Explain why we can encounter a Goto terminator that we want to promote

src/librustc_mir/transform/promote_consts.rs

@@ -332,7 +332,13 @@ impl<'a, 'tcx> Promoter<'a, 'tcx> {
                             let operand = Operand::Copy(promoted_place(ty, span));
                             mem::replace(&mut args[index], operand)
                         }
-                        // already promoted out
+                        // We expected a `TerminatorKind::Call` for which we'd like to promote an
+                        // argument. Since `qualify_consts` saw a `TerminatorKind::Call` here, but
+                        // we are seeing a `Goto`, that means that the `promote_temps` method
+                        // already promoted this call away entirely. This case occurs when calling
+                        // a function requiring a constant argument and as that constant value
+                        // providing a value whose computation contains another call to a function
+                        // requiring a constant argument.
                         TerminatorKind::Goto { .. } => return,
                         _ => bug!()
                     }