Rollup merge of rust-lang#68314 - oli-obk:true_unwind, r=eddyb

Stop treating `FalseEdges` and `FalseUnwind` as having semantic value for const eval

This change does not expose loops or conditions to stable const fns because we check those at the HIR level and in the regular const validity checks.

cc @ecstatic-morse

r? @eddyb

Author: tmandry

src/librustc_mir/transform/qualify_min_const_fn.rs

@@ -309,21 +309,22 @@ fn check_terminator(
 ) -> McfResult {
     let span = terminator.source_info.span;
     match &terminator.kind {
-        TerminatorKind::Goto { .. } | TerminatorKind::Return | TerminatorKind::Resume => Ok(()),
+        TerminatorKind::FalseEdges { .. }
+        | TerminatorKind::FalseUnwind { .. }
+        | TerminatorKind::Goto { .. }
+        | TerminatorKind::Return
+        | TerminatorKind::Resume => Ok(()),
 
         TerminatorKind::Drop { location, .. } => check_place(tcx, location, span, def_id, body),
         TerminatorKind::DropAndReplace { location, value, .. } => {
             check_place(tcx, location, span, def_id, body)?;
             check_operand(tcx, value, span, def_id, body)
         }
 
-        TerminatorKind::FalseEdges { .. } | TerminatorKind::SwitchInt { .. }
-            if !feature_allowed(tcx, def_id, sym::const_if_match) =>
-        {
+        TerminatorKind::SwitchInt { .. } if !feature_allowed(tcx, def_id, sym::const_if_match) => {
             Err((span, "loops and conditional expressions are not stable in const fn".into()))
         }
 
-        TerminatorKind::FalseEdges { .. } => Ok(()),
         TerminatorKind::SwitchInt { discr, switch_ty: _, values: _, targets: _ } => {
             check_operand(tcx, discr, span, def_id, body)
         }
@@ -367,13 +368,5 @@ fn check_terminator(
         TerminatorKind::Assert { cond, expected: _, msg: _, target: _, cleanup: _ } => {
             check_operand(tcx, cond, span, def_id, body)
         }
-
-        TerminatorKind::FalseUnwind { .. } if feature_allowed(tcx, def_id, sym::const_loop) => {
-            Ok(())
-        }
-
-        TerminatorKind::FalseUnwind { .. } => {
-            Err((span, "loops are not allowed in const fn".into()))
-        }
     }
 }