How does Rust infer resultant types from From::<>::from()?

Question

In this snippet from Hyper\'s example, there\'s a bit of code that I\'ve annotated with types that compiles successfully:

.map_err(|x: std::io::Error| -> hype         


        

Answer 1

Type information in Rust can flow backwards.

The return type of the closure is specified to be hyper::Error. Therefore, the result of the block must be hyper::Error, therefore the result of From::from must be hyper::Error.

If you wanted to, you could use ...

::::from(x)

... which would be the even more fully qualified version. But with the closure return type there, it's unnecessary.