Function format_number 

pub fn format_number(
    value: f64,
    locale: &str,
    decimals: Option<usize>,
) -> String

Format a number with the decimal point and thousands separator of a locale. Currency symbols live in format_currency.

Only the base language counts, so "en-US" acts as "en". An unknown locale falls back to "en" and logs a warning.

decimals of None keeps the input’s own precision. Some(n) pads or cuts the fraction to exactly n digits.

Separators by language:

• en, ja, zh, ko, th: . decimal, , groups
• de, es, it, nl, pt, pl, sv, nb, nn, da, fi, el, tr: , decimal, . groups
• fr, ru, cs, sk, bg, uk, hu: , decimal, space groups

As a Tera filter it takes the same locale and decimals arguments, and passes non-numeric input through unchanged:

{{ 1234567.89 | format_number(locale="de") }}            {# → 1.234.567,89 #}
{{ 1234.5    | format_number(locale="en", decimals=2) }} {# → 1,234.50 #}

use rustango::humanize::format_number;
assert_eq!(format_number(1234567.89, "en", None),    "1,234,567.89");
assert_eq!(format_number(1234567.89, "de", None),    "1.234.567,89");
assert_eq!(format_number(1234567.89, "fr", None),    "1 234 567,89");
assert_eq!(format_number(1234.5,     "en", Some(2)), "1,234.50");
assert_eq!(format_number(0.0,        "en", Some(0)), "0");
assert_eq!(format_number(-1234.5,    "en", None),    "-1,234.5");