Auto merge of #60669 - c410-f3r:attrs-fn, r=petrochenkov

Allow attributes in formal function parameters

Implements https://github.com/rust-lang/rust/issues/60406.

This is my first contribution to the compiler and since this is a large and complex project, I am not fully aware of the consequences of the changes I have made.

**TODO**

- [x] Forbid some built-in attributes.
- [x] Expand cfg/cfg_attr

src/test/ui/parser/fn-arg-doc-comment.rs

@@ -1,20 +1,20 @@
 pub fn f(
     /// Comment
-    //~^ ERROR documentation comments cannot be applied to method arguments
+    //~^ 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 method arguments
+    //~^ 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,
 ) {}
 
-fn foo(#[allow(dead_code)] id: i32) {}
-//~^ ERROR attributes cannot be applied to method arguments
-//~| NOTE attributes are not allowed here
-
 fn bar(id: #[allow(dead_code)] i32) {}
-//~^ ERROR attributes cannot be applied to a method argument's type
+//~^ ERROR attributes cannot be applied to a function parameter's type
 //~| NOTE attributes are not allowed here
 
 fn main() {
@@ -26,10 +26,6 @@ fn main() {
     //~| ERROR mismatched types
     //~| NOTE expected u8, found reference
     //~| NOTE expected
-    foo("");
-    //~^ ERROR mismatched types
-    //~| NOTE expected i32, found reference
-    //~| NOTE expected
     bar("");
     //~^ ERROR mismatched types
     //~| NOTE expected i32, found reference

src/test/ui/parser/fn-arg-doc-comment.stderr

@@ -1,26 +1,38 @@
-error: documentation comments cannot be applied to method arguments
+error: attributes cannot be applied to a function parameter's type
+  --> $DIR/fn-arg-doc-comment.rs:16:12
+   |
+LL | fn bar(id: #[allow(dead_code)] i32) {}
+   |            ^^^^^^^^^^^^^^^^^^^ attributes are not allowed here
+
+error: documentation comments cannot be applied to function parameters
   --> $DIR/fn-arg-doc-comment.rs:2:5
    |
 LL |     /// Comment
    |     ^^^^^^^^^^^ doc comments are not allowed here
 
-error: documentation comments cannot be applied to method arguments
-  --> $DIR/fn-arg-doc-comment.rs:6:5
+error: documentation comments cannot be applied to function parameters
+  --> $DIR/fn-arg-doc-comment.rs:8:5
    |
 LL |     /// Other
    |     ^^^^^^^^^ doc comments are not allowed here
 
-error: attributes cannot be applied to method arguments
-  --> $DIR/fn-arg-doc-comment.rs:12:8
+error[E0658]: attributes on function parameters are unstable
+  --> $DIR/fn-arg-doc-comment.rs:2:5
    |
-LL | fn foo(#[allow(dead_code)] id: i32) {}
-   |        ^^^^^^^^^^^^^^^^^^^ attributes are not allowed here
+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: attributes cannot be applied to a method argument's type
-  --> $DIR/fn-arg-doc-comment.rs:16:12
+error[E0658]: attributes on function parameters are unstable
+  --> $DIR/fn-arg-doc-comment.rs:8:5
    |
-LL | fn bar(id: #[allow(dead_code)] i32) {}
-   |            ^^^^^^^^^^^^^^^^^^^ attributes are not allowed here
+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
@@ -43,15 +55,6 @@ LL |     f("", "");
 error[E0308]: mismatched types
   --> $DIR/fn-arg-doc-comment.rs:29:9
    |
-LL |     foo("");
-   |         ^^ expected i32, found reference
-   |
-   = note: expected type `i32`
-              found type `&'static str`
-
-error[E0308]: mismatched types
-  --> $DIR/fn-arg-doc-comment.rs:33:9
-   |
 LL |     bar("");
    |         ^^ expected i32, found reference
    |
@@ -60,4 +63,5 @@ LL |     bar("");
 
 error: aborting due to 8 previous errors
 
-For more information about this error, try `rustc --explain E0308`.
+Some errors have detailed explanations: E0308, E0658.
+For more information about an error, try `rustc --explain E0308`.

