by dathinab | 2025-07-14 11:58:21+00:00
> They're not;

it's complicated, and simple

the simple answer is rust compiler times are not dominated by the borrow checker at all, so "it's fast" and you can say it's not overly related to their being borrow checking

the other simple answer is that a simple reasonable well implemented borrow checker is pretty much always fast

to complicated answer is that rusts borrow checker isn't simple as there are a _huge lot_ of code a simple borrow checker wouldn't allow which is safe and people want to write and the borrow checker rust needs to run to support all that edge cases has to basically run a constraint solver. (Which a) is a thing which in O notation is quite slow and b) is a thing CS has researched optimizations and heuristics for since decades so it is often quite fast in practice.) And as far as I remember rust currently does (did? wanted to?) run this in two layers, the simple checker checks most code and the more powerful on only engages for the cases where the simple checker fails. But , like mentioned, as the compilation still isn't dominated by the borrow checker this doesn't exactly mean its slow.

So the borrow checker isn't an issue and if you create a C-like language with a rust like borrow checker it will compile speedily, at least theoretically, if you then also have a tone of code gen and large compilation units you might run into similar issues as rust does ;)

Also recently most of the "especially bad cases" project in rust have run into (wrt. compiler times, AFIK) all had the same kind of pattern: A enormous huge amount of code (often auto generated, often even huge before monomorphization) being squeezed into very few (often one single) LLVM compilation unit leading to both LLVM struggling hard with optimizations and then the linker drowning, too. And here is the thing, that can happen to you in C too and then your compilation times will be terrible, too. Through people tend to very rarely run into it in C.

> not low-hanging fruit, requiring major architectural changes, so it'd require a large investment of resources which no one has put up.

it still happens from time to time (e.g. polenious) and then there are still many "hard" but very useful improvements which don't require any large scale architectural changes and also some bigger issues which wouldn't be fixed by large scale architectural improvements. So not sure if we are anywhere close to needing a large scale architectural overall in rustc, probably not.

E.g. in a somewhat recent article about way too long rust compiler times many HN comments thought that rustc had some major architectural issues wrt. parallelization, but the issue was that rust failed to properly subsection the massive auto-generated crate when handing code units to LLVM and that isn't an architectural issue. Or e.g. not replacing LLVM with cranelift (if viable) for the change->unit test loop is a good example for a change which can largely improve dev experience/decrease compiler times for the place where it matters the most (technically it does change the architecture of the stack, and needed many many small changes to allow a non LLVM backend, but it's not "a major rewrite(architectural) change" in the rustc compiler code).