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One `@derive` line gives a type random test inputs, so a property test finds the case you missed

Hand-picked examples test the cases you thought of. A property test states a rule, here "parsing a printed price gives the price back", and checks it on many generated values. @derive(Arbitrary, Debug, Eq) writes the three impls the test needs: random Price values, printing one when it fails, and comparing two. When the rule breaks, the test fails with the seed that reproduces it, and hd test also shrinks the failing input to a small one, such as Price { cents: -1000 }. The derivations are written once, at the type, not per test.

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@derive(Arbitrary, Debug, Eq) # ← random Prices, printed and compared for freestruct Price: cents: i32tests: it_prop("parsing a printed price gives it back", prop=fn!(price: Price): assert_equal(parse(print(price)), .Some(price), reason="round trip") )# Press Test. Then drop the padding: change `pad(abs % 100)` to# `abs % 100`, so $10.00 prints as "10.0", and press Test again:# assertion-failed: round trip: actual Option.None,# expected Option.Some(Price { cents: -1000 })# The property also catches i32 MIN, which hand-picked examples would never# try: `0 - cents` overflows there, so `print` works in `i64`.# ── plumbing ──use std.testing.{Arbitrary, it_prop}fn print(price: Price) -> string: sign := if price.cents < 0: "-" else: "" wide := i64(price.cents) abs := if wide < 0: 0 - wide else: wide "$sign${abs / 100}.${pad(abs % 100)}"fn pad(n: i64) -> string: if n < 10: "0$n" else: n.to_string()fn parse(text: string) -> Price?: negative := text.starts_with("-") body := if negative: text.slice(1, text.len()) else: text let .Some((whole, fraction)) = body.split_once(".") else: return .None if fraction.len() != 2: return .None wide := i64(whole.parse::[i32]().ok()?) * 100 + i64(fraction.parse::[i32]().ok()?) cents := if negative: 0 - wide else: wide if cents < -2147483648 || cents > 2147483647: .None else: .Some(Price { cents: i32(cents) })

One @derive line gives a type random test inputs, so a property test finds the case you missed

One `@derive` line gives a type random test inputs, so a property test finds the case you missed