5 ms·

I wonder why procmacros are slow. Can the compiler interpret them or does it have to go to all the work of compiling them before they can run?

Philpax 1y ago
It has to compile them; you can see the compiled binaries in your `target` directory. Rust doesn't have an interpreter for the full language, only for the `const` subset, so it can't interpret them. There have been some proposals to compile the proc-macros to WASM and share those alongside the code in crates.io, but nothing substantial has come of it.
- LoganDark 1y ago
  > Rust doesn't have an interpreter for the full language Ever since a while ago, rustc uses Miri for const evaluation. So there are a lot of things it can do that it used to not be able to do. But, yes, const evaluation is limited to things that are part of the `const` subset.
  - LegionMammal978 1y ago
    As far as I'm aware, it's always been the other way around: Miri adds some features on top of rustc's const-evaluation code. The limitations of the latter are mainly self-imposed, due to the issues of exposing the different runtime models to programs. (E.g., you don't want to create allocations in const code that get deallocated at runtime.) Indeed, since 2019, the full functionality can be exposed with the unstable -Zunleash-the-miri-inside-of-you flag [0]. [0] https://github.com/rust-lang/rust/pull/56123 https://github.com/rust-lang/rust/pull/56123
    - LoganDark 1y ago
      I'm pretty sure we're in agreement? Miri is capable of a lot more but there are self-imposed limitations on what you're allowed to do in const.
- LegionMammal978 1y ago
  It does have an interpreter for the full language, that's what Miri uses [0]. In fact, Miri doesn't even have its own evaluator, it just adds additional features to the rustc const-evaluation. The big limitations are that it doesn't have much support for syscalls or other calls into non-Rust code, and it emulates all multithreaded code on a single thread. [0] https://github.com/rust-lang/miri https://github.com/rust-lang/miri
  - Philpax 1y ago
    Fair enough!
- pjmlp 1y ago
  That is an implementation detail, but yeah so far other areas have been given more priority.

Philpax 1y ago

It has to compile them; you can see the compiled binaries in your `target` directory. Rust doesn't have an interpreter for the full language, only for the `const` subset, so it can't interpret them. There have been some proposals to compile the proc-macros to WASM and share those alongside the code in crates.io, but nothing substantial has come of it.

LoganDark 1y ago
> Rust doesn't have an interpreter for the full language Ever since a while ago, rustc uses Miri for const evaluation. So there are a lot of things it can do that it used to not be able to do. But, yes, const evaluation is limited to things that are part of the `const` subset.
- LegionMammal978 1y ago
  As far as I'm aware, it's always been the other way around: Miri adds some features on top of rustc's const-evaluation code. The limitations of the latter are mainly self-imposed, due to the issues of exposing the different runtime models to programs. (E.g., you don't want to create allocations in const code that get deallocated at runtime.) Indeed, since 2019, the full functionality can be exposed with the unstable -Zunleash-the-miri-inside-of-you flag [0]. [0] https://github.com/rust-lang/rust/pull/56123 https://github.com/rust-lang/rust/pull/56123
  - LoganDark 1y ago
    I'm pretty sure we're in agreement? Miri is capable of a lot more but there are self-imposed limitations on what you're allowed to do in const.
LegionMammal978 1y ago
It does have an interpreter for the full language, that's what Miri uses [0]. In fact, Miri doesn't even have its own evaluator, it just adds additional features to the rustc const-evaluation. The big limitations are that it doesn't have much support for syscalls or other calls into non-Rust code, and it emulates all multithreaded code on a single thread. [0] https://github.com/rust-lang/miri https://github.com/rust-lang/miri
- Philpax 1y ago
  Fair enough!
pjmlp 1y ago
That is an implementation detail, but yeah so far other areas have been given more priority.