WinDbg does not understand niche-layout enums #84671
Assignees
Labels
A-debuginfo
Area: Debugging information in compiled programs (DWARF, PDB, etc.)
A-layout
Area: Memory layout of types
C-bug
Category: This is a bug.
O-windows-msvc
Toolchain: MSVC, Operating system: Windows
Comments
wesleywiser
added
A-debuginfo
Dylan-DPC-zz
pushed a commit
to Dylan-DPC-zz/rust
that referenced
this issue
Jun 3, 2021
…elwoerister
Improve debugging experience for enums on windows-msvc
This PR makes significant improvements over the status quo of debugging enums on the windows-msvc platform with either WinDbg or Visual Studio in three ways:
1. Improves the debugger experience for directly tagged enums.
2. Fixes a bug which caused the debugger to sometimes show the wrong debug info for niche layout enums. For example, `Option<&u32>` could sometimes use the debug info for `Option<&f64>` instead leading to nonsensical variable values in the debugger.
3. Significantly improves the debugger experience for niche-layout enums.
Let's look at a few examples:
```rust
pub enum CStyleEnum {
Base = 2,
Exponent = 16,
}
pub enum NicheLayoutEnum {
Tag1,
Data { my_data: CStyleEnum },
Tag2,
Tag3,
Tag4,
}
pub enum OtherEnum<T> {
Case1(T),
Case2(T),
}
fn main() {
let a = Some(CStyleEnum::Base);
let b = Option::<CStyleEnum>::None;
let c = NicheLayoutEnum::Tag1;
let d = NicheLayoutEnum::Data { my_data: CStyleEnum::Exponent };
let e = NicheLayoutEnum::Tag2;
let f = Some(&1u32);
let g = Option::<&'static u32>::None;
let h = Some(&2u64);
let i = Option::<&'static u64>::None;
let j = Some(12u32);
let k = Option::<u32>::None;
let l = Some(12.34f64);
let m = Option::<f64>::None;
let n = CStyleEnum::Base;
let o = CStyleEnum::Exponent;
let p = Some("IAMA optional string!".to_string());
let q = OtherEnum::Case1(42u32);
}
```
This is what WinDbg Preview shows using the latest rustc nightly:
Most of the variables don't show a meaningful value expect for a few cases that we have targeted natvis definitions covering. Even worse, drilling into many of these variables shows information that can be difficult to interpret without an understanding of the layout of Rust types:
With the changes in this PR, we're able to write two natvis definitions that cover all enum cases generally. After building with these changes, WinDbg now shows this instead:
Drilling into the same variables, we can see much more useful information:
Fixes rust-lang#84670
Fixes rust-lang#84671
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
WinDbg does not understand niche-layout enums and the information provided is not very useful unless you understand how niche-layout works. Even then, aside from simple cases like the
0niche for references, it can be non-trivial to decode what variant of the enum is active when either the niche is not 0 or the enum contains multiple tag only variants, not just one likeOptiondoes.Having the debuginfo more like the direct-tag layout case would help.
The text was updated successfully, but these errors were encountered: