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Correct reasoning behind writeback ref removal

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The ref was actually always necessary.
This clarifies some incorrect thinking in the original code that might
have led to errors in the future to do with unsizing.
This commit is contained in:
Isaac van Bakel 2018-01-06 19:04:09 +00:00
parent 2133ace207
commit 6132806d36
1 changed files with 28 additions and 32 deletions
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60
src/librustc_typeck/check/writeback.rs
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@ -169,41 +169,37 @@ impl<'cx, 'gcx, 'tcx> WritebackCx<'cx, 'gcx, 'tcx> {
if let hir::ExprIndex(ref base, ref index) = e.node {
let mut tables = self.fcx.tables.borrow_mut();
let base_ty = {
let base_ty = tables.expr_ty_adjusted(&base);
// When unsizing, the final type of the expression is taken
// from the first argument of the indexing operator, which
// is a &self, and has to be deconstructed
if let ty::TyRef(_, ref ref_to) = base_ty.sty {
ref_to.ty
} else {
base_ty
}
};
match tables.expr_ty_adjusted(&base).sty {
// All valid indexing looks like this
ty::TyRef(_, ty::TypeAndMut { ty: ref base_ty, .. }) => {
let index_ty = tables.expr_ty_adjusted(&index);
let index_ty = self.fcx.resolve_type_vars_if_possible(&index_ty);
let index_ty = tables.expr_ty_adjusted(&index);
let index_ty = self.fcx.resolve_type_vars_if_possible(&index_ty);
if base_ty.builtin_index().is_some()
&& index_ty == self.fcx.tcx.types.usize {
// Remove the method call record
tables.type_dependent_defs_mut().remove(e.hir_id);
tables.node_substs_mut().remove(e.hir_id);
if base_ty.builtin_index().is_some()
&& index_ty == self.fcx.tcx.types.usize {
// Remove the method call record
tables.type_dependent_defs_mut().remove(e.hir_id);
tables.node_substs_mut().remove(e.hir_id);
tables.adjustments_mut().get_mut(base.hir_id).map(|a| {
// Discard the need for a mutable borrow
match a.pop() {
// Extra adjustment made when indexing causes a drop
// of size information - we need to get rid of it
// Since this is "after" the other adjustment to be
// discarded, we do an extra `pop()`
Some(Adjustment { kind: Adjust::Unsize, .. }) => {
// So the borrow discard actually happens here
a.pop();
},
_ => {}
tables.adjustments_mut().get_mut(base.hir_id).map(|a| {
// Discard the need for a mutable borrow
match a.pop() {
// Extra adjustment made when indexing causes a drop
// of size information - we need to get rid of it
// Since this is "after" the other adjustment to be
// discarded, we do an extra `pop()`
Some(Adjustment { kind: Adjust::Unsize, .. }) => {
// So the borrow discard actually happens here
a.pop();
},
_ => {}
}
});
}
});
},
// Might encounter non-valid indexes at this point, so there
// has to be a fall-through
_ => {},
}
}
}