src/test/ui/rfc-2565-param-attrs/param-attrs-2018.rs

@@ -0,0 +1,8 @@
+// edition:2018
+
+#![feature(param_attrs)]
+
+trait Trait2015 { fn foo(#[allow(C)] i32); }
+//~^ ERROR expected one of `:` or `@`, found `)`
+
+fn main() {}

src/test/ui/rfc-2565-param-attrs/param-attrs-2018.stderr

@@ -0,0 +1,18 @@
+error: expected one of `:` or `@`, found `)`
+  --> $DIR/param-attrs-2018.rs:5:41
+   |
+LL | trait Trait2015 { fn foo(#[allow(C)] i32); }
+   |                                         ^ expected one of `:` or `@` here
+   |
+   = note: anonymous parameters are removed in the 2018 edition (see RFC 1685)
+help: if this was a parameter name, give it a type
+   |
+LL | trait Trait2015 { fn foo(#[allow(C)] i32: TypeName); }
+   |                                      ^^^^^^^^^^^^^
+help: if this is a type, explicitly ignore the parameter name
+   |
+LL | trait Trait2015 { fn foo(#[allow(C)] _: i32); }
+   |                                      ^^^^^^
+
+error: aborting due to previous error
+

src/test/ui/rfc-2565-param-attrs/param-attrs-allowed.rs

@@ -0,0 +1,225 @@
+// compile-flags: --cfg something
+// compile-pass
+
+#![feature(param_attrs)]
+
+extern "C" {
+    fn ffi(
+        #[allow(C)] a: i32,
+        #[cfg(something)] b: i32,
+        #[cfg_attr(something, cfg(nothing))] c: i32,
+        #[deny(C)] d: i32,
+        #[forbid(C)] #[warn(C)] ...
+    );
+}
+
+type FnType = fn(
+    #[allow(C)] a: i32,
+    #[cfg(something)] b: i32,
+    #[cfg_attr(something, cfg(nothing))] c: i32,
+    #[deny(C)] d: i32,
+    #[forbid(C)] #[warn(C)] e: i32
+);
+
+pub fn foo(
+    #[allow(C)] a: i32,
+    #[cfg(something)] b: i32,
+    #[cfg_attr(something, cfg(nothing))] c: i32,
+    #[deny(C)] d: i32,
+    #[forbid(C)] #[warn(C)] e: i32
+) {}
+
+// self, &self and &mut self
+
+struct SelfStruct {}
+impl SelfStruct {
+    fn foo(
+        #[allow(C)] self,
+        #[cfg(something)] a: i32,
+        #[cfg_attr(something, cfg(nothing))]
+        #[deny(C)] b: i32,
+    ) {}
+}
+
+struct RefStruct {}
+impl RefStruct {
+    fn foo(
+        #[allow(C)] &self,
+        #[cfg(something)] a: i32,
+        #[cfg_attr(something, cfg(nothing))]
+        #[deny(C)] b: i32,
+    ) {}
+}
+trait RefTrait {
+    fn foo(
+        #[forbid(C)] &self,
+        #[warn(C)] a: i32
+    ) {}
+}
+impl RefTrait for RefStruct {
+    fn foo(
+        #[forbid(C)] &self,
+        #[warn(C)] a: i32
+    ) {}
+}
+
+struct MutStruct {}
+impl MutStruct {
+    fn foo(
+        #[allow(C)] &mut self,
+        #[cfg(something)] a: i32,
+        #[cfg_attr(something, cfg(nothing))]
+        #[deny(C)] b: i32,
+    ) {}
+}
+trait MutTrait {
+    fn foo(
+        #[forbid(C)] &mut self,
+        #[warn(C)] a: i32
+    ) {}
+}
+impl MutTrait for MutStruct {
+    fn foo(
+        #[forbid(C)] &mut self,
+        #[warn(C)] a: i32
+    ) {}
+}
+
+// self: Self, self: &Self and self: &mut Self
+
+struct NamedSelfSelfStruct {}
+impl NamedSelfSelfStruct {
+    fn foo(
+        #[allow(C)] self: Self,
