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improve const infer error #78249

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Merged
merged 2 commits into from
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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25 changes: 8 additions & 17 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 @@ -175,19 +175,15 @@ impl<'tcx> UnifyKey for ty::ConstVid<'tcx> {
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) {
fn unify_values(&value1: &Self, &value2: &Self) -> Result<Self, Self::Error> {
Ok(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)
}
(ConstVariableValue::Unknown { .. }, ConstVariableValue::Known { .. }) => {
(value2.val, value2.origin.span)
}
(ConstVariableValue::Known { .. }, ConstVariableValue::Unknown { .. }) => value1,
(ConstVariableValue::Unknown { .. }, ConstVariableValue::Known { .. }) => value2,

// If both sides are *unknown*, it hardly matters, does it?
(
Expand All @@ -200,16 +196,11 @@ 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)
ConstVarValue {
val: ConstVariableValue::Unknown { universe },
origin: value1.origin,
}
}
};

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