RFC 2700: Associated constants on integer primitive types

libs (const)

Summary

Add the relevant associated constants to the numeric types in the standard library, and consider a timeline for the deprecation of the corresponding (and originally intended to be temporary) primitive numeric modules and associated functions.

Motivation

All programming languages with bounded integers provide numeric constants for their maximum and minimum extents. In Rust, these constants were stabilized in the eleventh hour before Rust 1.0 (literally the day before the branching of 1.0-beta on April 1, 2015), with some known-to-be-undesirable properties. In particular, associated consts were yet to be implemented (these landed, amusingly, one month after 1.0-beta and two weeks before 1.0-stable), and so each of the twelve numeric types were given their own top-level modules in the standard library, whose contents are exclusively these constants (all related non-constants being defined in inherent impls directly on each type). However, in the even-eleventh-er hour before 1.0-beta, it was realized that this solution did not work for anyone seeking to reference these constants when working with types such as c_int, which are defined as type aliases and can thus access inherent impls but not modules that merely happen to be named the same as the original type; as a result, an emergency PR also added redundant max_value and min_value inherent functions as a last-second workaround. The PR itself notes how distasteful this remedy is:

It's unfortunate to freeze these as methods, but when we can provide inherent associated constants these methods can be deprecated. [aturon, Apr 1, 2015]

Meanwhile, the author of the associated consts patch despairs of just barely missing the deadline:

@nikomatsakis The original motivation for trying to get this in before the beta was to get rid of all the functions that deal with constants in Int/Float, and then to get rid of all the modules like std::i64 that just hold constants as well. We could have dodged most of the issues (ICEs and generic code design) by using inherent impls instead of associating the constants with traits. But since #23549 came in a bit earlier and stabilized a bunch more of those constants before the beta, whereas this hasn't landed yet, blegh. [quantheory, Apr 1, 2015]

Anticipating the situation, an issue was filed in the RFCs repo regarding moving the contents of these modules into associated consts:

I think it's a minor enough breaking change to move the constants and deprecate the modules u8, u16, etc. Not so sure about removing these modules entirely, I'd appreciate that, but it'll break all the code use-ing them. [petrochenkov, Apr 29, 2015]

Finally, so obvious was this solution that the original RFC for associated items used the numeric constants as the only motivating example for the feature of associated consts:

For example, today's Rust includes a variety of numeric traits, including Float, which must currently expose constants as static functions [...] Associated constants would allow the consts to live directly on the traits

Despite the obvious intent, 1.0 came and went and there were plenty of other things to occupy everyone's attention. Now, two days shy of Rust's fourth anniversary, let's re-examine the situation. We propose to deprecate all of the aforementioned functions and constants in favor of associated constants defined on the appropriate types, and to additionally deprecate all constants living directly in the i8, i16, i32, i64, i128, isize, u8, u16, u32, u64, u128, usize, f32 and f64 modules in std. Advantages of this:

  1. Consistency with the rest of the language. As demonstrated by the above quotes, associated consts have been the natural way to express these concepts in Rust since before associated consts were even implemented; this approach satisfies the principle of least surprise.

  2. Documentation. On the front page of the standard library API docs, 12 of the 60 modules in the standard library (20%) are the aforementioned numeric modules which exist only to namespace two constants each. This number will increase as new numeric primitives are added to Rust, as already seen with i128 and u128. Although deprecated modules cannot be easily removed from std, they can be removed from the documentation, making the stdlib API docs less cluttered and easier to navigate.

  3. Beginner ease. For a beginner, finding two identical ways to achieve something immediately raises the question of "why", to which the answer here is ultimately uninteresting (and mildly embarrassing). Even then the question of "which one to use" remains unanswered; neither current approach is more idiomatic than the other. As noted, deprecated items can be removed from the documentation, thereby decreasing the likelihood of head-scratching and incredulous sidelong glances from people new to Rust.

  4. Removal of ambiguity between primitive types and their identically-named modules. Currently if you import an integer module and access constants in the module and methods on the type, one has no apparent indication as to what comes from where:

use std::u32;
assert_eq!(u32::MAX, u32::max_value());

The fact that this sort of shadowing of primitive types works in the first place is surprising even to experienced Rust programmers; the fact that such a pattern is seemingly encouraged by the standard library is even more of a surprise. By making this change we would be able to remove all modules in the standard library whose names shadow integral types.

  1. Removal of a frustrating papercut. Even experienced Rust programmers are prone to trip over this and curse at having to be reminded of a bizarre and jarring artifact of Rust 1.0. By removing these artifacts we can make the experience of using Rust more universally pleasant.

Guide-level explanation

  1. Add the following associated constants to the relevant types in standard library, with their definitions taken from the corresponding legacy module-level constants:

    • i8::{MAX, MIN}
    • i16::{MAX, MIN}
    • i32::{MAX, MIN}
    • i64::{MAX, MIN}
    • i128::{MAX, MIN}
    • isize::{MAX, MIN}
    • u8::{MAX, MIN}
    • u16::{MAX, MIN}
    • u32::{MAX, MIN}
    • u64::{MAX, MIN}
    • u128::{MAX, MIN}
    • usize::{MAX, MIN}
    • f32::{DIGITS, EPSILON, INFINITY, MANTISSA_DIGITS, MAX, MAX_10_EXP, MAX_EXP, MIN, MIN_10_EXP, MIN_EXP, MIN_POSITIVE, NAN, NEG_INFINITY, RADIX}
    • f64::{DIGITS, EPSILON, INFINITY, MANTISSA_DIGITS, MAX, MAX_10_EXP, MAX_EXP, MIN, MIN_10_EXP, MIN_EXP, MIN_POSITIVE, NAN, NEG_INFINITY, RADIX}

  2. Redefine the following module-level constants in terms of the associated constants added in step 1:

  3. At a future point to be determined (see "Unresolved questions" below), deprecate the items listed in step 2. Additionally, deprecate the following associated functions:

  4. Following step 3, the following modules will be made hidden from the front page of the stdlib documentation, as they no longer contain any non-deprecated items: std::{i8, i16, i32, i64, i128, isize, u8, u16, u32, u64, u128, usize} (note that this does not apply to either of std::{f32, f64}; see the Alternatives section below)

Drawbacks

  1. Deprecation warnings, although these can be easily addressed.
  2. Because associated items cannot be directly imported, code of the form use std::i32::MAX; foo(MAX, MAX); will most likely be changed to foo(i32::MAX, i32::MAX), which may be marginally more verbose. However, given how many MAX and MIN constants there are in the stdlib, it is easy to argue that such unprefixed constants in the wild would be confusing, and ought to be avoided in the first place. In any case, users desperate for such behavior will be trivially capable of replacing use std::i32::MAX; with const MAX: i32 = i32::MAX;.

Unresolved questions

How long should we go before issuing a deprecation warning? At the extreme end of the scale we could wait until the next edition of Rust is released, and have the legacy items only issue deprecation warnings when opting in to the new edition; this would limit disruption only to people opting in to a new edition (and, being merely an trivially-addressed deprecation, would constitute far less of a disruption than any ordinary edition-related change; any impact of the deprecation would be mere noise in light of the broader edition-related impacts). However long it takes, it is the opinion of the author that deprecation should happen eventually, as we should not give the impression that it is the ideal state of things that there should exist three ways of finding the maximum value of an integer type; we expect experienced users to intuitively reach for the new way proposed in this RFC as the "natural" way these constants ought to be implemented, but for the sake of new users it would be a pedagogical wart to allow all three to exist without explicitly calling out the preferred one.

Alternatives

Copyright 2022 Nick Cameron and the RFC authors.