RFC: Make the as keyword consider Into Trait implementations

text/0000-as-keyword-consider-into-trait.md

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+- Feature Name: `as_keyword_consider_into_trait`
+- Start Date: 2018-01-21
+- RFC PR:
+- Rust Issue:
+
+# Summary
+[summary]: #summary
+
+Permit to use the common `as` keyword with any type that implement the `Into` Trait,
+allowing explicit conversions whose primitives already benefit, more visible than simple function calls.
+
+# Motivation
+[motivation]: #motivation
+
+Many operators are allowed to be implemented permitting a much better programming flow:
+- the `+`, `-`, `*`, `\` operators are the result of the `Add`, `Sub`, `Mul`, `Div` Traits implementations.
+- the `+=`, `-=`, `*=`, `\=` operators are the result of the `AddAssign`, `SubAssign`, `MulAssign`, `DivAssign` Traits implementations.
+- the `container[index]` operator is the result of the `Index` and `IndexMut` Traits implementations.
+- and many others can be found on the [`std::ops`](https://doc.rust-lang.org/std/ops/) documentation page.
+
+The `as` operator is actually reserved to primitives only but these given primitives already implement `Into` Traits for all possible conversions:
+- the `u32` implement `Into<u64>` in the form of an [`impl From<u32> for u64`](https://doc.rust-lang.org/std/primitive.u64.html#impl-From%3Cu32%3E).
+- the `u8` implement `Into<u16>` in the form of an [`impl From<u8> for u16`](https://doc.rust-lang.org/std/primitive.u16.html#impl-From%3Cu8%3E).
+
+All of these conversions can be found in the [`libcore/num/mod.rs` file](https://github.com/rust-lang/rust/blob/d9d5c667d819ce400fc7adb09dcd6482b0aa519e/src/libcore/num/mod.rs#L3343-L3400).
+
+Some special primitives like `usize` and `isize` doesn't implement many `From` Traits because these are [the representation of the address register](https://users.rust-lang.org/t/cant-convert-usize-to-u64/6243) and it can break compilation on some architectures.
+
+Adding this feature to the compiler will probably unify the [actual conversion error detections](https://users.rust-lang.org/t/cant-convert-usize-to-u64/6243/8) of the `as` keyword and the `into` method.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+The actual design is not far from what we already know of the `as` keyword.
+It doesn't add new grammar to the language, it add more freedom to the actual syntax:
+
+```rust
+struct Foo(i32);
+
+struct Bar(i32);
+
+impl Into<Bar> for Foo {
+    fn into(self) -> Bar {
+        Bar(self.0)
+    }
+}
+
+// this old syntax
+let x = Foo(42);
+let y: Bar = x.into();
+
+// is equivalent to the new one
+let x = Foo(42);
+let y = x as Bar;
+```
+
+The actual syntax **is not deprecated in any way**, it has to mimic the `Add::add` or the `Deref::deref` methods and permit the user to directly call these methods or use the operators as he likes.
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+The `as` keyword is use to reflect the actual `Into::into` method call, if the `Into` Trait is implemented for the type to be converted, the compiler needs to emit an error informing the user that the conversion is not possible.
+
+The actual `as` keyword is reserved for primitives to convert into other primitives but it shouldn't be a breaking change to change the behavior and to work with any type implementing `Into` because the primitives already implement these Traits.
+
+The internal desing could be a simple syntax sugar to something like that:
+
+```rust
+// this actual syntax
+let x = Foo(42);
+let y = x as Bar;
+
+// is a syntax sugar for
+let x = Foo(42);
+let y = Into::<Bar>::into(x);
+```
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+The `Into` Trait is not in the `std::ops` page and there is a reason why,
+it's probably not considered overridable to be used on an operator/keyword like the `as` one.
+
+The `as` keyword differ from the `Into::into` named method, it can be confusing.
+
+# Rationale and alternatives
+[alternatives]: #alternatives
+
+With this design we doesn't introduce a new syntax, we just enlarge its possibilities.
+
+The `as` keyword can be confusing with the `Into::into` named method,
+we can add a new `into` keyword and/or deprecate the `as` one.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+What about `Copy` on the actual types ? Can it be a problem ?
+
+Is the actual restriction on primitives due to the fact that they implement `Copy` and the conversion is low performance impact ?
+
+How can we handle generic type conversions ?
+Do we need to disallow them and only accpet non-generic ones ?
+
+```rust
+struct Foo<T>(T);
+
+struct Bar<T>(T);
+
+impl Into<Bar<i64>> for Foo<i32> {
+    fn into(self) -> Bar<i64> {
+        Bar(self.0 as i64)
+    }
+}
+
+// this old syntax
+let x = Foo(42_i32);
+let y: Bar<i64> = x.into();
+
+// is equivalent to the new one
+let x = Foo(42);
+let y = x as Bar<i64>;
+
+// we can let the type inference guess the generic type
+let x = Foo(42_i32);
+let y: Bar<_> = x.into();
+
+// equivalent to
+let x = Foo(42);
+let y = x as Bar<_>;
+```