Implement immediate-abort by hooking #[rustc_panic_entrypoint] and lint for functions that look like a panic entrypoint

compiler/rustc_attr_parsing/src/attributes/rustc_internal.rs

@@ -1256,6 +1256,15 @@ impl<S: Stage> SingleAttributeParser<S> for RustcDefPathParser {
     }
 }
 
+pub(crate) struct RustcPanicEntrypointParser;
+
+impl<S: Stage> NoArgsAttributeParser<S> for RustcPanicEntrypointParser {
+    const PATH: &[Symbol] = &[sym::rustc_panic_entrypoint];
+    const ON_DUPLICATE: OnDuplicate<S> = OnDuplicate::Error;
+    const ALLOWED_TARGETS: AllowedTargets = AllowedTargets::AllowList(&[Allow(Target::Fn)]);
+    const CREATE: fn(Span) -> AttributeKind = |_| AttributeKind::RustcPanicEntrypoint;
+}
+
 pub(crate) struct RustcStrictCoherenceParser;
 
 impl<S: Stage> NoArgsAttributeParser<S> for RustcStrictCoherenceParser {

compiler/rustc_attr_parsing/src/context.rs

@@ -311,6 +311,7 @@ attribute_parsers!(
         Single<WithoutArgs<RustcNonnullOptimizationGuaranteedParser>>,
         Single<WithoutArgs<RustcNounwindParser>>,
         Single<WithoutArgs<RustcOffloadKernelParser>>,
+        Single<WithoutArgs<RustcPanicEntrypointParser>>,
         Single<WithoutArgs<RustcParenSugarParser>>,
         Single<WithoutArgs<RustcPassByValueParser>>,
         Single<WithoutArgs<RustcPassIndirectlyInNonRusticAbisParser>>,

compiler/rustc_codegen_cranelift/src/base.rs

@@ -14,6 +14,7 @@ use rustc_middle::ty::layout::{FnAbiOf, HasTypingEnv as _};
 use rustc_middle::ty::print::with_no_trimmed_paths;
 use rustc_session::config::OutputFilenames;
 use rustc_span::Symbol;
+use rustc_target::spec::PanicStrategy;
 
 use crate::constant::ConstantCx;
 use crate::debuginfo::{FunctionDebugContext, TypeDebugContext};
@@ -384,6 +385,11 @@ fn codegen_fn_body(fx: &mut FunctionCx<'_, '_, '_>, start_block: Block) {
                 fx.bcx.switch_to_block(failure);
                 fx.bcx.ins().nop();
 
+                if fx.tcx.sess.panic_strategy() == PanicStrategy::ImmediateAbort {
+                    fx.bcx.ins().trap(TrapCode::user(1 /* unreachable */).unwrap());
+                    continue;
+                }
+
                 match &**msg {
                     AssertKind::BoundsCheck { len, index } => {
                         let len = codegen_operand(fx, len).load_scalar(fx);
@@ -1058,6 +1064,10 @@ pub(crate) fn codegen_panic_nounwind<'tcx>(
     msg_str: &str,
     span: Span,
 ) {
+    if fx.tcx.sess.panic_strategy() == PanicStrategy::ImmediateAbort {
+        fx.bcx.ins().trap(TrapCode::user(1 /* unreachable */).unwrap());
+    }
+
     let msg_ptr = crate::constant::pointer_for_anonymous_str(fx, msg_str);
     let msg_len = fx.bcx.ins().iconst(fx.pointer_type, i64::try_from(msg_str.len()).unwrap());
     let args = [msg_ptr, msg_len];

compiler/rustc_codegen_ssa/src/codegen_attrs.rs

@@ -1,6 +1,6 @@
 use rustc_abi::{Align, ExternAbi};
 use rustc_hir::attrs::{
-    AttributeKind, EiiImplResolution, InlineAttr, Linkage, RtsanSetting, UsedBy,
+    AttributeKind, EiiImplResolution, InlineAttr, Linkage, OptimizeAttr, RtsanSetting, UsedBy,
 };
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::{DefId, LOCAL_CRATE, LocalDefId};
@@ -15,7 +15,7 @@ use rustc_middle::ty::{self as ty, TyCtxt};
 use rustc_session::lint;
 use rustc_session::parse::feature_err;
 use rustc_span::{Span, sym};
-use rustc_target::spec::Os;
+use rustc_target::spec::{Os, PanicStrategy};
 
