Implement 8 and optionally 16 bit integers with build.rs generated enum

Cargo.toml

@@ -14,6 +14,7 @@ license = "MIT OR Apache-2.0"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [features]
+enum_repr_16 = []
 default = ["std"]
 std = []
 

build.rs

@@ -0,0 +1,127 @@
+#[derive(Clone, Copy)]
+enum BitInfo {
+    Unsigned {
+        prim_max: u64,
+    },
+    Signed {
+        signed_max: i64,
+        signed_min: i64,
+        prim_max: i64,
+    },
+}
+
+impl BitInfo {
+    fn prim_max(&self) -> i64 {
+        match self {
+            Self::Unsigned { prim_max } => *prim_max as i64,
+            Self::Signed { prim_max, .. } => *prim_max,
+        }
+    }
+}
+
+struct EnumInfo {
+    ty_name: &'static str,
+    name: &'static str,
+    bit_info: BitInfo,
+}
+
+const FILES: &[EnumInfo] = &[
+    EnumInfo {
+        ty_name: "u8",
+        name: "u8_repr.rs",
+        bit_info: BitInfo::Unsigned {
+            prim_max: u8::MAX as _,
+        },
+    },
+    EnumInfo {
+        ty_name: "i8",
+        name: "i8_repr.rs",
+        bit_info: BitInfo::Signed {
+            signed_max: i8::MAX as _,
+            signed_min: i8::MIN as _,
+            prim_max: u8::MAX as _,
+        },
+    },
+    #[cfg(feature = "enum_repr_16")]
+    EnumInfo {
+        ty_name: "u16",
+        name: "u16_repr.rs",
+        bit_info: BitInfo::Unsigned {
+            prim_max: u16::MAX as _,
+        },
+    },
+    #[cfg(feature = "enum_repr_16")]
+    EnumInfo {
+        ty_name: "i16",
+        name: "i16_repr.rs",
+        bit_info: BitInfo::Signed {
+            signed_max: i16::MAX as _,
+            signed_min: i16::MIN as _,
+            prim_max: u16::MAX as _,
+        },
+    },
+];
+
+fn generate_variants(
+    generated_file: &mut impl std::fmt::Write,
+    repr_name: &str,
+    bit_info: BitInfo,
+) {
+    write!(
+        generated_file,
+        "#[derive(Clone, Copy, PartialEq, Eq, Hash)]
+        #[allow(dead_code)]
+        pub(crate) enum {repr_name} {{",
+    )
+    .unwrap();
+
+    match bit_info {
+        BitInfo::Unsigned { prim_max } => {
+            for i in 0..prim_max {
+                write!(generated_file, "V{i}={i},").unwrap()
+            }
+        }
+        BitInfo::Signed {
+            signed_max,
+            signed_min,
+            prim_max,
+        } => {
+            for i in 0..signed_max {
+                write!(generated_file, "V{i}={i},").unwrap();
+            }
+
+            for (i, v) in (signed_max..prim_max).zip(signed_min..0) {
+                write!(generated_file, "MV{i}={v},").unwrap();
+            }
+        }
+    }
+
+    write!(generated_file, "}}").unwrap();
+}
+
+fn generate_impl(generated_file: &mut impl std::fmt::Write, repr_name: &str, ty_name: &str) {
+    write!(
+        generated_file,
+        "impl {repr_name} {{
+            pub(crate) const fn new(value: {ty_name}) -> Option<Self> {{
+                unsafe {{ std::mem::transmute(value) }}
+            }}
+        }}"
+    )
+    .unwrap()
+}
+
+fn main() {
+    let out_dir = std::env::var("OUT_DIR").unwrap();
+
+    for file in FILES {
+        let mut generated_file = String::with_capacity(file.bit_info.prim_max() as usize);
+
+        let repr_name = format!("{}Repr", file.ty_name.to_uppercase());
+
+        generate_variants(&mut generated_file, &repr_name, file.bit_info);
+        generate_impl(&mut generated_file, &repr_name, file.ty_name);
+
+        std::fs::write(format!("{out_dir}/{}", file.name), generated_file).unwrap();
+    }
+}

