src/libsyntax_ext/deriving/cmp/ord.rs

use crate::deriving::path_std;
use crate::deriving::generic::*;
use crate::deriving::generic::ty::*;

use syntax::ast::{self, Expr, MetaItem};
use syntax::ext::base::{Annotatable, ExtCtxt};
use syntax::ptr::P;
use syntax::symbol::sym;
use syntax_pos::Span;

pub fn expand_deriving_ord(cx: &mut ExtCtxt<'_>,
                           span: Span,
                           mitem: &MetaItem,
                           item: &Annotatable,
                           push: &mut dyn FnMut(Annotatable)) {
    let inline = cx.meta_word(span, sym::inline);
    let attrs = vec![cx.attribute(inline)];
    let trait_def = TraitDef {
        span,
        attributes: Vec::new(),
        path: path_std!(cx, cmp::Ord),
        additional_bounds: Vec::new(),
        generics: LifetimeBounds::empty(),
        is_unsafe: false,
        supports_unions: false,
        methods: vec![MethodDef {
                          name: "cmp",
                          generics: LifetimeBounds::empty(),
                          explicit_self: borrowed_explicit_self(),
                          args: vec![(borrowed_self(), "other")],
                          ret_ty: Literal(path_std!(cx, cmp::Ordering)),
                          attributes: attrs,
                          is_unsafe: false,
                          unify_fieldless_variants: true,
                          combine_substructure: combine_substructure(Box::new(|a, b, c| {
                              cs_cmp(a, b, c)
                          })),
                      }],
        associated_types: Vec::new(),
    };

    trait_def.expand(cx, mitem, item, push)
}


pub fn ordering_collapsed(cx: &mut ExtCtxt<'_>,
                          span: Span,
                          self_arg_tags: &[ast::Ident])
                          -> P<ast::Expr> {
    let lft = cx.expr_ident(span, self_arg_tags[0]);
    let rgt = cx.expr_addr_of(span, cx.expr_ident(span, self_arg_tags[1]));
    cx.expr_method_call(span, lft, cx.ident_of("cmp"), vec![rgt])
}

pub fn cs_cmp(cx: &mut ExtCtxt<'_>, span: Span, substr: &Substructure<'_>) -> P<Expr> {
    let test_id = cx.ident_of("cmp").gensym();
    let equals_path = cx.path_global(span, cx.std_path(&[sym::cmp, sym::Ordering, sym::Equal]));

    let cmp_path = cx.std_path(&[sym::cmp, sym::Ord, sym::cmp]);

    // Builds:
    //
    // match ::std::cmp::Ord::cmp(&self_field1, &other_field1) {
    // ::std::cmp::Ordering::Equal =>
    // match ::std::cmp::Ord::cmp(&self_field2, &other_field2) {
    // ::std::cmp::Ordering::Equal => {
    // ...
    // }
    // cmp => cmp
    // },
    // cmp => cmp
    // }
    //
    cs_fold(// foldr nests the if-elses correctly, leaving the first field
            // as the outermost one, and the last as the innermost.
            false,
            |cx, span, old, self_f, other_fs| {
        // match new {
        //     ::std::cmp::Ordering::Equal => old,
        //     cmp => cmp
        // }

        let new = {
            let other_f = match other_fs {
                [o_f] => o_f,
                _ => cx.span_bug(span, "not exactly 2 arguments in `derive(Ord)`"),
            };

            let args = vec![
                    cx.expr_addr_of(span, self_f),
                    cx.expr_addr_of(span, other_f.clone()),
                ];

            cx.expr_call_global(span, cmp_path.clone(), args)
        };

        let eq_arm = cx.arm(span,
                            vec![cx.pat_path(span, equals_path.clone())],
                            old);
        let neq_arm = cx.arm(span,
                             vec![cx.pat_ident(span, test_id)],
                             cx.expr_ident(span, test_id));

        cx.expr_match(span, new, vec![eq_arm, neq_arm])
    },
            cx.expr_path(equals_path.clone()),
            Box::new(|cx, span, (self_args, tag_tuple), _non_self_args| {
        if self_args.len() != 2 {
            cx.span_bug(span, "not exactly 2 arguments in `derive(Ord)`")
        } else {
            ordering_collapsed(cx, span, tag_tuple)
        }
    }),
            cx,
            span,
            substr)
}