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

1 files changed, 8 insertions, 0 deletions

diff --git a/compiler/rustc_middle/src/query/mod.rs b/compiler/rustc_middle/src/query/mod.rs
index 088b5d4ec96..3f61d7cc66f 100644
--- a/compiler/rustc_middle/src/query/mod.rs
+++ b/compiler/rustc_middle/src/query/mod.rs
@@ -2315,6 +2315,14 @@ rustc_queries! {
         desc { "whether the item should be made inlinable across crates" }
         separate_provide_extern
     }
+
+    /// Check the signature of this function as well as all the call expressions inside of it
+    /// to ensure that any target features required by the ABI are enabled.
+    /// Should be called on a fully monomorphized instance.
+    query check_feature_dependent_abi(key: ty::Instance<'tcx>) {
+        desc { "check for feature-dependent ABI" }
+        cache_on_disk_if { true }
+    }
 }
 
 rustc_query_append! { define_callbacks! }