Can show operands at runtime when 'attempt to multiply with overflow' panic ?

Is it possible to show the two operands at runtime when this panic occurs ?
thread 'main' panicked at 'attempt to multiply with overflow', src/main.rs:7
So maybe we can see 2*122 in the panic message?

example code

code with backtrace

output

  Compiling playground v0.0.1 (file:///playground)
    Finished dev [unoptimized + debuginfo] target(s) in 0.62 secs
     Running `target/debug/playground`
thread 'main' panicked at 'attempt to multiply with overflow', src/main.rs:9:20
stack backtrace:
   0: std::sys::imp::backtrace::tracing::imp::unwind_backtrace
             at /checkout/src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::sys_common::backtrace::_print
             at /checkout/src/libstd/sys_common/backtrace.rs:71
   2: std::panicking::default_hook::{{closure}}
             at /checkout/src/libstd/sys_common/backtrace.rs:60
             at /checkout/src/libstd/panicking.rs:380
   3: std::panicking::default_hook
             at /checkout/src/libstd/panicking.rs:396
   4: std::panicking::rust_panic_with_hook
             at /checkout/src/libstd/panicking.rs:610
   5: std::panicking::begin_panic
             at /checkout/src/libstd/panicking.rs:571
   6: std::panicking::begin_panic_fmt
             at /checkout/src/libstd/panicking.rs:521
   7: rust_begin_unwind
             at /checkout/src/libstd/panicking.rs:497
   8: core::panicking::panic_fmt
             at /checkout/src/libcore/panicking.rs:71
   9: core::panicking::panic
             at /checkout/src/libcore/panicking.rs:51
  10: playground::main
             at src/main.rs:9
  11: __rust_maybe_catch_panic
             at /checkout/src/libpanic_unwind/lib.rs:98
  12: std::rt::lang_start
             at /checkout/src/libstd/panicking.rs:458
             at /checkout/src/libstd/panic.rs:361
             at /checkout/src/libstd/rt.rs:61
  13: main
  14: __libc_start_main
  15: _start

---

EDIT: what I tried and had no effect whatsoever:

diff --git a/src/librustc_const_math/int.rs b/src/librustc_const_math/int.rs
index 65471416e8..1c05352aec 100644
--- a/src/librustc_const_math/int.rs
+++ b/src/librustc_const_math/int.rs
@@ -329,7 +329,10 @@ impl ::std::fmt::Display for ConstInt {
 macro_rules! overflowing {
     ($e:expr, $err:expr) => {{
         if $e.1 {
-            return Err(Overflow($err));
+            return {
+                eprintln!("!!!moo3:{:?}!{:?}!",$e,$err);
+                Err(Overflow($err))
+            }
         } else {
             $e.0
         }
@@ -341,6 +344,7 @@ macro_rules! impl_binop {
         impl ::std::ops::$op for ConstInt {
             type Output = Result<Self, ConstMathErr>;
             fn $func(self, rhs: Self) -> Result<Self, ConstMathErr> {
+                eprintln!("!!!moo4:{:?}!{:?}!",self,rhs);
                 match (self, rhs) {
                     (I8(a), I8(b)) => a.$checked_func(b).map(I8),
                     (I16(a), I16(b)) => a.$checked_func(b).map(I16),
@@ -428,7 +432,10 @@ fn check_division(
         (Usize(_), Usize(Us32(0))) => Err(zerr),
         (Usize(_), Usize(Us64(0))) => Err(zerr),
 
-        (I8(::std::i8::MIN), I8(-1)) => Err(Overflow(op)),
+        (I8(::std::i8::MIN), I8(-1)) => {
+            eprintln!("!!!moo2{:?}!",op);
+            Err(Overflow(op))
+        },
         (I16(::std::i16::MIN), I16(-1)) => Err(Overflow(op)),
         (I32(::std::i32::MIN), I32(-1)) => Err(Overflow(op)),
         (I64(::std::i64::MIN), I64(-1)) => Err(Overflow(op)),
diff --git a/src/librustc_mir/build/expr/as_rvalue.rs b/src/librustc_mir/build/expr/as_rvalue.rs
index 2512291f1a..f8fd8315c5 100644
--- a/src/librustc_mir/build/expr/as_rvalue.rs
+++ b/src/librustc_mir/build/expr/as_rvalue.rs
@@ -285,6 +285,7 @@ impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
                     bug!("MIR build_binary_op: {:?} is not checkable", op)
                 }
             });
+            eprintln!("!!!moo{:?}!",err);
 
             block = self.assert(block, Operand::Consume(of), false,
                                 AssertMessage::Math(err), span);

diff --git a/src/librustc_trans/mir/constant.rs b/src/librustc_trans/mir/constant.rs
index 53469689bc..783aea003d 100644
--- a/src/librustc_trans/mir/constant.rs
+++ b/src/librustc_trans/mir/constant.rs
@@ -932,6 +932,7 @@ pub fn const_scalar_checked_binop<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
                 bug!("Operator `{:?}` is not a checkable operator", op)
             }
         };
+        eprintln!("!!!moo5:{:?}!", result);
 
         let of = match result {
             Ok(_) => false,

[TimNN]

Regarding your patch: rustc's const evaluator has nothing whatsoever to do with the runtime panic message: Instead, when in debug mode, rustc will insert overflow checks for every multiplication / addition / subtraction.