+        #[cfg(something)] a: i32,
+        #[cfg_attr(something, cfg(nothing))]
+        #[deny(C)] b: i32,
+    ) {}
+}
+
+struct NamedSelfRefStruct {}
+impl NamedSelfRefStruct {
+    fn foo(
+        #[allow(C)] self: &Self,
+        #[cfg(something)] a: i32,
+        #[cfg_attr(something, cfg(nothing))]
+        #[deny(C)] b: i32,
+    ) {}
+}
+trait NamedSelfRefTrait {
+    fn foo(
+        #[forbid(C)] self: &Self,
+        #[warn(C)] a: i32
+    ) {}
+}
+impl NamedSelfRefTrait for NamedSelfRefStruct {
+    fn foo(
+        #[forbid(C)] self: &Self,
+        #[warn(C)] a: i32
+    ) {}
+}
+
+struct NamedSelfMutStruct {}
+impl NamedSelfMutStruct {
+    fn foo(
+        #[allow(C)] self: &mut Self,
+        #[cfg(something)] a: i32,
+        #[cfg_attr(something, cfg(nothing))]
+        #[deny(C)] b: i32,
+    ) {}
+}
+trait NamedSelfMutTrait {
+    fn foo(
+        #[forbid(C)] self: &mut Self,
+        #[warn(C)] a: i32
+    ) {}
+}
+impl NamedSelfMutTrait for NamedSelfMutStruct {
+    fn foo(
+        #[forbid(C)] self: &mut Self,
+        #[warn(C)] a: i32
+    ) {}
+}
+
+// &'a self and &'a mut self
+
+struct NamedLifetimeRefStruct {}
+impl NamedLifetimeRefStruct {
+    fn foo<'a>(
+        #[allow(C)] self: &'a Self,
+        #[cfg(something)] a: i32,
+        #[cfg_attr(something, cfg(nothing))]
+        #[deny(C)] b: i32,
+    ) {}
+}
+trait NamedLifetimeRefTrait {
+    fn foo<'a>(
+        #[forbid(C)] &'a self,
+        #[warn(C)] a: i32
+    ) {}
+}
+impl NamedLifetimeRefTrait for NamedLifetimeRefStruct {
+    fn foo<'a>(
+        #[forbid(C)] &'a self,
+        #[warn(C)] a: i32
+    ) {}
+}
+
+struct NamedLifetimeMutStruct {}
+impl NamedLifetimeMutStruct {
+    fn foo<'a>(
+        #[allow(C)] self: &'a mut Self,
+        #[cfg(something)] a: i32,
+        #[cfg_attr(something, cfg(nothing))]
+        #[deny(C)] b: i32,
+    ) {}
+}
+trait NamedLifetimeMutTrait {
+    fn foo<'a>(
+        #[forbid(C)] &'a mut self,
+        #[warn(C)] a: i32
+    ) {}
+}
+impl NamedLifetimeMutTrait for NamedLifetimeMutStruct {
+    fn foo<'a>(
+        #[forbid(C)] &'a mut self,
+        #[warn(C)] a: i32
+    ) {}
+}
+
+// Box<Self>
+
+struct BoxSelfStruct {}
+impl BoxSelfStruct {
+    fn foo(
+        #[allow(C)] self: Box<Self>,
+        #[cfg(something)] a: i32,
+        #[cfg_attr(something, cfg(nothing))]
+        #[deny(C)] b: i32,
+    ) {}
+}
+trait BoxSelfTrait {
+    fn foo(
+        #[forbid(C)] self: Box<Self>,
+        #[warn(C)] a: i32
+    ) {}
+}
+impl BoxSelfTrait for BoxSelfStruct {
+    fn foo(
+        #[forbid(C)] self: Box<Self>,
+        #[warn(C)] a: i32
+    ) {}
+}
+
+fn main() {
+    let _: unsafe extern "C" fn(_, _, _, ...) = ffi;
+    let _: fn(_, _, _, _) = foo;
+    let _: FnType = |_, _, _, _| {};
+    let c = |
+        #[allow(C)] a: u32,
+        #[cfg(something)] b: i32,
+        #[cfg_attr(something, cfg(nothing))]
+        #[deny(C)] c: i32,
+    | {};
+    let _ = c(1, 2);
+}

