Emit error when calling/declaring functions with unavailable vectors.

On some architectures, vector types may have a different ABI when
relevant target features are enabled.

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

This commit makes it an error 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.

Cargo.lock

@@ -4458,6 +4458,7 @@ dependencies = [
 name = "rustc_monomorphize"
 version = "0.0.0"
 dependencies = [
+ "rustc_abi",
  "rustc_data_structures",
  "rustc_errors",
  "rustc_fluent_macro",

compiler/rustc_monomorphize/Cargo.toml

@@ -5,6 +5,7 @@ edition = "2021"
 
 [dependencies]
 # tidy-alphabetical-start
+rustc_abi = { path = "../rustc_abi" }
 rustc_data_structures = { path = "../rustc_data_structures" }
 rustc_errors = { path = "../rustc_errors" }
 rustc_fluent_macro = { path = "../rustc_fluent_macro" }

compiler/rustc_monomorphize/messages.ftl

@@ -1,3 +1,10 @@
+monomorphize_abi_error_disabled_vector_type_call =
+  ABI error: this function call uses a {$required_feature} vector type, which is not enabled in the caller
+  .help = consider enabling it globally (-C target-feature=+{$required_feature}) or locally (#[target_feature(enable="{$required_feature}")])
+monomorphize_abi_error_disabled_vector_type_def =
+  ABI error: this function definition uses a {$required_feature} vector type, which is not enabled
+  .help = consider enabling it globally (-C target-feature=+{$required_feature}) or locally (#[target_feature(enable="{$required_feature}")])
+
 monomorphize_couldnt_dump_mono_stats =
     unexpected error occurred while dumping monomorphization stats: {$error}
 

compiler/rustc_monomorphize/src/collector.rs

@@ -205,6 +205,7 @@
 //! this is not implemented however: a mono item will be produced
 //! regardless of whether it is actually needed or not.
 
+mod abi_check;
 mod move_check;
 
 use rustc_data_structures::sync::{par_for_each_in, LRef, MTLock};
@@ -762,6 +763,7 @@ impl<'a, 'tcx> MirVisitor<'tcx> for MirUsedCollector<'a, 'tcx> {
                 self.used_mentioned_items.insert(MentionedItem::Fn(callee_ty));
                 let callee_ty = self.monomorphize(callee_ty);
                 self.check_fn_args_move_size(callee_ty, args, *fn_span, location);
+                abi_check::check_call_abi(tcx, callee_ty, *fn_span, self.body.source.instance);
                 visit_fn_use(self.tcx, callee_ty, true, source, &mut self.used_items)
             }
             mir::TerminatorKind::Drop { ref place, .. } => {
@@ -1199,6 +1201,7 @@ fn collect_items_of_instance<'tcx>(
     mentioned_items: &mut MonoItems<'tcx>,
     mode: CollectionMode,
 ) {
+    abi_check::check_instance_abi(tcx, instance);
     let body = tcx.instance_mir(instance.def);
     // Naively, in "used" collection mode, all functions get added to *both* `used_items` and
     // `mentioned_items`. Mentioned items processing will then notice that they have already been

