Consider infering floats from integer literals

[brson]

cc #183

[mdinger]

This would be nice for 1.0 or soon after. It would fix something first time users may easily run into on the rust-lang.org landing page. See rust-lang/rust#23994 for more details.

[crumblingstatue]

Just pointing out that this seems to harmonize with the 2017 goal of "lower learning curve".

In languages like C/C++/Java, it works using an integer constant when assigning values to floats.

In languages like python,javascript,etc, everything is a "float", and it just works using "whole numbers" without any decimal point notation.

This is also a nice ergonomic boost.

[leonardo-m]

I don't like Rust to infer floats from integer literals.

Think about this code that should work with your change:

fn main() {
    let x: f32 = 16_777_217;
    println!("{:?}", x);
    let y: f64 = 9_007_199_254_740_993;
    println!("{:?}", y);
}

It's going to print:

16777216
9007199254740992

Integer literals can't be represented exactly in f32/f64, so if you don't put a ".0" to them you are giving the programmer a false expectation of exactness. Copying the other languages is not an improvement here.

[crumblingstatue]

If someone is not familiar with the accuracy problems of floating point numbers, I don't think sticking a ".0" at the end will make them realize.

[petrochenkov]

In languages like C/C++/Java, it works using an integer constant when assigning values to floats.

That's why 0.0 for floats have to be enforced through style guides.
I prefer this to be done by the language itself.

[strega-nil]

@petrochenkov @crumblingstatue In those languages (okay I'm not sure about Java, but in C and C++), it is an integer type (usually int, unless it exceeds the value of INT_MAX, in which case it becomes a long, then a long long), and then is converted to a float or double. This means that C and C++ don't strictly have integer literals being floats, but just a language defined coercion.

[comex]

@leonardo-m In theory the compiler could produce an error in that case (if there is no point).

Arguably it should produce a warning even if there is one.

[lgarczyn]

It's going to print:

16777216
9007199254740992

Integer literals can't be represented exactly in f32/f64, so if you don't put a ".0" to them you are giving the programmer a false expectation of exactness. Copying the other languages is not an improvement here.

It would be trivial to add a warning when a loss of information may occur during the implicit cast of a const value. That chain of thought could also lead to small constant positive integers to be implicitly castable as just about any numerical value. In fact, this would be MUCH safer than developers getting into the habit of "as" casting everywhere.