Offer (opt-in) const-generic mappings. Mainly, a U<N> type alias.

.github/workflows/check.yml

@@ -21,6 +21,12 @@ jobs:
           - stable
           - beta
           - nightly
+        mb_const_generics:
+          - ""
+          - "--features const-generics"
+        exclude:
+          - mb_const_generics: "--features const-generics"
+            rust: 1.37.0
         include:
           - os: macos-latest
             rust: stable
@@ -36,11 +42,11 @@ jobs:
       - uses: actions-rs/cargo@v1
         with:
           command: test
-          args: --verbose --features "strict"
+          args: --verbose --features "strict" ${{ matrix.mb_const_generics }}
       - uses: actions-rs/cargo@v1
         with:
           command: doc
-          args: --features "strict"
+          args: --features "strict" ${{ matrix.mb_const_generics }}
 
   clippy:
     name: clippy + fmt

Cargo.toml

@@ -28,4 +28,5 @@ scale-info = { version = "1.0", default-features = false, optional = true }
   i128 = []
   strict = []
   force_unix_path_separator = []
+  const-generics = []
   scale_info = ["scale-info/derive"]

build/generic_const_mappings.rs

@@ -0,0 +1,91 @@
+use super::*;
+
+pub fn emit_impls() -> ::std::io::Result<()> {
+    let out_dir = ::std::env::var("OUT_DIR").unwrap();
+    let dest = ::std::path::Path::new(&out_dir).join("generic_const_mappings.rs");
+    println!(
+        "cargo:rustc-env=TYPENUM_BUILD_GENERIC_CONSTS={}",
+        dest.display()
+    );
+    let mut f = ::std::fs::File::create(&dest).unwrap();
+
+    #[allow(clippy::write_literal)]
+    write!(f, "{}", "\
+#[cfg(doc)]
+use generic_const_mappings::*;
+
+/// Module with some `const`-generics-friendly definitions, to help bridge the gap
+/// between those and `typenum` types.
+///
+///   - It requires the `const-generics` crate feature to be enabled.
+///
+/// The main type to use here is [`U`], although [`Const`] and [`ToUInt`] may be needed
+/// in a generic context.
+#[allow(warnings)] // script-generated code
+pub mod generic_const_mappings {
+    use crate::*;
+
+    /// The main mapping from a generic `const: usize` to a [`UInt`]: [`U<N>`] is expected to work like [`UN`].
+    ///
+    ///   - It requires the `const-generics` crate feature to be enabled.
+    ///
+    /// [`U<N>`]: `U`
+    /// [`UN`]: `U42`
+    ///
+    /// # Example
+    ///
+    /// ```rust
+    /// use typenum::*;
+    ///
+    /// assert_type_eq!(U<42>, U42);
+    /// ```
+    ///
+    /// This can even be used in a generic `const N: usize` context, provided the
+    /// genericity is guarded by a `where` clause:
+    ///
+    /// ```rust
+    /// use typenum::*;
+    ///
+    /// struct MyStruct<const N: usize>;
+    ///
+    /// trait MyTrait { type AssocType; }
+    ///
+    /// impl<const N: usize> MyTrait
+    ///     for MyStruct<N>
+    /// where
+    ///     Const<N> : ToUInt,
+    /// {
+    ///     type AssocType = U<N>;
+    /// }
+    ///
+    /// assert_type_eq!(<MyStruct<42> as MyTrait>::AssocType, U42);
+    /// ```
+    pub type U<const N: usize> = <Const<N> as ToUInt>::Output;
+
+    /// Used to allow the usage of [`U`] in a generic context.
+    pub struct Const<const N: usize>;
+
+    /// Used to allow the usage of [`U`] in a generic context.
+    pub trait ToUInt {
+        /// The [`UN`][`crate::U42`] type corresponding to `Self = Const<N>`.
+        type Output;
+    }
+\
+    ")?;
+
+    for uint in uints() {
+        write!(
+            f,
+            "
+    impl ToUInt for Const<{uint}> {{
+        type Output = U{uint};
+    }}
+\
+            ",
+            uint = uint,
+        )?;
+    }
+    write!(f, "}}")?;
+    f.flush()?;
+    Ok(())
+}

build/main.rs

@@ -4,6 +4,8 @@ use std::fs::File;
 use std::io::Write;
 use std::path::Path;
 
+#[cfg(feature = "const-generics")]
+mod generic_const_mappings;
 mod op;
 mod tests;
 
@@ -75,19 +77,22 @@ pub fn gen_int(i: i64) -> IntCode {
 )]
 pub fn no_std() {}
 
-// fixme: get a warning when testing without this
-#[allow(dead_code)]
-fn main() {
-    let highest: u64 = 1024;
-
+const HIGHEST: u64 = 1024;
+fn uints() -> impl Iterator<Item = u64> {
     // Use hardcoded values to avoid issues with cross-compilation.
     // See https://github.com/paholg/typenum/issues/162
     let first2: u32 = 11; // (highest as f64).log(2.0).round() as u32 + 1;
     let first10: u32 = 4; // (highest as f64).log(10.0) as u32 + 1;
-    let uints = (0..(highest + 1))
+    (0..(HIGHEST + 1))
         .chain((first2..64).map(|i| 2u64.pow(i)))
-        .chain((first10..20).map(|i| 10u64.pow(i)));
+        .chain((first10..20).map(|i| 10u64.pow(i)))
+}
 
+// fixme: get a warning when testing without this
+#[allow(dead_code)]
+fn main() {
+    println!("cargo:rerun-if-changed=build/main.rs"); // Allow caching the generation if `src/*` files change.
+
     let out_dir = env::var("OUT_DIR").unwrap();
     let dest = Path::new(&out_dir).join("consts.rs");
     #[cfg(not(feature = "force_unix_path_separator"))]
@@ -163,11 +168,11 @@ pub mod consts {{
     pub type True = B1;
     pub type False = B0;
 ",
-        highest = highest
+        highest = HIGHEST,
     )
     .unwrap();
 
-    for u in uints {
+    for u in uints() {
         writeln!(f, "    pub type U{} = {};", u, gen_uint(u)).unwrap();
         if u <= ::std::i64::MAX as u64 && u != 0 {
             let i = u as i64;
@@ -184,4 +189,7 @@ pub mod consts {{
     tests::build_tests().unwrap();
 
     op::write_op_macro().unwrap();
+
+    #[cfg(feature = "const-generics")]
+    generic_const_mappings::emit_impls().unwrap();
 }

src/lib.rs

@@ -71,12 +71,16 @@ use core::cmp::Ordering;
 mod generated {
     include!(concat!(env!("OUT_DIR"), "/op.rs"));
     include!(concat!(env!("OUT_DIR"), "/consts.rs"));
+    #[cfg(feature = "const-generics")]
+    include!(concat!(env!("OUT_DIR"), "/generic_const_mappings.rs"));
 }
 
 #[cfg(not(feature = "force_unix_path_separator"))]
 mod generated {
     include!(env!("TYPENUM_BUILD_OP"));
     include!(env!("TYPENUM_BUILD_CONSTS"));
+    #[cfg(feature = "const-generics")]
+    include!(env!("TYPENUM_BUILD_GENERIC_CONSTS"));
 }
 
 pub mod bit;
@@ -91,7 +95,6 @@ pub mod array;
 
 pub use crate::{
     array::{ATerm, TArr},
-    consts::*,
     generated::consts,
     int::{NInt, PInt},
     marker_traits::*,
@@ -100,6 +103,21 @@ pub use crate::{
     uint::{UInt, UTerm},
 };
 
+#[doc(no_inline)]
+#[rustfmt::skip]
+pub use consts::{
+    False, True, B0, B1,
+    U0, U1, U2, *,
+    N1, N2, Z0, P1, P2, *,
+};
+
+#[cfg(feature = "const-generics")]
+pub use crate::generated::generic_const_mappings;
+
+#[cfg(feature = "const-generics")]
+#[doc(no_inline)]
+pub use generic_const_mappings::{Const, ToUInt, U};
+
 /// A potential output from `Cmp`, this is the type equivalent to the enum variant
 /// `core::cmp::Ordering::Greater`.
 #[derive(Eq, PartialEq, Ord, PartialOrd, Clone, Copy, Hash, Debug, Default)]