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

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

Author: Dylan-DPC

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

@@ -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 { .. }

src/test/ui/consts/const-eval/unwind-abort.rs

@@ -0,0 +1,13 @@
+#![feature(unwind_attributes, const_panic)]
+
+#[unwind(aborts)]
+const fn foo() {
+    panic!() //~ evaluation of constant value failed
+}
+
+const _: () = foo(); //~ any use of this value will cause an error
+// Ensure that the CTFE engine handles calls to `#[unwind(aborts)]` gracefully
+
+fn main() {
+    let _ = foo();
+}

src/test/ui/consts/const-eval/unwind-abort.stderr

@@ -0,0 +1,21 @@
+error[E0080]: evaluation of constant value failed
+  --> $DIR/unwind-abort.rs:5:5
+   |
+LL |     panic!()
+   |     ^^^^^^^^ the evaluated program panicked at 'explicit panic', $DIR/unwind-abort.rs:5:5
+   |
+   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: any use of this value will cause an error
+  --> $DIR/unwind-abort.rs:8:15
+   |
+LL | const _: () = foo();
+   | --------------^^^^^-
+   |               |
+   |               referenced constant has errors
+   |
+   = note: `#[deny(const_err)]` on by default
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0080`.

src/test/ui/consts/unwind-abort.rs

@@ -0,0 +1,17 @@
+// check-pass
+
+#![feature(unwind_attributes, const_panic)]
+
+// `#[unwind(aborts)]` is okay for a `const fn`. We don't unwind in const-eval anyways.
+#[unwind(aborts)]
+const fn foo() {
+    panic!()
+}
+
+const fn bar() {
+    foo();
+}
+
+fn main() {
+    bar();
+}