Attempt to try to resolve blocking concerns

1 files changed, 144 insertions, 0 deletions

diff --git a/compiler/rustc_data_structures/src/tagged_ptr/impl_tag.rs b/compiler/rustc_data_structures/src/tagged_ptr/impl_tag.rs
index cb7f7d318dc..cafa91c8b8b 100644
--- a/compiler/rustc_data_structures/src/tagged_ptr/impl_tag.rs
+++ b/compiler/rustc_data_structures/src/tagged_ptr/impl_tag.rs
@@ -81,6 +81,7 @@
 ///     E::A,
 /// }
 /// ```
+#[cfg(bootstrap)]
 #[macro_export]
 macro_rules! impl_tag {
     (
@@ -140,5 +141,148 @@ macro_rules! impl_tag {
     };
 }
 
+/// Implements [`Tag`] for a given type.
+///
+/// You can use `impl_tag` on structs and enums.
+/// You need to specify the type and all its possible values,
+/// which can only be paths with optional fields.
+///
+/// [`Tag`]: crate::tagged_ptr::Tag
+///
+/// # Examples
+///
+/// Basic usage:
+///
+/// ```
+/// #![feature(macro_metavar_expr)]
+/// use rustc_data_structures::{impl_tag, tagged_ptr::Tag};
+///
+/// #[derive(Copy, Clone, PartialEq, Debug)]
+/// enum SomeTag {
+///     A,
+///     B,
+///     X { v: bool },
+///     Y(bool, bool),
+/// }
+///
+/// impl_tag! {
+///     // The type for which the `Tag` will be implemented
+///     impl Tag for SomeTag;
+///     // You need to specify all possible tag values:
+///     SomeTag::A, // 0
+///     SomeTag::B, // 1
+///     // For variants with fields, you need to specify the fields:
+///     SomeTag::X { v: true  }, // 2
+///     SomeTag::X { v: false }, // 3
+///     // For tuple variants use named syntax:
+///     SomeTag::Y { 0: true,  1: true  }, // 4
+///     SomeTag::Y { 0: false, 1: true  }, // 5
+///     SomeTag::Y { 0: true,  1: false }, // 6
+///     SomeTag::Y { 0: false, 1: false }, // 7
+/// }
+///
+/// // Tag values are assigned in order:
+/// assert_eq!(SomeTag::A.into_usize(), 0);
+/// assert_eq!(SomeTag::X { v: false }.into_usize(), 3);
+/// assert_eq!(SomeTag::Y(false, true).into_usize(), 5);
+///
+/// assert_eq!(unsafe { SomeTag::from_usize(1) }, SomeTag::B);
+/// assert_eq!(unsafe { SomeTag::from_usize(2) }, SomeTag::X { v: true });
+/// assert_eq!(unsafe { SomeTag::from_usize(7) }, SomeTag::Y(false, false));
+/// ```
+///
+/// Structs are supported:
+///
+/// ```
+/// #![feature(macro_metavar_expr)]
+/// # use rustc_data_structures::impl_tag;
+/// #[derive(Copy, Clone)]
+/// struct Flags { a: bool, b: bool }
+///
+/// impl_tag! {
+///     impl Tag for Flags;
+///     Flags { a: true,  b: true  },
+///     Flags { a: false, b: true  },
+///     Flags { a: true,  b: false },
+///     Flags { a: false, b: false },
+/// }
+/// ```
+///
+/// Not specifying all values results in a compile error:
+///
+/// ```compile_fail,E0004
+/// #![feature(macro_metavar_expr)]
+/// # use rustc_data_structures::impl_tag;
+/// #[derive(Copy, Clone)]
+/// enum E {
+///     A,
+///     B,
+/// }
+///
+/// impl_tag! {
+///     impl Tag for E;
+///     E::A,
+/// }
+/// ```
+#[cfg(not(bootstrap))]
+#[macro_export]
+macro_rules! impl_tag {
+    (
+        impl Tag for $Self:ty;
+        $(
+            $($path:ident)::* $( { $( $fields:tt )* })?,
+        )*
+    ) => {
+        // Safety:
+        // `bits_for_tags` is called on the same `${index()}`-es as
+        // `into_usize` returns, thus `BITS` constant is correct.
+        unsafe impl $crate::tagged_ptr::Tag for $Self {
+            const BITS: u32 = $crate::tagged_ptr::bits_for_tags(&[
+                $(
+                    ${index()},
+                    $( ${ignore($path)} )*
+                )*
+            ]);
+
+            #[inline]
+            fn into_usize(self) -> usize {
+                // This forbids use of repeating patterns (`Enum::V`&`Enum::V`, etc)
+                // (or at least it should, see <https://github.com/rust-lang/rust/issues/110613>)
+                #[forbid(unreachable_patterns)]
+                match self {
+                    // `match` is doing heavy lifting here, by requiring exhaustiveness
+                    $(
+                        $($path)::* $( { $( $fields )* } )? => ${index()},
+                    )*
+                }
+            }
+
+            #[inline]
+            unsafe fn from_usize(tag: usize) -> Self {
+                match tag {
+                    $(
+                        ${index()} => $($path)::* $( { $( $fields )* } )?,
+                    )*
+
+                    // Safety:
+                    // `into_usize` only returns `${index()}` of the same
+                    // repetition as we are filtering above, thus if this is
+                    // reached, the safety contract of this function was
+                    // already breached.
+                    _ => unsafe {
+                        debug_assert!(
+                            false,
+                            "invalid tag: {tag}\
+                             (this is a bug in the caller of `from_usize`)"
+                        );
+                        std::hint::unreachable_unchecked()
+                    },
+                }
+            }
+
+        }
+    };
+}
+
 #[cfg(test)]
 mod tests;