Add RFC 'closure_to_fn_coercion'

text/0000-closure-to-fn-coercion.md

@@ -0,0 +1,155 @@
+- Feature Name: closure_to_fn_coercion
+- Start Date: 2016-03-25
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+
+A non-capturing (that is, does not `Clone` or `move` any local variables) should be
+coercable to a function pointer (`fn`).
+
+# Motivation
+[motivation]: #motivation
+
+Currently in rust, it is impossible to bind anything but a pre-defined function
+as a function pointer. When dealing with closures, one must either rely upon
+rust's type-inference capabilities, or use the `Fn` trait to abstract for any
+closure with a certain type signature.
+
+What is not possible, though, is to define a function while at the same time
+binding it to a function pointer.
+
+This is mainly used for convenience purposes, but in certain situations
+the lack of ability to do so creates a significant amount of boilerplate code.
+For example, when attempting to create an array of small, simple, but unique functions,
+it would be necessary to pre-define each and every function beforehand:
+
+```rust
+fn inc_0(var: &mut u32) {}
+fn inc_1(var: &mut u32) { *var += 1; }
+fn inc_2(var: &mut u32) { *var += 2; }
+fn inc_3(var: &mut u32) { *var += 3; }
+
+const foo: [fn(&mut u32); 4] = [
+  inc_0,
+  inc_1,
+  inc_2,
+  inc_3,
+];
+```
+
+This is a trivial example, and one that might not seem too consequential, but the
+code doubles with every new item added to the array. With very many elements,
+the duplication begins to seem unwarranted.
+
+Another option, of course, is to use an array of `Fn` instead of `fn`:
+
+```rust
+const foo: [&'static Fn(&mut u32); 4] = [
+  &|var: &mut u32| {},
+  &|var: &mut u32| *var += 1,
+  &|var: &mut u32| *var += 2,
+  &|var: &mut u32| *var += 3,
+];
+```
+
+And this seems to fix the problem. Unfortunately, however, looking closely one
+can see that because we use the `Fn` trait, an extra layer of indirection
+is added when attempting to run `foo[n](&mut bar)`.
+
+Rust must use dynamic dispatch because a closure is secretly a struct that
+contains references to captured variables, and the code within that closure
+must be able to access those references stored in the struct.
+
+In the above example, though, no variables are captured by the closures,
+so in theory nothing would stop the compiler from treating them as anonymous
+functions. By doing so, unnecessary indirection would be avoided. In situations
+where this function pointer array is particularly hot code, the optimization
+would be appreciated.
+
+# Detailed design
+[design]: #detailed-design
+
+In C++, non-capturing lambdas (the C++ equivalent of closures) "decay" into function pointers
+when they do not need to capture any variables. This is used, for example, to pass a lambda
+into a C function:
+
+```cpp
+void foo(void (*foobar)(void)) {
+    // impl
+}
+void bar() {
+    foo([]() { /* do something */ });
+}
+```
+
+With this proposal, rust users would be able to do the same:
+
+```rust
+fn foo(foobar: fn()) {
+    // impl
+}
+fn bar() {
+    foo(|| { /* do something */ });
+}
+```
+
+Using the examples within ["Motivation"](#motivation), the code array would
+be simplified to no performance detriment:
+
+```rust
+const foo: [fn(&mut u32); 4] = [
+  |var: &mut u32| {},
+  |var: &mut u32| *var += 1,
+  |var: &mut u32| *var += 2,
+  |var: &mut u32| *var += 3,
+];
+```
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+To a rust user, there is no drawback to this new coercion from closures to `fn` types.
+
+The only drawback is that it would add some amount of complexity to the type system.
+
+# Alternatives
+[alternatives]: #alternatives
+
+## Anonymous function syntax
+
+With this alternative, rust users would be able to directly bind a function
+to a variable, without needing to give the function a name.
+
+```rust
+let foo = fn() { /* do something */ };
+foo();
+```
+
+```rust
+const foo: [fn(&mut u32); 4] = [
+  fn(var: &mut u32) {},
+  fn(var: &mut u32) { *var += 1 },
+  fn(var: &mut u32) { *var += 2 },
+  fn(var: &mut u32) { *var += 3 },
+];
+```
+
+This isn't ideal, however, because it would require giving new semantics
+to `fn` syntax.
+
+## Aggressive optimization
+
+This is possibly unrealistic, but an alternative would be to continue encouraging
+the use of closures with the `Fn` trait, but conduct heavy optimization to determine
+when the used closure is "trivial" and does not need indirection.
+
+Of course, this would probably significantly complicate the optimization process, and
+would have the detriment of not being easily verifiable by the programmer without
+checking the disassembly of their program.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+None

text/0401-coercions.md

@@ -154,6 +154,9 @@ Coercion is allowed between the following types:
 
 * `&mut T` to `*mut T`
 
+* `T` to `fn` if `T` is a closure that does not capture any local variables
+  in its environment.
+
 * `T` to `U` if `T` implements `CoerceUnsized<U>` (see below) and `T = Foo<...>`
   and `U = Foo<...>` (for any `Foo`, when we get HKT I expect this could be a
   constraint on the `CoerceUnsized` trait, rather than being checked here)
@@ -338,7 +341,7 @@ and where unsize_kind(`T`) is the kind of the unsize info
 in `T` - the vtable for a trait definition (e.g. `fmt::Display` or
 `Iterator`, not `Iterator<Item=u8>`) or a length (or `()` if `T: Sized`).
 
-Note that lengths are not adjusted when casting raw slices - 
+Note that lengths are not adjusted when casting raw slices -
 `T: *const [u16] as *const [u8]` creates a slice that only includes
 half of the original memory.
 
@@ -441,4 +444,9 @@ Specifically for the DST custom coercions, the compiler could throw an error if
 it finds a user-supplied implementation of the `Unsize` trait, rather than
 silently ignoring them.
 
-# Unresolved questions
+# Amendments
+
+* Updated by [#1558](https://github.com/rust-lang/rfcs/pull/1558), which allows
+  coercions from a non-capturing closure to a function pointer.
+
+# Unresolved questions