[rvrb]

it can narrow the payload: https://zigbin.io/7cb79d

I think the post would be more helpful if it had a concrete use case. let's say a contrived bytecode VM:

  dispatch: switch (instruction) {
      inline .load, .load0, .load1, .load2, .load3 => |_, tag| {
          const slot = switch (tag) {
              .load => self.read(u8),
              else => @intFromEnum(tag) - @intFromEnum(.load0),
          };
          self.push(self.locals[slot]);
          continue :dispatch self.read(Instruction);
      },
      // ...
  }

"because comptime", this is effectively the same runtime performance as the common:

  dispatch: switch (instruction) {
      .load => {
          self.push(self.locals[self.read(u8)]);
          continue :dispatch self.read(Instruction);
      },
      .load0 => {
          self.push(self.locals[0]);
          continue :dispatch self.read(Instruction);
      },
      .load1 => {
          self.push(self.locals[1]);
          continue :dispatch self.read(Instruction);
      },
      .load2 => {
          self.push(self.locals[2]);
          continue :dispatch self.read(Instruction);
      },
      .load3 => {
          self.push(self.locals[3]);
          continue :dispatch self.read(Instruction);
      },
      // ...
  }

and this is in a situation where this level of performance optimization is actually valuable to spend time on. it's nice that Zig lets you achieve it while reusing the logic.