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Avoid some unnecessary query invocations. #121387

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merged 1 commit into from
Mar 26, 2024
Merged

Avoid some unnecessary query invocations. #121387

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10 changes: 8 additions & 2 deletions compiler/rustc_hir_analysis/src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -102,8 +102,9 @@ use rustc_errors::ErrorGuaranteed;
use rustc_hir as hir;
use rustc_hir::def::DefKind;
use rustc_middle::middle;
use rustc_middle::mir::interpret::GlobalId;
use rustc_middle::query::Providers;
use rustc_middle::ty::{Ty, TyCtxt};
use rustc_middle::ty::{self, Ty, TyCtxt};
use rustc_middle::util;
use rustc_session::parse::feature_err;
use rustc_span::{symbol::sym, Span};
Expand Down Expand Up @@ -187,7 +188,12 @@ pub fn check_crate(tcx: TyCtxt<'_>) -> Result<(), ErrorGuaranteed> {
let def_kind = tcx.def_kind(item_def_id);
match def_kind {
DefKind::Static { .. } => tcx.ensure().eval_static_initializer(item_def_id),
DefKind::Const => tcx.ensure().const_eval_poly(item_def_id.into()),
DefKind::Const if tcx.generics_of(item_def_id).params.is_empty() => {
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this impacts behavior for consts with where-bounds, do we get a similar perf improvement if we continue using const_eval_poly but change it to use self.tcx.param_env_with_reveal_all(def_id)?

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@oli-obk oli-obk Feb 27, 2024 •
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this is only an issue with trivial_bounds, which are checked on MIR in mir_drops_elaborated_and_const_checked. If they don't hold, we destroy the MIR body which will fail const eval even if the constant is never called. Not ideal I guess. I'll ponder on how to get perf and no issues at the same time 😆

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Hmm... I guess we already fail it today:

#![feature(trivial_bounds)]
#![feature(generic_const_items)]
const FOO: () = () where str: Sized;

with exactly the error I assumed

error[E0080]: evaluation of constant value failed
 --> src/lib.rs:3:1
  |
3 | const FOO: () = () where str: Sized;
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ entering unreachable code

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how does this work with

trait Trait {
    const ASSOC: u32;
}

impl Trait for ()
where
    for<'a> &'a (): Trait,
{
    const ASSOC: u32 = <&'static ()>::ASSOC;
}

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@oli-obk oli-obk Mar 19, 2024 •
edited
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That's not a DefKind::Const, but a DefKind::AssocConst.

I can't come up with any problematic example using generic const items, but I think your example can be rewritten with them

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does

#![feature(generic_const_items)]
trait Trait {
    const ASSOC: u32;
}

const ASSOC: u32 = <&'static ()>::ASSOC where for<'a> &'a (): Trait;

go from pass to error with this PR?

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nope, it still passes

let instance = ty::Instance::new(item_def_id.into(), ty::GenericArgs::empty());
let cid = GlobalId { instance, promoted: None };
let param_env = ty::ParamEnv::reveal_all();
tcx.ensure().eval_to_const_value_raw(param_env.and(cid));
}
_ => (),
}
});
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14 changes: 14 additions & 0 deletions tests/ui/generic-const-items/hkl_where_bounds.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,14 @@
//! Test that nonsense bounds prevent consts from being evaluated at all.
//@ check-pass

#![feature(generic_const_items)]
#![allow(incomplete_features)]
trait Trait {
const ASSOC: u32;
}

// rustfmt eats the where bound
#[rustfmt::skip]
const ASSOC: u32 = <&'static ()>::ASSOC where for<'a> &'a (): Trait;

fn main() {}
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