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Rollup merge of #77512 - ecstatic-morse:const-checking-allow-abort, r=RalfJung

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Allow `Abort` terminators in all const-contexts

We never unwind during const-eval, so we basically have these semantics already. Also I just figured out that these only appear along the cleanup path, which doesn't get const-checked. In other words, this doesn't actually change behavior: the `check-pass` test I added compiles just fine on nightly.

r? @RalfJung
cc @rust-lang/wg-const-eval
This commit is contained in:
Dylan DPC 2020-10-05 02:29:40 +02:00 • committed by GitHub
commit 5fbb41108a
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5 changed files with 62 additions and 17 deletions
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12
compiler/rustc_mir/src/transform/check_consts/ops.rs
View file

@ -41,18 +41,6 @@ pub trait NonConstOp: std::fmt::Debug {
fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> DiagnosticBuilder<'tcx>;
}
#[derive(Debug)]
pub struct Abort;
impl NonConstOp for Abort {
fn status_in_item(&self, ccx: &ConstCx<'_, '_>) -> Status {
mcf_status_in_item(ccx)
}
fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> DiagnosticBuilder<'tcx> {
mcf_build_error(ccx, span, "abort is not stable in const fn")
}
}
#[derive(Debug)]
pub struct FloatingPointOp;
impl NonConstOp for FloatingPointOp {

16
compiler/rustc_mir/src/transform/check_consts/validation.rs
View file

@ -434,11 +434,13 @@ impl Visitor<'tcx> for Validator<'mir, 'tcx> {
fn visit_basic_block_data(&mut self, bb: BasicBlock, block: &BasicBlockData<'tcx>) {
trace!("visit_basic_block_data: bb={:?} is_cleanup={:?}", bb, block.is_cleanup);
// Just as the old checker did, we skip const-checking basic blocks on the unwind path.
// These blocks often drop locals that would otherwise be returned from the function.
// We don't const-check basic blocks on the cleanup path since we never unwind during
// const-eval: a panic causes an immediate compile error. In other words, cleanup blocks
// are unreachable during const-eval.
//
// FIXME: This shouldn't be unsound since a panic at compile time will cause a compiler
// error anyway, but maybe we should do more here?
// We can't be more conservative (e.g., by const-checking cleanup blocks anyways) because
// locals that would never be dropped during normal execution are sometimes dropped during
// unwinding, which means backwards-incompatible live-drop errors.
if block.is_cleanup {
return;
}
@ -874,12 +876,16 @@ impl Visitor<'tcx> for Validator<'mir, 'tcx> {
}
TerminatorKind::InlineAsm { .. } => self.check_op(ops::InlineAsm),
TerminatorKind::Abort => self.check_op(ops::Abort),
TerminatorKind::GeneratorDrop | TerminatorKind::Yield { .. } => {
self.check_op(ops::Generator(hir::GeneratorKind::Gen))
}
TerminatorKind::Abort => {
// Cleanup blocks are skipped for const checking (see `visit_basic_block_data`).
span_bug!(self.span, "`Abort` terminator outside of cleanup block")
}
TerminatorKind::Assert { .. }
| TerminatorKind::FalseEdge { .. }
| TerminatorKind::FalseUnwind { .. }