src/test/ui/rfc-2565-param-attrs/param-attrs-builtin-attrs.rs

@@ -0,0 +1,145 @@
+#![feature(param_attrs)]
+
+extern "C" {
+    fn ffi(
+        /// Foo
+        //~^ ERROR documentation comments cannot be applied to function
+        #[test] a: i32,
+        //~^ ERROR The attribute `test` is currently unknown to the compiler and may have
+        /// Bar
+        //~^ ERROR documentation comments cannot be applied to function
+        #[must_use]
+        //~^ ERROR allow, cfg, cfg_attr, deny, forbid, and warn are the only allowed built-in
+        /// Baz
+        //~^ ERROR documentation comments cannot be applied to function
+        #[no_mangle] b: i32,
+        //~^ ERROR allow, cfg, cfg_attr, deny, forbid, and warn are the only allowed built-in
+    );
+}
+
+type FnType = fn(
+    /// Foo
+    //~^ ERROR documentation comments cannot be applied to function
+    #[test] a: u32,
+    //~^ ERROR The attribute `test` is currently unknown to the compiler and may have
+    /// Bar
+    //~^ ERROR documentation comments cannot be applied to function
+    #[must_use]
+    //~^ ERROR allow, cfg, cfg_attr, deny, forbid, and warn are the only allowed built-in
+    /// Baz
+    //~^ ERROR documentation comments cannot be applied to function
+    #[no_mangle] b: i32,
+    //~^ ERROR allow, cfg, cfg_attr, deny, forbid, and warn are the only allowed built-in
+);
+
+pub fn foo(
+    /// Foo
+    //~^ ERROR documentation comments cannot be applied to function
+    #[test] a: u32,
+    //~^ ERROR The attribute `test` is currently unknown to the compiler and may have
+    /// Bar
+    //~^ ERROR documentation comments cannot be applied to function
+    #[must_use]
+    //~^ ERROR allow, cfg, cfg_attr, deny, forbid, and warn are the only allowed built-in
+    /// Baz
+    //~^ ERROR documentation comments cannot be applied to function
+    #[no_mangle] b: i32,
+    //~^ ERROR allow, cfg, cfg_attr, deny, forbid, and warn are the only allowed built-in
+) {}
+
+struct SelfStruct {}
+impl SelfStruct {
+    fn foo(
+        /// Foo
+        //~^ ERROR documentation comments cannot be applied to function
+        self,
+        /// Bar
+        //~^ ERROR documentation comments cannot be applied to function
+        #[test] a: i32,
+        //~^ ERROR The attribute `test` is currently unknown to the compiler and may have
+        /// Baz
+        //~^ ERROR documentation comments cannot be applied to function
+        #[must_use]
+        //~^ ERROR allow, cfg, cfg_attr, deny, forbid, and warn are the only allowed built-in
+        /// Qux
+        //~^ ERROR documentation comments cannot be applied to function
+        #[no_mangle] b: i32,
+        //~^ ERROR allow, cfg, cfg_attr, deny, forbid, and warn are the only allowed built-in
+    ) {}
+}
+
+struct RefStruct {}
+impl RefStruct {
+    fn foo(
+        /// Foo
+        //~^ ERROR documentation comments cannot be applied to function
+        &self,
+        /// Bar
+        //~^ ERROR documentation comments cannot be applied to function
+        #[test] a: i32,
+        //~^ ERROR The attribute `test` is currently unknown to the compiler and may have
+        /// Baz
+        //~^ ERROR documentation comments cannot be applied to function
+        #[must_use]
+        //~^ ERROR allow, cfg, cfg_attr, deny, forbid, and warn are the only allowed built-in
+        /// Qux
+        //~^ ERROR documentation comments cannot be applied to function
+        #[no_mangle] b: i32,
+        //~^ ERROR allow, cfg, cfg_attr, deny, forbid, and warn are the only allowed built-in
+    ) {}
+}
+trait RefTrait {
+    fn foo(
+        /// Foo
+        //~^ ERROR documentation comments cannot be applied to function
+        &self,
+        /// Bar
+        //~^ ERROR documentation comments cannot be applied to function
+        #[test] a: i32,
+        //~^ ERROR The attribute `test` is currently unknown to the compiler and may have
+        /// Baz
+        //~^ ERROR documentation comments cannot be applied to function
+        #[must_use]
+        //~^ ERROR allow, cfg, cfg_attr, deny, forbid, and warn are the only allowed built-in
+        /// Qux
+        //~^ ERROR documentation comments cannot be applied to function
+        #[no_mangle] b: i32,
+        //~^ ERROR allow, cfg, cfg_attr, deny, forbid, and warn are the only allowed built-in
+    ) {}
+}
+impl RefTrait for RefStruct {
+    fn foo(
+        /// Foo
+        //~^ ERROR documentation comments cannot be applied to function
+        &self,
+        /// Bar
+        //~^ ERROR documentation comments cannot be applied to function
+        #[test] a: i32,
+        //~^ ERROR The attribute `test` is currently unknown to the compiler and may have
+        /// Baz
+        //~^ ERROR documentation comments cannot be applied to function
+        #[must_use]
+        //~^ ERROR allow, cfg, cfg_attr, deny, forbid, and warn are the only allowed built-in
+        /// Qux
+        //~^ ERROR documentation comments cannot be applied to function
+        #[no_mangle] b: i32,
+        //~^ ERROR allow, cfg, cfg_attr, deny, forbid, and warn are the only allowed built-in
+    ) {}
+}
+
+fn main() {
+    let _ = |
+        /// Foo
+        //~^ ERROR documentation comments cannot be applied to function
+        #[test] a: u32,
+        //~^ ERROR The attribute `test` is currently unknown to the compiler and may have
+        /// Bar
+        //~^ ERROR documentation comments cannot be applied to function
+        #[must_use]
+        //~^ ERROR allow, cfg, cfg_attr, deny, forbid, and warn are the only allowed built-in
+        /// Baz
+        //~^ ERROR documentation comments cannot be applied to function
+        #[no_mangle] b: i32
+        //~^ ERROR allow, cfg, cfg_attr, deny, forbid, and warn are the only allowed built-in
+    | {};
+}

