The compiler could do that automatically. Suppose you're operating on the control flow graph and you are to specialise a basic block in some type environment. You can cache the result so that the next time the same basic block is specialised in the same type environment you look up the result in the cache and create a call to that existing basic block.

  pub fn big_function<T: Into<i32>>(x: T) {
      let y: i32 = x.into();
      ... code that uses y but not x ....
  }

In the first line of the function you have the environment {T: Into<i32>, x: T}. After the let you have the environment {T: Into<i32>, x: T, y: i32}. If you keyed the cache on the full type environment you wouldn't solve the issue because the code that only uses y would still get specialised to the {T: Into<i32>, x: T} too. However, you could detect that that code doesn't actually use T and x, so that you can specialise it to the type environment {y: i32} only.

That detection can happen as a side effect of specialising that code to some particular {T: Into<i32>, x: T, y: i32} for the first time. As you specialise that code you record which parts of the type environment actually got used, and you use only those parts as a key in the cache. The type environment object itself could take care of recording what the compiler looked up in it. Another advantage of doing it this way, rather than analysing the code ahead of time, is that it can handle cases where a particular type variable does or doesn't get used depending on what type some other type variable is instantiated to. You could also use the same system to avoid duplicating code that only relies on particular aspects of a type. For instance, a function that permutes the values in a &mut[T] might only care about the size of T and not about the precise type T, so that all its specialisations to T of 4 byte size can call into the same code.

Another thing you'd probably want to do is integrate a basic form of constant propagation & dead code elimination during monomorphisation, so that you don't spend a lot of time monomorphising code that ends up dead for particular type instantiations.