Thanks @TimNN ! Any idea where to look for the code that inserts overflow checks?

So I tried this:

            CheckedBinaryOp(bin_op, ref left, ref right) => {
                // Due to the extra boolean in the result, we can never reuse the `dest.layout`.
                let left = self.read_immediate(self.eval_operand(left, None)?)?;
                let layout = if binop_right_homogeneous(bin_op) { Some(left.layout) } else { None };
                let right = self.read_immediate(self.eval_operand(right, layout)?)?;
                self.binop_with_overflow(                                       
                    bin_op,
                    left,
                    right,
                    dest,
                )?;
                eprintln!("!{:?}!{:?}!{:?}!{:?}!", left,bin_op,right,dest);
            }

in interpret/step.rs

but I see no output from that eprintln! with this:

fn main() {
    //let innum: i8 = 122;
    let in_num:i8=122;

    let a = 2_i8.checked_mul(in_num);
    let b= 2_i8.overflowing_mul(in_num);
    println!("{:?} / {:?}", a, b);
    println!("{:?}", 2*in_num);
    let c:i8=2*in_num;
}

(used stage1 compiler, meanwhile rustc (stage2?) is still compiling)
there are output from that eprintln! while it's still compiling, one example for !Mul!:

!ImmTy { immediate: Scalar(Scalar(Bits { size: 8, bits: 16 })), layout: TyLayout { ty: usize, details: LayoutDetails { variants: Single { index: 0 }, fields: Union(0), abi: Scalar(Scalar { value: Int(I64, false), valid_range: 0..=18446744073709551615 }), align: AbiAndPrefAlign { abi: Align { pow2: 3 }, pref: Align { pow2: 3 } }, size: Size { raw: 8 } } } }!Mul!ImmTy { immediate: Scalar(Scalar(Bits { size: 8, bits: 16 })), layout: TyLayout { ty: usize, details: LayoutDetails { variants: Single { index: 0 }, fields: Union(0), abi: Scalar(Scalar { value: Int(I64, false), valid_range: 0..=18446744073709551615 }), align: AbiAndPrefAlign { abi: Align { pow2: 3 }, pref: Align { pow2: 3 } }, size: Size { raw: 8 } } } }!PlaceTy { place: Local { frame: 0, local: _1 }, layout: TyLayout { ty: (usize, bool), details: LayoutDetails { variants: Single { index: 0 }, fields: Arbitrary { offsets: [Size { raw: 0 }, Size { raw: 8 }], memory_index: [0, 1] }, abi: ScalarPair(Scalar { value: Int(I64, false), valid_range: 0..=18446744073709551615 }, Scalar { value: Int(I8, false), valid_range: 0..=1 }), align: AbiAndPrefAlign { abi: Align { pow2: 3 }, pref: Align { pow2: 3 } }, size: Size { raw: 16 } } } }!

but the above would've shown me a true/false somewhere in dest(likely this Scalar { value: Int(I8, false), valid_range: 0..=1 }) seen above) as per this:

    /// Applies the binary operation `op` to the two operands and writes a tuple of the result
    /// and a boolean signifying the potential overflow to the destination.
    pub fn binop_with_overflow(
        &mut self,
        op: mir::BinOp,
        left: ImmTy<'tcx, M::PointerTag>,
        right: ImmTy<'tcx, M::PointerTag>,
        dest: PlaceTy<'tcx, M::PointerTag>,
    ) -> EvalResult<'tcx> {
        let (val, overflowed) = self.binary_op_imm(op, left, right)?;
        let val = Immediate::ScalarPair(val.into(), Scalar::from_bool(overflowed).into());
        self.write_immediate(val, dest)
    }

I'm still not sure exactly where, since I don't understand most of what's going on :D

But there are still more places to look:

