Why can't I compile this?

[Seungwoo Kang]

        use once_cell::unsync::Lazy;
        let lazy  = Lazy::new(|| "sd".to_uppercase());

Above code returns following error:

error[E0282]: type annotations needed for `once_cell::unsync::Lazy<T, [closure@perfkit_config\src\storage.rs:65:31: 65:53]>`
  --> perfkit_config\src\storage.rs:65:13
   |
65 |         let lazy  = Lazy::new(|| "sd".to_uppercase());
   |             ^^^^
   |
help: consider giving `lazy` an explicit type, where the type for type parameter `T` is specified
   |
65 |         let lazy: once_cell::unsync::Lazy<T, [closure@perfkit_config\src\storage.rs:65:31: 65:53]>  = Lazy::new(|| "sd".to_uppercase());
   |                 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

In Cargo:

[dependencies]
once_cell = "1.15.0"

(Can't compile example one neither, which replaces "sd" to captured 'hello')

It seems the compiler should deduce type 'String' from return value of given closure, but it doesn't. Am I doing something wrong?

I'm using rustc 1.63.0 on Windows.

[Alex Kladov]

Heh, that's an interesteen puzzle, see rust-lang/rust#103718.

We might consider doing the same for once cell, once our MSRV allows.

For your specific case, the fix would be

let lazy: Lazy<String, _>  = Lazy::new(|| "sd".to_uppercase());

[Seungwoo Kang]

Oh, thanks for your reply. Now it compiles!