ring/polyfill/cold_error.rs
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101
// Copyright 2024 Brian Smith.
//
// Permission to use, copy, modify, and/or distribute this software for any
// purpose with or without fee is hereby granted, provided that the above
// copyright notice and this permission notice appear in all copies.
//
// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
// SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
// OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
// CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
/// Reduces boilerplate for defining error types where we want the compiler to
/// optimize for the non-error path by assuming constructing an error is
/// unlikely/cold code.
///
/// WARNING: Every struct/variant must contain some *non-constant* value so
/// that the "invariant code" pass of the compiler doesn't recognize the
/// constructor as being "invariant code" and optimizing it away;
/// although such optimization would be nice to take advantage of, it
/// seems to lose the `#[cold]` attribute.
///
/// Constructor functions ar marked `pub(super)` to ensure that instances can
/// only be constructed from within the enclosing module (and its submodules).
///
/// XXX: #[inline(never)] is required to avoid the (MIR?) optimizer inlining
/// away the function call and losing the `#[cold]` attribute in the process.
/// We'd otherwise maybe prefer all constructors to be inline.
///
/// The type is defined in its own submodule `#mod_name` to hide the
/// variant/struct constructor, ensuring instances are only constructed
/// through the generated `$constructor` functions. The constructor methods
/// work around the lack of the ability to mark an enum variant `#[cold]` and
/// `#[inline(never)]`.
macro_rules! cold_exhaustive_error {
// struct
{
struct $mod_name:ident::$Error:ident with $vis:vis constructor {
$field:ident: $ValueType:ty
}
} => {
mod $mod_name {
#[allow(unused_imports)]
use super::*; // So `$ValueType` is in scope.
pub struct $Error { #[allow(dead_code)] $field: $ValueType }
impl $Error {
#[cold]
#[inline(never)]
$vis fn new($field: $ValueType) -> Self {
Self { $field }
}
}
}
};
// struct with default constructor visibility.
{
struct $mod_name:ident::$Error:ident {
$field:ident: $ValueType:ty
}
} => {
cold_exhaustive_error! {
struct $mod_name::$Error with pub(super) constructor {
$field: $ValueType
}
}
};
// enum
{
enum $mod_name:ident::$Error:ident {
$(
$constructor:ident => $Variant:ident($ValueType:ty),
)+
}
} => {
mod $mod_name {
#[allow(unused_imports)]
use super::*; // So `$ValueType` is in scope.
pub enum $Error {
$(
$Variant(#[allow(dead_code)] $ValueType)
),+
}
impl $Error {
$(
#[cold]
#[inline(never)]
pub(super) fn $constructor(value: $ValueType) -> Self {
Self::$Variant(value)
}
)+
}
}
};
}