Codegen const panic messages as function calls

This skips emitting extra arguments at every callsite (of which there
can be many).

Author: Mark-Simulacrum

compiler/rustc_codegen_cranelift/src/base.rs

@@ -369,8 +369,14 @@ fn codegen_fn_body(fx: &mut FunctionCx<'_, '_, '_>, start_block: Block) {
                         );
                     }
                     _ => {
-                        let msg_str = msg.description();
-                        codegen_panic(fx, msg_str, source_info);
+                        let location = fx.get_caller_location(source_info).load_scalar(fx);
+
+                        codegen_panic_inner(
+                            fx,
+                            msg.description(),
+                            &[location],
+                            Some(source_info.span),
+                        );
                     }
                 }
             }
@@ -954,20 +960,6 @@ pub(crate) fn codegen_operand<'tcx>(
     }
 }
 
-pub(crate) fn codegen_panic<'tcx>(
-    fx: &mut FunctionCx<'_, '_, 'tcx>,
-    msg_str: &str,
-    source_info: mir::SourceInfo,
-) {
-    let location = fx.get_caller_location(source_info).load_scalar(fx);
-
-    let msg_ptr = fx.anonymous_str(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, location];
-
-    codegen_panic_inner(fx, rustc_hir::LangItem::Panic, &args, Some(source_info.span));
-}
-
 pub(crate) fn codegen_panic_nounwind<'tcx>(
     fx: &mut FunctionCx<'_, '_, 'tcx>,
     msg_str: &str,

compiler/rustc_codegen_ssa/src/mir/block.rs

@@ -651,10 +651,8 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                 (LangItem::PanicMisalignedPointerDereference, vec![required, found, location])
             }
             _ => {
-                let msg = bx.const_str(msg.description());
-                // It's `pub fn panic(expr: &str)`, with the wide reference being passed
-                // as two arguments, and `#[track_caller]` adds an implicit third argument.
-                (LangItem::Panic, vec![msg.0, msg.1, location])
+                // It's `pub fn panic_...()` and `#[track_caller]` adds an implicit argument.
+                (msg.description(), vec![location])
             }
         };
 

compiler/rustc_hir/src/lang_items.rs

@@ -246,6 +246,25 @@ language_item_table! {
     PanicImpl,               sym::panic_impl,          panic_impl,                 Target::Fn,             GenericRequirement::None;
     PanicCannotUnwind,       sym::panic_cannot_unwind, panic_cannot_unwind,        Target::Fn,             GenericRequirement::Exact(0);
     PanicInCleanup,          sym::panic_in_cleanup,    panic_in_cleanup,           Target::Fn,             GenericRequirement::Exact(0);
+    /// Constant panic messages, used for codegen of MIR asserts.
+    PanicAddOverflow,        sym::panic_const_add_overflow, panic_const_add_overflow, Target::Fn, GenericRequirement::None;
+    PanicSubOverflow,        sym::panic_const_sub_overflow, panic_const_sub_overflow, Target::Fn, GenericRequirement::None;
+    PanicMulOverflow,        sym::panic_const_mul_overflow, panic_const_mul_overflow, Target::Fn, GenericRequirement::None;
+    PanicDivOverflow,        sym::panic_const_div_overflow, panic_const_div_overflow, Target::Fn, GenericRequirement::None;
+    PanicRemOverflow,        sym::panic_const_rem_overflow, panic_const_rem_overflow, Target::Fn, GenericRequirement::None;
+    PanicNegOverflow,        sym::panic_const_neg_overflow, panic_const_neg_overflow, Target::Fn, GenericRequirement::None;
+    PanicShrOverflow,        sym::panic_const_shr_overflow, panic_const_shr_overflow, Target::Fn, GenericRequirement::None;
+    PanicShlOverflow,        sym::panic_const_shl_overflow, panic_const_shl_overflow, Target::Fn, GenericRequirement::None;
+    PanicDivZero,            sym::panic_const_div_by_zero, panic_const_div_by_zero, Target::Fn, GenericRequirement::None;
+    PanicRemZero,            sym::panic_const_rem_by_zero, panic_const_rem_by_zero, Target::Fn, GenericRequirement::None;
+    PanicCoroutineResumed, sym::panic_const_coroutine_resumed, panic_const_coroutine_resumed, Target::Fn, GenericRequirement::None;
+    PanicAsyncFnResumed, sym::panic_const_async_fn_resumed, panic_const_async_fn_resumed, Target::Fn, GenericRequirement::None;
+    PanicAsyncGenFnResumed, sym::panic_const_async_gen_fn_resumed, panic_const_async_gen_fn_resumed, Target::Fn, GenericRequirement::None;
+    PanicGenFnNone, sym::panic_const_gen_fn_none, panic_const_gen_fn_none, Target::Fn, GenericRequirement::None;
+    PanicCoroutineResumedPanic, sym::panic_const_coroutine_resumed_panic, panic_const_coroutine_resumed_panic, Target::Fn, GenericRequirement::None;
+    PanicAsyncFnResumedPanic, sym::panic_const_async_fn_resumed_panic, panic_const_async_fn_resumed_panic, Target::Fn, GenericRequirement::None;
+    PanicAsyncGenFnResumedPanic, sym::panic_const_async_gen_fn_resumed_panic, panic_const_async_gen_fn_resumed_panic, Target::Fn, GenericRequirement::None;
+    PanicGenFnNonePanic, sym::panic_const_gen_fn_none_panic, panic_const_gen_fn_none_panic, Target::Fn, GenericRequirement::None;
     /// libstd panic entry point. Necessary for const eval to be able to catch it
     BeginPanic,              sym::begin_panic,         begin_panic_fn,             Target::Fn,             GenericRequirement::None;
 

