Add support for use Trait::func

text/0000-import-trait-methods.md

@@ -0,0 +1,238 @@
+- Feature Name: `import-trait-methods`
+- Start Date: 2024-03-19
+- RFC PR: [rust-lang/rfcs#0000](https://github.com/rust-lang/rfcs/pull/0000)
+- Rust Issue: [rust-lang/rust#0000](https://github.com/rust-lang/rust/issues/0000)
+
+# Summary
+[summary]: #summary
+
+Allow importing methods from traits and then using them like regular functions.
+
+# Motivation
+[motivation]: #motivation
+
+There has for a long time been a desire to shorten the duplication needed to access certain methods, such as `Default::default`. Codebases like [Bevy](https://github.com/bevyengine/bevy/blob/7c7d1e8a6442a4258896b6c605beb1bf50399396/crates/bevy_utils/src/default.rs#L27) provide wrapper methods to shorten this call, and a previous, now-rejected, [RFC](https://github.com/rust-lang/rust/pull/73001) aimed to provide this method as part of the standard library. This RFC was rejected with a note that there is a desire for a more general capability to import any trait method.
+
+Additionally, if you pull in a crate like [num_traits](https://docs.rs/num-traits/latest/num_traits/), then this feature will allow access to numeric methods such as `sin` using the infix syntax that is more common in mathematics. More generally, it will make infix calls to a trait method shorter without having to write a wrapper function.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+Importing a method from a trait is the same as importing a method from any module:
+```rust
+use Default::default;
+```
+
+Once you have done this, the method is made available in your current scope just like any other regular function.
+
+```rust
+struct S {
+    a: HashMap<i32, i32>,
+}
+
+impl S {
+    fn new() -> S {
+        S {
+            a: default()
+        }
+    }
+}
+```
+
+You can also use this with trait methods that take a `self` argument:
+
+```rust
+use num_traits::float::Float::{sin, cos}
+
+fn eulers_formula(theta: f64) -> (f64, f64) {
+    (cos(theta), sin(theta))
+}
+```
+
+Importing a method from a trait does not import the trait. If you want to call `self` methods on a trait or `impl` it, then you can import the trait as well as methods in the trait:
+
+```rust
+mod a {
+    pub trait A {
+        fn new() -> Self;
+        fn do_something(&self);
+    }
+}
+
+mod b {
+    use super::a::A::{self, new}
+
+    struct B();
+
+    impl A for B {
+        fn new() -> Self {
+            B()
+        }
+
+        fn do_something(&self) {
+        }
+    }
+
+    fn f() {
+        let b: B = new();
+        b.do_something();
+    }
+}
+```
+
+Trait methods can also be renamed when they are imported using the usual `as` syntax:
+```rust
+use Default::default as gimme
+
+struct S {
+    a: HashMap<i32, i32>,
+}
+
+impl S {
+    fn new() -> S {
+        S {
+            a: gimme()
+        }
+    }
+}
+```
+
+You can also import a parent trait method from a sub-trait:
+
+```rust
+use num_traits::float::Float::zero;
+
+fn main() {
+    let x : f64 = zero();
+    println!("{}",x);
+}
+```
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+When 
+
+```rust
+use Trait::method as m;
+```
+occurs, we first find the supertrait in which `method` occurs.
+A new item `m` is made available in the function namespace of the current module. Any attempts to call this item are treated calling the (super-)trait method explicitly qualified. As always, the `as` qualifier is optional, in which case the name of the new item is identical with the name of the method in the trait. In other words, the example:
+
+```rust
+use Default::default;
+
+struct S {
+    a: HashMap<i32, i32>,
+}
+
+impl S {
+    fn new() -> S {
+        S {
+            a: default()
+        }
+    }
+}
+```
+desugars to 
+```rust
+struct S {
+    a: HashMap<i32, i32>,
+}
+
+impl S {
+    fn new() -> S {
+        S {
+            a: Default::default()
+        }
+    }
+}
+```
+And a call
+```rust
+use Trait::method as m;
+m(x, y, z);
+```
+desugars to
+```rust
+Trait::method(x, y, z);
+```
+
+Additionally, the syntax
+```rust
+use some_module::Trait::self;
+```
+is sugar for
+```rust
+use some_module::Trait;
+```
+
+Finally, given traits 
+```rust
+trait Super {
+    fn f();
+}
+
+trait Sub : Super {
+}
+```
+the usage
+```rust
+use module::Sub::f;
+f();
+```
+desugars to
+```rust
+use module::Sub::f;
+Super::f();
+```
+**not** `Sub::f();` as that desugaring will cause compiler errors, see https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=51bef9ba69ce1fc20248e987bf106bd4.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+Calls to `default` are less explicit than calls to `Default::default`, likewise for any other trait.
+
+# Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+## Why is this design the best in the space of possible designs?
+
+This design is minimalist, it adds no extra syntax, instead providing a natural extension of existing syntax to support a feature that is frequently requested. Users might very well already expect this feature, with this exact syntax, to be present in Rust, and surprised when it isn't.
+
+## What other designs have been considered and what is the rationale for not choosing them?
+
+In [Zulip](https://rust-lang.zulipchat.com/#narrow/stream/213817-t-lang/topic/Writing.20an.20RFC.20for.20.60use.20Default.3A.3Adefault.60/near/427795694), there was some discussion of whether `use Trait::method` should bring `Trait` into scope. There are three possibilities:
+
+1. It does not - this may be unexpected, but maybe not
+2. It does - then `value.other_method_from_the_same_trait()` will work as well, this may be unexpected too
+3. It does, but only for method, that's something new for the language (need new more fine-grained tracking of traits in scope)
+
+Option 1 is what is proposed here. It has the simplest semantics, and I believe it best matches the user intent when they import a trait method; the desire is to make that method available as-if it were a regular function. Furthermore, it is more minimalist than the other two options in the sense that you can get to option 2 simply by importing the trait also. Option 3 seems like extra complexity for almost no added value.
+
+An earlier version of this RFC proposed not allowing `use Trait::super_trait_method`. This was changed because comments indicated this usecase would be common.
+
+## What is the impact of not doing this?
+
+Users of the language continue to create helper methods to access trait methods with infix syntax.
+
+## If this is a language proposal, could this be done in a library or macro instead? Does the proposed change make Rust code easier or harder to read, understand, and maintain?
+
+A library solution has already been rejected for this. This solves the same problem as a library solution in a much more general way, that doesn't require adding new library methods every time we want infix/shorthand access to trait method names.
+
+# Prior art
+[prior-art]: #prior-art
+
+As mentioned in [motivation], there was a rejected [RFC](https://github.com/rust-lang/rust/pull/73001) for adding a method `std::default::default` to make calling `Default::default` less repetitive. This RFC was rejected, with a desire to see something like what this RFC proposes replace it.
+
+[This issue](https://github.com/rust-lang/rfcs/issues/1995) also lists some further motivation for this feature.
+
+# Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+- Is specifying this in terms of desugaring sufficient to give the desired semantics?
+
+# Future possibilities
+[future-possibilities]: #future-possibilities
+
+This RFC does not propose the ability to import `Type::method` where `method` is contained in an `impl` block. Such a feature would be a natural extension of this work, and would enable numeric features like that discussed in [motivation] without the need for the [num_traits](https://docs.rs/num-traits/latest/num_traits/) crate. This feature is not proposed in this RFC since initial investigations revealed that it would be [difficult](https://rust-lang.zulipchat.com/#narrow/stream/213817-t-lang/topic/Writing.20an.20RFC.20for.20.60use.20Default.3A.3Adefault.60/near/427804375) to implement in today's rustc.