 use crate::errors;
 use crate::target_features::{
@@ -292,6 +292,9 @@ fn process_builtin_attrs(
                 codegen_fn_attrs.patchable_function_entry =
                     Some(PatchableFunctionEntry::from_prefix_and_entry(*prefix, *entry));
             }
+            AttributeKind::RustcPanicEntrypoint => {
+                codegen_fn_attrs.flags |= CodegenFnAttrFlags::PANIC_ENTRYPOINT
+            }
             _ => {}
         }
     }
@@ -384,6 +387,28 @@ fn apply_overrides(tcx: TyCtxt<'_>, did: LocalDefId, codegen_fn_attrs: &mut Code
             codegen_fn_attrs.flags |= CodegenFnAttrFlags::NO_MANGLE;
         }
     }
+
+    if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::PANIC_ENTRYPOINT) {
+        // Panic entrypoints are always cold.
+        //
+        // If we have immediate-abort enabled, we want panic entrypoints to be inlined. We have a
+        // MIR transform that tries to patch out immediate-abort panic entrypoint calls, but it can
+        // miss indirect calls. In such cases, this should encourage the optimizer to clean up the
+        // mess that we missed.
+        //
+        // When the panic strategies that support panic messages are enabled, we want panic
+        // entrypoints outlined and optimized for size.
+        // Most panic entrypoints want #[track_caller] but not all, so we do not add it.
+        // FIXME: It seems unlikely that all choices of #[track_caller] were deliberate, but it can
+        // have a code size impact, so it makes sense to apply judiciously.
+        codegen_fn_attrs.flags |= CodegenFnAttrFlags::COLD;
+        if tcx.sess.panic_strategy() == PanicStrategy::ImmediateAbort {
+            codegen_fn_attrs.inline = InlineAttr::Always;
+        } else {
+            codegen_fn_attrs.inline = InlineAttr::Never;
+            codegen_fn_attrs.optimize = OptimizeAttr::Size;
+        }
+    }
 }
 
 #[derive(Diagnostic)]

compiler/rustc_codegen_ssa/src/mir/block.rs

@@ -14,6 +14,7 @@ use rustc_middle::{bug, span_bug};
 use rustc_session::config::OptLevel;
 use rustc_span::{Span, Spanned};
 use rustc_target::callconv::{ArgAbi, ArgAttributes, CastTarget, FnAbi, PassMode};
+use rustc_target::spec::PanicStrategy;
 use tracing::{debug, info};
 
 use super::operand::OperandRef;
@@ -175,22 +176,22 @@ impl<'a, 'tcx> TerminatorCodegenHelper<'tcx> {
         if let Some(instance) = instance
             && is_call_from_compiler_builtins_to_upstream_monomorphization(tcx, instance)
         {
-            if destination.is_some() {
+            if instance.def.is_panic_entrypoint(tcx) {
+                info!(
+                    "compiler_builtins call to panic entrypoint {:?} replaced with abort",
+                    instance.def_id()
+                );
+                bx.abort();
+                bx.unreachable();
+                return MergingSucc::False;
+            } else {
                 let caller_def = fx.instance.def_id();
                 let e = CompilerBuiltinsCannotCall {
                     span: tcx.def_span(caller_def),
                     caller: with_no_trimmed_paths!(tcx.def_path_str(caller_def)),
                     callee: with_no_trimmed_paths!(tcx.def_path_str(instance.def_id())),
                 };
                 tcx.dcx().emit_err(e);
-            } else {
-                info!(
-                    "compiler_builtins call to diverging function {:?} replaced with abort",
-                    instance.def_id()
-                );
-                bx.abort();
-                bx.unreachable();
-                return MergingSucc::False;
             }
         }
 
@@ -735,6 +736,12 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         bx.switch_to_block(panic_block);
         self.set_debug_loc(bx, terminator.source_info);
 
+        if bx.tcx().sess.panic_strategy() == PanicStrategy::ImmediateAbort {
+            bx.abort();
+            bx.unreachable();
+            return MergingSucc::False;
+        }
+
         // Get the location information.
         let location = self.get_caller_location(bx, terminator.source_info).immediate();
 

compiler/rustc_feature/src/builtin_attrs.rs

@@ -1323,6 +1323,11 @@ pub static BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         "`#[rustc_non_const_trait_method]` should only used by the standard library to mark trait methods \
         as non-const to allow large traits an easier transition to const"
     ),
+    rustc_attr!(
+        rustc_panic_entrypoint, AttributeType::Normal, template!(Word),
+        WarnFollowing, EncodeCrossCrate::Yes,
+        "`#[rustc_panic_entrypoint]` makes this function patchable by panic=immediate-abort",
+    ),
 
