Make all methods of `std::net::Ipv6Addr` const #76206

CDirkx · 2020-09-01T17:05:09Z

Make the following methods of std::net::Ipv6Addr unstable const under the const_ipv6 feature:

segments
is_unspecified
is_loopback
is_global (unstable)
is_unique_local
is_unicast_link_local_strict
is_documentation
multicast_scope
is_multicast
to_ipv4_mapped
to_ipv4

This would make all methods of Ipv6Addr const.

Changed the implementation of is_unspecified and is_loopback to use a match instead of ==, all other methods did not require a change.

All these methods are dependent on segments, the current implementation of which requires unstable const_fn_transmute (PR#75085).

Part of #76205

Make the following methods of `std::net::Ipv6Addr` unstable const under the `const_ipv6` feature: - `segments` - `is_unspecified` - `is_loopback` - `is_global` (unstable) - `is_unique_local` - `is_unicast_link_local_strict` - `is_documentation` - `multicast_scope` - `is_multicast` - `to_ipv4_mapped` - `to_ipv4` Changed the implementation of `is_unspecified` and `is_loopback` to use a `match` instead of `==`. Part of rust-lang#76205

rust-highfive · 2020-09-01T17:05:13Z

r? @Mark-Simulacrum

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Mark-Simulacrum · 2020-09-01T17:16:20Z

r? @dtolnay, needs libs FCP

Mark-Simulacrum · 2020-09-01T17:17:06Z

Oh, actually, missed that this is just unstable, so r? @ecstatic-morse

CDirkx · 2020-09-01T17:44:42Z

Yeah it's unstable for now since segments depends on const_fn_transmute as an optimization. However, interestingly, this also applies to new, which uses #[allow_internal_unstable(const_fn_transmute)], since new was already stabilized as const.

To stabilize these methods as const requires either

wait for const_fn_transmute to stabilize
temporarily revert PR#75085 (remove the transmute, possibly leads to worse performance)
#[allow_internal_unstable(const_fn_transmute)] on segments (like new)

ecstatic-morse · 2020-09-01T18:05:20Z

library/std/src/net/ip.rs

-    pub fn is_unspecified(&self) -> bool {
-        self.segments() == [0, 0, 0, 0, 0, 0, 0, 0]
+    pub const fn is_unspecified(&self) -> bool {
+        matches!(self.segments(), [0, 0, 0, 0, 0, 0, 0, 0])


As a general rule, matches! won't optimize as well as comparison to an array. I don't think the performance of is_unspecified and is_loopback is terribly important for real code, so this is fine for now. It would be good to look into constifying slice equality, making memcmp callable in a const context, or generating better code for array patterns in match statements.

Yeah, which is a shame. I was looking into this for the Ipv4Addr methods as well, most of which can be written very elegantly using matches!. If performance is a concern, the same implementation could be used as in Ipv4Addr::is_global:

u128::from_be_bytes(self.octets()) == 0

Yeah, let's do it that way. Instead of comparing with an integer literal, however, you should convert the LOOPBACK and UNSPECIFIED associated constants to a u128 and compare with those.

ecstatic-morse · 2020-09-01T18:12:13Z

allow_internal_unstable would be fine on segments by the same logic it is okay on new: There is a const-stable alternative implementation. However, I think it's fine to have these be unstably const for a little while.

Ping me once there's tests for these methods and I'll approve this. FWIW, I also find writing "is usable in a const context" tests dull, but c'est la vie.

CDirkx · 2020-09-01T18:40:28Z

That's alright. About those tests (also relevant for the other PRs), right now they live under src/test/ui/consts, as that is where const-option.rs already was. However I noticed that there is src/test/ui/consts/std, so I'm going to create the tests for this PR under there. After that, shall I also move the tests from other PRs there, to collect the library interface const tests and separate them from the const implementation tests? Maybe also move the const tests for NonZeroX and the like.

ecstatic-morse · 2020-09-01T19:00:55Z

Having them live under std seems clearly better, and I would happily sign off on it. OTOH, src/test/ui/consts is already small enough that ./x.py test -- src/test/ui/consts runs pretty quickly, so it's not terribly urgent.

… from `matches!` to `u128` comparison Done because `matches!` doesn't optimize well with array comparisons

CDirkx · 2020-09-01T20:18:02Z

@ecstatic-morse Tests added :)

ecstatic-morse · 2020-09-01T20:38:42Z

Thanks!

@bors r+ rollup

bors · 2020-09-01T20:38:44Z

📌 Commit 9afe97c has been approved by ecstatic-morse

@ghost

Rollup of 14 pull requests Successful merges: - rust-lang#74880 (Add trailing comma support to matches macro) - rust-lang#76074 (Add new `-Z dump-mir-spanview` option) - rust-lang#76088 (Add more examples to lexicographic cmp on Iterators.) - rust-lang#76099 (Add info about `!` and `impl Trait`) - rust-lang#76126 (Use "Fira Sans" for crate list font) - rust-lang#76132 (Factor out StmtKind::MacCall fields into `MacCallStmt` struct) - rust-lang#76143 (Give a better error message for duplicate built-in macros) - rust-lang#76158 (Stabilise link-self-contained option) - rust-lang#76201 (Move to intra-doc links for library/core/src/panic.rs) - rust-lang#76206 (Make all methods of `std::net::Ipv6Addr` const) - rust-lang#76207 (# Move to intra-doc links for library/core/src/clone.rs) - rust-lang#76212 (Document lint missing_doc_code_examples is nightly-only) - rust-lang#76218 (lexer: Tiny improvement to shebang detection) - rust-lang#76221 (Clean up header in `iter` docs for `for` loops) Failed merges: r? @ghost