src/test/ui/rfc-2565-param-attrs/param-attrs-builtin-attrs.stderr

@@ -0,0 +1,339 @@
+error: documentation comments cannot be applied to function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:5:9
+   |
+LL |         /// Foo
+   |         ^^^^^^^ doc comments are not allowed here
+
+error: documentation comments cannot be applied to function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:9:9
+   |
+LL |         /// Bar
+   |         ^^^^^^^ doc comments are not allowed here
+
+error: allow, cfg, cfg_attr, deny, forbid, and warn are the only allowed built-in attributes in function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:11:9
+   |
+LL |         #[must_use]
+   |         ^^^^^^^^^^^
+
+error: documentation comments cannot be applied to function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:13:9
+   |
+LL |         /// Baz
+   |         ^^^^^^^ doc comments are not allowed here
+
+error: allow, cfg, cfg_attr, deny, forbid, and warn are the only allowed built-in attributes in function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:15:9
+   |
+LL |         #[no_mangle] b: i32,
+   |         ^^^^^^^^^^^^
+
+error: documentation comments cannot be applied to function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:21:5
+   |
+LL |     /// Foo
+   |     ^^^^^^^ doc comments are not allowed here
+
+error: documentation comments cannot be applied to function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:25:5
+   |
+LL |     /// Bar
+   |     ^^^^^^^ doc comments are not allowed here
+
+error: allow, cfg, cfg_attr, deny, forbid, and warn are the only allowed built-in attributes in function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:27:5
+   |
+LL |     #[must_use]
+   |     ^^^^^^^^^^^
+
+error: documentation comments cannot be applied to function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:29:5
+   |
+LL |     /// Baz
+   |     ^^^^^^^ doc comments are not allowed here
+
+error: allow, cfg, cfg_attr, deny, forbid, and warn are the only allowed built-in attributes in function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:31:5
+   |
+LL |     #[no_mangle] b: i32,
+   |     ^^^^^^^^^^^^
+
+error: documentation comments cannot be applied to function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:36:5
+   |
+LL |     /// Foo
+   |     ^^^^^^^ doc comments are not allowed here
+
+error: documentation comments cannot be applied to function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:40:5
+   |
+LL |     /// Bar
+   |     ^^^^^^^ doc comments are not allowed here
+
+error: allow, cfg, cfg_attr, deny, forbid, and warn are the only allowed built-in attributes in function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:42:5
+   |
+LL |     #[must_use]
+   |     ^^^^^^^^^^^
+
+error: documentation comments cannot be applied to function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:44:5
+   |
+LL |     /// Baz
+   |     ^^^^^^^ doc comments are not allowed here
+
+error: allow, cfg, cfg_attr, deny, forbid, and warn are the only allowed built-in attributes in function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:46:5
+   |
+LL |     #[no_mangle] b: i32,
+   |     ^^^^^^^^^^^^
+
+error: documentation comments cannot be applied to function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:53:9
+   |
+LL |         /// Foo
+   |         ^^^^^^^ doc comments are not allowed here
+
+error: documentation comments cannot be applied to function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:56:9
+   |
+LL |         /// Bar
+   |         ^^^^^^^ doc comments are not allowed here
+
+error: documentation comments cannot be applied to function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:60:9
+   |
+LL |         /// Baz
+   |         ^^^^^^^ doc comments are not allowed here
+
+error: allow, cfg, cfg_attr, deny, forbid, and warn are the only allowed built-in attributes in function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:62:9
+   |
+LL |         #[must_use]
+   |         ^^^^^^^^^^^
+
+error: documentation comments cannot be applied to function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:64:9
+   |
+LL |         /// Qux
+   |         ^^^^^^^ doc comments are not allowed here
+
+error: allow, cfg, cfg_attr, deny, forbid, and warn are the only allowed built-in attributes in function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:66:9
+   |
+LL |         #[no_mangle] b: i32,
+   |         ^^^^^^^^^^^^
+
+error: documentation comments cannot be applied to function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:74:9
+   |
+LL |         /// Foo
+   |         ^^^^^^^ doc comments are not allowed here
+
+error: documentation comments cannot be applied to function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:77:9
+   |
+LL |         /// Bar
+   |         ^^^^^^^ doc comments are not allowed here
+
+error: documentation comments cannot be applied to function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:81:9
+   |
+LL |         /// Baz
+   |         ^^^^^^^ doc comments are not allowed here
+
+error: allow, cfg, cfg_attr, deny, forbid, and warn are the only allowed built-in attributes in function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:83:9
+   |
+LL |         #[must_use]
+   |         ^^^^^^^^^^^
+
+error: documentation comments cannot be applied to function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:85:9
+   |
+LL |         /// Qux
+   |         ^^^^^^^ doc comments are not allowed here
+
+error: allow, cfg, cfg_attr, deny, forbid, and warn are the only allowed built-in attributes in function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:87:9
+   |
+LL |         #[no_mangle] b: i32,
+   |         ^^^^^^^^^^^^
+
+error: documentation comments cannot be applied to function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:93:9
+   |
+LL |         /// Foo
+   |         ^^^^^^^ doc comments are not allowed here
+
+error: documentation comments cannot be applied to function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:96:9
+   |
+LL |         /// Bar
+   |         ^^^^^^^ doc comments are not allowed here
+
