Tracking issue for a minimal subset of RFC 911, const fn #53555

Also cc: @eddyb, @RalfJung, @est31, @durka, @mark-i-m, @japaric

Some concerns noted over at #24111:

• design (const fn tracking issue (RFC 911) #24111 (comment))
• priority (const fn tracking issue (RFC 911) #24111 (comment))
• runtime-pointer-addresses (const fn tracking issue (RFC 911) #24111 (comment))
• everything-const (const fn tracking issue (RFC 911) #24111 (comment))
• parallel-const-traits resolved by const fn tracking issue (RFC 911) #24111 (comment)

runtime-pointer-addresses (#24111 (comment))

does not apply here, because we neither stabilize union field accesses which could be used to transmute between any pointer and an integer, nor do we stabilize casting *const T to usize.

priority (#24111 (comment))

The 2018 edition takes precedence over this feature. Since it has been minimized to a very small surface area which will be enforced by a feature whitelist, it seems plausible that we can get everyone on board to at least stabilize it shortly after the 2018 edition has been released.

design (#24111 (comment))
everything-const (#24111 (comment))

Since we are punting on (almost) all design issues by using a minimal scheme, the only design question left is whether we want to start attaching the const keyword to many functions.

Alternate designs:

• infer the constness (deemed not feasible right now and a semver footgun)
• inverting the system by marking functions as notconst (massive breaking change)
• using a #[const] attribute

Wrt the "everything function is getting const attached to it" worry: The current design is entirely forward compatible with allowing const mod, const impl to allow marking entire code regions as const. Although that seems like it could benefit from the attribute, as you could write #![const] at the top of the crate root and have everything be const.

because we neither stabilize union field accesses which could be used to transmute between any pointer and an integer

We do not? I see you sneakily added that to the first post.^^

These are already stable for const. Why not allow them in const fn?

@RalfJung

Why not allow the in const fn?

@oli-obk added that part, but I think about it like this: given union field accesses, you can perform &[mut] T -> usize and so you may violate our plans for CTFE correctness per your blog post. Now, we can say that the operation *(const/mut) T -> usize is UB in const fn, but I would like to wait with that because:

1. It gives us more time to develop our story around the rules of unsafe within const fn.
2. We need to make sure that our teaching material is up to the task so that users will not do &T -> usize and think that's OK.

All in all, I think that if we are more conservative here and punt on this, we can ship a more minimal const fn faster.

I see. I don't like these inconsistencies but oh well. Looks like qualify_consts will be cleaned up... another time.^^

In terms of teaching, again we have the same problem for const... I thought the sanity check might help (unlike a const fn, a const has a very simple "observable behavior" so we can actually test for these things statically) but it seems it does not complain about

union Trans { x: &'static i32, y: usize }
const BAR : usize = unsafe { Trans { x: &0 }.y };

First, a nit:

const fns with type parameters with bounds (including where clauses) (lifetimes are OK) either on themselves or on the scope they are contained within (e.g. impls and such...)

Presumably something like T: Sized is ok -- or, more to the point, const fn foo<T>(t: T) -> T { t }...? So is it just that Sized trait is allowed?

But, more generally, I think that this proposal does a good job of outlining the surface area of the language to be stabilized, but I'd like to see a better outline of what the implications are regarding the "const system in general".

One key question here is the interaction of const fn and promotion, I think, since that is the main place where we can find ourselves in tricky questions (e.g., even something like const fn foo(a: u32, b: u32) -> { a + b } can be problematic if a call to it is promoted). Can someone (e.g., @oli-obk) write out the conditions in which calls to const fn can currently be promoted, if any? I don't recall. =)

I think by now I am mostly bought into the general framing from Ralf's blog post, but I'd like to also know if there are alternative approaches that we are ruling out by stabilizing const fn.

It seems like one of the key bits that I could imagine being controversial was the idea that a const fn could do things that we cannot statically guarantee are valid (e.g., in unsafe code) but that we could still promote calls to that const fn. If we wind up hitting errors at compilation time, that is then a violation of the const fn declaration analogous to hitting UB at runtime. As I said, I think I am mostly bought into this framing, but I would like to clarify if we are losing room to maneuever here in any way.

To expand a bit on something:

Presumably something like T: Sized is ok -- or, more to the point, const fn foo(t: T) -> T { t }...? So is it just that Sized trait is allowed?

I had initially considered suggesting that we allow auto traits as well, but I do not think this is a good idea. For one thing, unlike Sized ,auto traits may have custom impls, and that might constrain us in ways that we do not want. e.g., you can have

unsafe impl<T: Display> Send for Foo<T> { }

Now, if you have a const fn foo<U: Send>(...) where U = Foo<T>, and we accept it, we will therefore be saying that T: Display is provable in a const context. But if we wanted to change how trait resolution works in a const context to consider only impls that are marked const or some such thing, that could be problematic.

Since Sized does not permit custom impls, we don't have this problem. Similarly, we could permit lifetime bounds (T: 'a).

But we could still write a function that takes two fn ptrs and compares them? It couldn't be called (currently), but we should rule out the binop as well.

@RalfJung Attempting to define

const fn cmp(x: fn(), y: fn()) -> bool {
    x == y
}

with #![feature(min_const_fn)] gives "error: function pointers in const fn are unstable".

If we have that in a test, I am happy :)

@oli-obk given that we landed #53851, should we move to stabilize this?

However, I'd like to see a test ensuring that

   const FOO: NonZeroU8 = unsafe { NonZeroU8::new_unchecked(0) };

is caught by the sanity check. Currently nightly accepts that constant without complaining.

@RalfJung , it would appear that post-#54835 nightly still accepts this code without warning. Did you intend for this code not to work?

Nightly on the playground gives me

error[E0080]: this constant likely exhibits undefined behavior
 --> src/main.rs:3:1
  |
3 | const FOO: NonZeroU8 = unsafe { NonZeroU8::new_unchecked(0) };
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered 0, but expected something greater or equal to 1
  |
  = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior

https://play.rust-lang.org/?gist=8544bb6dd824c73bd0ea8466aad29ae9&version=nightly&mode=debug&edition=2015

And there's tests as well: https://github.com/rust-lang/rust/blob/master/src/test/ui/consts/const-eval/ub-nonnull.rs

This can been fixed with #54762

Any change we can get this into beta? I'd really love to do some 2018 edition proving and cleanup of some embedded crates but without min_const_fn available on beta, realistic checks are somewhat limited.

It will be in the next beta. Until then you could just use nightly without any feature gates

@oli-obk That's what I'm doing at the moment but I thought the point of the beta was to test Edition features and nightly might behave differently. I guess I'm looking forward then to trying the next beta and everything working fine just like on nighty. ;)

Documentation was done in rust-lang/reference#440. Since everything is done, I'll close this out.

@oli-obk > It will be in the next beta. Until then you could just use nightly without any feature gates

# rustc +beta --version
rustc 1.30.0-beta.15 (590121930 2018-10-12)
# cargo +beta build --release
...
error[E0658]: const fn is unstable (see issue #53555)
  --> /Users/egger/OSS/bare-metal/src/lib.rs:22:5
   |
22 | /     pub const unsafe fn new(address: usize) -> Self {
23 | |         Peripheral {
24 | |             address: address as *mut T,
25 | |         }
26 | |     }
   | |_____^

@therealprof "the next beta" means the 1.31 one, which will arrive some time around the time 1.30 stable ships(Oct 25).