/home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust 
$ grep -nrIFw CheckedBinaryOp
src/librustc/ich/impls_mir.rs:347:            mir::Rvalue::CheckedBinaryOp(op, ref operand1, ref operand2) => {
src/librustc/mir/tcx.rs:238:            Rvalue::CheckedBinaryOp(op, ref lhs, ref rhs) => {
src/librustc/mir/mod.rs:2191:    CheckedBinaryOp(BinOp, Operand<'tcx>, Operand<'tcx>),
src/librustc/mir/mod.rs:2330:            CheckedBinaryOp(ref op, ref a, ref b) => {
src/librustc/mir/mod.rs:3318:            CheckedBinaryOp(op, ref rhs, ref lhs) => {
src/librustc/mir/mod.rs:3319:                CheckedBinaryOp(op, rhs.fold_with(folder), lhs.fold_with(folder))
src/librustc/mir/mod.rs:3355:            BinaryOp(_, ref rhs, ref lhs) | CheckedBinaryOp(_, ref rhs, ref lhs) => {
src/librustc/mir/visit.rs:626:                    Rvalue::CheckedBinaryOp(_bin_op,
src/librustc_mir/borrow_check/nll/type_check/mod.rs:2005:            | Rvalue::CheckedBinaryOp(..)
src/librustc_mir/borrow_check/nll/type_check/mod.rs:2022:            | Rvalue::CheckedBinaryOp(..)
src/librustc_mir/borrow_check/nll/invalidation.rs:371:            | Rvalue::CheckedBinaryOp(_bin_op, ref operand1, ref operand2) => {
src/librustc_mir/borrow_check/mod.rs:1206:            | Rvalue::CheckedBinaryOp(_bin_op, ref operand1, ref operand2) => {
src/librustc_mir/build/expr/as_rvalue.rs:416:                Rvalue::CheckedBinaryOp(op, lhs, rhs),
src/librustc_mir/dataflow/move_paths/builder.rs:309:            Rvalue::CheckedBinaryOp(ref _binop, ref lhs, ref rhs) => {
src/librustc_mir/interpret/step.rs:163:            CheckedBinaryOp(bin_op, ref left, ref right) => {
src/librustc_mir/transform/lower_128bit.rs:75:                            | Rvalue::CheckedBinaryOp(_, lhs, rhs) => (place, lhs, rhs),
src/librustc_mir/transform/lower_128bit.rs:157:        StatementKind::Assign(_, box Rvalue::CheckedBinaryOp(bin_op, ref lhs, _)) => {
src/librustc_mir/transform/const_prop.rs:399:            Rvalue::CheckedBinaryOp(op, ref left, ref right) |
src/librustc_mir/transform/const_prop.rs:454:                let val = if let Rvalue::CheckedBinaryOp(..) = *rvalue {
src/librustc_mir/transform/qualify_consts.rs:586:            Rvalue::CheckedBinaryOp(..) |
src/librustc_mir/transform/qualify_min_const_fn.rs:172:        Rvalue::BinaryOp(_, lhs, rhs) | Rvalue::CheckedBinaryOp(_, lhs, rhs) => {
src/librustc_codegen_ssa/mir/rvalue.rs:435:            mir::Rvalue::CheckedBinaryOp(op, ref lhs, ref rhs) => {
src/librustc_codegen_ssa/mir/rvalue.rs:716:            mir::Rvalue::CheckedBinaryOp(..) |

I put an eprintln! and got no output:

    /// Applies the binary operation `op` to the two operands and writes a tuple of the result
    /// and a boolean signifying the potential overflow to the destination.
    pub fn binop_with_overflow(
        &mut self,
        op: mir::BinOp,
        left: ImmTy<'tcx, M::PointerTag>,
        right: ImmTy<'tcx, M::PointerTag>,
        dest: PlaceTy<'tcx, M::PointerTag>,
    ) -> EvalResult<'tcx> {
        let (val, overflowed) = self.binary_op_imm(op, left, right)?;
        let val = Immediate::ScalarPair(val.into(), Scalar::from_bool(overflowed).into());
        eprintln!("{:?}",val);                                                  
        self.write_immediate(val, dest)
    }

in interpret/operator.rs
maybe i'm looking in the wrong place!

ok, i might be on to something, with this patch:

diff --git a/src/librustc/mir/interpret/error.rs b/src/librustc/mir/interpret/error.rs
index c3fe5d773a..f8c87f3393 100644
--- a/src/librustc/mir/interpret/error.rs
+++ b/src/librustc/mir/interpret/error.rs
@@ -413,7 +413,25 @@ impl<'tcx, O> EvalErrorKind<'tcx, O> {
                 "referenced constant has errors",
             Overflow(mir::BinOp::Add) => "attempt to add with overflow",
             Overflow(mir::BinOp::Sub) => "attempt to subtract with overflow",
-            Overflow(mir::BinOp::Mul) => "attempt to multiply with overflow",
+            Overflow(mir::BinOp::Mul) => {
+                //sys_common::backtrace::print(, PrintFormat::Short);
+                //extern crate sys;//HOW to?!
+                //use sys::backtrace;
+                //extern crate backtrace;
+                //use backtrace;
+                //#[cfg(feature = "backtrace")] //this is never true here!
+                {
+                    //extern crate sys_common;
+                    //use sys_common::rwlock::RWLock;
+                    //use sys_common::thread_info;
+                    //use sys_common::util;
+                    //use sys_common::backtrace;
+                    use backtrace::Backtrace;
+                    eprintln!("{:?}", Backtrace::new()); //thanks to WindowsBunny on irc for the idea!
+                }
+                "wuuuuuuattttttt??!attempt to multiply with overflow"
+            }
+                ,
             Overflow(mir::BinOp::Div) => "attempt to divide with overflow",
             Overflow(mir::BinOp::Rem) => "attempt to calculate the remainder with overflow",
             OverflowNeg => "attempt to negate with overflow",

