add rustc_abi debugging attribute

compiler/rustc_feature/src/builtin_attrs.rs

@@ -814,6 +814,7 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     rustc_attr!(TEST, rustc_strict_coherence, Normal, template!(Word), WarnFollowing),
     rustc_attr!(TEST, rustc_variance, Normal, template!(Word), WarnFollowing),
     rustc_attr!(TEST, rustc_layout, Normal, template!(List: "field1, field2, ..."), WarnFollowing),
+    rustc_attr!(TEST, rustc_abi, Normal, template!(List: "field1, field2, ..."), WarnFollowing),
     rustc_attr!(TEST, rustc_regions, Normal, template!(Word), WarnFollowing),
     rustc_attr!(
         TEST, rustc_error, Normal,

compiler/rustc_interface/src/passes.rs

@@ -22,7 +22,7 @@ use rustc_middle::query::{ExternProviders, Providers};
 use rustc_middle::ty::{self, GlobalCtxt, RegisteredTools, TyCtxt};
 use rustc_mir_build as mir_build;
 use rustc_parse::{parse_crate_from_file, parse_crate_from_source_str, validate_attr};
-use rustc_passes::{self, hir_stats, layout_test};
+use rustc_passes::{self, abi_test, hir_stats, layout_test};
 use rustc_plugin_impl as plugin;
 use rustc_resolve::Resolver;
 use rustc_session::code_stats::VTableSizeInfo;
@@ -818,6 +818,7 @@ fn analysis(tcx: TyCtxt<'_>, (): ()) -> Result<()> {
     }
 
     sess.time("layout_testing", || layout_test::test_layout(tcx));
+    sess.time("abi_testing", || abi_test::test_abi(tcx));
 
     // Avoid overwhelming user with errors if borrow checking failed.
     // I'm not sure how helpful this is, to be honest, but it avoids a

compiler/rustc_passes/messages.ftl

@@ -7,6 +7,9 @@
 passes_abi =
     abi: {$abi}
 
+passes_abi_of =
+    fn_abi_of_instance({$fn_name}) = {$fn_abi}
+
 passes_align =
     align: {$align}
 

compiler/rustc_passes/src/abi_test.rs

@@ -0,0 +1,93 @@
+use rustc_ast::Attribute;
+use rustc_hir::def::DefKind;
+use rustc_hir::def_id::DefId;
+use rustc_middle::ty::layout::{FnAbiError, LayoutError};
+use rustc_middle::ty::{self, GenericArgs, Instance, TyCtxt};
+use rustc_span::source_map::Spanned;
+use rustc_span::symbol::sym;
+
+use crate::errors::{AbiOf, UnrecognizedField};
+
+pub fn test_abi(tcx: TyCtxt<'_>) {
+    if !tcx.features().rustc_attrs {
+        // if the `rustc_attrs` feature is not enabled, don't bother testing ABI
+        return;
+    }
+    for id in tcx.hir().items() {
+        match tcx.def_kind(id.owner_id) {
+            DefKind::Fn => {
+                for attr in tcx.get_attrs(id.owner_id, sym::rustc_abi) {
+                    dump_abi_of(tcx, id.owner_id.def_id.into(), attr);
+                }
+            }
+            DefKind::Impl { .. } => {
+                // To find associated functions we need to go into the child items here.
+                for &id in tcx.associated_item_def_ids(id.owner_id) {
+                    if matches!(tcx.def_kind(id), DefKind::AssocFn) {
+                        for attr in tcx.get_attrs(id, sym::rustc_abi) {
+                            dump_abi_of(tcx, id, attr);
+                        }
+                    }
+                }
+            }
+            _ => {}
+        }
+    }
+}
+
+fn dump_abi_of(tcx: TyCtxt<'_>, item_def_id: DefId, attr: &Attribute) {
+    let param_env = tcx.param_env(item_def_id);
+    let args = GenericArgs::identity_for_item(tcx, item_def_id);
+    let instance = match Instance::resolve(tcx, param_env, item_def_id, args) {
+        Ok(Some(instance)) => instance,
+        Ok(None) => {
+            // Not sure what to do here, but `LayoutError::Unknown` seems reasonable?
+            let ty = tcx.type_of(item_def_id).instantiate_identity();
+            tcx.sess.emit_fatal(Spanned {
+                node: LayoutError::Unknown(ty).into_diagnostic(),
+
+                span: tcx.def_span(item_def_id),
+            });
+        }
+        Err(_guaranteed) => return,
+    };
+    match tcx.fn_abi_of_instance(param_env.and((instance, /* extra_args */ ty::List::empty()))) {
+        Ok(abi) => {
+            // Check out the `#[rustc_abi(..)]` attribute to tell what to dump.
+            // The `..` are the names of fields to dump.
+            let meta_items = attr.meta_item_list().unwrap_or_default();
+            for meta_item in meta_items {
+                match meta_item.name_or_empty() {
+                    sym::debug => {
+                        let fn_name = tcx.item_name(item_def_id);
+                        tcx.sess.emit_err(AbiOf {
+                            span: tcx.def_span(item_def_id),
+                            fn_name,
+                            fn_abi: format!("{:#?}", abi),
+                        });
+                    }
+
+                    name => {
+                        tcx.sess.emit_err(UnrecognizedField { span: meta_item.span(), name });
+                    }
+                }
+            }
+        }
+
+        Err(FnAbiError::Layout(layout_error)) => {
+            tcx.sess.emit_fatal(Spanned {
+                node: layout_error.into_diagnostic(),
+                span: tcx.def_span(item_def_id),
+            });
+        }
+        Err(FnAbiError::AdjustForForeignAbi(e)) => {
+            // Sadly there seems to be no `into_diagnostic` for this case... and I am not sure if
+            // this can even be reached. Anyway this is a perma-unstable debug attribute, an ICE
+            // isn't the worst thing. Also this matches what codegen does.
+            span_bug!(
+                tcx.def_span(item_def_id),
+                "error computing fn_abi_of_instance, cannot adjust for foreign ABI: {e:?}",
+            )
+        }
+    }
+}

