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The LLVM-based Rust compiler uses a lot of unstable/nightly-only Rust features internally. So even if this project got to the point where it could compile all s

The LLVM-based Rust compiler uses a lot of unstable/nightly-only Rust features internally. So even if this project got to the point where it could compile all stable Rust programs, I think it would take quite a bit more work than that to be able to compile `rustc` itself. (It might be that the unstable stuff is mostly in the standard library and not the compiler itself? Does it make a difference?)

steveklabnik 6y ago
> It might be that the unstable stuff is mostly in the standard library and not the compiler itself? Does it make a difference? It might. There are at least two major things off the top of my head, regarding libstd: 1. specialization is needed for performance around String and &str 2. const generics are needed to support some trait implementations We currently allow some stuff like this to leak through, in a sense, when we're sure that we're actually going to be making things stable someday. An alternative compiler could patch out 1, and accept slower code, but 2 would require way more effort. There has been some discussion about trying to remove unstable features from the compiler itself, specifically to make it easier to contribute to, but it unlikely that it will be completely removed from the current implementation of libstd for some time.
- CodesInChaos 6y ago
  Doesn't min_const_generics (stable once 1.51 releases on 2021-03-25) cover everything std needs (implementing traits for arbitrary array sizes)?
  - steveklabnik 6y ago
    I believe that it does, yes. We haven't hit that in stable yet, though, so I am speaking purely in the present. You're right that it's looking good for stabilization in a few months, but anything can happen, in theory. I'm more trying to illustrate the point, that significant features can depend on something that's unstable currently. Those impls landed well before it was even marked as being stable in nightly.
deleted 6y ago
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cbmuser 6y ago
Isn‘t mrustc already able to compile rustc?
- steveklabnik 6y ago
  Yes.
  - oconnor663 6y ago
    Oh that's impressive. I didn't know it was that far along.
    - steveklabnik 6y ago
      It managed to do it for the first time Dec 24 2017 https://www.reddit.com/r/rust/comments/7lu6di/mrustc_alternate_rust_compiler_in_c_now_broken/ https://www.reddit.com/r/rust/comments/7lu6di/mrustc_alterna... > It's managed to build rustc from a source tarball, and use that rustc as stage0 for a full bootstrap pass. Even better, from my two full attempts, the resultant stage3 files have been binary identical to the same source archive built with the downloaded stage0.

steveklabnik 6y ago

> It might be that the unstable stuff is mostly in the standard library and not the compiler itself? Does it make a difference? It might. There are at least two major things off the top of my head, regarding libstd: 1. specialization is needed for performance around String and &str 2. const generics are needed to support some trait implementations We currently allow some stuff like this to leak through, in a sense, when we're sure that we're actually going to be making things stable someday. An alternative compiler could patch out 1, and accept slower code, but 2 would require way more effort. There has been some discussion about trying to remove unstable features from the compiler itself, specifically to make it easier to contribute to, but it unlikely that it will be completely removed from the current implementation of libstd for some time.

CodesInChaos 6y ago
Doesn't min_const_generics (stable once 1.51 releases on 2021-03-25) cover everything std needs (implementing traits for arbitrary array sizes)?
- steveklabnik 6y ago
  I believe that it does, yes. We haven't hit that in stable yet, though, so I am speaking purely in the present. You're right that it's looking good for stabilization in a few months, but anything can happen, in theory. I'm more trying to illustrate the point, that significant features can depend on something that's unstable currently. Those impls landed well before it was even marked as being stable in nightly.

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cbmuser 6y ago

Isn‘t mrustc already able to compile rustc?

steveklabnik 6y ago
Yes.
- oconnor663 6y ago
  Oh that's impressive. I didn't know it was that far along.
  - steveklabnik 6y ago
    It managed to do it for the first time Dec 24 2017 https://www.reddit.com/r/rust/comments/7lu6di/mrustc_alternate_rust_compiler_in_c_now_broken/ https://www.reddit.com/r/rust/comments/7lu6di/mrustc_alterna... > It's managed to build rustc from a source tarball, and use that rustc as stage0 for a full bootstrap pass. Even better, from my two full attempts, the resultant stage3 files have been binary identical to the same source archive built with the downloaded stage0.