docpos - compact-documenting items after defining them: functions, structs, enums…

(a fork of the "passively-maintained" roxygen with a few more supported items and allowing for a pos-position)

The #[docpos] attribute allows you to add doc-comments after an item, not before (as in regular Rust) in enums and structs, and also document function parameters (a compile error in current Rust). Generic lifetimes, types, and constants of the function can also be documented.

You can now write a much more readable and compact table:

use std::path::PathBuf;
use docpos::*;

#[docpos] pub struct StructyPos { /// "inner" scruct docs
  pub field1       :        String  ,/// pos-doc for `field1` (in regular Rust this would be a doc for `field2_longer`)
  pub field2_longer: Option<String> ,/// pos-doc for `field2_longer`
                                     /// pos-doc for `field2_longer` line 2
                                     ///! pre-doc for `paths` at `field2_longer` (after `///!`)
  pub paths        : Vec   <PathBuf>, // no doc comments allowed here, use `///!` in the previous field
}

instead of

use std::path::PathBuf;

/// "Outer" scruct docs wasting a line
pub struct StructyPre {
  /// pre-doc for `field1`, another line wasted on a short comment
  pub field1: String	,
  /// pre-doc for `field2_longer`, breaking the vertical flow of fields
  /// pre-doc for `field2_longer`, making the disconnect bigger
  pub field2_longer: Option<String>,
  /// pre-doc for `paths`
  pub paths: Vec<PathBuf>,
  // oh, and it's not vertically aligned anymore since you can't use an elastic-tabstoppy plugin to help with that
}

Macro can be used with an explicit argument #[docpos(struct)] (enum,fn) or let the macro try each supported type via #[docpos], though the latter will generate errors for each type.

Similarly, for a function, you can add doc-comments to parameters:

use docpos::*;

#[docpos]
fn sum_image_rows_pos( /// sum the rows of an image
  image_data: &    [f32], /// the image data in row-major format
  nrows     :       u32 , /// the number of rows in the image
  ncols     :       u32 , /// the number of columns in the image
  // everything after ///! will become part of the last parameter's comments!
                          ///! an out buffer into which the resulting
                          /// sums are placed. Must have space 
                          /// for exactly `nrows` elements
  sums      : &mut [f32], // doc comments are illegal here, so use "///!"-split comment syntax from ↑
) -> Result<(),String> {} 

(Or use the main roxygen's crate macro roxygen for reguler pre-doc-comment support)

You have to document at least one parameter (or generic), but you don't have to document all of them. The example above will produce documentation as if you had written a doc comment for the function like so:

/// sum the rows of an image
///
/// **Parameters**:
///
/// * `image_data`: the image data in row-major format
/// * `nrows`: the number of rows in the image
/// * `ncols`: the number of columns in the image
/// * `sums`: an out buffer into which the resulting
///    sums are placed. Must have space
///    for exactly `nrows` elements
fn sum_image_rows_pos(
    image_data: &[f32],
    nrows: u32,
    ncols: u32,
    sums: &mut [f32],
) -> Result<(), String> {}

Enums

#[docpos(enum)] // or #[docpos] for auto-detection
enum EnumyPos { /// enumPos line1
                /// enumPos line2
  V1	,/// v1→v1 line 1
    	 /// v1→v1 line 2
  V2	,/// v2→v2 line 1
  V3	,// won't be shown
  V4	,// won't be shown
    	 ///! v4→v5 line 1
  V5	,//
}

Replaces

/// enumPos line1
/// enumPos line2
enum EnumyPos {
    /// v1→v1 line 1
    /// v1→v1 line 2
    V1,
    /// v2→v2 line 1
    V2,
    V3,
    V4,
    /// v4→v5 line 1
    V5,
}

Though here the expansion results is exactly this, without a separate section like for function parameters, since even when all docs are removed by the macro for manual formatting, the default Variants section remains (use #[docpos(enum_sect)] to force an explicit section)

⚠️ Renaming the macros exported from this crate (use ... as ...) or renaming the crate itself (in your Cargo.toml) will make all of this stop working properly.

Placing the Parameters-Section

By default, the section documenting the parameters will go at the end of the top-level function documentation. However, this crate allows to explicitly place the section by using a custom attribute like so:

use docpos::roxygen;

#[roxygen]
/// long documention
/// ...
#[docpos::parameters_section]
/// # Examples
/// ...
fn foo(
  /// some docs
  first: i32,
  second: f32
  )
{}

Considerations

It's a long standing issue whether and how to add this capability to rustdoc. Firstly, there's no general consensus on how exactly to document function parameters. However, I've seen the presented style used a lot, with minor variations. Secondly, the standard library doesn't need this style of documentation at all. So before you stick this macro on every function, do consider

• taking inspiration from how the standard library deals with function parameters,
• using fewer function parameters,
• using more descriptive parameters names,
• using types to communicate intent,
• sticking function parameters in a struct.

Here is an elegant way, how the example above can be reworked without using per parameter documentation:

/// Sums the rows of an image.
///
/// The rows of `image_data`, an `nrows` by `ncols`
/// matrix in row-major ordering, are summed into `sums`
/// which must have exactly `nrows` elements.
fn sum_image_rows(
  image_data: &[f32],
  nrows: u32,
  ncols: u32,
  sums: &mut [f32]) -> Result<(),String> {
    todo!()
}

All that being said, I've realized that sometimes I still want to document function parameters.

Compile Times

Macros will always increase your compile time to some degree, but I don't think this is a giant issue (after the docpos dependency itself was compiled, that is): firstly, this macro is to be used sparingly. Secondly, this macro just does some light parsing and shuffling around of the documentation tokens. It introduces no additional code. Thus, it doesn't make your actual code more or less complex and should not affect compile times much (after this crate was compiled once), but I haven't measured it... so take it with a grain of sodium-chloride.

Known issues

• the default doc sections (e.g., Variants for enums) persists even when it has no docs, so structs and enums don't add their own section unless explicitly called with enum_sect, struct_sect
• unrealized potential to waste less space in the output: short comments could've been "inlined" to the function signature as regular comments instead of having the whole new section
• rather limited support: functions (parameters and generics), structs (fields), enums (variants)
• invalid enum variant's visibility qualifiers enum Vis {pub V1} are not rejected due to syn design decision