Like this:

  'a: {
      let a_var: u32 = 42;
      let a_ref: &'a u32 = &a_var;
  } //    ^^^^^ only valid in 'a: {}

That doesn't compile with "use of undeclared lifetime name `'a`"

You can name loop labels like this, but you cannot use them to introduce actual lifetimes, that’s right.