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When type-checking binops, LHS of assign-op like += is invariant.

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Therefore we cannot coerce it to a supertype the same way that we can
the LHS of `+`.

Addresses issue 52126.
This commit is contained in:
Felix S. Klock II 2018-07-19 15:10:14 +02:00
parent 4260c8b1e4
commit c83602a702
1 changed files with 18 additions and 11 deletions
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29
src/librustc_typeck/check/op.rs
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@ -165,18 +165,25 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
op,
is_assign);
let lhs_needs = match is_assign {
IsAssign::Yes => Needs::MutPlace,
IsAssign::No => Needs::None
let lhs_ty = match is_assign {
IsAssign::No => {
// Find a suitable supertype of the LHS expression's type, by coercing to
// a type variable, to pass as the `Self` to the trait, avoiding invariant
// trait matching creating lifetime constraints that are too strict.
// E.g. adding `&'a T` and `&'b T`, given `&'x T: Add<&'x T>`, will result
// in `&'a T <: &'x T` and `&'b T <: &'x T`, instead of `'a = 'b = 'x`.
let lhs_ty = self.check_expr_with_needs(lhs_expr, Needs::None);
let fresh_var = self.next_ty_var(TypeVariableOrigin::MiscVariable(lhs_expr.span));
self.demand_coerce(lhs_expr, lhs_ty, fresh_var, AllowTwoPhase::No)
}
IsAssign::Yes => {
// rust-lang/rust#52126: We have to use strict
// equivalence on the LHS of an assign-op like `+=`;
// overwritten or mutably-borrowed places cannot be
// coerced to a supertype.
self.check_expr_with_needs(lhs_expr, Needs::MutPlace)
}
};
// Find a suitable supertype of the LHS expression's type, by coercing to
// a type variable, to pass as the `Self` to the trait, avoiding invariant
// trait matching creating lifetime constraints that are too strict.
// E.g. adding `&'a T` and `&'b T`, given `&'x T: Add<&'x T>`, will result
// in `&'a T <: &'x T` and `&'b T <: &'x T`, instead of `'a = 'b = 'x`.
let lhs_ty = self.check_expr_with_needs(lhs_expr, lhs_needs);
let fresh_var = self.next_ty_var(TypeVariableOrigin::MiscVariable(lhs_expr.span));
let lhs_ty = self.demand_coerce(lhs_expr, lhs_ty, fresh_var, AllowTwoPhase::No);
let lhs_ty = self.resolve_type_vars_with_obligations(lhs_ty);
// NB: As we have not yet type-checked the RHS, we don't have the