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

compiler/rustc_target/src/target_features.rs

@@ -524,6 +524,13 @@ pub fn all_known_features() -> impl Iterator<Item = (&'static str, Stability)> {
         .map(|(f, s, _)| (f, s))
 }
 
+// These arrays represent the least-constraining feature that is required for vector types up to a
+// certain size to have their "proper" ABI on each architecture.
+// Note that they must be kept sorted by vector size.
+const X86_FEATURES_FOR_CORRECT_VECTOR_ABI: &'static [(u64, &'static str)] =
+    &[(128, "sse"), (256, "avx"), (512, "avx512f")];
+const AARCH64_FEATURES_FOR_CORRECT_VECTOR_ABI: &'static [(u64, &'static str)] = &[(128, "neon")];
+
 impl super::spec::Target {
     pub fn supported_target_features(
         &self,
@@ -545,6 +552,16 @@ impl super::spec::Target {
         }
     }
 
+    // Returns None if we do not support ABI checks on the given target yet.
+    pub fn features_for_correct_vector_abi(&self) -> Option<&'static [(u64, &'static str)]> {
+        match &*self.arch {
+            "x86" | "x86_64" => Some(X86_FEATURES_FOR_CORRECT_VECTOR_ABI),
+            "aarch64" => Some(AARCH64_FEATURES_FOR_CORRECT_VECTOR_ABI),
+            // FIXME: add support for non-tier1 architectures
+            _ => None,
+        }
+    }
+
     pub fn tied_target_features(&self) -> &'static [&'static [&'static str]] {
         match &*self.arch {
             "aarch64" | "arm64ec" => AARCH64_TIED_FEATURES,