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Comment by emillon | Hacker News Reader

The big problem for me is the compilation time, which is very slow (this may or may not be because the compiler itself is written in Rust).

I dedicated a few days to hacking on rustc this summer, and while the tooling and community are very nice, the compilation times are really high. Git pull && make ? 20 min. Edit a file && make ? 5 min. IIRC the major was that metadata got serialized/unserialized all the time, but anyway it's one of the priorities. I'll hack again on it with pleasure when I'll have more time.

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> this may or may not be because the compiler itself is written in Rust

Well, due to the nature of self-hosting compilers, it's certainly greatly exacerbated by being written in Rust. :)

To wit, a self-hosting compiler must actually compile itself no less than three times to ensure that the generated artifacts "converge" upon a single point. This is as true for Rust as it is for GCC.

Three times, so:

    1. Old version compiles new version
    2. New version, compiled by old version, compiles new version
    3. New compiled by new-compiled-by-old compiles new version

I hadn't heard of this before and initially didn't see why #3 is special. But I guess the idea is, the output object code is a function of both the input source code and the compiler version. And those two are the same for #2 and #3. So you expect the output of both #2 and #3 to be identical, and if it's not, that's a bug in the compiler. Is that how it works?

Yes, the third compile is effectively a single very large test.

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