[steveklabnik]

Yes, this idea is sort of similar, just like, more complicated in a sense than the clean design, which is what Zig does.

Incidentally, you might want to look at gc-sections, which Rust already does. As well as https://rust-lang.github.io/rust-project-goals/2025h2/relink... which is kinda related.

The sort of key here is understanding that "produce a library" means that every public item is "used" in the sense of "do we need to compile it." The real trick is to do demand-driven compilation starting from the actual final program's needs, and this is inherently at odds with the idea of producing standalone libraries and combining them into the final artifact.

[saghm]

Makes sense! I think what's most surprising to me about the library compilation model is that for the most part, it doesn't seem like it's actually that useful to how Rust does things by default. Without something like sccache, the intermediate libraries aren't going to be reused across all of my projects, so unless I'm compiling multiple binaries in the same project, I'm not really getting much out of having the entire library sitting there in my target directory rather than just the subset that I'm using. I'm guessing this is related to what you mean by potentially being able to do something differently if there were more time before 1.0?

[steveklabnik]

I think that there is a general desire to work with the system, and since that is how other systems languages have generally done it, doing it the same way feels good. You don't necessarily want to innovate on everything all at once.

It is true that for various reasons, Rust can't take as much advantage as say, C can.

> I'm guessing this is related to what you mean by potentially being able to do something differently if there were more time before 1.0?

I mean more traditional language design things, but sure, this too.

[saghm]

Interesting. My perception for why other systems languages generally compile things to libraries is that they tend to use dynamic linking from a single instance of libraries on a system. I would have expected that when static linking is the default, the argument for having standalone intermediate libraries is much weaker, since the deviation from the other systems languages has already been decided. Maybe I'm missing something about how choosing static linking by default but still supporting dynamic linking requires still having a library as the output always.

[steveklabnik]

> My perception for why other systems languages generally compile things to libraries is that they tend to use dynamic linking from a single instance of libraries on a system.

This is both true and not the whole story. In C, the file (okay if you want to get REALLY technical it's not the file but I'm talking about 99.9% of computers and not some old mainframe platforms) is the compilation unit. You pass a .c in, you get a .o (or whatever for your platform) out. Producing a final binary is where the static vs dynamic choice really comes into play, but you end up with these intermediate artifacts because that is how the language is defined.

> I would have expected that when static linking is the default, the argument for having standalone intermediate libraries is much weaker,

It is weaker, sure. But that doesn't mean there aren't other advantages. For example, you can more easily parallelize a large workload by breaking it up into multiple intermediate libraries, and compiling those simultaneously, whereas doing it all in one compilation/translation unit requires compiler support for said parallelization. Especially if you're already writing a batch compiler, this is a much easier win than rearchitecting the whole thing.

[skavi]

> The real trick is to do demand-driven compilation starting from the actual final program's needs

this is not so far off from hint-mostly-unused, no?

[NobodyNada]

hint-mostly-unused defers codegen of a crate's functions until compiling a dependent crate where those functions are actually called. Therefore, unused functions will not need to be codegen'd.

The downside is that functions which are called from multiple dependent crates will need to be codegen'd in each of their dependents, so this can increase compile times if the crate is not "mostly unused."

So it's not quite as powerful as full demand-driven compilation, because of how Rust separates the compilation process into separate crates.

[IshKebab]

Can't you just cache them?

[steveklabnik]

In my understanding, it's similar, yeah. I don't know enough about how hint-mostly-unused works internally to really speak to the specific differences here.