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Add comments to explain the test case and the special treatment

This commit is contained in:
Ding Xiang Fei 2020-07-31 02:16:42 +08:00
parent ddbe69a5b2
commit 3b2642ffa7
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2 changed files with 22 additions and 2 deletions
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14
src/librustc_mir/transform/promote_consts.rs
View File

@ -506,6 +506,10 @@ impl<'tcx> Validator<'_, 'tcx> {
match *elem {
ProjectionElem::Deref => {
let mut not_promotable = true;
// This is a special treatment for cases like *&STATIC where STATIC is a
// global static variable.
// This pattern is generated only when global static variables are directly
// accessed and is qualified for promotion safely.
if let TempState::Defined { location, .. } = self.temps[local] {
let def_stmt =
self.body[location.block].statements.get(location.statement_index);
@ -517,7 +521,15 @@ impl<'tcx> Validator<'_, 'tcx> {
{
if let Some(did) = c.check_static_ptr(self.tcx) {
if let Some(hir::ConstContext::Static(..)) = self.const_kind {
if !self.tcx.is_thread_local_static(did) {
// The `is_empty` predicate is introduced to exclude the case
// where the projection operations are [ .field, * ].
// The reason is because promotion will be illegal if field
// accesses preceed the dereferencing.
// Discussion can be found at
// https://github.com/rust-lang/rust/pull/74945#discussion_r463063247
// There may be opportunity for generalization, but this needs to be
// accounted for.
if proj_base.is_empty() && !self.tcx.is_thread_local_static(did) {
not_promotable = false;
}
}

10
src/test/ui/statics/static-promotion.rs
View File

@ -1,4 +1,12 @@
// run-pass
// check-pass
// Use of global static variables in literal values should be allowed for
// promotion.
// This test is to demonstrate the issue raised in
// https://github.com/rust-lang/rust/issues/70584
// Literal values were previously promoted into local static values when
// other global static variables are used.
struct A<T: 'static>(&'static T);
struct B<T: 'static + ?Sized> {