doc_for

[WARNING] This crate is still in development, and the API is subject to BREAKING CHANGES.

📖 Get the documentation comment for structs, enums and unions, in a zero-cost fashion.

🪄 Features

• Zero-cost: All work is done at compile-time
• Simple: Just derive the DocFor trait and use the doc_for! macro

🤔 Usage

Get the documentation comment for a type

First, bring DocFor and doc_for! into scope:

use doc_for::{DocFor, doc_for};

Then, derive the DocFor trait for your struct:

# use doc_for::{DocFor, doc_for};
#
/// Some documentation
#[derive(DocFor)]
struct MyStruct {
    field: i32,
}

Finally, use the doc_for! macro to get the documentation comment, which returns an Option<&'static str>:

# use doc_for::{DocFor, doc_for};
#
# /// Some documentation
# #[derive(DocFor)]
# struct MyStruct {
#     field: i32,
# }
assert_eq!(doc_for!(MyStruct).unwrap(), " Some documentation");

Note that the leading spaces are preserved. Multi-line comments are also supported:

# use doc_for::{DocFor, doc_for};
#
/// Some documentation
/// that spans multiple lines
///
/// Additional information
#[derive(DocFor)]
struct MyStruct {
    field: i32,
}
assert_eq!(doc_for!(MyStruct).unwrap(), r#" Some documentation
 that spans multiple lines

 Additional information"#);

If the type does not have a documentation comment, doc_for! will return None:

# use doc_for::{DocFor, doc_for};
#
// No documentation comment here
#[derive(DocFor)]
struct MyStruct {
    field: i32,
}
assert!(doc_for!(MyStruct).is_none());

Also works with tuple structs, enums and unions:

# use doc_for::{DocFor, doc_for};
#
/// Tuple struct documentation
#[derive(DocFor)]
struct MyTupleStruct(i32);
assert_eq!(doc_for!(MyTupleStruct).unwrap(), " Tuple struct documentation");

/// Enum documentation
#[derive(DocFor)]
enum MyEnum {
    Variant,
}
assert_eq!(doc_for!(MyEnum).unwrap(), " Enum documentation");

/// Union documentation
#[derive(DocFor)]
union MyUnion {
    field: i32,
}
assert_eq!(doc_for!(MyUnion).unwrap(), " Union documentation");

Get the documentation comment for fields and variants

Same as before, bring DocFor and doc_for! into scope and derive the DocFor trait for your struct:

use doc_for::{DocFor, doc_for};

#[derive(DocFor)]
struct MyStruct {
    /// Field documentation
    field: i32,
    not_documented: i32,
}

Then, use the doc_for! macro to get the documentation comment. If the field does not have a documentation comment, doc_for! will return None:

# use doc_for::{DocFor, doc_for};
#
# #[derive(DocFor)]
# struct MyStruct {
#     /// Field documentation
#     field: i32,
#     not_documented: i32,
# }
assert_eq!(doc_for!(MyStruct, field).unwrap(), " Field documentation");
assert!(doc_for!(MyStruct, not_documented).is_none());
// Won't compile due to `Field or variant does not exist`
// assert_eq!(doc_for!(MyStruct, non_existent), None);

If the field or variant does not exist, doc_for! will panic, thus failing the compilation:

# use doc_for::{DocFor, doc_for};
#
# #[derive(DocFor)]
# struct MyStruct {
#     /// Field documentation
#     field: i32,
#     not_documented: i32,
# }
// Won't compile due to `Field or variant does not exist`
assert_eq!(doc_for!(MyStruct, non_existent), None);

Similarly, it also works with union fields (not listed here), enum variants and tuple struct fields:

# use doc_for::{DocFor, doc_for};
#
#[derive(DocFor)]
enum MyEnum {
    /// Variant documentation
    Variant,
    NotDocumented,
}
assert_eq!(doc_for!(MyEnum, Variant).unwrap(), " Variant documentation");
assert!(doc_for!(MyEnum, NotDocumented).is_none());
// Won't compile due to `Field or variant does not exist`
// assert_eq!(doc_for!(MyEnum, NonExistent), None);

#[derive(DocFor)]
struct MyTupleStruct(
    /// Tuple struct field documentation
    i32,
    i32,
);
assert_eq!(doc_for!(MyTupleStruct, 0).unwrap(), " Tuple struct field documentation");
assert!(doc_for!(MyTupleStruct, 1).is_none());
// Won't compile due to `Field or variant does not exist`
// assert_eq!(doc_for!(MyTupleStruct, 2), None);