+error: documentation comments cannot be applied to function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:100:9
+   |
+LL |         /// Baz
+   |         ^^^^^^^ doc comments are not allowed here
+
+error: allow, cfg, cfg_attr, deny, forbid, and warn are the only allowed built-in attributes in function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:102:9
+   |
+LL |         #[must_use]
+   |         ^^^^^^^^^^^
+
+error: documentation comments cannot be applied to function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:104:9
+   |
+LL |         /// Qux
+   |         ^^^^^^^ doc comments are not allowed here
+
+error: allow, cfg, cfg_attr, deny, forbid, and warn are the only allowed built-in attributes in function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:106:9
+   |
+LL |         #[no_mangle] b: i32,
+   |         ^^^^^^^^^^^^
+
+error: documentation comments cannot be applied to function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:112:9
+   |
+LL |         /// Foo
+   |         ^^^^^^^ doc comments are not allowed here
+
+error: documentation comments cannot be applied to function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:115:9
+   |
+LL |         /// Bar
+   |         ^^^^^^^ doc comments are not allowed here
+
+error: documentation comments cannot be applied to function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:119:9
+   |
+LL |         /// Baz
+   |         ^^^^^^^ doc comments are not allowed here
+
+error: allow, cfg, cfg_attr, deny, forbid, and warn are the only allowed built-in attributes in function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:121:9
+   |
+LL |         #[must_use]
+   |         ^^^^^^^^^^^
+
+error: documentation comments cannot be applied to function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:123:9
+   |
+LL |         /// Qux
+   |         ^^^^^^^ doc comments are not allowed here
+
+error: allow, cfg, cfg_attr, deny, forbid, and warn are the only allowed built-in attributes in function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:125:9
+   |
+LL |         #[no_mangle] b: i32,
+   |         ^^^^^^^^^^^^
+
+error: documentation comments cannot be applied to function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:132:9
+   |
+LL |         /// Foo
+   |         ^^^^^^^ doc comments are not allowed here
+
+error: documentation comments cannot be applied to function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:136:9
+   |
+LL |         /// Bar
+   |         ^^^^^^^ doc comments are not allowed here
+
+error: allow, cfg, cfg_attr, deny, forbid, and warn are the only allowed built-in attributes in function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:138:9
+   |
+LL |         #[must_use]
+   |         ^^^^^^^^^^^
+
+error: documentation comments cannot be applied to function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:140:9
+   |
+LL |         /// Baz
+   |         ^^^^^^^ doc comments are not allowed here
+
+error: allow, cfg, cfg_attr, deny, forbid, and warn are the only allowed built-in attributes in function parameters
+  --> $DIR/param-attrs-builtin-attrs.rs:142:9
+   |
+LL |         #[no_mangle] b: i32
+   |         ^^^^^^^^^^^^
+
+error[E0658]: The attribute `test` is currently unknown to the compiler and may have meaning added to it in the future
+  --> $DIR/param-attrs-builtin-attrs.rs:7:9
+   |
+LL |         #[test] a: i32,
+   |         ^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/29642
+   = help: add #![feature(custom_attribute)] to the crate attributes to enable
+
+error[E0658]: The attribute `test` is currently unknown to the compiler and may have meaning added to it in the future
+  --> $DIR/param-attrs-builtin-attrs.rs:23:5
+   |
+LL |     #[test] a: u32,
+   |     ^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/29642
+   = help: add #![feature(custom_attribute)] to the crate attributes to enable
+
+error[E0658]: The attribute `test` is currently unknown to the compiler and may have meaning added to it in the future
+  --> $DIR/param-attrs-builtin-attrs.rs:38:5
+   |
+LL |     #[test] a: u32,
+   |     ^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/29642
+   = help: add #![feature(custom_attribute)] to the crate attributes to enable
+
+error[E0658]: The attribute `test` is currently unknown to the compiler and may have meaning added to it in the future
+  --> $DIR/param-attrs-builtin-attrs.rs:58:9
+   |
+LL |         #[test] a: i32,
+   |         ^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/29642
+   = help: add #![feature(custom_attribute)] to the crate attributes to enable
+
+error[E0658]: The attribute `test` is currently unknown to the compiler and may have meaning added to it in the future
+  --> $DIR/param-attrs-builtin-attrs.rs:79:9
+   |
+LL |         #[test] a: i32,
+   |         ^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/29642
+   = help: add #![feature(custom_attribute)] to the crate attributes to enable
+
+error[E0658]: The attribute `test` is currently unknown to the compiler and may have meaning added to it in the future
+  --> $DIR/param-attrs-builtin-attrs.rs:98:9
+   |
+LL |         #[test] a: i32,
+   |         ^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/29642
+   = help: add #![feature(custom_attribute)] to the crate attributes to enable
+
+error[E0658]: The attribute `test` is currently unknown to the compiler and may have meaning added to it in the future
+  --> $DIR/param-attrs-builtin-attrs.rs:117:9
+   |
+LL |         #[test] a: i32,
+   |         ^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/29642
+   = help: add #![feature(custom_attribute)] to the crate attributes to enable
+
+error[E0658]: The attribute `test` is currently unknown to the compiler and may have meaning added to it in the future
+  --> $DIR/param-attrs-builtin-attrs.rs:134:9
+   |
+LL |         #[test] a: u32,
+   |         ^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/29642
+   = help: add #![feature(custom_attribute)] to the crate attributes to enable
+
+error: aborting due to 52 previous errors
+
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/rfc-2565-param-attrs/param-attrs-cfg.rs

