Lint integer variable fallback to i32

[tamuhey]

From https://stackoverflow.com/questions/65867874/how-to-deny-int-variable-without-explicit-type-annotation-in-rust?noredirect=1#comment116458771_65867874

Related: https://github.com/rust-lang/rfcs/blob/master/text/0212-restore-int-fallback.md

What it does

Reports integer variable fallback to i32, e.g.

let mut counter = 0; # Warning, because the type of `counter` is ambiguous and fallback to `i32`
let mut counter: u64 = 0; # OK
let mut counter = 0usize # OK

The below code should also be accepted because the variable is inferred though it is not annotated:

fn f(x: usize);
let mut counter = 0; # OK, because the variable `counter` is inferred as `usize` later.
f(counter)

Categories (optional)

• Kind: See https://github.com/rust-lang/rust-clippy/blob/master/README.md#clippy for list of lint kinds

maybe pedantic

What is the advantage of the recommended code over the original code

I want this lint rule because it can cause overflow unintendedly:

let mut counter = 0;
for _ in 0..1_000_000_000_000usize {
    counter += 1;  # overflow!
    ...
}

Drawbacks

May feel tedious

Example

let counter = 0;

Could be written as:

let counter = 0usize;
let counter: usize = 0;
// or make `couter` to be inferred as `usize`
f(counter); // f: (usize) -> (); for example.

[Y-Nak]

This issue seems duplicate of #6064.