Allow Unreachable terminators through min_const_fn checks

src/librustc_mir/transform/qualify_min_const_fn.rs

@@ -337,6 +337,9 @@ fn check_terminator(
             check_operand(tcx, discr, span, def_id, body)
         }
 
+        // FIXME(ecstaticmorse): We probably want to allow `Unreachable` unconditionally.
+        TerminatorKind::Unreachable if tcx.features().const_if_match => Ok(()),
+
         | TerminatorKind::Abort | TerminatorKind::Unreachable => {
             Err((span, "const fn with unreachable code is not stable".into()))
         }

src/test/ui/consts/control-flow/exhaustive-c-like-enum-match.rs

@@ -0,0 +1,27 @@
+// Test for <https://github.com/rust-lang/rust/issues/66756>
+
+// check-pass
+
+#![feature(const_if_match)]
+
+enum E {
+    A,
+    B,
+    C
+}
+
+const fn f(e: E) {
+    match e {
+        E::A => {}
+        E::B => {}
+        E::C => {}
+    }
+}
+
+const fn g(e: E) {
+    match e {
+        _ => {}
+    }
+}
+
+fn main() {}