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, 5 insertions, 11 deletions

diff --git a/src/test/codegen/avr/avr-func-addrspace.rs b/src/test/codegen/avr/avr-func-addrspace.rs
index a038dfe76f7..cbbcfad3ef4 100644
--- a/src/test/codegen/avr/avr-func-addrspace.rs
+++ b/src/test/codegen/avr/avr-func-addrspace.rs
@@ -19,6 +19,8 @@ pub trait Sized { }
 pub trait Copy { }
 #[lang = "receiver"]
 pub trait Receiver { }
+#[lang = "tuple_trait"]
+pub trait Tuple { }
 
 pub struct Result<T, E> { _a: T, _b: E }
 
@@ -29,7 +31,7 @@ impl Copy for &usize {}
 pub unsafe fn drop_in_place<T: ?Sized>(_: *mut T) {}
 
 #[lang = "fn_once"]
-pub trait FnOnce<Args> {
+pub trait FnOnce<Args: Tuple> {
     #[lang = "fn_once_output"]
     type Output;
 
@@ -37,24 +39,16 @@ pub trait FnOnce<Args> {
 }
 
 #[lang = "fn_mut"]
-pub trait FnMut<Args> : FnOnce<Args> {
+pub trait FnMut<Args: Tuple> : FnOnce<Args> {
     extern "rust-call" fn call_mut(&mut self, args: Args) -> Self::Output;
 }
 
 #[lang = "fn"]
-pub trait Fn<Args>: FnOnce<Args> {
+pub trait Fn<Args: Tuple>: FnOnce<Args> {
     /// Performs the call operation.
     extern "rust-call" fn call(&self, args: Args) -> Self::Output;
 }
 
-impl<'a, A, R> FnOnce<A> for &'a fn(A) -> R {
-    type Output = R;
-
-    extern "rust-call" fn call_once(self, args: A) -> R {
-        (*self)(args)
-    }
-}
-
 pub static mut STORAGE_FOO: fn(&usize, &mut u32) -> Result<(), ()> = arbitrary_black_box;
 pub static mut STORAGE_BAR: u32 = 12;