compiler: limit in-HIR ABI support checks to stable ABIs

We already gated unstable, unsupported ABIs during AST lowering.
If we check all ABIs for target support during HIR lowering, then we
risk duplicate errors in code that uses unstable extern "{abi}"s.

Limiting the ABI support checks to stable ABIs reduces this, and
remains correct because of gating all unstable ABIs already.
This code complication can be reduced when we switch the FCW to error.

Author: workingjubilee

Cargo.lock

@@ -3815,6 +3815,7 @@ dependencies = [
  "rustc_abi",
  "rustc_arena",
  "rustc_ast",
+ "rustc_ast_lowering",
  "rustc_attr_data_structures",
  "rustc_data_structures",
  "rustc_errors",

compiler/rustc_ast_lowering/src/stability.rs

@@ -20,10 +20,10 @@ pub(crate) fn extern_abi_enabled(
     features: &rustc_feature::Features,
     span: Span,
     abi: ExternAbi,
-) -> Result<(), UnstableAbi> {
+) -> Result<ExternAbi, UnstableAbi> {
     extern_abi_stability(abi).or_else(|unstable @ UnstableAbi { feature, .. }| {
         if features.enabled(feature) || span.allows_unstable(feature) {
-            Ok(())
+            Ok(abi)
         } else {
             Err(unstable)
         }
@@ -81,7 +81,7 @@ impl fmt::Display for UnstableAbi {
     }
 }
 
-pub fn extern_abi_stability(abi: ExternAbi) -> Result<(), UnstableAbi> {
+pub fn extern_abi_stability(abi: ExternAbi) -> Result<ExternAbi, UnstableAbi> {
     match abi {
         // stable ABIs
         ExternAbi::Rust
@@ -94,7 +94,7 @@ pub fn extern_abi_stability(abi: ExternAbi) -> Result<(), UnstableAbi> {
         | ExternAbi::Win64 { .. }
         | ExternAbi::SysV64 { .. }
         | ExternAbi::System { .. }
-        | ExternAbi::EfiApi => Ok(()),
+        | ExternAbi::EfiApi => Ok(abi),
         ExternAbi::Unadjusted => {
             Err(UnstableAbi { abi, feature: sym::abi_unadjusted, explain: GateReason::ImplDetail })
         }

compiler/rustc_hir_analysis/Cargo.toml

@@ -13,6 +13,7 @@ itertools = "0.12"
 rustc_abi = { path = "../rustc_abi" }
 rustc_arena = { path = "../rustc_arena" }
 rustc_ast = { path = "../rustc_ast" }
+rustc_ast_lowering = { path = "../rustc_ast_lowering" }
 rustc_attr_data_structures = { path = "../rustc_attr_data_structures" }
 rustc_data_structures = { path = "../rustc_data_structures" }
 rustc_errors = { path = "../rustc_errors" }

compiler/rustc_hir_analysis/src/check/check.rs

@@ -2,6 +2,7 @@ use std::cell::LazyCell;
 use std::ops::ControlFlow;
 
 use rustc_abi::FieldIdx;
+use rustc_ast_lowering::stability::extern_abi_stability;
 use rustc_attr_data_structures::ReprAttr::ReprPacked;
 use rustc_data_structures::unord::{UnordMap, UnordSet};
 use rustc_errors::MultiSpan;
@@ -35,25 +36,33 @@ use {rustc_attr_data_structures as attrs, rustc_hir as hir};
 use super::compare_impl_item::check_type_bounds;
 use super::*;
 
-pub fn check_abi(tcx: TyCtxt<'_>, span: Span, abi: ExternAbi) {
-    if !tcx.sess.target.is_abi_supported(abi) {
-        struct_span_code_err!(
-            tcx.dcx(),
-            span,
-            E0570,
-            "`{abi}` is not a supported ABI for the current target",
-        )
-        .emit();
+/// check if a stable ABI passes e.g. target support
+///
+/// we are gating unstable target-unsupported ABIs in rustc_ast_lowering which is more foolproof.
+/// but until we finish the FCW associated with check_abi_fn_ptr we can't simply consolidate them.
+pub fn gate_stable_abi(tcx: TyCtxt<'_>, span: Span, abi: ExternAbi) {
+    if let Ok(abi) = extern_abi_stability(abi) {
+        if !tcx.sess.target.is_abi_supported(abi) {
+            struct_span_code_err!(
+                tcx.dcx(),
+                span,
+                E0570,
+                "`{abi}` is not a supported ABI for the current target",
+            )
+            .emit();
+        }
     }
 }
 