compiler/rustc_middle/src/mir/terminator.rs

@@ -149,44 +149,45 @@ impl<O> AssertKind<O> {
         matches!(self, OverflowNeg(..) | Overflow(Add | Sub | Mul | Shl | Shr, ..))
     }
 
-    /// Get the message that is printed at runtime when this assertion fails.
+    /// Get the lang item that is invoked to print a static message when this assert fires.
     ///
     /// The caller is expected to handle `BoundsCheck` and `MisalignedPointerDereference` by
     /// invoking the appropriate lang item (panic_bounds_check/panic_misaligned_pointer_dereference)
-    /// instead of printing a static message.
-    pub fn description(&self) -> &'static str {
+    /// instead of printing a static message. Those have dynamic arguments that aren't present for
+    /// the rest of the messages here.
+    pub fn description(&self) -> LangItem {
         use AssertKind::*;
         match self {
-            Overflow(BinOp::Add, _, _) => "attempt to add with overflow",
-            Overflow(BinOp::Sub, _, _) => "attempt to subtract with overflow",
-            Overflow(BinOp::Mul, _, _) => "attempt to multiply with overflow",
-            Overflow(BinOp::Div, _, _) => "attempt to divide with overflow",
-            Overflow(BinOp::Rem, _, _) => "attempt to calculate the remainder with overflow",
-            OverflowNeg(_) => "attempt to negate with overflow",
-            Overflow(BinOp::Shr, _, _) => "attempt to shift right with overflow",
-            Overflow(BinOp::Shl, _, _) => "attempt to shift left with overflow",
+            Overflow(BinOp::Add, _, _) => LangItem::PanicAddOverflow,
+            Overflow(BinOp::Sub, _, _) => LangItem::PanicSubOverflow,
+            Overflow(BinOp::Mul, _, _) => LangItem::PanicMulOverflow,
+            Overflow(BinOp::Div, _, _) => LangItem::PanicDivOverflow,
+            Overflow(BinOp::Rem, _, _) => LangItem::PanicRemOverflow,
+            OverflowNeg(_) => LangItem::PanicNegOverflow,
+            Overflow(BinOp::Shr, _, _) => LangItem::PanicShrOverflow,
+            Overflow(BinOp::Shl, _, _) => LangItem::PanicShlOverflow,
             Overflow(op, _, _) => bug!("{:?} cannot overflow", op),
-            DivisionByZero(_) => "attempt to divide by zero",
-            RemainderByZero(_) => "attempt to calculate the remainder with a divisor of zero",
-            ResumedAfterReturn(CoroutineKind::Coroutine(_)) => "coroutine resumed after completion",
+            DivisionByZero(_) => LangItem::PanicDivZero,
+            RemainderByZero(_) => LangItem::PanicRemZero,
+            ResumedAfterReturn(CoroutineKind::Coroutine(_)) => LangItem::PanicCoroutineResumed,
             ResumedAfterReturn(CoroutineKind::Desugared(CoroutineDesugaring::Async, _)) => {
-                "`async fn` resumed after completion"
+                LangItem::PanicAsyncFnResumed
             }
             ResumedAfterReturn(CoroutineKind::Desugared(CoroutineDesugaring::AsyncGen, _)) => {
-                "`async gen fn` resumed after completion"
+                LangItem::PanicAsyncGenFnResumed
             }
             ResumedAfterReturn(CoroutineKind::Desugared(CoroutineDesugaring::Gen, _)) => {
-                "`gen fn` should just keep returning `None` after completion"
+                LangItem::PanicGenFnNone
             }
-            ResumedAfterPanic(CoroutineKind::Coroutine(_)) => "coroutine resumed after panicking",
+            ResumedAfterPanic(CoroutineKind::Coroutine(_)) => LangItem::PanicCoroutineResumedPanic,
             ResumedAfterPanic(CoroutineKind::Desugared(CoroutineDesugaring::Async, _)) => {
-                "`async fn` resumed after panicking"
+                LangItem::PanicAsyncFnResumedPanic
             }
             ResumedAfterPanic(CoroutineKind::Desugared(CoroutineDesugaring::AsyncGen, _)) => {
-                "`async gen fn` resumed after panicking"
+                LangItem::PanicAsyncGenFnResumedPanic
             }
             ResumedAfterPanic(CoroutineKind::Desugared(CoroutineDesugaring::Gen, _)) => {
-                "`gen fn` should just keep returning `None` after panicking"
+                LangItem::PanicGenFnNonePanic
             }
 