If you don't care about the Option

The doc! macro is basically doc_for! with unwrap:

use doc_for::{DocFor, doc};

#[derive(DocFor)]
struct MyStruct {
    /// Field documentation
    field: i32,
    not_documented: i32,
}

assert_eq!(doc!(MyStruct, field), " Field documentation");

...So it panics and fails the compilation if the requested type or field is not documented:

# use doc_for::{DocFor, doc};
#
# #[derive(DocFor)]
# struct MyStruct {
#    /// Field documentation
#    field: i32,
#    not_documented: i32,
# }
#
// Won't compile due to `The type is not documented`
println!("{}", doc!(MyStruct));

# use doc_for::{DocFor, doc};
#
# #[derive(DocFor)]
# struct MyStruct {
#    /// Field documentation
#    field: i32,
#    not_documented: i32,
# }
#
// Won't compile due to `The field is not documented`
println!("{}", doc!(MyStruct, not_documented));

Of course, trying to access a non-existent field or variant will also fail the compilation:

# use doc_for::{DocFor, doc};
#
# #[derive(DocFor)]
# struct MyStruct {
#    /// Field documentation
#    field: i32,
#    not_documented: i32,
# }
#
// Won't compile due to `Field or variant does not exist`
println!("{}", doc!(MyStruct, non_existent));

Get the documentation comment for an enum variant

This time, bring DocDyn into scope:

use doc_for::DocDyn;

Then, derive the DocDyn trait for your enum:

# use doc_for::DocDyn;
#
#[derive(DocDyn)]
enum MyEnum {
    /// Variant documentation
    Variant,
    NotDocumented,
}

Finally, call the doc_dyn method on the enum variant:

# use doc_for::DocDyn;
#
# #[derive(DocDyn)]
# enum MyEnum {
#     /// Variant documentation
#     Variant,
#     NotDocumented,
# }
assert_eq!(MyEnum::Variant.doc_dyn().unwrap(), " Variant documentation");
assert!(MyEnum::NotDocumented.doc_dyn().is_none());

Note that this method is not zero-cost, as it matches the enum variant at runtime.

⚙️ Implementation

DocFor and doc_for!

The doc_for crate provides a DocFor trait and a doc_for! macro:

• The DocFor trait requires an associated constant DOC to be implemented for the type
• Deriving the DocFor trait sets the DOC constant as the documentation comment of the type, and generates a const fn doc_for_field(name) -> Option<&'static str> function
  • Currently Rust doesn't support constant functions in traits, so the doc_for_field function is implemented directly on the annotated type
  • If the annotated type is a struct, union or enum, the name parameter accepts a &'static str
  • If the annotated type is a tuple struct, the name parameter accepts an usize
• If given a type, the doc_for! macro retrieves the value of this constant; If given a type and a field name, the doc_for! macro calls the doc_for_field function with the given field name

Using these APIs is zero-cost, as all the work is done at compile-time:

• When compiled, types that derive DocFor will have their documentation comments inlined as associated constants or in constant functions
• Calls to doc_for! will be replaced with the value of the associated constant or the result of the constant function

DocDyn and doc_dyn

The doc_for crate also provides a DocDyn trait and a doc_dyn method:

• The DocDyn trait requires a doc_dyn method to be implemented for the type, which returns an Option<&'static str>
• Deriving the DocDyn trait generates a doc_dyn method, which returns the documentation comment that matches the variant of the enum

This method is not zero-cost, as it matches the enum variant at runtime.

✅ TODO

• Strip each line of the documentation comment, via a strip attribute
• Access module documentation (e.g. doc_for!(my_module))
• Access trait documentation (e.g. doc_for!(MyTrait))
• Access sub-item documentation
  • Access field documentation (e.g. doc_for!(MyStruct, field) or doc_for!(MyUnion, field))
  • Access tuple struct field documentation (e.g. doc_for!(MyTupleStruct, 0))
  • Access enum variant documentation (statically) (e.g. doc_for!(MyEnum, Variant))
  • Access enum variant documentation (dynamically) (e.g. doc_for!(my_enum_variant))
  • Access method documentation (e.g. doc_for!(MyStruct, method))
  • Access associated constant documentation (e.g. doc_for!(MyStruct, CONSTANT))
  • Access associated type documentation (e.g. doc_for!(MyStruct, Type))