kexplain/
lib.rs

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extern crate proc_macro;
use proc_macro::TokenStream;
use proc_macro2::{Ident, Span};
use quote::{quote, ToTokens};
use syn::{parse_macro_input, parse_quote, ItemFn};

/// Unwrap the Option value or break.
macro_rules! or_continue {
    ( $wrapper:expr ) => {
        match $wrapper {
            Some(v) => v,
            None => continue,
        }
    };
}

fn has_attr(attrs: &[syn::Attribute], attr_name: &str) -> bool {
    attrs.iter().any(|a| {
        a.parse_meta()
            .ok()
            .map(|meta| meta.path().is_ident(attr_name))
            .unwrap_or(false)
    })
}

fn has_skip_attr(attrs: &[syn::Attribute]) -> bool {
    has_attr(attrs, "skip")
}

fn has_no_expr_attr(attrs: &[syn::Attribute]) -> bool {
    has_attr(attrs, "no_expr")
}

fn find_ident(pat: &syn::Pat) -> Option<&Ident> {
    match pat {
        syn::Pat::Ident(pat_ident) => Some(&pat_ident.ident),
        _ => None,
    }
}

#[proc_macro_attribute]
/// ```
/// use kexplain::explain;
/// 
/// #[explain]
/// fn foo(a: u32, b: f64) -> u32 {
///     let _x = a * b as u32;
///     #[no_expr]
///     let x = a * b as u32;
///     #[skip]
///     let _y = a * b as u32;
///     x * 3
/// }
/// 
/// struct Foo;
/// 
/// impl Foo {
///     #[explain]
///     fn bar(&self, a: u32, b: f64) -> u32 {
///         let _x = a * b as u32;
///         #[no_expr]
///         let x = a * b as u32;
///         #[skip]
///         let _y = a * b as u32;
///         x * 3
///     }
/// }
/// 
/// fn main() {
///     assert_eq!(6, foo(1, 2.));
///     assert_eq!(6, foo_explain(1, 2., |name, expr, value| {
///         println!("{name} {expr:?} {value}");
///     }));
///     assert_eq!(6, Foo.bar(1, 2.));
///     assert_eq!(6, Foo.bar_explain(1, 2., |name, expr, value| {
///         println!("{name} {expr:?} {value}");
///     }));
/// }
/// ```
///
/// Example stdout:
/// ```text
/// STDOUT:
/// ┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈
/// a None 1
/// b None 2
/// _x Some("a * b as u32") 2
/// x None 2
///  None 6
/// a None 1
/// b None 2
/// _x Some("a * b as u32") 2
/// x None 2
///  None 6
/// ┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈
/// ```
///
/// See the `tests` for more examples.
pub fn explain(_attr: TokenStream, item: TokenStream) -> TokenStream {
    let mut function = parse_macro_input!(item as ItemFn);
    let mut new_function = function.clone();

    // TODO wish I could use Span::def_site() but needs nightly
    let callback = Ident::new("callback", Span::call_site());
    let callback_arg: syn::FnArg = parse_quote! {
        mut #callback: impl FnMut(&str, Option<&str>, &dyn std::fmt::Display)
    };

    new_function.sig.inputs.push(callback_arg);

    // TODO wish I could use Span::def_site() but needs nightly
    new_function.sig.ident = Ident::new(
        &format!("{}_explain", function.sig.ident),
        Span::call_site(),
    );

    let new_body = &mut new_function.block;
    new_body.stmts.clear();
    for arg in function.sig.inputs.iter() {
        match arg {
            syn::FnArg::Typed(pattype) if !has_skip_attr(&pattype.attrs) => {
                let ident = or_continue!(find_ident(&pattype.pat));
                let ident_str = ident.to_string();
                let ident_str = ident_str.as_str();
                new_body.stmts.push(parse_quote! {
                    #callback(#ident_str, None, &#ident);
                });
            }
            syn::FnArg::Receiver(_receiver) => (),
            syn::FnArg::Typed(_) => (),
        }
    }
    for stmt in function.block.stmts.iter_mut() {
        match stmt {
            syn::Stmt::Local(local) => {
                let should_skip = has_skip_attr(&local.attrs);
                let skip_expression = has_no_expr_attr(&local.attrs);
                local.attrs.clear();
                new_body.stmts.push(syn::Stmt::Local(local.clone()));
                if should_skip {
                    continue;
                }
                let expr = &or_continue!(local.init.as_ref()).1;
                let ident = or_continue!(find_ident(&local.pat));
                let ident_str = ident.to_string();
                let ident_str = ident_str.as_str();
                let expr_str = expr.to_token_stream().to_string();
                let expr_str = expr_str.as_str();
                let expr_expr: syn::Expr = if skip_expression {
                    parse_quote! { None }
                } else {
                    parse_quote! { Some(#expr_str) }
                };
                new_body.stmts.push(parse_quote! {
                    #callback(#ident_str, #expr_expr, &#ident);
                });
            }
            // syn::Stmt::Item(_item) => (),
            // syn::Stmt::Expr(_expr) => (),
            // syn::Stmt::Semi(_expr, _semi) => (),
            _ => {
                new_body.stmts.push(stmt.clone());
            }
        }
    }

    *new_body = parse_quote! {
        {
            let result = #new_body;
            #callback("", None, &result);
            result
        }
    };

    (quote! {
        #function
        #new_function
    })
    .into()
}