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

2 files changed, 32 insertions, 0 deletions

diff --git a/library/alloc/src/boxed.rs b/library/alloc/src/boxed.rs
index b7e7d5a38a5..66f4c19e0f9 100644
--- a/library/alloc/src/boxed.rs
+++ b/library/alloc/src/boxed.rs
@@ -158,6 +158,8 @@ use core::hash::{Hash, Hasher};
 #[cfg(not(no_global_oom_handling))]
 use core::iter::FromIterator;
 use core::iter::{FusedIterator, Iterator};
+#[cfg(not(bootstrap))]
+use core::marker::Tuple;
 use core::marker::{Destruct, Unpin, Unsize};
 use core::mem;
 use core::ops::{
@@ -1979,6 +1981,7 @@ impl<I: ExactSizeIterator + ?Sized, A: Allocator> ExactSizeIterator for Box<I, A
 #[stable(feature = "fused", since = "1.26.0")]
 impl<I: FusedIterator + ?Sized, A: Allocator> FusedIterator for Box<I, A> {}
 
+#[cfg(bootstrap)]
 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
 impl<Args, F: FnOnce<Args> + ?Sized, A: Allocator> FnOnce<Args> for Box<F, A> {
     type Output = <F as FnOnce<Args>>::Output;
@@ -1988,6 +1991,17 @@ impl<Args, F: FnOnce<Args> + ?Sized, A: Allocator> FnOnce<Args> for Box<F, A> {
     }
 }
 
+#[cfg(not(bootstrap))]
+#[stable(feature = "boxed_closure_impls", since = "1.35.0")]
+impl<Args: Tuple, F: FnOnce<Args> + ?Sized, A: Allocator> FnOnce<Args> for Box<F, A> {
+    type Output = <F as FnOnce<Args>>::Output;
+
+    extern "rust-call" fn call_once(self, args: Args) -> Self::Output {
+        <F as FnOnce<Args>>::call_once(*self, args)
+    }
+}
+
+#[cfg(bootstrap)]
 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
 impl<Args, F: FnMut<Args> + ?Sized, A: Allocator> FnMut<Args> for Box<F, A> {
     extern "rust-call" fn call_mut(&mut self, args: Args) -> Self::Output {
@@ -1995,6 +2009,15 @@ impl<Args, F: FnMut<Args> + ?Sized, A: Allocator> FnMut<Args> for Box<F, A> {
     }
 }
 
+#[cfg(not(bootstrap))]
+#[stable(feature = "boxed_closure_impls", since = "1.35.0")]
+impl<Args: Tuple, F: FnMut<Args> + ?Sized, A: Allocator> FnMut<Args> for Box<F, A> {
+    extern "rust-call" fn call_mut(&mut self, args: Args) -> Self::Output {
+        <F as FnMut<Args>>::call_mut(self, args)
+    }
+}
+
+#[cfg(bootstrap)]
 #[stable(feature = "boxed_closure_impls", since = "1.35.0")]
 impl<Args, F: Fn<Args> + ?Sized, A: Allocator> Fn<Args> for Box<F, A> {
     extern "rust-call" fn call(&self, args: Args) -> Self::Output {
@@ -2002,6 +2025,14 @@ impl<Args, F: Fn<Args> + ?Sized, A: Allocator> Fn<Args> for Box<F, A> {
     }
 }
 
+#[cfg(not(bootstrap))]
+#[stable(feature = "boxed_closure_impls", since = "1.35.0")]
+impl<Args: Tuple, F: Fn<Args> + ?Sized, A: Allocator> Fn<Args> for Box<F, A> {
+    extern "rust-call" fn call(&self, args: Args) -> Self::Output {
+        <F as Fn<Args>>::call(self, args)
+    }
+}
+
 #[unstable(feature = "coerce_unsized", issue = "27732")]
 impl<T: ?Sized + Unsize<U>, U: ?Sized, A: Allocator> CoerceUnsized<Box<U, A>> for Box<T, A> {}
 
diff --git a/library/alloc/src/lib.rs b/library/alloc/src/lib.rs
index ce36b116f13..008926666c1 100644
--- a/library/alloc/src/lib.rs
+++ b/library/alloc/src/lib.rs
@@ -150,6 +150,7 @@
 #![feature(trusted_len)]
 #![feature(trusted_random_access)]
 #![feature(try_trait_v2)]
+#![cfg_attr(not(bootstrap), feature(tuple_trait))]
 #![feature(unchecked_math)]
 #![feature(unicode_internals)]
 #![feature(unsize)]