Comparison with f64.nan

Built from the repository at commit a8614dc.

A comparison == or != with f64.nan on either side. nan is not equal to any number, itself included, so x == f64.nan is always false and x != f64.nan always true. The warning points at the comparison, and the program still builds. To test whether x is nan, call x.is_nan().

Why

x == f64.nan reads as the test that x is nan, and it never is one: IEEE 754 defines every comparison with nan as false, except !=. The same holds in Rust, C and Python. When the other side is an f64, the safe fix writes x.is_nan(), or !x.is_nan() for !=, with parentheses around an x that needs them. When it is an f64?, there is no fix, because x.is_nan() would not check: unwrap the value first.

Broken

fn main() {
    let x = 0.5
    print(x == f64.nan)
}

What sisu says

warning[W0005]: comparison with `f64.nan` is always false
 --> W0005.sisu:3:11
  |
3 |     print(x == f64.nan)
  |           ^^^^^^^^^^^^
  |
  = help: `nan` is not equal to any number, itself included; test for it with `is_nan()`
help: test it with `is_nan()`
  |
3 -     print(x == f64.nan)
3 +     print(x.is_nan())
  |

sisu: 1 warning
sisu: for more on a code, run `sisu explain <code>`: W0005

Fixed

fn main() {
    let x = 0.5
    print(x.is_nan())
}

Related

• E0105: a type used as a value.