Allow Abort terminators in a const-context

These appear along the cleanup path inside functions with
`#[unwind(aborts)]`. We don't const-check the cleanup path anyways,
since const-eval already has "abort-on-panic" semantics and there's
often drops that would otherwise be forbidden, so the check wasn't
really preventing anything anyways.

compiler/rustc_mir/src/transform/check_consts/ops.rs

@@ -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 {

compiler/rustc_mir/src/transform/check_consts/validation.rs

@@ -874,13 +874,13 @@ 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::Assert { .. }
+            TerminatorKind::Abort
+            | TerminatorKind::Assert { .. }
             | TerminatorKind::FalseEdge { .. }
             | TerminatorKind::FalseUnwind { .. }
             | TerminatorKind::Goto { .. }