I'm getting the stacktrace during compile-only, for this program:

fn main() {
//    let in_num:i32=1289346750; // 4CD9DEBE
//    println!("{:?}", 2*in_num);
    //let innum: i8 = 122;
    let in_num:i8=122;

    //let a = 2_i8.checked_mul(in_num);
    let a=0;
    //let b= 2_i8.overflowing_mul(in_num);
    let b=0;
    println!("{:?} / {:?}", a, b);
    let _c:i8=2*in_num;
    //println!("{:?}", 2*in_num);
}

looks like this:

/tmp/tr 
$ time /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/build/x86_64-unknown-linux-gnu/stage1/bin/rustc --edition=2018 -C debuginfo=2 /tmp/tr/src/main.rs -v && RUST_LOG=trace RUST_BACKTRACE=1 ./main 
stack backtrace:
   0: <rustc::mir::interpret::error::EvalErrorKind<'tcx, O>>::description::h93cabbaeb77cf8e9 (0x7ff4add1b014)
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/librustc/mir/interpret/error.rs:430
   1: rustc_codegen_ssa::mir::block::<impl rustc_codegen_ssa::mir::FunctionCx<'a, 'tcx, Bx>>::codegen_terminator::hf5867a49a7c0092c (0x7ff4ade85850)
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/librustc_codegen_ssa/mir/block.rs:396
   2: rustc_codegen_ssa::mir::block::<impl rustc_codegen_ssa::mir::FunctionCx<'a, 'tcx, Bx>>::codegen_block::hd71f5a610ab19665 (0x7ff4ade821fe)
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/librustc_codegen_ssa/mir/block.rs:38
      rustc_codegen_ssa::mir::codegen_mir::h97380ecbeb32f317
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/librustc_codegen_ssa/mir/mod.rs:343
   3: rustc_codegen_ssa::base::codegen_instance::h08fe7402fba94e2a (0x7ff4add0c5d2)
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/librustc_codegen_ssa/base.rs:436
   4: rustc_codegen_ssa::mono_item::MonoItemExt::define::hc1b44ea0cbf6e3a8 (0x7ff4adda9047)
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/librustc_codegen_ssa/mono_item.rs:43
   5: rustc_codegen_llvm::base::compile_codegen_unit::module_codegen::he0cfdc914df0c847 (0x7ff4addbe1bd)
             at src/librustc_codegen_llvm/base.rs:178
   6: rustc::dep_graph::graph::DepGraph::with_task_impl::h8ca210522f66e976 (0x7ff4ade33d9c)
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/librustc/dep_graph/graph.rs:295
      rustc::dep_graph::graph::DepGraph::with_task::h9677e9916ca4c2fb
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/librustc/dep_graph/graph.rs:181
   7: rustc_codegen_llvm::base::compile_codegen_unit::h2c505b8e6ff00db0 (0x7ff4addbddfa)
             at src/librustc_codegen_llvm/base.rs:145
   8: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_ssa::traits::backend::ExtraBackendMethods>::compile_codegen_unit::h92d011270b11d0bf (0x7ff4adc16af1)
             at src/librustc_codegen_llvm/lib.rs:144
   9: rustc_codegen_ssa::base::codegen_crate::hdac3c97bf6d7fbca (0x7ff4add0a243)
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/librustc_codegen_ssa/base.rs:687
  10: <rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_utils::codegen_backend::CodegenBackend>::codegen_crate::h888e69a0f96218ba (0x7ff4adc170d0)
             at src/librustc_codegen_llvm/lib.rs:304
  11: rustc_driver::driver::phase_4_codegen::{{closure}}::h227d1332dd6978c2 (0x7ff4bccf2cdc)
             at src/librustc_driver/driver.rs:1349
      rustc::util::common::time_ext::h194714b7b1369f7c
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/librustc/util/common.rs:150
      rustc::util::common::time::ha11a3c04898eac7a
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/librustc/util/common.rs:144
  12: rustc_driver::driver::phase_4_codegen::h6bcca110e885caa9 (0x7ff4bcce18e0)
             at src/librustc_driver/driver.rs:1349
  13: rustc_driver::driver::compile_input::{{closure}}::h231c43f5b8e6c3e9 (0x7ff4bcdf8c50)
             at src/librustc_driver/driver.rs:316
  14: rustc_driver::driver::phase_3_run_analysis_passes::{{closure}}::h8f36925bf5962022 (0x7ff4bcdf2b0d)
             at src/librustc_driver/driver.rs:1332
      rustc::ty::context::tls::enter_global::{{closure}}::{{closure}}::hef4ec43c7065bde2
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/librustc/ty/context.rs:1963
      rustc::ty::context::tls::enter_context::{{closure}}::he652dca2629d0c4f
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/librustc/ty/context.rs:1929
      rustc_rayon_core::tlv::with::h1749e7b1cef75566
             at /home/xftroxgpx/.cargo/registry/src/github.com-1ecc6299db9ec823/rustc-rayon-core-0.1.1/src/tlv.rs:19
  15: rustc::ty::context::tls::set_tlv::hc87f5ce672c3fe3c (0x7ff4bcc8a35f)
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/librustc/ty/context.rs:1838
      rustc::ty::context::tls::enter_context::h540c161a5c470d51
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/librustc/ty/context.rs:1928
      rustc::ty::context::tls::enter_global::{{closure}}::h79bd1d338d355e69
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/librustc/ty/context.rs:1962
      rustc::ty::context::tls::with_thread_locals::{{closure}}::{{closure}}::h36513f108ba283a4
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/librustc/ty/context.rs:1917
