impl Fn(T) argument type is not infered from return type

[LHolten]

I tried this code:

struct Acceptor<T>(Box<dyn Fn(T)>);

impl<T: 'static> Acceptor<T> {
    fn new(f: impl 'static + Fn(T)) -> Self {
        Self(Box::new(f))
    }

    fn new_fn(f: fn(T)) -> Self {
        Self(Box::new(f))
    }
}

fn test_ok1() -> Acceptor<String> {
    Acceptor(Box::new(|x| {
        println!("{}", x.len());
    }))
}

fn test_ok2() -> Acceptor<String> {
    Acceptor::new_fn(|x| {
        println!("{}", x.len());
    })
}

fn test_ok3() -> Acceptor<String> {
    Acceptor::new(|x| {
        println!("{}", x);
    })
}

fn test_ok4() -> Acceptor<String> {
    Acceptor::<String>::new(|x| {
        println!("{}", x.len());
    })
}

// This is the only one that fails
fn test_fail() -> Acceptor<String> {
    Acceptor::new(|x| {
        println!("{}", x.len());
    })
}

I expected all of the test functions to compile, but the last one fails to infer the type of the variable x.

Error message:

error[E0282]: type annotations needed
  --> src/lib.rs:39:20
   |
39 |     Acceptor::new(|x| {
   |                    ^
40 |         println!("{}", x.len());
   |                        - type must be known at this point
   |
help: consider giving this closure parameter an explicit type
   |
39 |     Acceptor::new(|x: /* Type */| {
   |                     ++++++++++++

Meta

Reproduces on nightly too.

rustc --version --verbose:

rustc 1.85.0 (4d91de4e4 2025-02-17)
binary: rustc
commit-hash: 4d91de4e48198da2e33413efdcd9cd2cc0c46688
commit-date: 2025-02-17
host: x86_64-unknown-linux-gnu
release: 1.85.0
LLVM version: 19.1.7

[bjorn3]

When a method call is typechecked, the receiver type must already be known by that point to know which method to resolve the method call to in the first place. In the case of test_fail this type only gets backpropagated from the return type through Acceptor::new after typechecking the method call was attempted. And as such you get an error. Rustc doesn't go back and try to resolve the method again after it failed to resolve it the first time.

[LHolten]

@bjorn3 Do you maybe know why Acceptor::new_fn typechecks, while Acceptor::new fails, with the exact same calling code?
(respective examples test_ok2 and test_fail)