diff --git a/text/2344-const-looping.md b/text/2344-const-looping.md new file mode 100644 index 00000000000..5e9e1a159f4 --- /dev/null +++ b/text/2344-const-looping.md @@ -0,0 +1,107 @@ +- Feature Name: `const_looping` +- Start Date: 2018-02-18 +- RFC PR: [rust-lang/rfcs#2344](https://github.com/rust-lang/rfcs/pull/2344) +- Rust Issue: [rust-lang/rust#52000](https://github.com/rust-lang/rust/issues/52000) + +# Summary +[summary]: #summary + +Allow the use of `loop`, `while` and `while let` during constant evaluation. +`for` loops are technically allowed, too, but can't be used in practice because +each iteration calls `iterator.next()`, which is not a `const fn` and thus can't +be called within constants. Future RFCs (like +https://github.com/rust-lang/rfcs/pull/2237) might lift that restriction. + +# Motivation +[motivation]: #motivation + +Any iteration is expressible with recursion. Since we already allow recursion +via const fn and termination of said recursion via `if` or `match`, all code +enabled by const recursion is already legal now. Some algorithms are better +expressed as imperative loops and a lot of Rust code uses loops instead of +recursion. Allowing loops in constants will allow more functions to become const +fn without requiring any changes. + +# Guide-level explanation +[guide-level-explanation]: #guide-level-explanation + +If you previously had to write functional code inside constants, you can now +change it to imperative code. For example if you wrote a fibonacci like + +```rust +const fn fib(n: u128) -> u128 { + match n { + 0 => 1, + 1 => 1, + n => fib(n - 1) + fib(n + 1) + } +} +``` + +which takes exponential time to compute a fibonacci number, you could have +changed it to the functional loop + +```rust +const fn fib(n: u128) -> u128 { + const fn helper(n: u128, a: u128, b: u128, i: u128) -> u128 { + if i <= n { + helper(n, b, a + b, i + 1) + } else { + b + } + } + helper(n, 1, 1, 2) +} +``` + +but now you can just write it as an imperative loop, which also finishes in +linear time. + +```rust +const fn fib(n: u128) -> u128 { + let mut a = 1; + let mut b = 1; + let mut i = 2; + while i <= n { + let tmp = a + b; + a = b; + b = tmp; + i += 1; + } + b +} +``` + +# Reference-level explanation +[reference-level-explanation]: #reference-level-explanation + +A loop in MIR is a cyclic graph of `BasicBlock`s. Evaluating such a loop is no +different from evaluating a linear sequence of `BasicBlock`s, except that +termination is not guaranteed. To ensure that the compiler never hangs +indefinitely, we count the number of terminators processed and whenever we reach +a fixed limit, we report a lint mentioning that we cannot guarantee that the +evaluation will terminate and reset the counter to zero. This lint should recur +in a non-annoying amount of time (e.g. at least 30 seconds between occurrences). +This means that there's an internal deterministic counter (for the terminators) and +a timestamp of the last (if any) loop warning emission. Both the counter needs to reach +its limit and 30 seconds have to have passed since the last warning emission in order +for a new warning to be emitted. + +# Drawbacks +[drawbacks]: #drawbacks + +* Infinite loops will cause the compiler to never finish if the lint is not denied + +# Rationale and alternatives +[alternatives]: #alternatives + +- Do nothing, users can keep using recursion + +# Unresolved questions +[unresolved]: #unresolved-questions + +* Should we add a true recursion check that hashes the interpreter state and + detects if it has reached the same state again? + * This will slow down const evaluation enormously and for complex iterations + is essentially useless because it'll take forever (e.g. counting from 0 to + `u64::max_value()`)