      <std::thread::local::LocalKey<T>>::try_with::hb5b48892a35874da
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/libstd/thread/local.rs:299
      <std::thread::local::LocalKey<T>>::with::hb27a948451c920d5
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/libstd/thread/local.rs:245
      rustc::ty::context::tls::with_thread_locals::{{closure}}::hb1b4ea8acb560389
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/librustc/ty/context.rs:1909
      <std::thread::local::LocalKey<T>>::try_with::hfee4f67c590f43fa
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/libstd/thread/local.rs:299
      <std::thread::local::LocalKey<T>>::with::hf7d2ceb7d643b7ad
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/libstd/thread/local.rs:245
  16: rustc::ty::context::tls::with_thread_locals::hc28c29965271ab67 (0x7ff4bcd721e4)
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/librustc/ty/context.rs:1901
      rustc::ty::context::tls::enter_global::h72db5c2568d3abbb
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/librustc/ty/context.rs:1940
  17: rustc::ty::context::TyCtxt::create_and_enter::h26fa6283efd9a6b7 (0x7ff4bcd8c37d)
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/librustc/ty/context.rs:1260
  18: rustc_driver::driver::phase_3_run_analysis_passes::he0bcbc46efa21744 (0x7ff4bcd03052)
             at src/librustc_driver/driver.rs:1229
  19: rustc_driver::driver::compile_input::h4d4398317b2c6d58 (0x7ff4bccdfb97)
             at src/librustc_driver/driver.rs:275
  20: rustc_driver::run_compiler_with_pool::he0ab58fb4a355771 (0x7ff4bce01fac)
             at src/librustc_driver/lib.rs:527
  21: rustc_driver::run_compiler::{{closure}}::h1e4c126fef6bd8dc (0x7ff4bcd9fa05)
             at src/librustc_driver/lib.rs:449
      rustc_driver::driver::spawn_thread_pool::{{closure}}::{{closure}}::hd8a1f072d8753d2c
             at src/librustc_driver/driver.rs:86
      rustc_rayon_core::thread_pool::ThreadPool::install::{{closure}}::h7a43dce80642a2b4
             at /home/xftroxgpx/.cargo/registry/src/github.com-1ecc6299db9ec823/rustc-rayon-core-0.1.1/src/thread_pool/mod.rs:158
      rustc_rayon_core::registry::Registry::in_worker_cold::{{closure}}::he1599453f909cbdd
             at /home/xftroxgpx/.cargo/registry/src/github.com-1ecc6299db9ec823/rustc-rayon-core-0.1.1/src/registry.rs:359
      <rustc_rayon_core::job::StackJob<L, F, R> as rustc_rayon_core::job::Job>::execute::{{closure}}::hc7a75fba1110d7b4
             at /home/xftroxgpx/.cargo/registry/src/github.com-1ecc6299db9ec823/rustc-rayon-core-0.1.1/src/job.rs:121
      <std::panic::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once::h0fe22c82f5272699
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/libstd/panic.rs:309
  22: std::panicking::try::do_call::hb69d76ef89a9d93d (0x7ff4bcd74e53)
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/libstd/panicking.rs:297
  23: __rust_maybe_catch_panic (0x7ff4bcbb7ff0)
             at src/libpanic_unwind/lib.rs:92
  24: std::panicking::try::hfe22bc5b1e02df82 (0x7ff4bcd74aae)
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/libstd/panicking.rs:276
  25: std::panic::catch_unwind::hab370608fba46ba6 (0x7ff4bcda25f8)
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/libstd/panic.rs:388
  26: rustc_rayon_core::unwind::halt_unwinding::hee06b0c95c109e81 (0x7ff4bcc9a478)
             at /home/xftroxgpx/.cargo/registry/src/github.com-1ecc6299db9ec823/rustc-rayon-core-0.1.1/src/unwind.rs:18
  27: <rustc_rayon_core::job::StackJob<L, F, R> as rustc_rayon_core::job::Job>::execute::h693c3cfaa60523e8 (0x7ff4bcd1d4f0)
             at /home/xftroxgpx/.cargo/registry/src/github.com-1ecc6299db9ec823/rustc-rayon-core-0.1.1/src/job.rs:121
  28: rustc_rayon_core::job::JobRef::execute::ha52f551ecea678cd (0x7ff4b8702df3)
             at /home/xftroxgpx/.cargo/registry/src/github.com-1ecc6299db9ec823/rustc-rayon-core-0.1.1/src/job.rs:62
      rustc_rayon_core::registry::WorkerThread::execute::hc89f8cf7c1d41e9b
             at /home/xftroxgpx/.cargo/registry/src/github.com-1ecc6299db9ec823/rustc-rayon-core-0.1.1/src/registry.rs:609
      rustc_rayon_core::registry::WorkerThread::wait_until_cold::h5177fb21147a739c
             at /home/xftroxgpx/.cargo/registry/src/github.com-1ecc6299db9ec823/rustc-rayon-core-0.1.1/src/registry.rs:589
  29: rustc_rayon_core::registry::WorkerThread::wait_until::h12fac8c0f1ed4bb8 (0x7ff4b87039db)
             at /home/xftroxgpx/.cargo/registry/src/github.com-1ecc6299db9ec823/rustc-rayon-core-0.1.1/src/registry.rs:565
      rustc_rayon_core::registry::main_loop::{{closure}}::hb6a51d03c20496ef
             at /home/xftroxgpx/.cargo/registry/src/github.com-1ecc6299db9ec823/rustc-rayon-core-0.1.1/src/registry.rs:701
  30: rustc_driver::driver::spawn_thread_pool::{{closure}}::{{closure}}::{{closure}}::{{closure}}::{{closure}}::{{closure}}::{{closure}}::h41edfded97a513b4 (0x7ff4bce0b6df)
             at src/librustc_driver/driver.rs:100
      <scoped_tls::ScopedKey<T>>::set::hb491eb13ee4c3b95
