Replace libbacktrace with a native Rust library

[bstrie]

One of our last remaining C dependencies is libbacktrace. unwind-rs, from the gimli project, looks to be a promising replacement: https://github.com/gimli-rs/unwind-rs . Evaluate its suitability and continue the inexorable oxidation of the compiler.

[alexcrichton]

Note that I don't think this is quite right in the sense that, despite the name, we don't actually use libbacktrace for unwinding. Instead we primarily use it for symbolication of a backtrace (mapping from an address to symbol name, file name, line number, etc). We also have an unwinder but that comes from a variety of sources and is often way trickier to get right on all the platforms.

A good incremental first step would be indeed to replace libbacktrace, but probably with something like addr2line instead of for unwinding functionality.

[bstrie]

Updated the title to be more general.

[est31]

Hmm so I've tried including addr2line as a crate in the Cargo.toml of libstd. Trying to compile gives me a bunch of errors like these, for a bunch of crates:

error[E0463]: can't find crate for core

Now it seems to me like that this problem is worked around for currently present crates by adding shims for them in src/rustc/*_shim. I am not sure for how many crates I'd need a shim, but the "can't find crate for core" is shown to me 6 times so it's probably at least 6 crates, but maybe more because cargo didn't compile them all. They are also probably deep in the hierarchy.

@alexcrichton what do you think we should do here? Can this be fixed by an unstable feature in cargo that we enable in libstd's Cargo.toml?

[fitzgen]

@est31, FYI Alex is AFK for the next month to month and a half or so.

[est31]

@fitzgen thanks for pointing this out.

I'm right now in contact with @Mark-Simulacrum about ways to fix the issue. Turns out that they already have a draft version of a cargo change lying around that's very similar to the one I had in mind. I only need to rebase it.

[est31]

So I've taken @Mark-Simulacrum 's patch for cargo ( https://github.com/Mark-Simulacrum/cargo/tree/implicit-deps ) and rebased it and changed it to use env vars. And I also changed bootstrap to generate such an env var during build (without using most of @Mark-Simulacrum 's patch on that). Then I discovered that now every single invocation of cargo that goes not through Builder::cargo in bootstrap/builder.rs causes Cargo.lock to be changed because then Cargo doesn't get the list of overrides... This already happens at least once every time during the normal build as bootstrap.py calls cargo run on bootstrap. That can be fixed by removing bootstrap from the workspace but there are far far more places that use bare cargo and I don't think we want to track down every single use of it. It makes life harder for any outside contributor who wants to e.g. cargo update a given crate or such. So I've shelved my work onto these two branches (cargo, rustc).

Changing the feature to be cargo-centric like enabling it via directives in Cargo.toml is a bit complicated and I don't know much how Cargo works internally.

@Mark-Simulacrum suggested that the easiest fix would be to run sth like cargo build -p alloc -p core etc before we compile addr2line. That's the next option I explore.