compiler/rustc_monomorphize/src/collector/abi_check.rs

@@ -0,0 +1,96 @@
+use rustc_abi::Abi;
+use rustc_middle::ty::{self, Instance, InstanceKind, Ty, TyCtxt};
+use rustc_span::{def_id::DefId, Span, Symbol};
+use rustc_target::abi::call::FnAbi;
+
+use crate::errors::{AbiErrorDisabledVectorTypeCall, AbiErrorDisabledVectorTypeDef};
+
+fn do_check_abi<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    abi: &FnAbi<'tcx, Ty<'tcx>>,
+    target_feature_def: DefId,
+    emit_err: impl Fn(&'static str),
+) {
+    // This check is a no-op on non-x86, at least for now.
+    if tcx.sess.target.arch != "x86" && tcx.sess.target.arch != "x86_64" {
+        return;
+    }
+    let codegen_attrs = tcx.codegen_fn_attrs(target_feature_def);
+    for arg_abi in abi.args.iter().chain(std::iter::once(&abi.ret)) {
+        let size = arg_abi.layout.size;
+        if matches!(arg_abi.layout.abi, Abi::Vector { .. }) {
+            let required_feature = match size.bits() {
+                x if x <= 128 => "sse",
+                x if x <= 256 => "avx",
+                x if x <= 512 => "avx512f",
+                _ => {
+                    panic!("Unknown vector size for x86: {}; arg = {:?}", size.bits(), arg_abi)
+                }
+            };
+            let required_feature_sym = Symbol::intern(required_feature);
+            if !tcx.sess.unstable_target_features.contains(&required_feature_sym)
+                && !codegen_attrs.target_features.contains(&required_feature_sym)
+            {
+                emit_err(required_feature);
+            }
+        }
+    }
+}
+
+/// Checks that the ABI of a given instance of a function does not contain vector-passed arguments
+/// or return values for which the corresponding target feature is not enabled.
+pub fn check_instance_abi<'tcx>(tcx: TyCtxt<'tcx>, instance: Instance<'tcx>) {
+    let InstanceKind::Item(item_def) = instance.def else {
+        return;
+    };
+
+    let param_env = tcx.param_env(item_def);
+    let Ok(abi) = tcx.fn_abi_of_instance(param_env.and((instance, ty::List::empty()))) else {
+        // an error will be reported somewhere else if we cannot determine the ABI of this
+        // function.
+        return;
+    };
+    do_check_abi(tcx, abi, item_def, |required_feature| {
+        tcx.dcx().emit_err(AbiErrorDisabledVectorTypeDef {
+            span: tcx.def_span(item_def),
+            required_feature,
+        });
+    })
+}
+
+/// Checks that a call expression does not try to pass a vector-passed argument which requires a
+/// target feature that the caller does not have, as doing so causes UB because of ABI mismatch.
+pub fn check_call_abi<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    ty: Ty<'tcx>,
+    span: Span,
+    caller: InstanceKind<'tcx>,
+) {
+    let InstanceKind::Item(caller_def) = caller else {
+        return;
+    };
+    let param_env = tcx.param_env(caller_def);
+    let callee_abi = match *ty.kind() {
+        ty::FnPtr(sig) => tcx.fn_abi_of_fn_ptr(param_env.and((sig, ty::List::empty()))),
+        ty::FnDef(def_id, args) => {
+            // Intrinsics are handled separately by the compiler.
+            if tcx.intrinsic(def_id).is_some() {
+                return;
+            }
+            let Ok(Some(instance)) = ty::Instance::try_resolve(tcx, param_env, def_id, args) else {
+                return;
+            };
+            tcx.fn_abi_of_instance(param_env.and((instance, ty::List::empty())))
+        }
+        _ => {
+            panic!("Invalid function call");
+        }
+    };
+
+    let Ok(callee_abi) = callee_abi else {
+        return;
+    };
+    do_check_abi(tcx, callee_abi, caller_def, |required_feature| {
+        tcx.dcx().emit_err(AbiErrorDisabledVectorTypeCall { span, required_feature });
+    })
+}

compiler/rustc_monomorphize/src/errors.rs

@@ -91,3 +91,21 @@ pub struct StartNotFound;
 pub struct UnknownCguCollectionMode<'a> {
     pub mode: &'a str,
 }
+
+#[derive(Diagnostic)]
+#[diag(monomorphize_abi_error_disabled_vector_type_def)]
+#[help]
+pub struct AbiErrorDisabledVectorTypeDef<'a> {
+    #[primary_span]
+    pub span: Span,
+    pub required_feature: &'a str,
+}
+
+#[derive(Diagnostic)]
+#[diag(monomorphize_abi_error_disabled_vector_type_call)]
+#[help]
+pub struct AbiErrorDisabledVectorTypeCall<'a> {
+    #[primary_span]
+    pub span: Span,
+    pub required_feature: &'a str,
+}