-pub fn check_abi_fn_ptr(tcx: TyCtxt<'_>, hir_id: hir::HirId, span: Span, abi: ExternAbi) {
-    if !tcx.sess.target.is_abi_supported(abi) {
-        tcx.node_span_lint(UNSUPPORTED_FN_PTR_CALLING_CONVENTIONS, hir_id, span, |lint| {
-            lint.primary_message(format!(
-                "the calling convention {abi} is not supported on this target"
-            ));
-        });
+pub fn gate_stable_fn_ptr_abi(tcx: TyCtxt<'_>, hir_id: hir::HirId, span: Span, abi: ExternAbi) {
+    if let Ok(abi) = extern_abi_stability(abi) {
+        if !tcx.sess.target.is_abi_supported(abi) {
+            tcx.node_span_lint(UNSUPPORTED_FN_PTR_CALLING_CONVENTIONS, hir_id, span, |lint| {
+                lint.primary_message(format!(
+                    "the calling convention {abi} is not supported on this target"
+                ));
+            });
+        }
     }
 }
 
@@ -779,7 +788,7 @@ pub(crate) fn check_item_type(tcx: TyCtxt<'_>, def_id: LocalDefId) {
             let hir::ItemKind::ForeignMod { abi, items } = it.kind else {
                 return;
             };
-            check_abi(tcx, it.span, abi);
+            gate_stable_abi(tcx, it.span, abi);
 
             for item in items {
                 let def_id = item.id.owner_id.def_id;

compiler/rustc_hir_analysis/src/check/mod.rs

@@ -72,7 +72,7 @@ pub mod wfcheck;
 
 use std::num::NonZero;
 
-pub use check::{check_abi, check_abi_fn_ptr};
+pub use check::{gate_stable_abi, gate_stable_fn_ptr_abi};
 use rustc_abi::{ExternAbi, VariantIdx};
 use rustc_data_structures::fx::{FxHashSet, FxIndexMap};
 use rustc_errors::{Diag, ErrorGuaranteed, pluralize, struct_span_code_err};

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

@@ -50,7 +50,7 @@ use rustc_trait_selection::traits::wf::object_region_bounds;
 use rustc_trait_selection::traits::{self, ObligationCtxt};
 use tracing::{debug, instrument};
 
-use crate::check::check_abi_fn_ptr;
+use crate::check::gate_stable_fn_ptr_abi;
 use crate::errors::{AmbiguousLifetimeBound, BadReturnTypeNotation};
 use crate::hir_ty_lowering::errors::{GenericsArgsErrExtend, prohibit_assoc_item_constraint};
 use crate::hir_ty_lowering::generics::{check_generic_arg_count, lower_generic_args};
@@ -2739,7 +2739,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         if let hir::Node::Ty(hir::Ty { kind: hir::TyKind::BareFn(bare_fn_ty), span, .. }) =
             tcx.hir_node(hir_id)
         {
-            check_abi_fn_ptr(tcx, hir_id, *span, bare_fn_ty.abi);
+            gate_stable_fn_ptr_abi(tcx, hir_id, *span, bare_fn_ty.abi);
         }
 
         // reject function types that violate cmse ABI requirements

compiler/rustc_hir_typeck/src/lib.rs

@@ -48,7 +48,7 @@ use rustc_errors::{Applicability, ErrorGuaranteed, pluralize, struct_span_code_e
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::{HirId, HirIdMap, Node};
-use rustc_hir_analysis::check::check_abi;
+use rustc_hir_analysis::check::gate_stable_abi;
 use rustc_hir_analysis::hir_ty_lowering::HirTyLowerer;
 use rustc_infer::traits::{ObligationCauseCode, ObligationInspector, WellFormedLoc};
 use rustc_middle::query::Providers;
@@ -149,7 +149,7 @@ fn typeck_with_inspect<'tcx>(
             tcx.fn_sig(def_id).instantiate_identity()
         };
 
-        check_abi(tcx, span, fn_sig.abi());
+        gate_stable_abi(tcx, span, fn_sig.abi());
 
         // Compute the function signature from point of view of inside the fn.
         let mut fn_sig = tcx.liberate_late_bound_regions(def_id.to_def_id(), fn_sig);