In Rust Book Brown University Chapter 10.2 2nd Quiz Question 1 states:

Determine whether the program will pass the compiler. If it passes, write the expected output of the program if it were executed.

use std::fmt::Display;
fn displayable<T: Display>(t: T) -> impl Display { t }
fn main() {
  let s = String::from("hello");
  let mut s2 = displayable(s);
  s2.push_str(" world");
  println!("{s2}");
}

Spoler alert, it doesn’t compile.

It’s explained that it could compile if the return type was changed to T instead of impl Display.

This left me curious, I’m thinking the compiler will always be able to infer the type. The reason being that the return type of a function cannot be conditional (also stated in ch. 10.2) and thus the return type is either

• 1. Constant, always the same independent of any of the argument types
• 2. Same as one of the arguments

I am wrong or are you simply forced to be explicit about the return type event though it could be inferred?