Stabilize `param_attrs` in Rust 1.39.0
# Stabilization proposal
I propose that we stabilize `#![feature(param_attrs)]`.
Tracking issue: #60406
Version: 1.39 (2019-09-26 => beta, 2019-11-07 => stable).
## What is stabilized
It is now possible to add outer attributes like `#[cfg(..)]` on formal parameters of functions, closures, and function pointer types. For example:
```rust
fn len(
#[cfg(windows)] slice: &[u16],
#[cfg(not(windows))] slice: &[u8],
) -> usize {
slice.len()
}
```
## What isn't stabilized
* Documentation comments like `/// Doc` on parameters.
* Code expansion of a user-defined `#[proc_macro_attribute]` macro used on parameters.
* Built-in attributes other than `cfg`, `cfg_attr`, `allow`, `warn`, `deny`, and `forbid`. Currently, only the lints `unused_variables` and `unused_mut` have effect and may be controlled on parameters.
## Motivation
The chief motivations for stabilizing `param_attrs` include:
* Finer conditional compilation with `#[cfg(..)]` and linting control of variables.
* Richer macro DSLs created by users.
* External tools and compiler internals can take advantage of the additional information that the parameters provide.
For more examples, see the [RFC][rfc motivation].
## Reference guide
In the grammar of function and function pointer, the grammar of variadic tails (`...`) and parameters are changed respectively from:
```rust
FnParam = { pat:Pat ":" }? ty:Type;
VaradicTail = "...";
```
into:
```rust
FnParam = OuterAttr* { pat:Pat ":" }? ty:Type;
VaradicTail = OuterAttr* "...";
```
The grammar of a closure parameter is changed from:
```rust
ClosureParam = pat:Pat { ":" ty:Type }?;
```
into:
```rust
ClosureParam = OuterAttr* pat:Pat { ":" ty:Type }?;
```
More generally, where there's a list of formal (value) parameters separated or terminated by `,` and delimited by `(` and `)`. Each parameter in that list may optionally be prefixed by `OuterAttr+`.
Note that in all cases, `OuterAttr*` applies to the whole parameter and not just the pattern. This distinction matters in pretty printing and in turn for macros.
## History
* On 2018-10-15, @Robbepop proposes [RFC 2565][rfc], "Attributes in formal function parameter position".
* On 2019-04-30, [RFC 2565][rfc] is merged and the tracking issue is made.
* On 2019-06-12, a partial implementation was completed. The implementation was done in [#60669][60669] by @c410-f3r and the PR was reviewed by @petrochenkov and @Centril.
* On 2019-07-29, [#61238][61238] was fixed in [#61856][61856]. The issue fixed was that lint attributes on function args had no effect. The PR was written by @c410-f3r and reviewed by @matthewjasper, @petrochenkov, and @oli-obk.
* On 2019-08-02, a bug [#63210][63210] was filed wherein the attributes on formal parameters would not be passed to macros. The issue was about forgetting to call the relevant method in `fn print_arg` in the pretty printer. In [#63212][63212], written by @Centril on 2019-08-02 and reviewed by @davidtwco, the issue aforementioned was fixed.
* This PR stabilizes `param_attrs`.
## Tests
* [On Rust 2018, attributes aren't permitted on function parameters without a pattern in trait definitions.](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2565-param-attrs/param-attrs-2018.rs)
* [All attributes that should be allowed. This includes `cfg`, `cfg_attr`, and lints check attributes.](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2565-param-attrs/param-attrs-allowed.rs)
* [Built-in attributes, which should be forbidden, e.g., `#[test]`, are.](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2565-param-attrs/param-attrs-builtin-attrs.rs)
* [`cfg` and `cfg_attr` are properly evaluated.](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2565-param-attrs/param-attrs-cfg.rs)
* [`unused_mut`](46f405ec4d/src/test/ui/rfc-2565-param-attrs/param-attrs-cfg.rs) and [`unused_variables`](https://github.com/rust-lang/rust/blob/master/src/test/ui/lint/lint-unused-variables.rs) are correctly applied to parameter patterns.
* [Pretty printing takes formal parameter attributes into account.](https://github.com/rust-lang/rust/blob/master/src/test/ui/rfc-2565-param-attrs/param-attrs-pretty.rs)
## Possible future work
* Custom attributes inside function parameters aren't currently supported but it is something being worked on internally.
* Since documentation comments are syntactic sugar for `#[doc(...)]`, it is possible to allow literal `/// Foo` comments on function parameters.
[rfc motivation]: https://github.com/rust-lang/rfcs/blob/master/text/2565-formal-function-parameter-attributes.md#motivation
[rfc]: https://github.com/rust-lang/rfcs/pull/2565
[60669]: https://github.com/rust-lang/rust/pull/60669
[61856]: https://github.com/rust-lang/rust/pull/61856
[63210]: https://github.com/rust-lang/rust/issues/63210
[61238]: https://github.com/rust-lang/rust/issues/61238
[63212]: https://github.com/rust-lang/rust/pull/63212
This report is a collaborative work with @Centril.
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