable [E0658]
 
 fn main() { foo(); }

tests/ui/imports/import-trait-method.stderr

@@ -1,9 +1,13 @@
-error[E0253]: `foo` is not directly importable
+error[E0658]: `use` associated items of traits is unstable
   --> $DIR/import-trait-method.rs:5:5
    |
 LL | use Foo::foo;
-   |     ^^^^^^^^ cannot be imported directly
+   |     ^^^^^^^^
+   |
+   = 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 1 previous error
 
-For more information about this error, try `rustc --explain E0253`.
+For more information about this error, try `rustc --explain E0658`.

tests/ui/use/import_trait_associated_functions-2015.rs

@@ -0,0 +1,61 @@
+//@ edition:2015
+//@ check-pass
+#![feature(import_trait_associated_functions)]
+
+use std::collections::HashMap;
+
+use A::{DEFAULT, new};
+use std::default::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() {}

tests/ui/use/import_trait_associated_functions.rs

@@ -0,0 +1,61 @@
+//@ edition:2018
+//@ 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() {}