compiler/rustc_passes/src/errors.rs

@@ -913,6 +913,15 @@ pub struct LayoutOf {
     pub ty_layout: String,
 }
 
+#[derive(Diagnostic)]
+#[diag(passes_abi_of)]
+pub struct AbiOf {
+    #[primary_span]
+    pub span: Span,
+    pub fn_name: Symbol,
+    pub fn_abi: String,
+}
+
 #[derive(Diagnostic)]
 #[diag(passes_unrecognized_field)]
 pub struct UnrecognizedField {

compiler/rustc_passes/src/layout_test.rs

@@ -11,16 +11,17 @@ use rustc_target::abi::{HasDataLayout, TargetDataLayout};
 use crate::errors::{Abi, Align, HomogeneousAggregate, LayoutOf, Size, UnrecognizedField};
 
 pub fn test_layout(tcx: TyCtxt<'_>) {
-    if tcx.features().rustc_attrs {
+    if !tcx.features().rustc_attrs {
         // if the `rustc_attrs` feature is not enabled, don't bother testing layout
-        for id in tcx.hir().items() {
-            if matches!(
-                tcx.def_kind(id.owner_id),
-                DefKind::TyAlias { .. } | DefKind::Enum | DefKind::Struct | DefKind::Union
-            ) {
-                for attr in tcx.get_attrs(id.owner_id, sym::rustc_layout) {
-                    dump_layout_of(tcx, id.owner_id.def_id, attr);
-                }
+        return;
+    }
+    for id in tcx.hir().items() {
+        if matches!(
+            tcx.def_kind(id.owner_id),
+            DefKind::TyAlias { .. } | DefKind::Enum | DefKind::Struct | DefKind::Union
+        ) {
+            for attr in tcx.get_attrs(id.owner_id, sym::rustc_layout) {
+                dump_layout_of(tcx, id.owner_id.def_id, attr);
             }
         }
     }

compiler/rustc_passes/src/lib.rs

@@ -24,6 +24,7 @@ use rustc_errors::{DiagnosticMessage, SubdiagnosticMessage};
 use rustc_fluent_macro::fluent_messages;
 use rustc_middle::query::Providers;
 
+pub mod abi_test;
 mod check_attr;
 mod check_const;
 pub mod dead;

compiler/rustc_span/src/symbol.rs

@@ -1281,6 +1281,7 @@ symbols! {
         rust_eh_catch_typeinfo,
         rust_eh_personality,
         rustc,
+        rustc_abi,
         rustc_allocator,
         rustc_allocator_zeroed,
         rustc_allow_const_fn_unstable,