Auto merge of #127731 - veluca93:abi_checks, r=RalfJung

Emit future-incompatibility lint when calling/declaring functions with vectors that require missing target feature

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 https://github.com/rust-lang/lang-team/issues/235, this turns out to very easily lead to unsound code.

This commit makes it a post-monomorphization 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. 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.

r? RalfJung

Part of https://github.com/rust-lang/rust/issues/116558

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 std::path::PathBuf;
@@ -766,6 +767,7 @@ 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_site_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, .. } => {
@@ -1207,6 +1209,9 @@ fn collect_items_of_instance<'tcx>(
     mentioned_items: &mut MonoItems<'tcx>,
     mode: CollectionMode,
 ) {
+    // Check the instance for feature-dependent ABI.
+    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 +1,111 @@
+//! This module ensures that if a function's ABI requires a particular target feature,
+//! that target feature is enabled both on the callee and all callers.
+use rustc_hir::CRATE_HIR_ID;
+use rustc_middle::ty::{self, Instance, InstanceKind, ParamEnv, Ty, TyCtxt};
+use rustc_session::lint::builtin::ABI_UNSUPPORTED_VECTOR_TYPES;
+use rustc_span::def_id::DefId;
+use rustc_span::{Span, Symbol};
+use rustc_target::abi::call::{FnAbi, PassMode};
+use rustc_target::abi::{Abi, RegKind};
+
+use crate::errors::{AbiErrorDisabledVectorTypeCall, AbiErrorDisabledVectorTypeDef};
+
+fn uses_vector_registers(mode: &PassMode, abi: &Abi) -> bool {
+    match mode {
+        PassMode::Ignore | PassMode::Indirect { .. } => false,
+        PassMode::Cast { pad_i32: _, cast } => {
+            cast.prefix.iter().any(|r| r.is_some_and(|x| x.kind == RegKind::Vector))
+                || cast.rest.unit.kind == RegKind::Vector
+        }
+        PassMode::Direct(..) | PassMode::Pair(..) => matches!(abi, Abi::Vector { .. }),
+    }
+}
+
+fn do_check_abi<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    abi: &FnAbi<'tcx, Ty<'tcx>>,
+    target_feature_def: DefId,
+    emit_err: impl Fn(&'static str),
+) {
+    let Some(feature_def) = tcx.sess.target.features_for_correct_vector_abi() else {
+        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 uses_vector_registers(&arg_abi.mode, &arg_abi.layout.abi) {
+            // Find the first feature that provides at least this vector size.
+            let feature = match feature_def.iter().find(|(bits, _)| size.bits() <= *bits) {
+                Some((_, feature)) => feature,
+                None => {
+                    emit_err("<no available feature for this size>");
+                    continue;
+                }
+            };
+            let feature_sym = Symbol::intern(feature);
+            if !tcx.sess.unstable_target_features.contains(&feature_sym)
+                && !codegen_attrs.target_features.iter().any(|x| x.name == feature_sym)
+            {
+                emit_err(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(super) fn check_instance_abi<'tcx>(tcx: TyCtxt<'tcx>, instance: Instance<'tcx>) {
+    let param_env = ParamEnv::reveal_all();
+    let Ok(abi) = tcx.fn_abi_of_instance(param_env.and((instance, ty::List::empty()))) else {
+        // An error will be reported during codegen if we cannot determine the ABI of this
+        // function.
+        return;
+    };
+    do_check_abi(tcx, abi, instance.def_id(), |required_feature| {
+        let span = tcx.def_span(instance.def_id());
+        tcx.emit_node_span_lint(
+            ABI_UNSUPPORTED_VECTOR_TYPES,
+            CRATE_HIR_ID,
+            span,
+            AbiErrorDisabledVectorTypeDef { span, 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(super) fn check_call_site_abi<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    ty: Ty<'tcx>,
+    span: Span,
+    caller: InstanceKind<'tcx>,
+) {
+    let param_env = ParamEnv::reveal_all();
+    let callee_abi = match *ty.kind() {
+        ty::FnPtr(..) => tcx.fn_abi_of_fn_ptr(param_env.and((ty.fn_sig(tcx), ty::List::empty()))),
+        ty::FnDef(def_id, args) => {
+            // Intrinsics are handled separately by the compiler.
+            if tcx.intrinsic(def_id).is_some() {
+                return;
+            }
+            let instance = ty::Instance::expect_resolve(tcx, param_env, def_id, args, span);
+            tcx.fn_abi_of_instance(param_env.and((instance, ty::List::empty())))
+        }
+        _ => {
+            panic!("Invalid function call");
+        }
+    };
+
+    let Ok(callee_abi) = callee_abi else {
+        // ABI failed to compute; this will not get through codegen.
+        return;
+    };
+    do_check_abi(tcx, callee_abi, caller.def_id(), |required_feature| {
+        tcx.emit_node_span_lint(
+            ABI_UNSUPPORTED_VECTOR_TYPES,
+            CRATE_HIR_ID,
+            span,
+            AbiErrorDisabledVectorTypeCall { span, required_feature },
+        );
+    })
+}

compiler/rustc_monomorphize/src/errors.rs

@@ -92,3 +92,21 @@ pub(crate) struct StartNotFound;
 pub(crate) struct UnknownCguCollectionMode<'a> {
     pub mode: &'a str,
 }
+
+#[derive(LintDiagnostic)]
+#[diag(monomorphize_abi_error_disabled_vector_type_def)]
+#[help]
+pub(crate) struct AbiErrorDisabledVectorTypeDef<'a> {
+    #[label]
+    pub span: Span,
+    pub required_feature: &'a str,
+}
+
+#[derive(LintDiagnostic)]
+#[diag(monomorphize_abi_error_disabled_vector_type_call)]
+#[help]
+pub(crate) struct AbiErrorDisabledVectorTypeCall<'a> {
+    #[label]
+    pub span: Span,
+    pub required_feature: &'a str,
+}