Add very basic "comptime" fn implementation #148820
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Some changes occurred in compiler/rustc_attr_parsing Some changes occurred in compiler/rustc_passes/src/check_attr.rs Some changes occurred in compiler/rustc_hir/src/attrs Some changes occurred to the CTFE machinery Some changes occurred in src/tools/rustfmt cc @rust-lang/rustfmt |
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…d in comptime fn
…rgetted at what it does
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Some changes occurred to MIR optimizations cc @rust-lang/wg-mir-opt |
| Comptime, | ||
| Const, |
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Seems worth doc comments distinguishing these two.
| let fn_span = self.token.span; | ||
| let header = self.parse_fn_front_matter(vis, case, FrontMatterParsingMode::Function)?; // `const ... fn` | ||
| let mut header = self.parse_fn_front_matter(vis, case, FrontMatterParsingMode::Function)?; // `const ... fn` | ||
| if let Some(attr) = attrs.iter().find(|attr| attr.has_name(sym::rustc_comptime)) { |
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This doesn't account for #[cfg_attr(…, rustc_comptime)]. I would just leave the parser and ast::Const alone and do the lowering to hir::Constness (which would still have Comptime) in AST lowering. This should simplify things a lot.
Even if you anticipate it being turned into a keyword / qualifier eventually, you can't assume that and it would be super easy anyway to update the parser & AST once it's truly necessary.
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The job Click to see the possible cause of the failure (guessed by this bot) |
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| #[derive(Copy, Clone, PartialEq, Eq, Debug, Encodable, Decodable, HashStable_Generic)] | ||
| pub enum Constness { |
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Personally speaking I would rename the variants like so:
pub enum Constness {
Always, // `#[rustc_comptime]`
Maybe, // `const`
Never,
}to mirror BoundConstness which is Always (for const), Maybe (for [const]) and Never. This makes it crystal clear what they represent and leaves no room for ambiguity.
If it were up to me, I would even rename Constness to ItemConstness if traits will be allowed to be compiletime, too. Otherwise, I would split it into FnConstness and TraitConstness.
| ), | ||
| rustc_attr!( | ||
| rustc_comptime, Normal, template!(Word), ErrorFollowing, | ||
| EncodeCrossCrate::Yes, |
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It this necessary? The other queries would store this information
| if let hir::Constness::Comptime = self.tcx.constness(callee_did) { | ||
| match const_context { | ||
| Some(hir::ConstContext::Const { .. } | hir::ConstContext::Static(_)) => {} | ||
| Some(hir::ConstContext::ConstFn) | None => { | ||
| self.dcx().span_err(span, "comptime fns can only be called at compile time"); | ||
| } | ||
| } |
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This doesn't enforce cross crate if I use a comptime function in fn if my crate doesn't enable const_trait_impl.
Would it make sense to also add logic to constck? (check_const.rs)
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Well I'm working on turning this check on unconditionally. I can just move the comptime check before the feature gate check
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Implements functions that cannot be called at runtime (and thus also not from normal const functions, as those could be called from runtime).
This is done via the internal attribute
rustc_comptimethat can be added to normal functions, turning them into compile-time-only functions.Because @fee1-dead and @compiler-errors did amazing work, we even get trait bounds that work inside comptime fns: via unconditionally-const
const Traitbounds.Use cases are
project goal issue: rust-lang/rust-project-goals#406
no tracking issue until we have a feature gate and some sort of syntax
cc @scottmcm as the T-lang goal champion