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Constify implementations of (Try)From for int types #86840

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Merged
merged 3 commits into from
Aug 11, 2021
Merged

Constify implementations of (Try)From for int types #86840

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eefd790
impl const From<num> for num
usbalbin Dec 27, 2020
09928a9
Add tests
usbalbin Mar 13, 2021
c8bf5ed
Add test for int to float
usbalbin Aug 7, 2021
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15 changes: 10 additions & 5 deletions library/core/src/convert/num.rs
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Expand Up @@ -45,7 +45,8 @@ impl_float_to_int!(f64 => u8 u16 u32 u64 u128 usize i8 i16 i32 i64 i128 isize);
macro_rules! impl_from {
($Small: ty, $Large: ty, #[$attr:meta], $doc: expr) => {
#[$attr]
impl From<$Small> for $Large {
#[rustc_const_unstable(feature = "const_num_from_num", issue = "87852")]
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Stability attributes do not usually work on trait impls. Do they work for impl const?

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Yes. See this.

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That tests the case where we call a trait fn on a concrete type.

But there is another way to use a trait impl: to satisfy a bound.

const fn const_context_generic<T: MyTrait>() {
  T::func();
}

const_context_generic::<Unstable>();

Is that properly checked?

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That tests the case where we call a trait fn on a concrete type.

But there is another way to use a trait impl: to satisfy a bound.

const fn const_context_generic<T: MyTrait>() {
  T::func();
}

const_context_generic::<Unstable>();

Is that properly checked?

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You are right, those are probably not. I guess it is fine to let this PR pass and I will come up with a fix in the future. (and we need to wait for #87375, as that introduces a check on const bounds to error on previous code that should not be accepted) Trait bounds on const functons are gated behind a feature anyways, so even if this would allow one to use an unstable library feature without having it enabled, it would still require users to use a nightly compiler anyways.

impl const From<$Small> for $Large {
// Rustdocs on the impl block show a "[+] show undocumented items" toggle.
// Rustdocs on functions do not.
#[doc = $doc]
Expand Down Expand Up @@ -172,7 +173,8 @@ impl_from! { f32, f64, #[stable(feature = "lossless_float_conv", since = "1.6.0"
macro_rules! try_from_unbounded {
($source:ty, $($target:ty),*) => {$(
#[stable(feature = "try_from", since = "1.34.0")]
impl TryFrom<$source> for $target {
#[rustc_const_unstable(feature = "const_num_from_num", issue = "87852")]
impl const TryFrom<$source> for $target {
type Error = TryFromIntError;

/// Try to create the target number type from a source
Expand All @@ -190,7 +192,8 @@ macro_rules! try_from_unbounded {
macro_rules! try_from_lower_bounded {
($source:ty, $($target:ty),*) => {$(
#[stable(feature = "try_from", since = "1.34.0")]
impl TryFrom<$source> for $target {
#[rustc_const_unstable(feature = "const_num_from_num", issue = "87852")]
impl const TryFrom<$source> for $target {
type Error = TryFromIntError;

/// Try to create the target number type from a source
Expand All @@ -212,7 +215,8 @@ macro_rules! try_from_lower_bounded {
macro_rules! try_from_upper_bounded {
($source:ty, $($target:ty),*) => {$(
#[stable(feature = "try_from", since = "1.34.0")]
impl TryFrom<$source> for $target {
#[rustc_const_unstable(feature = "const_num_from_num", issue = "87852")]
impl const TryFrom<$source> for $target {
type Error = TryFromIntError;

/// Try to create the target number type from a source
Expand All @@ -234,7 +238,8 @@ macro_rules! try_from_upper_bounded {
macro_rules! try_from_both_bounded {
($source:ty, $($target:ty),*) => {$(
#[stable(feature = "try_from", since = "1.34.0")]
impl TryFrom<$source> for $target {
#[rustc_const_unstable(feature = "const_num_from_num", issue = "87852")]
impl const TryFrom<$source> for $target {
type Error = TryFromIntError;

/// Try to create the target number type from a source
Expand Down
1 change: 1 addition & 0 deletions library/core/src/lib.rs
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Expand Up @@ -99,6 +99,7 @@
#![feature(const_slice_from_raw_parts)]
#![feature(const_slice_ptr_len)]
#![feature(const_swap)]
#![feature(const_trait_impl)]
#![feature(const_type_id)]
#![feature(const_type_name)]
#![feature(const_unreachable_unchecked)]
Expand Down
2 changes: 2 additions & 0 deletions library/core/tests/lib.rs
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Expand Up @@ -13,6 +13,8 @@
#![feature(const_ptr_read)]
#![feature(const_ptr_write)]
#![feature(const_ptr_offset)]
#![feature(const_trait_impl)]
#![feature(const_num_from_num)]
#![feature(core_intrinsics)]
#![feature(core_private_bignum)]
#![feature(core_private_diy_float)]
Expand Down
25 changes: 25 additions & 0 deletions library/core/tests/num/const_from.rs
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@@ -0,0 +1,25 @@
#[test]
fn from() {
use core::convert::TryFrom;
use core::num::TryFromIntError;

// From
const FROM: i64 = i64::from(1i32);
assert_eq!(FROM, 1i64);

// From int to float
const FROM_F64: f64 = f64::from(42u8);
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cc @rust-lang/wg-const-eval this is an interesting edge case around floats. We forbid floats in const fn, but those checks only happen at the declaration site, and libcore uses the feature gate that allows them. So now we can invoke const fns (only within items, not within other const fn) that work with floats.

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Yeah, we should really work towards getting rid of "things that are allowed in const but forbidden in const fn"...

assert_eq!(FROM_F64, 42f64);

// Upper bounded
const U8_FROM_U16: Result<u8, TryFromIntError> = u8::try_from(1u16);
assert_eq!(U8_FROM_U16, Ok(1u8));

// Both bounded
const I8_FROM_I16: Result<i8, TryFromIntError> = i8::try_from(1i16);
assert_eq!(I8_FROM_I16, Ok(1i8));

// Lower bounded
const I16_FROM_U16: Result<i16, TryFromIntError> = i16::try_from(1u16);
assert_eq!(I16_FROM_U16, Ok(1i16));
}
2 changes: 2 additions & 0 deletions library/core/tests/num/mod.rs
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Expand Up @@ -27,6 +27,8 @@ mod u64;
mod u8;

mod bignum;

mod const_from;
mod dec2flt;
mod flt2dec;
mod int_log;
Expand Down