assert_eq! output is very unhelpful for floats

[nagisa]

assert_eq!(1E-7, 0.0);

will output

assertion failed: (left == right) && (right == left) (left: 0f64, right: 0f64)' …

Note that values displayed are pretty useless.

[kmcallister]

How about

trait DebugCompare<RHS> {
    pub describe_difference(&self, rhs: &RHS) -> Option<Cow<str>>;
}

macro_rules! assert_eq {
    ($left:expr, $right:expr) => {{
        match (&($left), &($right)) {
            (left_val, right_val) => if let Some(desc) = left_val.describe_difference(right_val) {
                panic!("assert_eq! failed: difference between {:?} and {:?} is {}",
                    *left_val, *right_val, desc);
            },
        }
    }}
}

Then DebugCompare for f64 can give the numerical difference, DebugCompare for Expr can do an AST diff (???), etc.

[lifthrasiir]

With #24612 this prints the following, thus I believe this is fixed by now.

thread '<main>' panicked at 'assertion failed: `(left == right)` (left: `0.0000001`, right: `0`)', t.rs:1

[bstrie]

Closed by #24612.