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Optimize impl Hash for ObligationCauseData by not hashing ObligationCauseCode variant fields #90996

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Merged
merged 1 commit into from
Nov 19, 2021
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Optimize impl Hash for ObligationCauseData by not hashing ObligationCauseCode variant fields #90996

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11 changes: 10 additions & 1 deletion compiler/rustc_middle/src/traits/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -23,6 +23,7 @@ use smallvec::SmallVec;

use std::borrow::Cow;
use std::fmt;
use std::hash::{Hash, Hasher};
use std::ops::Deref;

pub use self::select::{EvaluationCache, EvaluationResult, OverflowError, SelectionCache};
Expand Down Expand Up @@ -108,7 +109,7 @@ impl Deref for ObligationCause<'tcx> {
}
}

#[derive(Clone, Debug, PartialEq, Eq, Hash, Lift)]
#[derive(Clone, Debug, PartialEq, Eq, Lift)]
pub struct ObligationCauseData<'tcx> {
pub span: Span,

Expand All @@ -123,6 +124,14 @@ pub struct ObligationCauseData<'tcx> {
pub code: ObligationCauseCode<'tcx>,
}

impl Hash for ObligationCauseData<'_> {
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For next time: I suggest leaving a brief comment explaining why this custom hash is used, because it's non-obvious and no fun to have to dig through version control history to understand why.

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Please open an issue so it's not forgotten.

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I'm not opposed to adding comments but

no fun to have to dig through version control history

seems a bit subjective, for me that's 3 clicks which is hardly onerous. So that didn't quite meet my threshold of adding a comment.

fn hash<H: Hasher>(&self, state: &mut H) {
self.body_id.hash(state);
self.span.hash(state);
std::mem::discriminant(&self.code).hash(state);
}
}

impl<'tcx> ObligationCause<'tcx> {
#[inline]
pub fn new(
Expand Down