     BuiltinAttribute {
         name: sym::rustc_diagnostic_item,

compiler/rustc_hir/src/attrs/data_structures.rs

@@ -1508,6 +1508,9 @@ pub enum AttributeKind {
     /// Represents `#[rustc_offload_kernel]`
     RustcOffloadKernel,
 
+    /// Represents `#[rustc_panic_entrypoint]`
+    RustcPanicEntrypoint,
+
     /// Represents `#[rustc_paren_sugar]`.
     RustcParenSugar(Span),
 

compiler/rustc_hir/src/attrs/encode_cross_crate.rs

@@ -166,6 +166,7 @@ impl AttributeKind {
             RustcObjcClass { .. } => No,
             RustcObjcSelector { .. } => No,
             RustcOffloadKernel => Yes,
+            RustcPanicEntrypoint => No,
             RustcParenSugar(..) => No,
             RustcPassByValue(..) => Yes,
             RustcPassIndirectlyInNonRusticAbis(..) => No,

compiler/rustc_hir/src/lang_items.rs

@@ -235,6 +235,7 @@ language_item_table! {
     FnMut,                   sym::fn_mut,              fn_mut_trait,               Target::Trait,          GenericRequirement::Exact(1);
     FnOnce,                  sym::fn_once,             fn_once_trait,              Target::Trait,          GenericRequirement::Exact(1);
 
+    AbortIntrinsic,          sym::abort_intrinsic,     abort_intrinsic,            Target::Fn,             GenericRequirement::Exact(0);
     AsyncFn,                 sym::async_fn,            async_fn_trait,             Target::Trait,          GenericRequirement::Exact(1);
     AsyncFnMut,              sym::async_fn_mut,        async_fn_mut_trait,         Target::Trait,          GenericRequirement::Exact(1);
     AsyncFnOnce,             sym::async_fn_once,       async_fn_once_trait,        Target::Trait,          GenericRequirement::Exact(1);

compiler/rustc_lint/src/internal.rs

@@ -4,11 +4,14 @@
 use rustc_ast as ast;
 use rustc_ast::{Pat, PatKind, Path};
 use rustc_hir as hir;
+use rustc_hir::attrs::{AttributeKind, InlineAttr};
 use rustc_hir::def::Res;
 use rustc_hir::def_id::DefId;
-use rustc_hir::{Expr, ExprKind, HirId, find_attr};
+use rustc_hir::intravisit::FnKind;
+use rustc_hir::{Body, Expr, ExprKind, FnDecl, HirId, find_attr};
 use rustc_middle::ty::{self, GenericArgsRef, PredicatePolarity};
 use rustc_session::{declare_lint_pass, declare_tool_lint};
+use rustc_span::def_id::LocalDefId;
 use rustc_span::hygiene::{ExpnKind, MacroKind};
 use rustc_span::{Span, sym};
 
@@ -713,3 +716,69 @@ impl EarlyLintPass for BadUseOfFindAttr {
         }
     }
 }
+
+declare_tool_lint! {
+    /// The `missing_panic_entrypoint` lint detects forgotten use of #[rustc_panic_entrypoint].
+    ///
+    /// This lint is intended to ensure that panic=immediate-abort can function as designed,
+    /// because it uses #[rustc_panic_entrypoint] to locate functions that should be outlined
+    /// for other panic modes, and be deleted entirely when immediate-abort is enabled.
+    ///
+    /// The current design of this lint is expected to have a lot of false positives outside of core
+    /// and alloc.
+    pub rustc::MISSING_PANIC_ENTRYPOINT,
+    Allow,
+    "detects missing #[rustc_panic_entrypoint]",
+    report_in_external_macro: true
+}
+
+declare_lint_pass!(MissingPanicEntrypoint => [MISSING_PANIC_ENTRYPOINT]);
+
+fn has_panic_entrypoint(attrs: &[hir::Attribute]) -> bool {
+    attrs
+        .iter()
+        .any(|attr| matches!(attr, hir::Attribute::Parsed(AttributeKind::RustcPanicEntrypoint)))
+}
+
+fn has_inline_encouragement(attrs: &[hir::Attribute]) -> bool {
+    attrs.iter().any(|attr| {
+        matches!(
+            attr,
+            hir::Attribute::Parsed(hir::attrs::AttributeKind::Inline(
+                InlineAttr::Hint | InlineAttr::Always | InlineAttr::Force { .. },
+                _
+            ))
+        )
+    })
+}
+
+fn has_rustc_intrinsic(attrs: &[hir::Attribute]) -> bool {
+    attrs.iter().any(|attr| matches!(attr, hir::Attribute::Parsed(AttributeKind::RustcIntrinsic)))
+}
+
+impl<'tcx> LateLintPass<'tcx> for MissingPanicEntrypoint {
+    fn check_fn(
+        &mut self,
+        cx: &LateContext<'tcx>,
+        _: FnKind<'tcx>,
+        fn_decl: &'tcx FnDecl<'tcx>,
+        _: &'tcx Body<'tcx>,
+        span: Span,
+        def_id: LocalDefId,
+    ) {
+        if matches!(fn_decl.output, hir::FnRetTy::Return(hir::Ty { kind: hir::TyKind::Never, .. }))
+        {
+            let attrs = cx.tcx.hir_attrs(cx.tcx.local_def_id_to_hir_id(def_id));
+            if has_rustc_intrinsic(attrs) {
+                return;
+            }
+            if !has_inline_encouragement(attrs) && !has_panic_entrypoint(attrs) {
+                cx.emit_span_lint(
+                    MISSING_PANIC_ENTRYPOINT,
+                    span,
+                    crate::lints::MissingPanicEntrypoint,
+                );
+            }
+        }
+    }
+}

