Rollup merge of #67131 - Centril:item-merge, r=petrochenkov

Merge `TraitItem` & `ImplItem into `AssocItem`

In this PR we:

- Merge `{Trait,Impl}Item{Kind?}` into `AssocItem{Kind?}` as discussed in https://github.com/rust-lang/rust/issues/65041#issuecomment-538105286.

   - This is done by using the cover grammar of both forms.

   - In particular, it requires that we syntactically allow (under `#[cfg(FALSE)]`):

      - `default`ness on `trait` items,

      - `impl` items without a body / definition (`const`, `type`, and `fn`),

      - and associated `type`s in `impl`s with bounds, e.g., `type Foo: Ord;`.

   - The syntactic restrictions are replaced by semantic ones in `ast_validation`.

- Move syntactic restrictions around C-variadic parameters from the parser into `ast_validation`:

    - `fn`s in all contexts now syntactically allow `...`,

    - `...` can occur anywhere in the list syntactically (`fn foo(..., x: usize) {}`),

    - and `...` can be the sole parameter (`fn foo(...) {}`.

r? @petrochenkov

1 files changed, 11 insertions, 5 deletions

diff --git a/src/librustc_error_codes/error_codes/E0743.md b/src/librustc_error_codes/error_codes/E0743.md
index aaf19d8478c..1780fe59cbd 100644
--- a/src/librustc_error_codes/error_codes/E0743.md
+++ b/src/librustc_error_codes/error_codes/E0743.md
@@ -1,11 +1,17 @@
-C-variadic has been used on a non-foreign function.
+The C-variadic type `...` has been nested inside another type.
 
 Erroneous code example:
 
 ```compile_fail,E0743
-fn foo2(x: u8, ...) {} // error!
+#![feature(c_variadic)]
+
+fn foo2(x: u8, y: &...) {} // error!
 ```
 
-Only foreign functions can use C-variadic (`...`). It is used to give an
-undefined number of parameters to a given function (like `printf` in C). The
-equivalent in Rust would be to use macros directly.
+Only foreign functions can use the C-variadic type (`...`).
+In such functions, `...` may only occur non-nested.
+That is, `y: &'a ...` is not allowed.
+
+A C-variadic type is used to give an undefined number
+of parameters to a given function (like `printf` in C).
+The equivalent in Rust would be to use macros directly.