             at /home/xftroxgpx/.cargo/registry/src/github.com-1ecc6299db9ec823/scoped-tls-0.1.2/src/lib.rs:155
  31: rustc_driver::driver::spawn_thread_pool::{{closure}}::{{closure}}::{{closure}}::{{closure}}::{{closure}}::{{closure}}::he70cf8f3dbad9ef5 (0x7ff4bcc8629b)
             at src/librustc_driver/driver.rs:99
      rustc::ty::context::tls::with_thread_locals::{{closure}}::{{closure}}::h6074966d5788cc51
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/librustc/ty/context.rs:1917
      <std::thread::local::LocalKey<T>>::try_with::h077abb1178ec3502
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/libstd/thread/local.rs:299
      <std::thread::local::LocalKey<T>>::with::he2ee3cda01983d7a
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/libstd/thread/local.rs:245
      rustc::ty::context::tls::with_thread_locals::{{closure}}::h01bfe6c1c47008ff
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/librustc/ty/context.rs:1909
      <std::thread::local::LocalKey<T>>::try_with::h4fcbc8d2e276cb37
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/libstd/thread/local.rs:299
      <std::thread::local::LocalKey<T>>::with::h07c04aa6c28bd983
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/libstd/thread/local.rs:245
  32: rustc_driver::driver::spawn_thread_pool::{{closure}}::{{closure}}::{{closure}}::{{closure}}::{{closure}}::h6a1d65875583578a (0x7ff4bce0b776)
             at src/librustc_driver/driver.rs:98
      <scoped_tls::ScopedKey<T>>::set::h385c43b72356bc8f
             at /home/xftroxgpx/.cargo/registry/src/github.com-1ecc6299db9ec823/scoped-tls-0.1.2/src/lib.rs:155
      rustc_driver::driver::spawn_thread_pool::{{closure}}::{{closure}}::{{closure}}::{{closure}}::h597bf30d030c4095
             at src/librustc_driver/driver.rs:97
      <scoped_tls::ScopedKey<T>>::set::hc0e454a26a369f5e
             at /home/xftroxgpx/.cargo/registry/src/github.com-1ecc6299db9ec823/scoped-tls-0.1.2/src/lib.rs:155
  33: rustc_driver::driver::spawn_thread_pool::{{closure}}::{{closure}}::{{closure}}::he26ea67043a46132 (0x7ff4bce1976d)
             at src/librustc_driver/driver.rs:96
      rustc_rayon_core::thread_pool::ThreadPool::scoped_pool::{{closure}}::ha394f83af0bf888f
             at /home/xftroxgpx/.cargo/registry/src/github.com-1ecc6299db9ec823/rustc-rayon-core-0.1.1/src/thread_pool/mod.rs:103
  34: __rust_maybe_catch_panic (0x7ff4bcbb7ff0)
             at src/libpanic_unwind/lib.rs:92
  35: std::panicking::try::h0324edf9a21513db (0x7ff4b86fcf5f)
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/libstd/panicking.rs:276
      std::panic::catch_unwind::hac46a414d53b9e0d
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/libstd/panic.rs:388
      rustc_rayon_core::unwind::halt_unwinding::h2b6105fb03509a92
             at /home/xftroxgpx/.cargo/registry/src/github.com-1ecc6299db9ec823/rustc-rayon-core-0.1.1/src/unwind.rs:18
      rustc_rayon_core::registry::main_loop::h1fda886cbb064140
             at /home/xftroxgpx/.cargo/registry/src/github.com-1ecc6299db9ec823/rustc-rayon-core-0.1.1/src/registry.rs:705
      rustc_rayon_core::registry::Registry::new::{{closure}}::hdc57e38d2eea6137
             at /home/xftroxgpx/.cargo/registry/src/github.com-1ecc6299db9ec823/rustc-rayon-core-0.1.1/src/registry.rs:139
      std::sys_common::backtrace::__rust_begin_short_backtrace::h3ee2e7302fbd8cab
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/libstd/sys_common/backtrace.rs:135
  36: std::thread::Builder::spawn_unchecked::{{closure}}::{{closure}}::h4d46a00a4516e335 (0x7ff4b86fddfb)
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/libstd/thread/mod.rs:469
      <std::panic::AssertUnwindSafe<F> as core::ops::function::FnOnce<()>>::call_once::h88071d6ae379fc37
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/libstd/panic.rs:309
      std::panicking::try::do_call::hd9ecda59b94bddcc
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/libstd/panicking.rs:297
  37: __rust_maybe_catch_panic (0x7ff4bcbb7ff0)
             at src/libpanic_unwind/lib.rs:92
  38: std::panicking::try::h3d46a5104c396178 (0x7ff4b870035f)
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/libstd/panicking.rs:276
      std::panic::catch_unwind::hc10ac859fe269334
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/libstd/panic.rs:388
      std::thread::Builder::spawn_unchecked::{{closure}}::hed00129e1c5572f2
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/libstd/thread/mod.rs:468
      <F as alloc::boxed::FnBox<A>>::call_box::h3d507a5e4bf27391
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/liballoc/boxed.rs:734
  39: std::sys_common::thread::start_thread::h7c24e4f48ce975ba (0x7ff4bcba8d06)
             at src/libstd/sys_common/thread.rs:14
  40: std::sys::unix::thread::Thread::new::thread_start::h966b57701b78ecf1 (0x7ff4bcb9c3f5)
             at src/libstd/sys/unix/thread.rs:81
  41: start_thread (0x7ff4b862c73e)
             at /usr/src/debug/glibc/nptl/pthread_create.c:486
  42: __GI___clone (0x7ff4bca33bc2)
  43: <unknown> (0x0)