compiler/rustc_lint/src/lib.rs

@@ -667,6 +667,8 @@ fn register_internals(store: &mut LintStore) {
     store.register_early_pass(|| Box::new(ImplicitSysrootCrateImport));
     store.register_lints(&BadUseOfFindAttr::lint_vec());
     store.register_early_pass(|| Box::new(BadUseOfFindAttr));
+    store.register_lints(&MissingPanicEntrypoint::lint_vec());
+    store.register_late_mod_pass(|_| Box::new(MissingPanicEntrypoint));
     store.register_group(
         false,
         "rustc::internal",

compiler/rustc_lint/src/lints.rs

@@ -3973,3 +3973,7 @@ pub(crate) struct MalformedOnConstAttrLint {
 #[diag("`Eq::assert_receiver_is_total_eq` should never be implemented by hand")]
 #[note("this method was used to add checks to the `Eq` derive macro")]
 pub(crate) struct EqInternalMethodImplemented;
+
+#[derive(Diagnostic)]
+#[diag("this function could have the attribute #[rustc_panic_entrypoint]")]
+pub(crate) struct MissingPanicEntrypoint;

compiler/rustc_middle/src/middle/codegen_fn_attrs.rs

@@ -210,6 +210,7 @@ bitflags::bitflags! {
         /// As such, we must make sure these symbols really do exist in the final binary/library.
         /// This flag is put on both the implementations of EIIs and the foreign item they implement.
         const EXTERNALLY_IMPLEMENTABLE_ITEM = 1 << 18;
+        const PANIC_ENTRYPOINT = 1 << 19;
     }
 }
 rustc_data_structures::external_bitflags_debug! { CodegenFnAttrFlags }

compiler/rustc_middle/src/ty/instance.rs

@@ -343,6 +343,10 @@ impl<'tcx> InstanceKind<'tcx> {
             | InstanceKind::VTableShim(..) => true,
         }
     }
+
+    pub fn is_panic_entrypoint(&self, tcx: TyCtxt<'_>) -> bool {
+        tcx.codegen_instance_attrs(*self).flags.contains(CodegenFnAttrFlags::PANIC_ENTRYPOINT)
+    }
 }
 
 fn type_length<'tcx>(item: impl TypeVisitable<TyCtxt<'tcx>>) -> usize {

compiler/rustc_mir_transform/src/lib.rs

@@ -177,6 +177,7 @@ declare_passes! {
     mod remove_unneeded_drops : RemoveUnneededDrops;
     mod remove_zsts : RemoveZsts;
     mod required_consts : RequiredConstsVisitor;
+    mod panic_entrypoints : PanicEntrypoints;
     mod post_analysis_normalize : PostAnalysisNormalize;
     mod sanity_check : SanityCheck;
     // This pass is public to allow external drivers to perform MIR cleanup
@@ -705,7 +706,7 @@ pub(crate) fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'
             &check_null::CheckNull,
             &check_enums::CheckEnums,
             // Before inlining: trim down MIR with passes to reduce inlining work.
-
+            &panic_entrypoints::PanicEntrypoints,
             // Has to be done before inlining, otherwise actual call will be almost always inlined.
             // Also simple, so can just do first.
             &lower_slice_len::LowerSliceLenCalls,