|
| 1 | +/// Implements [`Tag`] for a given type. |
| 2 | +/// |
| 3 | +/// You can use `impl_tag` on structs and enums. |
| 4 | +/// You need to specify the type and all its possible values, |
| 5 | +/// which can only be paths with optional fields. |
| 6 | +/// |
| 7 | +/// [`Tag`]: crate::tagged_ptr::Tag |
| 8 | +/// |
| 9 | +/// # Examples |
| 10 | +/// |
| 11 | +/// Basic usage: |
| 12 | +/// |
| 13 | +/// ``` |
| 14 | +/// #![feature(macro_metavar_expr)] |
| 15 | +/// use rustc_data_structures::{impl_tag, tagged_ptr::Tag}; |
| 16 | +/// |
| 17 | +/// #[derive(Copy, Clone, PartialEq, Debug)] |
| 18 | +/// enum SomeTag { |
| 19 | +/// A, |
| 20 | +/// B, |
| 21 | +/// X { v: bool }, |
| 22 | +/// Y(bool, bool), |
| 23 | +/// } |
| 24 | +/// |
| 25 | +/// impl_tag! { |
| 26 | +/// // The type for which the `Tag` will be implemented |
| 27 | +/// impl Tag for SomeTag; |
| 28 | +/// // You need to specify all possible tag values: |
| 29 | +/// SomeTag::A, // 0 |
| 30 | +/// SomeTag::B, // 1 |
| 31 | +/// // For variants with fields, you need to specify the fields: |
| 32 | +/// SomeTag::X { v: true }, // 2 |
| 33 | +/// SomeTag::X { v: false }, // 3 |
| 34 | +/// // For tuple variants use named syntax: |
| 35 | +/// SomeTag::Y { 0: true, 1: true }, // 4 |
| 36 | +/// SomeTag::Y { 0: false, 1: true }, // 5 |
| 37 | +/// SomeTag::Y { 0: true, 1: false }, // 6 |
| 38 | +/// SomeTag::Y { 0: false, 1: false }, // 7 |
| 39 | +/// } |
| 40 | +/// |
| 41 | +/// // Tag values are assigned in order: |
| 42 | +/// assert_eq!(SomeTag::A.into_usize(), 0); |
| 43 | +/// assert_eq!(SomeTag::X { v: false }.into_usize(), 3); |
| 44 | +/// assert_eq!(SomeTag::Y(false, true).into_usize(), 5); |
| 45 | +/// |
| 46 | +/// assert_eq!(unsafe { SomeTag::from_usize(1) }, SomeTag::B); |
| 47 | +/// assert_eq!(unsafe { SomeTag::from_usize(2) }, SomeTag::X { v: true }); |
| 48 | +/// assert_eq!(unsafe { SomeTag::from_usize(7) }, SomeTag::Y(false, false)); |
| 49 | +/// ``` |
| 50 | +/// |
| 51 | +/// Structs are supported: |
| 52 | +/// |
| 53 | +/// ``` |
| 54 | +/// #![feature(macro_metavar_expr)] |
| 55 | +/// # use rustc_data_structures::impl_tag; |
| 56 | +/// #[derive(Copy, Clone)] |
| 57 | +/// struct Flags { a: bool, b: bool } |
| 58 | +/// |
| 59 | +/// impl_tag! { |
| 60 | +/// impl Tag for Flags; |
| 61 | +/// Flags { a: true, b: true }, |
| 62 | +/// Flags { a: false, b: true }, |
| 63 | +/// Flags { a: true, b: false }, |
| 64 | +/// Flags { a: false, b: false }, |
| 65 | +/// } |
| 66 | +/// ``` |
| 67 | +/// |
| 68 | +/// Not specifying all values results in a compile error: |
| 69 | +/// |
| 70 | +/// ```compile_fail,E0004 |
| 71 | +/// #![feature(macro_metavar_expr)] |
| 72 | +/// # use rustc_data_structures::impl_tag; |
| 73 | +/// #[derive(Copy, Clone)] |
| 74 | +/// enum E { |
| 75 | +/// A, |
| 76 | +/// B, |
| 77 | +/// } |
| 78 | +/// |
| 79 | +/// impl_tag! { |
| 80 | +/// impl Tag for E; |
| 81 | +/// E::A, |
| 82 | +/// } |
| 83 | +/// ``` |
| 84 | +#[macro_export] |
| 85 | +macro_rules! impl_tag { |
| 86 | + ( |
| 87 | + impl Tag for $Self:ty; |
| 88 | + $( |
| 89 | + $($path:ident)::* $( { $( $fields:tt )* })?, |
| 90 | + )* |
| 91 | + ) => { |
| 92 | + // Safety: |
| 93 | + // `bits_for_tags` is called on the same `${index()}`-es as |
| 94 | + // `into_usize` returns, thus `BITS` constant is correct. |
| 95 | + unsafe impl $crate::tagged_ptr::Tag for $Self { |
| 96 | + const BITS: u32 = $crate::tagged_ptr::bits_for_tags(&[ |
| 97 | + $( |
| 98 | + ${index()}, |
| 99 | + $( ${ignore(path)} )* |
| 100 | + )* |
| 101 | + ]); |
| 102 | + |
| 103 | + #[inline] |
| 104 | + fn into_usize(self) -> usize { |
| 105 | + // This forbids use of repeating patterns (`Enum::V`&`Enum::V`, etc) |
| 106 | + // (or at least it should, see <https://github.com/rust-lang/rust/issues/110613>) |
| 107 | + #[forbid(unreachable_patterns)] |
| 108 | + match self { |
| 109 | + // `match` is doing heavy lifting here, by requiring exhaustiveness |
| 110 | + $( |
| 111 | + $($path)::* $( { $( $fields )* } )? => ${index()}, |
| 112 | + )* |
| 113 | + } |
| 114 | + } |
| 115 | + |
| 116 | + #[inline] |
| 117 | + unsafe fn from_usize(tag: usize) -> Self { |
| 118 | + match tag { |
| 119 | + $( |
| 120 | + ${index()} => $($path)::* $( { $( $fields )* } )?, |
| 121 | + )* |
| 122 | + |
| 123 | + // Safety: |
| 124 | + // `into_usize` only returns `${index()}` of the same |
| 125 | + // repetition as we are filtering above, thus if this is |
| 126 | + // reached, the safety contract of this function was |
| 127 | + // already breached. |
| 128 | + _ => unsafe { |
| 129 | + debug_assert!( |
| 130 | + false, |
| 131 | + "invalid tag: {tag}\ |
| 132 | + (this is a bug in the caller of `from_usize`)" |
| 133 | + ); |
| 134 | + std::hint::unreachable_unchecked() |
| 135 | + }, |
| 136 | + } |
| 137 | + } |
| 138 | + |
| 139 | + } |
| 140 | + }; |
| 141 | +} |
| 142 | + |
| 143 | +#[cfg(test)] |
| 144 | +mod tests; |
0 commit comments