@@ -0,0 +1,79 @@
+// compile-flags: --cfg something
+
+#![feature(param_attrs)]
+#![deny(unused_variables)]
+
+extern "C" {
+    fn ffi(
+        #[cfg(nothing)] a: i32,
+        #[cfg(something)] b: i32,
+        #[cfg_attr(something, cfg(nothing))] c: i32,
+        #[cfg_attr(nothing, cfg(nothing))] ...
+    );
+}
+
+type FnType = fn(
+    #[cfg(nothing)] a: i32,
+    #[cfg(something)] b: i32,
+    #[cfg_attr(nothing, cfg(nothing))] c: i32,
+    #[cfg_attr(something, cfg(nothing))] d: i32,
+);
+
+fn foo(
+    #[cfg(nothing)] a: i32,
+    #[cfg(something)] b: i32,
+    //~^ ERROR unused variable: `b` [unused_variables]
+    #[cfg_attr(nothing, cfg(nothing))] c: i32,
+    //~^ ERROR unused variable: `c` [unused_variables]
+    #[cfg_attr(something, cfg(nothing))] d: i32,
+) {}
+
+struct RefStruct {}
+impl RefStruct {
+    fn bar(
+        &self,
+        #[cfg(nothing)] a: i32,
+        #[cfg(something)] b: i32,
+        //~^ ERROR unused variable: `b` [unused_variables]
+        #[cfg_attr(nothing, cfg(nothing))] c: i32,
+        //~^ ERROR unused variable: `c` [unused_variables]
+        #[cfg_attr(something, cfg(nothing))] d: i32,
+    ) {}
+}
+trait RefTrait {
+    fn bar(
+        &self,
+        #[cfg(nothing)] a: i32,
+        #[cfg(something)] b: i32,
+        //~^ ERROR unused variable: `b` [unused_variables]
+        #[cfg_attr(nothing, cfg(nothing))] c: i32,
+        //~^ ERROR unused variable: `c` [unused_variables]
+        #[cfg_attr(something, cfg(nothing))] d: i32,
+    ) {}
+}
+impl RefTrait for RefStruct {
+    fn bar(
+        &self,
+        #[cfg(nothing)] a: i32,
+        #[cfg(something)] b: i32,
+        //~^ ERROR unused variable: `b` [unused_variables]
+        #[cfg_attr(nothing, cfg(nothing))] c: i32,
+        //~^ ERROR unused variable: `c` [unused_variables]
+        #[cfg_attr(something, cfg(nothing))] d: i32,
+    ) {}
+}
+
+fn main() {
+    let _: unsafe extern "C" fn(_, ...) = ffi;
+    let _: fn(_, _) = foo;
+    let _: FnType = |_, _| {};
+    let c = |
+        #[cfg(nothing)] a: i32,
+        #[cfg(something)] b: i32,
+        //~^ ERROR unused variable: `b` [unused_variables]
+        #[cfg_attr(nothing, cfg(nothing))] c: i32,
+        //~^ ERROR unused variable: `c` [unused_variables]
+        #[cfg_attr(something, cfg(nothing))] d: i32,
+    | {};
+    let _ = c(1, 2);
+}

