const-gen

This is a crate for generating compile-time constants in your build.rs file. This crate supports converting types that are typically heap-allocated into fixed-size constants. It includes support for primitives, strings, vectors, maps, sets, and comes with a derive macro to allow implementation with structs and enums.

See this example: ```rust,ignore // build.rs

use const_gen::*; use std::{env, fs, path::Path};

// First, let's dummy up some structs. Enabling the "derive" // feature allows us to do this simply, but implementing the // CompileConst trait by hand is straightforward.

[derive(CompileConst)]

struct TestStruct { testu8: u8, testvec: Vec, }

[derive(CompileConst)]

struct TestTup(u8, u16);

[derive(CompileConst)]

[inherit_doc]

/// Example inherited documentation enum TestEnum { Variant1, Variant2(u8), Variant3 { named: u8 } }

fn main() { // Use the OUTDIR environment variable to get an // appropriate path. let outdir = env::varos("OUTDIR").unwrap(); let destpath = Path::new(&outdir).join("const_gen.rs");

// Now let's dummy up some data to use in our const 
// generation
let test_vec: Vec<u8> = vec!(1,2,3,4,5,10,4);
let test_struct = TestStruct
{ 
    test_u8: 12, 
    test_vec: vec!(String::from("Hello there.")) 
};
let test_tup_struct = TestTup(4, 55,);
let test_enum = TestEnum::Variant1;
let test_enum_tup = TestEnum::Variant2(23);
let test_enum_structlike = TestEnum::Variant3{ named: 78 };

// Now we'll generate the const declarations. We're also 
// going to test with some primitive types. 
let const_declarations = vec!
{
    // Here are type definitions for our enums and structs 
    // above. Attributes from build.rs will not be preserved, 
    // so we need to pass any we want in.
    const_definition!(#[derive(Debug)] pub TestStruct),
    const_definition!(#[derive(Debug)] TestTup),
    const_definition!(#[derive(Debug)] TestEnum),

    // And here are constant definitions for particular 
    // values.
    const_declaration!(TEST_U8 = 27u8),
    const_declaration!(#[doc = "Example inherited documentation"] TEST_F32 = 33.5f32),
    const_declaration!(TEST_VEC = test_vec),
    const_declaration!(TEST_STRING = "I'm a string!"),
    const_declaration!(TEST_COW = 
        std::borrow::Cow::from("Cow!")),
    const_declaration!(pub TEST_STRUCT = test_struct),
    const_declaration!(TEST_TUP_STRUCT = test_tup_struct),
    const_declaration!(TEST_ENUM = test_enum),
    const_declaration!(TEST_ENUM_TUP = test_enum_tup),
    const_declaration!(TEST_ENUM_STRUCTLIKE = 
        test_enum_structlike),

   // or generate statics with the static_declaration macro
   static_declaration!(pub static TEST_U8_STATIC = 27u8),
}.join("\n");

// Note: The `const_definition!` and `const_declaration!` 
// macros above are just simple wrappers for CompileConst 
// trait methods of the same name. Using those methods
// would entail the following sytax:
// TestStruct::const_definition("#[derive(Debug)]")
// test_struct.const_declaration("TEST_STRUCT")
// These may be preferable in cases where const names
// or type attributes have been procedurally generated
// somehow and need to be treated as strings.

// If the "phf" feature is enabled, this crate will also 
// support converting HashMap and HashSet types into 
// compile-time constant phf map and set types respectively.

// Lastly, output to the destination file.
fs::write(&dest_path, const_declarations).unwrap();

}

```

Now, in our main.rs file we can do something like this:

```rust,ignore

// Include our constants include!(concat!(env!("OUTDIR"), "/constgen.rs"));

// And that's it, we can access all of the const values below. // It plays quite well with rust-analyzer, etc fn main() { println!("{}", TESTU8); println!("{}", TESTF32); println!("{:?}", TESTVEC); println!("{}", TESTSTRING); println!("{}", TESTCOW); println!("{:?}", TESTSTRUCT); println!("{:?}", TESTTUPSTRUCT); println!("{:?}", TESTENUM); println!("{:?}", TESTENUMTUP); println!("{:?}", TESTENUM_STRUCTLIKE); } ```

The actual generated output looks like (an unformatted version of) this: ```rust,ignore

[derive(Debug)]

struct TestStruct { testu8: u8, testvec: &'static [&'static str], }

[derive(Debug)]

struct TestTup(u8, u16);

[derive(Debug)]

/// Example inherited documentation enum TestEnum { Variant1, Variant2(u8), Variant3 { named: u8 }, } const TESTU8: u8 = 27u8; /// Example inherited documentation const TESTF32: f32 = 33.5f32; const TESTVEC: &'static [u8] = &[1u8, 2u8, 3u8, 4u8, 5u8, 10u8, 4u8]; const TESTSTRING: &'static str = "I'm a string!"; const TESTCOW: &'static str = "Cow!"; const TESTSTRUCT: TestStruct = TestStruct { testu8: 12u8, testvec: &["Hello there."], }; const TESTTUPSTRUCT: TestTup = TestTup(4u8, 55u16); const TESTENUM: TestEnum = TestEnum::Variant1; const TESTENUMTUP: TestEnum = TestEnum::Variant2(23u8); const TESTENUMSTRUCTLIKE: TestEnum = TestEnum::Variant3 { named: 78u8 }; static TESTU8_STATIC: u8 = 27u8; ```

Out-of-the-box Implementations

The following table shows what types have implementations of the CompileConst trait already defined

|Type|Const Representation| --- | --- |\|phf::Map\

There is also a CompileConstArray trait which generates fixed-size arrays rather than static slices for the following

|Type|Const Array Representation| --- | --- |Vec\

Limitations

This crate will use the endianness, pointer widths, etc of the host machine rather than the target. Eg, doing things like calling to_ne_bytes on an integer and storing the results in a const will result in a byte representation that may not be equivalent to that same integer on the target machine.

Features

At the current time, all features are default.

phf

The phf feature implements the CompileConst trait for HashMaps and HashSets. It will generate a phf::Map for HashMap types and a phf::Set for HashSet types. Note that phf does NOT need to be included in your build dependencies, but it ought to be included in your runtime dependencies in order to use the constants.

derive

The derive feature adds #[derive(CompileConst)] for structs and enums. The requirement is that all members implement CompileConst as well. The #[inherit_docs] attribute may be added to cause generated definition to inherit rustdocs.

std

The std default feature can be disabled to use the alloc crate in place of std constructs.