Auto merge of #99943 - compiler-errors:tuple-trait, r=jackh726

Implement `std::marker::Tuple`, use it in `extern "rust-call"` and `Fn`-family traits

Implements rust-lang/compiler-team#537

I made a few opinionated decisions in this implementation, specifically:
1. Enforcing `extern "rust-call"` on fn items during wfcheck,
2. Enforcing this for all functions (not just ones that have bodies),
3. Gating this `Tuple` marker trait behind its own feature, instead of grouping it into (e.g.) `unboxed_closures`.

Still needing to be done:
1. Enforce that `extern "rust-call"` `fn`-ptrs are well-formed only if they have 1/2 args and the second one implements `Tuple`. (Doing this would fix ICE in #66696.)
2. Deny all explicit/user `impl`s of the `Tuple` trait, kinda like `Sized`.
3. Fixing `Tuple` trait built-in impl for chalk, so that chalkification tests are un-broken.

Open questions:
1. Does this need t-lang or t-libs signoff?

Fixes #99820

1 files changed, 9 insertions, 3 deletions

diff --git a/src/test/ui/error-codes/E0059.stderr b/src/test/ui/error-codes/E0059.stderr
index a1b8aeaedbb..f331d014226 100644
--- a/src/test/ui/error-codes/E0059.stderr
+++ b/src/test/ui/error-codes/E0059.stderr
@@ -1,8 +1,14 @@
-error[E0059]: cannot use call notation; the first type parameter for the function trait is neither a tuple nor unit
-  --> $DIR/E0059.rs:3:41
+error[E0059]: type parameter to bare `Fn` trait must be a tuple
+  --> $DIR/E0059.rs:3:11
    |
 LL | fn foo<F: Fn<i32>>(f: F) -> F::Output { f(3) }
-   |                                         ^^^^
+   |           ^^^^^^^ the trait `Tuple` is not implemented for `i32`
+   |
+note: required by a bound in `Fn`
+  --> $SRC_DIR/core/src/ops/function.rs:LL:COL
+   |
+LL | pub trait Fn<Args: Tuple>: FnMut<Args> {
+   |                    ^^^^^ required by this bound in `Fn`
 
 error: aborting due to previous error