             BoundsCheck { .. } | MisalignedPointerDereference { .. } => {
@@ -246,13 +247,37 @@ impl<O> AssertKind<O> {
             Overflow(BinOp::Shl, _, r) => {
                 write!(f, "\"attempt to shift left by `{{}}`, which would overflow\", {r:?}")
             }
+            Overflow(op, _, _) => bug!("{:?} cannot overflow", op),
             MisalignedPointerDereference { required, found } => {
                 write!(
                     f,
                     "\"misaligned pointer dereference: address must be a multiple of {{}} but is {{}}\", {required:?}, {found:?}"
                 )
             }
-            _ => write!(f, "\"{}\"", self.description()),
+            ResumedAfterReturn(CoroutineKind::Coroutine(_)) => {
+                write!(f, "\"coroutine resumed after completion\"")
+            }
+            ResumedAfterReturn(CoroutineKind::Desugared(CoroutineDesugaring::Async, _)) => {
+                write!(f, "\"`async fn` resumed after completion\"")
+            }
+            ResumedAfterReturn(CoroutineKind::Desugared(CoroutineDesugaring::AsyncGen, _)) => {
+                write!(f, "\"`async gen fn` resumed after completion\"")
+            }
+            ResumedAfterReturn(CoroutineKind::Desugared(CoroutineDesugaring::Gen, _)) => {
+                write!(f, "\"`gen fn` should just keep returning `None` after completion\"")
+            }
+            ResumedAfterPanic(CoroutineKind::Coroutine(_)) => {
+                write!(f, "\"coroutine resumed after panicking\"")
+            }
+            ResumedAfterPanic(CoroutineKind::Desugared(CoroutineDesugaring::Async, _)) => {
+                write!(f, "\"`async fn` resumed after panicking\"")
+            }
+            ResumedAfterPanic(CoroutineKind::Desugared(CoroutineDesugaring::AsyncGen, _)) => {
+                write!(f, "\"`async gen fn` resumed after panicking\"")
+            }
+            ResumedAfterPanic(CoroutineKind::Desugared(CoroutineDesugaring::Gen, _)) => {
+                write!(f, "\"`gen fn` should just keep returning `None` after panicking\"")
+            }
         }
     }
 

compiler/rustc_monomorphize/src/collector.rs

@@ -886,16 +886,17 @@ impl<'a, 'tcx> MirVisitor<'tcx> for MirUsedCollector<'a, 'tcx> {
                     }
                 }
             }
-            mir::TerminatorKind::Assert { ref msg, .. } => {
-                let lang_item = match &**msg {
-                    mir::AssertKind::BoundsCheck { .. } => LangItem::PanicBoundsCheck,
-                    mir::AssertKind::MisalignedPointerDereference { .. } => {
-                        LangItem::PanicMisalignedPointerDereference
-                    }
-                    _ => LangItem::Panic,
-                };
-                push_mono_lang_item(self, lang_item);
-            }
+            mir::TerminatorKind::Assert { ref msg, .. } => match &**msg {
+                mir::AssertKind::BoundsCheck { .. } => {
+                    push_mono_lang_item(self, LangItem::PanicBoundsCheck);
+                }
+                mir::AssertKind::MisalignedPointerDereference { .. } => {
+                    push_mono_lang_item(self, LangItem::PanicMisalignedPointerDereference);
+                }
+                _ => {
+                    push_mono_lang_item(self, msg.description());
+                }
+            },
             mir::TerminatorKind::UnwindTerminate(reason) => {
                 push_mono_lang_item(self, reason.lang_item());
             }

