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improve const infer error #78249

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Oct 24, 2020
Merged

improve const infer error #78249

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40ab18d
improve const infer error
lcnr Oct 22, 2020
e1c524c
review
lcnr Oct 23, 2020
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16 changes: 5 additions & 11 deletions compiler/rustc_middle/src/infer/unify_key.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -176,17 +176,17 @@ impl<'tcx> UnifyValue for ConstVarValue<'tcx> {
type Error = (&'tcx ty::Const<'tcx>, &'tcx ty::Const<'tcx>);

fn unify_values(value1: &Self, value2: &Self) -> Result<Self, Self::Error> {
let (val, span) = match (value1.val, value2.val) {
let (val, origin) = match (value1.val, value2.val) {
(ConstVariableValue::Known { .. }, ConstVariableValue::Known { .. }) => {
bug!("equating two const variables, both of which have known values")
}

// If one side is known, prefer that one.
(ConstVariableValue::Known { .. }, ConstVariableValue::Unknown { .. }) => {
(value1.val, value1.origin.span)
(value1.val, value1.origin)
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}
(ConstVariableValue::Unknown { .. }, ConstVariableValue::Known { .. }) => {
(value2.val, value2.origin.span)
(value2.val, value2.origin)
}

// If both sides are *unknown*, it hardly matters, does it?
Expand All @@ -200,17 +200,11 @@ impl<'tcx> UnifyValue for ConstVarValue<'tcx> {
// universe is the minimum of the two universes, because that is
// the one which contains the fewest names in scope.
let universe = cmp::min(universe1, universe2);
(ConstVariableValue::Unknown { universe }, value1.origin.span)
(ConstVariableValue::Unknown { universe }, value1.origin)
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And inline the ConstVarValue { .. } part here. I don't even think the span should be relevant here, so maybe DUMMY_SP would be more semantic.

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The span is relevant here. It isn't really deterministic which const infer we use when reporting errors afaict, so we pretty much require all const infer variables to have relevant origin data.

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Hmm, I was basing my assumption off

rust/compiler/rustc_infer/src/infer/type_variable.rs

Lines 482 to 494 in d1c2815

// If both sides are *unknown*, it hardly matters, does it?
(
&TypeVariableValue::Unknown { universe: universe1 },
&TypeVariableValue::Unknown { universe: universe2 },
) => {
// If we unify two unbound variables, ?T and ?U, then whatever
// value they wind up taking (which must be the same value) must
// be nameable by both universes. Therefore, the resulting
// universe is the minimum of the two universes, because that is
// the one which contains the fewest names in scope.
let universe = cmp::min(universe1, universe2);
Ok(TypeVariableValue::Unknown { universe })
}

which doesn't keep track of span information, but it's not important either way.

}
};

Ok(ConstVarValue {
origin: ConstVariableOrigin {
kind: ConstVariableOriginKind::ConstInference,
span: span,
},
val,
})
Ok(ConstVarValue { origin, val })
}
}

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2 changes: 1 addition & 1 deletion src/test/ui/const-generics/infer/issue-77092.stderr
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Expand Up @@ -2,7 +2,7 @@ error[E0282]: type annotations needed
--> $DIR/issue-77092.rs:13:26
|
LL | println!("{:?}", take_array_from_mut(&mut arr, i));
| ^^^^^^^^^^^^^^^^^^^ cannot infer the value of the constant `{_: usize}`
| ^^^^^^^^^^^^^^^^^^^ cannot infer the value of const parameter `N` declared on the function `take_array_from_mut`

error: aborting due to previous error

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