Difference in impl block with generics


struct Type<T>{}

What is the difference between:

impl<T> Type<T>{}


impl Type<T>{}

And where could I read about it in detail?
Thank you

If you omit the first <T> Rust thinks the remaining <T> refers to a concrete type, which is likely not what you want. But if you instead insert a known type it makes more sense:

use std::sync::Mutex;

struct MyMutex<T>(Mutex<T>);

impl<T> MyMutex<T> {
    pub fn lock(&self) {

impl MyMutex<String> {
    pub fn lock_and_set_string(&self, s: &str) {

There's a note about this in Rust By Example


You could read impl<T> as "for all types T, implement the following". So in a way, <T> written behind impl is some sort of universal quantifier.

Note that strictly speaking, the implementation is implicitly restricted to types T which are Sized. So if you want to implement something really for all types T, you would have to write impl<T: ?Sized>.


Thanks, I really like that answer.