Emit warning when calling/declaring functions with unavailable vectors.

On some architectures, vector types may have a different ABI depending
on whether the relevant target features are enabled. (The ABI when the
feature is disabled is often not specified, but LLVM implements some
de-facto ABI.)

As discussed in rust-lang/lang-team#235, this turns out to very easily
lead to unsound code.

This commit makes it a post-monomorphization future-incompat warning to
declare or call functions using those vector types in a context in which
the corresponding target features are disabled, if using an ABI for
which the difference is relevant. This ensures that these functions are
always called with a consistent ABI.

See the [nomination comment](https://github.com/rust-lang/rust/pull/127731#issuecomment-2288558187)
for more discussion.

Part of #116558

2 files changed, 8 insertions, 49 deletions

diff --git a/tests/crashes/131342-2.rs b/tests/crashes/131342-2.rs
deleted file mode 100644
index 79b6a837a49..00000000000
--- a/tests/crashes/131342-2.rs
+++ /dev/null
@@ -1,40 +0,0 @@
-//@ known-bug: #131342
-// see also: 131342.rs
-
-fn main() {
-    problem_thingy(Once);
-}
-
-struct Once;
-
-impl Iterator for Once {
-    type Item = ();
-}
-
-fn problem_thingy(items: impl Iterator) {
-    let peeker = items.peekable();
-    problem_thingy(&peeker);
-}
-
-trait Iterator {
-    type Item;
-
-    fn peekable(self) -> Peekable<Self>
-    where
-        Self: Sized,
-    {
-        loop {}
-    }
-}
-
-struct Peekable<I: Iterator> {
-    _peeked: I::Item,
-}
-
-impl<I: Iterator> Iterator for Peekable<I> {
-    type Item = I::Item;
-}
-
-impl<I: Iterator + ?Sized> Iterator for &I {
-    type Item = I::Item;
-}
diff --git a/tests/crashes/131342.rs b/tests/crashes/131342.rs
index 7f7ee9c9ac1..f4404092917 100644
--- a/tests/crashes/131342.rs
+++ b/tests/crashes/131342.rs
@@ -1,16 +1,15 @@
 //@ known-bug: #131342
-// see also: 131342-2.rs
 
 fn main() {
-  let mut items = vec![1, 2, 3, 4, 5].into_iter();
-  problem_thingy(&mut items);
+    let mut items = vec![1, 2, 3, 4, 5].into_iter();
+    problem_thingy(&mut items);
 }
 
 fn problem_thingy(items: &mut impl Iterator<Item = u8>) {
-  let mut peeker = items.peekable();
-  match peeker.peek() {
-    Some(_) => (),
-    None => return (),
-  }
-  problem_thingy(&mut peeker);
+    let mut peeker = items.peekable();
+    match peeker.peek() {
+        Some(_) => (),
+        None => return (),
+    }
+    problem_thingy(&mut peeker);
 }