src/enum_impl/mod.rs

@@ -0,0 +1,56 @@
+#[cfg(feature = "enum_repr_16")]
+pub(crate) mod i16_repr {
+    include!(concat!(env!("OUT_DIR"), "/i16_repr.rs"));
+}
+#[cfg(feature = "enum_repr_16")]
+pub(crate) mod u16_repr {
+    include!(concat!(env!("OUT_DIR"), "/u16_repr.rs"));
+}
+pub(crate) mod i8_repr {
+    include!(concat!(env!("OUT_DIR"), "/i8_repr.rs"));
+}
+pub(crate) mod u8_repr {
+    include!(concat!(env!("OUT_DIR"), "/u8_repr.rs"));
+}
+
+macro_rules! nonmax {
+    ( $nonmax: ident, $primitive: ident, $byte_repr: ident ) => {
+        /// An integer that is known not to equal its maximum value.
+        #[derive(Clone, Copy, PartialEq, Eq, Hash)]
+        #[repr(transparent)]
+        pub struct $nonmax($byte_repr);
+
+        impl $nonmax {
+            /// Creates a new non-max if the given value is not the maximum
+            /// value.
+            pub const fn new(value: $primitive) -> Option<Self> {
+                match $byte_repr::new(value) {
+                    Some(byte) => Some(Self(byte)),
+                    None => None,
+                }
+            }
+
+            /// Creates a new non-max without checking the value.
+            ///
+            /// # Safety
+            ///
+            /// The value must not equal the maximum representable value for the
+            /// primitive type.
+            #[inline]
+            pub const unsafe fn new_unchecked(value: $primitive) -> Self {
+                match Self::new(value) {
+                    Some(this) => this,
+                    None => unsafe { std::hint::unreachable_unchecked() },
+                }
+            }
+
+            /// Returns the value as a primitive type.
+            #[inline]
+            pub const fn get(&self) -> $primitive {
+                self.0 as $primitive
+            }
+        }
+    };
+}
+
+pub(crate) use nonmax;

src/lib.rs

@@ -56,6 +56,14 @@ will only require minor version bumps, but will need significant justification.
 #![forbid(missing_docs)]
 #![cfg_attr(not(feature = "std"), no_std)]
 
+#[macro_use]
+mod enum_impl;
+
+#[cfg(feature = "enum_repr_16")]
+use enum_impl::i16_repr::I16Repr;
+use enum_impl::i8_repr::I8Repr;
+use enum_impl::u8_repr::U8Repr;
+
 /// An error type returned when a checked integral type conversion fails (mimics [std::num::TryFromIntError])
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub struct TryFromIntError(());
@@ -401,14 +409,25 @@ macro_rules! nonmax_impls {
     };
 }
 
-nonmax_impls!(def, signed, NonMaxI8, NonZeroI8, i8);
+enum_impl::nonmax!(NonMaxI8, i8, I8Repr);
+enum_impl::nonmax!(NonMaxU8, u8, U8Repr);
+#[cfg(feature = "enum_repr_16")]
+enum_impl::nonmax!(NonMaxI16, i16, I16Repr);
+
+nonmax_impls!(signed, NonMaxI8, NonZeroI8, i8);
+#[cfg(feature = "enum_repr_16")]
+nonmax_impls!(signed, NonMaxI16, NonZeroI16, i16);
+#[cfg(not(feature = "enum_repr_16"))]
 nonmax_impls!(def, signed, NonMaxI16, NonZeroI16, i16);
 nonmax_impls!(def, signed, NonMaxI32, NonZeroI32, i32);
 nonmax_impls!(def, signed, NonMaxI64, NonZeroI64, i64);
 nonmax_impls!(def, signed, NonMaxI128, NonZeroI128, i128);
 nonmax_impls!(def, signed, NonMaxIsize, NonZeroIsize, isize);
 
-nonmax_impls!(def, unsigned, NonMaxU8, NonZeroU8, u8);
+nonmax_impls!(unsigned, NonMaxU8, NonZeroU8, u8);
+#[cfg(feature = "enum_repr_16")]
+nonmax_impls!(unsigned, NonMaxU16, NonZeroU16, u16);
+#[cfg(not(feature = "enum_repr_16"))]
 nonmax_impls!(def, unsigned, NonMaxU16, NonZeroU16, u16);
 nonmax_impls!(def, unsigned, NonMaxU32, NonZeroU32, u32);
 nonmax_impls!(def, unsigned, NonMaxU64, NonZeroU64, u64);