constify Option methods

[oli-obk]

r? @fee1-dead

tracking issue: #143956

these unblock making PartialOrd and Ord const without resorting to inlining some of these at call sites.

[oli-obk]

This pattern was introduced in #116481 because it optimizes better.

[fee1-dead]

I have a feeling this will nerdsnipe me hard.

Here's how code is currently generated

#![feature(const_destruct)] #![feature(const_trait_impl)] use std::marker::Destruct; pub const fn opt_or_else<T, F>(this: Option<T>, f: F) -> Option<T> where F: ~const Destruct + ~const FnOnce() -> Option<T>, T: ~const Destruct { match this { x @ Some(_) => x, None => f() } }

// MIR FOR CTFE fn opt_or_else(_1: Option<T>, _2: F) -> Option<T> { debug this => _1; debug f => _2; let mut _0: std::option::Option<T>; let mut _3: isize; let _4: std::option::Option<T>; let mut _5: F; let mut _6: (); let mut _7: bool; let mut _8: bool; scope 1 { debug x => _4; } bb0: { _8 = const false; _7 = const false; _8 = const true; _7 = const true; _3 = discriminant(_1); switchInt(move _3) -> [0: bb2, 1: bb3, otherwise: bb1]; } bb1: { unreachable; } bb2: { StorageLive(_5); _7 = const false; _5 = move _2; StorageLive(_6); _6 = (); ConstEvalCounter; _0 = <F as FnOnce<()>>::call_once(move _5, move _6) -> [return: bb4, unwind: bb12]; } bb3: { StorageLive(_4); _8 = const false; _4 = move _1; _0 = move _4; StorageDead(_4); goto -> bb9; } bb4: { StorageDead(_6); StorageDead(_5); goto -> bb9; } bb5: { switchInt(copy _8) -> [0: bb6, otherwise: bb10]; } bb6: { return; } bb7 (cleanup): { resume; } bb8: { drop(_2) -> [return: bb5, unwind: bb12]; } bb9: { switchInt(copy _7) -> [0: bb5, otherwise: bb8]; } bb10: { drop(_1) -> [return: bb6, unwind: bb7]; } bb11 (cleanup): { drop(_1) -> [return: bb7, unwind terminate(cleanup)]; } bb12 (cleanup): { switchInt(copy _8) -> [0: bb7, otherwise: bb11]; } }

looks like it tries to drop self at the None arm, which is just weird.

[fee1-dead]

r=me after nit

[fee1-dead]

I think we should put FnOnce first, here and elsewhere in the PR. the destruct bound is noisy and likely to make it harder for someone to read the code.

[oli-obk]

@bors r=fee1-dead

[bors]

📌 Commit 853333d has been approved by fee1-dead

It is now in the queue for this repository.