compiler/rustc_builtin_macros/src/deriving/cmp/partial_ord.rs

   1 use crate::deriving::generic::ty::*;
   2 use crate::deriving::generic::*;
   3 use crate::deriving::{path_std, pathvec_std};
   4 use rustc_ast::{ExprKind, ItemKind, MetaItem, PatKind};
   5 use rustc_expand::base::{Annotatable, ExtCtxt};
   6 use rustc_span::symbol::{sym, Ident};
   7 use rustc_span::Span;
   8 use thin_vec::thin_vec;
   9
  10 pub fn expand_deriving_partial_ord(
  11     cx: &mut ExtCtxt<'_>,
  12     span: Span,
  13     mitem: &MetaItem,
  14     item: &Annotatable,
  15     push: &mut dyn FnMut(Annotatable),
  16     is_const: bool,
  17 ) {
  18     let ordering_ty = Path(path_std!(cmp::Ordering));
  19     let ret_ty =
  20         Path(Path::new_(pathvec_std!(option::Option), vec![Box::new(ordering_ty)], PathKind::Std));
  21
  22     let attrs = thin_vec![cx.attr_word(sym::inline, span)];
  23
  24     // Order in which to perform matching
  25     let tag_then_data = if let Annotatable::Item(item) = item
  26         && let ItemKind::Enum(def, _) = &item.kind {
  27             let dataful: Vec<bool> = def.variants.iter().map(|v| !v.data.fields().is_empty()).collect();
  28             match dataful.iter().filter(|&&b| b).count() {
  29                 // No data, placing the tag check first makes codegen simpler
  30                 0 => true,
  31                 1..=2 => false,
  32                 _ => {
  33                     (0..dataful.len()-1).any(|i| {
  34                         if dataful[i] && let Some(idx) = dataful[i+1..].iter().position(|v| *v) {
  35                             idx >= 2
  36                         } else {
  37                             false
  38                         }
  39                     })
  40                 }
  41             }
  42         } else {
  43             true
  44         };
  45     let partial_cmp_def = MethodDef {
  46         name: sym::partial_cmp,
  47         generics: Bounds::empty(),
  48         explicit_self: true,
  49         nonself_args: vec![(self_ref(), sym::other)],
  50         ret_ty,
  51         attributes: attrs,
  52         fieldless_variants_strategy: FieldlessVariantsStrategy::Unify,
  53         combine_substructure: combine_substructure(Box::new(|cx, span, substr| {
  54             cs_partial_cmp(cx, span, substr, tag_then_data)
  55         })),
  56     };
  57
  58     let trait_def = TraitDef {
  59         span,
  60         path: path_std!(cmp::PartialOrd),
  61         skip_path_as_bound: false,
  62         additional_bounds: vec![],
  63         supports_unions: false,
  64         methods: vec![partial_cmp_def],
  65         associated_types: Vec::new(),
  66         is_const,
  67     };
  68     trait_def.expand(cx, mitem, item, push)
  69 }
  70
  71 fn cs_partial_cmp(
  72     cx: &mut ExtCtxt<'_>,
  73     span: Span,
  74     substr: &Substructure<'_>,
  75     tag_then_data: bool,
  76 ) -> BlockOrExpr {
  77     let test_id = Ident::new(sym::cmp, span);
  78     let equal_path = cx.path_global(span, cx.std_path(&[sym::cmp, sym::Ordering, sym::Equal]));
  79     let partial_cmp_path = cx.std_path(&[sym::cmp, sym::PartialOrd, sym::partial_cmp]);
  80
  81     // Builds:
  82     //
  83     // match ::core::cmp::PartialOrd::partial_cmp(&self.x, &other.x) {
  84     //     ::core::option::Option::Some(::core::cmp::Ordering::Equal) =>
  85     //         ::core::cmp::PartialOrd::partial_cmp(&self.y, &other.y),
  86     //     cmp => cmp,
  87     // }
  88     let expr = cs_fold(
  89         // foldr nests the if-elses correctly, leaving the first field
  90         // as the outermost one, and the last as the innermost.
  91         false,
  92         cx,
  93         span,
  94         substr,
  95         |cx, fold| match fold {
  96             CsFold::Single(field) => {
  97                 let [other_expr] = &field.other_selflike_exprs[..] else {
  98                         cx.span_bug(field.span, "not exactly 2 arguments in `derive(Ord)`");
  99                     };
 100                 let args = vec![field.self_expr.clone(), other_expr.clone()];
 101                 cx.expr_call_global(field.span, partial_cmp_path.clone(), args)
 102             }
 103             CsFold::Combine(span, mut expr1, expr2) => {
 104                 // When the item is an enum, this expands to
 105                 // ```
 106                 // match (expr2) {
 107                 //     Some(Ordering::Equal) => expr1,
 108                 //     cmp => cmp
 109                 // }
 110                 // ```
 111                 // where `expr2` is `partial_cmp(self_tag, other_tag)`, and `expr1` is a `match`
 112                 //  against the enum variants. This means that we begin by comparing the enum tags,
 113                 // before either inspecting their contents (if they match), or returning
 114                 // the `cmp::Ordering` of comparing the enum tags.
 115                 // ```
 116                 // match partial_cmp(self_tag, other_tag) {
 117                 //     Some(Ordering::Equal) => match (self, other)  {
 118                 //         (Self::A(self_0), Self::A(other_0)) => partial_cmp(self_0, other_0),
 119                 //         (Self::B(self_0), Self::B(other_0)) => partial_cmp(self_0, other_0),
 120                 //         _ => Some(Ordering::Equal)
 121                 //     }
 122                 //     cmp => cmp
 123                 // }
 124                 // ```
 125                 // If we have any certain enum layouts, flipping this results in better codegen
 126                 // ```
 127                 // match (self, other) {
 128                 //     (Self::A(self_0), Self::A(other_0)) => partial_cmp(self_0, other_0),
 129                 //     _ => partial_cmp(self_tag, other_tag)
 130                 // }
 131                 // ```
 132                 // Reference: https://github.com/rust-lang/rust/pull/103659#issuecomment-1328126354
 133
 134                 if !tag_then_data
 135                     && let ExprKind::Match(_, arms) = &mut expr1.kind
 136                     && let Some(last) = arms.last_mut()
 137                     && let PatKind::Wild = last.pat.kind {
 138                         last.body = expr2;
 139                         expr1
 140                 } else {
 141                     let eq_arm =
 142                         cx.arm(span, cx.pat_some(span, cx.pat_path(span, equal_path.clone())), expr1);
 143                     let neq_arm =
 144                         cx.arm(span, cx.pat_ident(span, test_id), cx.expr_ident(span, test_id));
 145                     cx.expr_match(span, expr2, vec![eq_arm, neq_arm])
 146                 }
 147             }
 148             CsFold::Fieldless => cx.expr_some(span, cx.expr_path(equal_path.clone())),
 149         },
 150     );
 151     BlockOrExpr::new_expr(expr)
 152 }