compiler/rustc_span/src/symbol.rs

@@ -1297,6 +1297,24 @@ symbols! {
         panic_abort,
         panic_bounds_check,
         panic_cannot_unwind,
+        panic_const_add_overflow,
+        panic_const_async_fn_resumed,
+        panic_const_async_fn_resumed_panic,
+        panic_const_async_gen_fn_resumed,
+        panic_const_async_gen_fn_resumed_panic,
+        panic_const_coroutine_resumed,
+        panic_const_coroutine_resumed_panic,
+        panic_const_div_by_zero,
+        panic_const_div_overflow,
+        panic_const_gen_fn_none,
+        panic_const_gen_fn_none_panic,
+        panic_const_mul_overflow,
+        panic_const_neg_overflow,
+        panic_const_rem_by_zero,
+        panic_const_rem_overflow,
+        panic_const_shl_overflow,
+        panic_const_shr_overflow,
+        panic_const_sub_overflow,
         panic_fmt,
         panic_handler,
         panic_impl,

library/core/src/panicking.rs

@@ -145,6 +145,57 @@ pub const fn panic(expr: &'static str) -> ! {
     panic_fmt(fmt::Arguments::new_const(&[expr]));
 }
 
+macro_rules! panic_const {
+    ($($lang:ident = $message:expr,)+) => {
+        #[cfg(not(bootstrap))]
+        pub mod panic_const {
+            use super::*;
+
+            $(
+                /// This is a panic called with a message that's a result of a MIR-produced Assert.
+                //
+                // never inline unless panic_immediate_abort to avoid code
+                // bloat at the call sites as much as possible
+                #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never), cold)]
+                #[cfg_attr(feature = "panic_immediate_abort", inline)]
+                #[track_caller]
+                #[rustc_const_unstable(feature = "panic_internals", issue = "none")]
+                #[lang = stringify!($lang)] // needed by codegen for panic on overflow and other `Assert` MIR terminators
+                pub const fn $lang() -> ! {
+                    // Use Arguments::new_v1 instead of format_args!("{expr}") to potentially
+                    // reduce size overhead. The format_args! macro uses str's Display trait to
+                    // write expr, which calls Formatter::pad, which must accommodate string
+                    // truncation and padding (even though none is used here). Using
+                    // Arguments::new_v1 may allow the compiler to omit Formatter::pad from the
+                    // output binary, saving up to a few kilobytes.
+                    panic_fmt(fmt::Arguments::new_const(&[$message]));
+                }
+            )+
+        }
+    }
+}
+
+panic_const! {
+    panic_const_add_overflow = "attempt to add with overflow",
+    panic_const_sub_overflow = "attempt to subtract with overflow",
+    panic_const_mul_overflow = "attempt to multiply with overflow",
+    panic_const_div_overflow = "attempt to divide with overflow",
+    panic_const_rem_overflow = "attempt to calculate the remainder with overflow",
+    panic_const_neg_overflow = "attempt to negate with overflow",
+    panic_const_shr_overflow = "attempt to shift right with overflow",
+    panic_const_shl_overflow = "attempt to shift left with overflow",
+    panic_const_div_by_zero = "attempt to divide by zero",
+    panic_const_rem_by_zero = "attempt to calculate the remainder with a divisor of zero",
+    panic_const_coroutine_resumed = "coroutine resumed after completion",
+    panic_const_async_fn_resumed = "`async fn` resumed after completion",
+    panic_const_async_gen_fn_resumed = "`async gen fn` resumed after completion",
+    panic_const_gen_fn_none = "`gen fn` should just keep returning `None` after completion",
+    panic_const_coroutine_resumed_panic = "coroutine resumed after panicking",
+    panic_const_async_fn_resumed_panic = "`async fn` resumed after panicking",
+    panic_const_async_gen_fn_resumed_panic = "`async gen fn` resumed after panicking",
+    panic_const_gen_fn_none_panic = "`gen fn` should just keep returning `None` after panicking",
+}
+
 /// Like `panic`, but without unwinding and track_caller to reduce the impact on codesize on the caller.
 /// If you want `#[track_caller]` for nicer errors, call `panic_nounwind_fmt` directly.
 #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never), cold)]

src/tools/miri/src/shims/panic.rs

@@ -256,8 +256,16 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
             }
 
             _ => {
-                // Forward everything else to `panic` lang item.
-                this.start_panic(msg.description(), unwind)?;
+                // Call the lang item associated with this message.
+                let fn_item = this.tcx.require_lang_item(msg.description(), None);
+                let instance = ty::Instance::mono(this.tcx.tcx, fn_item);
+                this.call_function(
+                    instance,
+                    Abi::Rust,
+                    &[],
+                    None,
+                    StackPopCleanup::Goto { ret: None, unwind },
+                )?;
             }
         }
         Ok(())