Rollup merge of #64010 - c410-f3r:stabilize-attrs-fn, r=Centril

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`](https://github.com/rust-lang/rust/blob/46f405ec4d7c6bf16fc2eaafe7541019f1da2996/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.

2 files changed, 7 insertions, 30 deletions

diff --git a/src/test/ui/parser/fn-arg-doc-comment.rs b/src/test/ui/parser/fn-arg-doc-comment.rs
index 4a4f959e213..995eb62d0bb 100644
--- a/src/test/ui/parser/fn-arg-doc-comment.rs
+++ b/src/test/ui/parser/fn-arg-doc-comment.rs
@@ -2,14 +2,10 @@ pub fn f(
     /// Comment
     //~^ ERROR documentation comments cannot be applied to function parameters
     //~| NOTE doc comments are not allowed here
-    //~| ERROR attributes on function parameters are unstable
-    //~| NOTE https://github.com/rust-lang/rust/issues/60406
     id: u8,
     /// Other
     //~^ ERROR documentation comments cannot be applied to function parameters
     //~| NOTE doc comments are not allowed here
-    //~| ERROR attributes on function parameters are unstable
-    //~| NOTE https://github.com/rust-lang/rust/issues/60406
     a: u8,
 ) {}
 
diff --git a/src/test/ui/parser/fn-arg-doc-comment.stderr b/src/test/ui/parser/fn-arg-doc-comment.stderr
index d8884de1fe8..669785af45f 100644
--- a/src/test/ui/parser/fn-arg-doc-comment.stderr
+++ b/src/test/ui/parser/fn-arg-doc-comment.stderr
@@ -1,5 +1,5 @@
 error: attributes cannot be applied to a function parameter's type
-  --> $DIR/fn-arg-doc-comment.rs:16:12
+  --> $DIR/fn-arg-doc-comment.rs:12:12
    |
 LL | fn bar(id: #[allow(dead_code)] i32) {}
    |            ^^^^^^^^^^^^^^^^^^^ attributes are not allowed here
@@ -11,31 +11,13 @@ LL |     /// Comment
    |     ^^^^^^^^^^^ doc comments are not allowed here
 
 error: documentation comments cannot be applied to function parameters
-  --> $DIR/fn-arg-doc-comment.rs:8:5
+  --> $DIR/fn-arg-doc-comment.rs:6:5
    |
 LL |     /// Other
    |     ^^^^^^^^^ doc comments are not allowed here
 
-error[E0658]: attributes on function parameters are unstable
-  --> $DIR/fn-arg-doc-comment.rs:2:5
-   |
-LL |     /// Comment
-   |     ^^^^^^^^^^^
-   |
-   = note: for more information, see https://github.com/rust-lang/rust/issues/60406
-   = help: add `#![feature(param_attrs)]` to the crate attributes to enable
-
-error[E0658]: attributes on function parameters are unstable
-  --> $DIR/fn-arg-doc-comment.rs:8:5
-   |
-LL |     /// Other
-   |     ^^^^^^^^^
-   |
-   = note: for more information, see https://github.com/rust-lang/rust/issues/60406
-   = help: add `#![feature(param_attrs)]` to the crate attributes to enable
-
 error[E0308]: mismatched types
-  --> $DIR/fn-arg-doc-comment.rs:22:7
+  --> $DIR/fn-arg-doc-comment.rs:18:7
    |
 LL |     f("", "");
    |       ^^ expected u8, found reference
@@ -44,7 +26,7 @@ LL |     f("", "");
               found type `&'static str`
 
 error[E0308]: mismatched types
-  --> $DIR/fn-arg-doc-comment.rs:22:11
+  --> $DIR/fn-arg-doc-comment.rs:18:11
    |
 LL |     f("", "");
    |           ^^ expected u8, found reference
@@ -53,7 +35,7 @@ LL |     f("", "");
               found type `&'static str`
 
 error[E0308]: mismatched types
-  --> $DIR/fn-arg-doc-comment.rs:29:9
+  --> $DIR/fn-arg-doc-comment.rs:25:9
    |
 LL |     bar("");
    |         ^^ expected i32, found reference
@@ -61,7 +43,6 @@ LL |     bar("");
    = note: expected type `i32`
               found type `&'static str`
 
-error: aborting due to 8 previous errors
+error: aborting due to 6 previous errors
 
-Some errors have detailed explanations: E0308, E0658.
-For more information about an error, try `rustc --explain E0308`.
+For more information about this error, try `rustc --explain E0308`.