src/test/ui/rfc-2565-param-attrs/param-attrs-cfg.stderr

@@ -0,0 +1,68 @@
+error: unused variable: `b`
+  --> $DIR/param-attrs-cfg.rs:24:23
+   |
+LL |     #[cfg(something)] b: i32,
+   |                       ^ help: consider prefixing with an underscore: `_b`
+   |
+note: lint level defined here
+  --> $DIR/param-attrs-cfg.rs:4:9
+   |
+LL | #![deny(unused_variables)]
+   |         ^^^^^^^^^^^^^^^^
+
+error: unused variable: `c`
+  --> $DIR/param-attrs-cfg.rs:26:40
+   |
+LL |     #[cfg_attr(nothing, cfg(nothing))] c: i32,
+   |                                        ^ help: consider prefixing with an underscore: `_c`
+
+error: unused variable: `b`
+  --> $DIR/param-attrs-cfg.rs:72:27
+   |
+LL |         #[cfg(something)] b: i32,
+   |                           ^ help: consider prefixing with an underscore: `_b`
+
+error: unused variable: `c`
+  --> $DIR/param-attrs-cfg.rs:74:44
+   |
+LL |         #[cfg_attr(nothing, cfg(nothing))] c: i32,
+   |                                            ^ help: consider prefixing with an underscore: `_c`
+
+error: unused variable: `b`
+  --> $DIR/param-attrs-cfg.rs:47:27
+   |
+LL |         #[cfg(something)] b: i32,
+   |                           ^ help: consider prefixing with an underscore: `_b`
+
+error: unused variable: `c`
+  --> $DIR/param-attrs-cfg.rs:49:44
+   |
+LL |         #[cfg_attr(nothing, cfg(nothing))] c: i32,
+   |                                            ^ help: consider prefixing with an underscore: `_c`
+
+error: unused variable: `b`
+  --> $DIR/param-attrs-cfg.rs:36:27
+   |
+LL |         #[cfg(something)] b: i32,
+   |                           ^ help: consider prefixing with an underscore: `_b`
+
+error: unused variable: `c`
+  --> $DIR/param-attrs-cfg.rs:38:44
+   |
+LL |         #[cfg_attr(nothing, cfg(nothing))] c: i32,
+   |                                            ^ help: consider prefixing with an underscore: `_c`
+
+error: unused variable: `b`
+  --> $DIR/param-attrs-cfg.rs:58:27
+   |
+LL |         #[cfg(something)] b: i32,
+   |                           ^ help: consider prefixing with an underscore: `_b`
+
+error: unused variable: `c`
+  --> $DIR/param-attrs-cfg.rs:60:44
+   |
+LL |         #[cfg_attr(nothing, cfg(nothing))] c: i32,
+   |                                            ^ help: consider prefixing with an underscore: `_c`
+
+error: aborting due to 10 previous errors
+

src/test/ui/rfc-2565-param-attrs/param-attrs-feature-gate.rs

@@ -0,0 +1,14 @@
+// gate-test-param_attrs
+
+fn foo(
+    /// Foo
+    //~^ 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
+    #[allow(C)] a: u8
+    //~^ ERROR attributes on function parameters are unstable
+    //~| NOTE https://github.com/rust-lang/rust/issues/60406
+) {}
+
+fn main() {}

src/test/ui/rfc-2565-param-attrs/param-attrs-feature-gate.stderr

@@ -0,0 +1,27 @@
+error: documentation comments cannot be applied to function parameters
+  --> $DIR/param-attrs-feature-gate.rs:4:5
+   |
+LL |     /// Foo
+   |     ^^^^^^^ doc comments are not allowed here
+
+error[E0658]: attributes on function parameters are unstable
+  --> $DIR/param-attrs-feature-gate.rs:4:5
+   |
+LL |     /// Foo
+   |     ^^^^^^^
+   |
+   = 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/param-attrs-feature-gate.rs:9:5
+   |
+LL |     #[allow(C)] a: u8
+   |     ^^^^^^^^^^^
+   |
+   = 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: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0658`.