cargo: Respect Meson's default_library instead of Cargo.toml's crate_type

[xclaesse]

When Cargo.toml has e.g. crate_type = ["staticlib", "cdylib"] it would make sense to use library(..., rust_abi: 'c') instead of both_libraries() to give shared and/or static decision to Meson instead of respecting Cargo.toml.

[sdroege]

Not that I disagree, but I'm curious why :)

Also AFAIU library would be more like crate_type = ["rlib", "dylib"] (Rust ABI) instead of crate_type = ["staticlib", "cdylib"] (C ABI). You probably know that but for linking Rust dependencies you need the former and not the latter, and the latter is on a level with an executable in the sense that it's a "final" build result that can't be directly linked as a Rust dependency anymore.

For anything that is listed as dependency inside Cargo.tomls, you'll have to ensure to build a rlib (or dynlib but that has all kinds of ABI-related questions).

[xclaesse]

@sdroege I was thinking doing this only for C ABI. If crate_type contains one of staticlib and/or cdylib, then use library(..., rust_abi: 'c'). Keep rlib, dylib and proc-macro unchanged.

[xclaesse]

That's @bonzini idea: #11708 (comment).

[sdroege]

I was thinking doing this only for C ABI. If crate_type contains one of staticlib and/or cdylib, then use library(..., rust_abi: 'c'). Keep rlib, dylib and proc-macro unchanged.

If crate_type contains both Rust and C ABI (as is done in gst-plugins-rs: cdylib and rlib), what would you do?

[bonzini]

If crate_type contains both Rust and C ABI (as is done in gst-plugins-rs: cdylib and rlib), what would you do?

cdylib and rlib would build static_library(rust_abi: 'rust') and shared_library(rust_abi: 'c').

But if you have both staticlib and cdylib, there's an argument for using library(rust_abi: 'c') instead of both_libraries(rust_abi: 'c').

(A lot less so for rlib and dylib, since in that case static vs. dynamic is controlled by -C prefer-dynamic rather than default_both_libraries. Therefore I would still build both of them and then let rustc pick the one it wants).

[xclaesse]

That would still build for both ABISs: only rlib for rust abi, but respect default_library for C abi.

That would allow for example to build static gst plugins for gst-full I think.

[xclaesse]

If crate_type contains both Rust and C ABI (as is done in gst-plugins-rs: cdylib and rlib), what would you do?

cdylib and rlib would build static_library(rust_abi: 'rust') and shared_library(rust_abi: 'c').

In the case crate_type = [cdylib, rlib] I propose to do static_library(rust_abi: 'rust') and library(rust_abi: 'c'). Basically if you have any of cdylib and staticlib then always let Meson decide whether to build static and/or shared.

#15207

In the GStreamer case, that would allow gst-full (-Ddefault_library=static) to static link Rust plugins as well. I'm sure there are plenty other roadblocks before that becomes reality, but that's a step toward that.

[bonzini]

Why not require [staticlib, cdylib] to force use of library (I am sure you have a practical reason but I am not sure what reason is it :))

[xclaesse]

Commented on the PR.