real	0m7.569s
user	0m1.512s
sys	0m2.920s
0 / 0
thread 'main' panicked at 'wuuuuuuattttttt??!attempt to multiply with overflow', /tmp/tr/src/main.rs:12:15
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:39
   1: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:70
             at src/libstd/sys_common/backtrace.rs:58
   2: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:200
   3: std::panicking::default_hook
             at src/libstd/panicking.rs:215
   4: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:478
   5: std::panicking::continue_panic_fmt
             at src/libstd/panicking.rs:385
   6: rust_begin_unwind
             at src/libstd/panicking.rs:312
   7: core::panicking::panic_fmt
             at src/libcore/panicking.rs:85
   8: core::panicking::panic
             at src/libcore/panicking.rs:49
   9: main::main
             at ./src/main.rs:12
  10: std::rt::lang_start::{{closure}}
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/libstd/rt.rs:64
  11: std::panicking::try::do_call
             at src/libstd/rt.rs:49
             at src/libstd/panicking.rs:297
  12: __rust_maybe_catch_panic
             at src/libpanic_unwind/lib.rs:92
  13: std::panicking::try
             at src/libstd/panicking.rs:276
  14: std::panic::catch_unwind
             at src/libstd/panic.rs:388
  15: std::rt::lang_start_internal
             at src/libstd/rt.rs:48
  16: std::rt::lang_start
             at /home/xftroxgpx/build/2nonpkgs/rust.stuff/rust/rust/src/libstd/rt.rs:64
  17: main
  18: __libc_start_main
             at ../csu/libc-start.c:308
  19: _start
             at ../sysdeps/x86_64/start.S:120

