Support separate PDB with MSVC staticlib

[joverwey]

Problem

When compiling staticlib crates using the MSVC toolchain, no PDBs are produced.

This makes it very hard to debug C++ programs that are statically linked to Rust libraries, especially in release mode.

Steps

cargo new --lib test

In Cargo.toml:

[lib]
path = "./src/lib.rs"
crate_type = ["staticlib"]

cargo build

Note that the output is test.lib

Now change Cargo.toml:

[lib]
path = "./src/lib.rs"
crate_type = ["cdylib"]

cargo build

Note that the output is test.lib + test.pdb

Possible Solution(s)

Is there a way to pass a flag to force creating the PDB. In Visual Studio it is /Zi.

Notes

It seems it is still possible to step into the code for debug builds, but not release builds.

Version

cargo 1.55.0 (32da73ab1 2021-08-23)
release: 1.55.0
commit-hash: 32da73ab19417aa89686e1d85c1440b72fdf877d
commit-date: 2021-08-23

[ehuss]

I have transferred this to the rust-lang repository, as cargo does not handle this (it is part of rustc).

My understanding is that staticlibs contain the debuginfo internally, so usually separate PDBs are not necessary. rustc does not have any option to create a PDB when generating a staticlib. Since generating PDBs are usually handled by the linker (link.exe), and staticlibs are not generated with a linker, this seems likely difficult to support. However, I'm not super familiar with this particular scenario.

[obsgolem]

I am currently running into this issue I think. The natvis files don't appear to be being embedded in the .lib file like they are in .pdbs, hence debugging something that links a rust library statically is not super pretty. I also have an issue where VS thinks that my source and the compiled code are different. I am not sure what is causing that, but I am currently suspecting some kind of issue in the debug info.