Rust did not choose to infer types in signatures for UX and stability reasons. If you infer types in signatures, then a change in the body of the function can change the type of the function, which makes dealing with backwards compatibility harder. Also, it leads to weird "errors at a distance."

I call this property "an accidental complexity" - when changing one piece of code causes failures in other, seemingly unrelated parts of the system. The property causes fear of introducing changes, and is usually a sign of tight coupling.

I tend to agree with the philosophy that a language should not require an IDE to be productive in.

With function signatures, I think it's particularly relevant for documentation as well. I should be able to look at a function signature in generated docs and know everything about what goes into and comes out of that function. For me the signature should be a complete contract, and changing code within a function body should not be able to change the signature.