Failed to evaluate constant value

[MahadMuhammad]

Failed to evaluate constant value E0080

• Fixing this issue, help gccrs to emit more user friendly error code and error emission similiar to rustc.
• You can view the same on compiler-explorer

---

I tried this code from E0080:

#![allow(unused)]
fn main() {
enum Enum {
    X = (1 << 500),
    Y = (1 / 0),
}
}

---

I expected to see this happen:

• Emit error message, similiar to rustc .i.e.,
  • evaluation of constant value failed
• Error message emitted by rustc:

error[E0080]: evaluation of constant value failed
 --> <source>:4:9
  |
4 |     X = (1 << 500),
  |         ^^^^^^^^^^ attempt to shift left by `500_i32`, which would overflow

error[E0080]: evaluation of constant value failed
 --> <source>:5:9
  |
5 |     Y = (1 / 0),
  |         ^^^^^^^ attempt to divide `1_isize` by zero

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0080`.
Compiler returned: 1

---

Instead, this happened:

• Compiled Successfully.
• This is the output of gccrs:

Compiler returned: 0

---

Meta

• What version of Rust GCC were you using, git sha b9566fd.
• gccrs (Compiler-Explorer-Build-gcc-2f91d511200bf85558c9013b09a458c06edd1e02-binutils-2.40) 13.0.1 20230417 (experimental)

---

[philberty]

There are a few parts to this issue:

1. We are not doing const-eval on the enum discriminant values during compilation of the type
2. GCC is not doing overflow checking on shift expression see: fold_convert_loc is simply folding the value and not marking TREE_OVERFLOW

We should probably be calling build2 instead of fold_build2 when inside a const context to get the constexpr code to do the overflow checking for us might fix this

[philberty]

We actually need to port over code from c/c-fold.cc to get support for detecting this case the default GCC middle end does not set TREE_OVERFLOW for us.

[dkm]

Side note, when looking at the precise version from Compiler-Explorer for nightly compilers, do NOT use the tooltip at the bottom. These are pre-computed offline and will not be updated along with the compiler every night. It only gets updated when we release a new version of compiler explorer (can be several times a week, sometimes more, sometimes less...).
If you need the exact version, use --version, or check the extra information in the assembly.