@ecstatic-morse

Make delegation methods of `std::net::IpAddr` unstably const Make the following methods of `std::net::IpAddr` unstable const under the `const_ip` feature: - `is_unspecified` - `is_loopback` - `is_global` - `is_multicast` Also adds a test for these methods in a const context. Possible because these methods delegate to the inner `Ipv4Addr` or `Ipv6Addr`, which were made const ([PR#76205](rust-lang#76142) and [PR#76206](rust-lang#76206)), and the recent stabilization of const control flow. Part of rust-lang#76205 r? @ecstatic-morse

Stabilize all stable methods of `Ipv4Addr`, `Ipv6Addr` and `IpAddr` as const This PR stabilizes all currently stable methods of `Ipv4Addr`, `Ipv6Addr` and `IpAddr` as const. Tracking issue: rust-lang#76205 `Ipv4Addr` (`const_ipv4`): - `octets` - `is_loopback` - `is_private` - `is_link_local` - `is_multicast` - `is_broadcast` - `is_docmentation` - `to_ipv6_compatible` - `to_ipv6_mapped` `Ipv6Addr` (`const_ipv6`): - `segments` - `is_unspecified` - `is_loopback` - `is_multicast` - `to_ipv4` `IpAddr` (`const_ip`): - `is_unspecified` - `is_loopback` - `is_multicast` ## Motivation The ip methods seem like prime candidates to be made const: their behavior is defined by an external spec, and based solely on the byte contents of an address. These methods have been made unstable const in the beginning of September, after the necessary const integer arithmetic was stabilized. There is currently a PR open (rust-lang#78802) to change the internal representation of `IpAddr{4,6}` from `libc` types to a byte array. This does not have any impact on the constness of the methods. ## Implementation Most of the stabilizations are straightforward, with the exception of `Ipv6Addr::segments`, which uses the unstable feature `const_fn_transmute`. The code could be rewritten to equivalent stable code, but this leads to worse code generation (rust-lang#75085). This is why `segments` gets marked with `#[rustc_allow_const_fn_unstable(const_fn_transmute)]`, like the already const-stable `Ipv6Addr::new`, the justification being that a const-stable alternative implementation exists rust-lang#76206 (comment). ## Future posibilities This PR const-stabilizes all currently stable ip methods, however there are also a number of unstable methods under the `ip` feature (rust-lang#27709). These methods are already unstable const. There is a PR open (rust-lang#76098) to stabilize those methods, which could include const-stabilization. However, stabilizing those methods as const is dependent on `Ipv4Addr::octets` and `Ipv6Addr::segments` (covered by this PR).

rust-highfive assigned Mark-Simulacrum Sep 1, 2020

rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Sep 1, 2020

CDirkx mentioned this pull request Sep 1, 2020

Constify std::net::IpAddr, Ipv4Addr, and Ipv6Addr #76205

Closed

jonas-schievink added A-const-fn Area: const fn foo(..) {..}. Pure functions which can be applied at compile time. T-libs-api Relevant to the library API team, which will review and decide on the PR/issue. labels Sep 1, 2020

rust-highfive assigned dtolnay and unassigned Mark-Simulacrum Sep 1, 2020

rust-highfive assigned ecstatic-morse and unassigned dtolnay Sep 1, 2020

ecstatic-morse reviewed Sep 1, 2020

View reviewed changes

Add test for Ipv6Addr methods in a const context

cd08def

CDirkx and others added 2 commits September 1, 2020 21:05

Change implementation of Ipv6Addr::is_unspecified and is_loopback…

a43dd4f

… from `matches!` to `u128` comparison Done because `matches!` doesn't optimize well with array comparisons

Add trailing newline to ipv6.rs

9afe97c

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Sep 1, 2020

tmandry mentioned this pull request Sep 2, 2020

Rollup of 14 pull requests #76231

Merged

bors merged commit 11ff32f into rust-lang:master Sep 2, 2020

CDirkx deleted the const-ipv6 branch September 3, 2020 21:57

CDirkx mentioned this pull request Sep 3, 2020

Make delegation methods of std::net::IpAddr unstably const #76304

Merged

CDirkx mentioned this pull request Nov 23, 2020

Stabilize all stable methods of Ipv4Addr, Ipv6Addr and IpAddr as const #79342

Merged

cuviper added this to the 1.48.0 milestone Nov 16, 2023

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Make all methods of `std::net::Ipv6Addr` const #76206

Make all methods of `std::net::Ipv6Addr` const #76206

CDirkx commented Sep 1, 2020

rust-highfive commented Sep 1, 2020

Mark-Simulacrum commented Sep 1, 2020

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CDirkx commented Sep 1, 2020

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Make all methods of std::net::Ipv6Addr const #76206

Make all methods of std::net::Ipv6Addr const #76206

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Make all methods of `std::net::Ipv6Addr` const #76206

Make all methods of `std::net::Ipv6Addr` const #76206

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