[dtolnay]

I agree that this would be desirable. Would love to see a PR with an implementation!

I found the code that codegens the panic into the executable:

rust/src/librustc_codegen_ssa/mir/block.rs

Lines 354 to 396 in ceb2512

	// Pass the condition through llvm.expect for branch hinting.
	let cond = bx.expect(cond, expected);
	// Create the failure block and the conditional branch to it.
	let lltarget = llblock(self, target);
	let panic_block = self.new_block("panic");
	if expected {
	bx.cond_br(cond, lltarget, panic_block.llbb());
	} else {
	bx.cond_br(cond, panic_block.llbb(), lltarget);
	}
	// After this point, bx is the block for the call to panic.
	bx = panic_block;
	self.set_debug_loc(&mut bx, terminator.source_info);
	// Get the location information.
	let loc = bx.sess().source_map().lookup_char_pos(span.lo());
	let filename = Symbol::intern(&loc.file.name.to_string()).as_str();
	let filename = bx.const_str_slice(filename);
	let line = bx.const_u32(loc.line as u32);
	let col = bx.const_u32(loc.col.to_usize() as u32 + 1);
	let align = tcx.data_layout.aggregate_align.abi
	.max(tcx.data_layout.i32_align.abi)
	.max(tcx.data_layout.pointer_align.abi);
	// Put together the arguments to the panic entry point.
	let (lang_item, args) = match *msg {
	EvalErrorKind::BoundsCheck { ref len, ref index } => {
	let len = self.codegen_operand(&mut bx, len).immediate();
	let index = self.codegen_operand(&mut bx, index).immediate();
	let file_line_col = bx.const_struct(&[filename, line, col], false);
	let file_line_col = bx.static_addr_of(
	file_line_col,
	align,
	Some("panic_bounds_check_loc")
	);
	(lang_items::PanicBoundsCheckFnLangItem,
	vec![file_line_col, index, len])
	}
	_ => {
	let str = msg.description();

and this very line(from the above):

rust/src/librustc_codegen_ssa/mir/block.rs

Line 396 in ceb2512

let str = msg.description();

is the one that gets the value of the error message string attempt to multiply with overflow which can be seen at:

rust/src/librustc/mir/interpret/error.rs

Line 416 in ceb2512

Overflow(mir::BinOp::Mul) => "attempt to multiply with overflow",

and is part of the panic message that happens at runtime.

So if I'm interpreting this correctly:

rust/src/librustc_codegen_ssa/mir/block.rs

Line 331 in ceb2512

mir::TerminatorKind::Assert { ref cond, expected, ref msg, target, cleanup } => {

then the condition whether or not it overflowed is already a bool(or possibly something else?) there, so the operands of the mul or even the mul operation itself cannot be accessed from here.

And since I basically don't understand 95% of what's going on or what most code does here, and I'm still only at chapter 10(is next) in my rustbook reading(altho many months ago I once I went as far as(but not including) that chapter on how to build your own grep tool - oh it's chapter 12), I'm going to conclude that it's basically impossible(if at least, for me) to show the lhs,op and rhs in the panic message, simply because they aren't accessible from the context where the if cond then panic block is being codegenned into the executable. (I've only some vague understanding of this, based mostly on deductions - could someone please recommend a link to read about this? uuuu, I found something and this)

EDIT: Ha! I might actually be (gladly!) wrong! due to my interpretation of this comment // Don't codegen the panic block if success if known. which to me means that the mul operation is also codegen-ned(a few lines later) thus the lhs,op and rhs would be accessible maybe they're somehow inside cond (whatever type/struct(?) that is it)
EDIT2: progressing here: https://gist.github.com/xftroxgpx/c34b67bb43ef3ca6fbd75971c460f541

[correabuscar]

Looks like this is already in stable rust 1.76.0 (at least), as I'm getting this output:

   Compiling playground v0.0.1 (/playground)
error: this arithmetic operation will overflow
 --> src/main.rs:7:20
  |
7 |     println!("{}", 2*innum);
  |                    ^^^^^^^ attempt to compute `2_i8 * 122_i8`, which would overflow
  |
  = note: `#[deny(arithmetic_overflow)]` on by default

error: could not compile `playground` (bin "playground") due to 1 previous error

Therefore the operands are shown, so this issue can be closed. Thanks all!