src/librustdoc/clean/simplify.rs

   1 //! Simplification of where-clauses and parameter bounds into a prettier and
   2 //! more canonical form.
   3 //!
   4 //! Currently all cross-crate-inlined function use `rustc_middle::ty` to reconstruct
   5 //! the AST (e.g., see all of `clean::inline`), but this is not always a
   6 //! non-lossy transformation. The current format of storage for where-clauses
   7 //! for functions and such is simply a list of predicates. One example of this
   8 //! is that the AST predicate of: `where T: Trait<Foo = Bar>` is encoded as:
   9 //! `where T: Trait, <T as Trait>::Foo = Bar`.
  10 //!
  11 //! This module attempts to reconstruct the original where and/or parameter
  12 //! bounds by special casing scenarios such as these. Fun!
  13
  14 use rustc_data_structures::fx::FxIndexMap;
  15 use rustc_hir::def_id::DefId;
  16 use rustc_middle::ty;
  17 use thin_vec::ThinVec;
  18
  19 use crate::clean;
  20 use crate::clean::GenericArgs as PP;
  21 use crate::clean::WherePredicate as WP;
  22 use crate::core::DocContext;
  23
  24 pub(crate) fn where_clauses(cx: &DocContext<'_>, clauses: Vec<WP>) -> ThinVec<WP> {
  25     // First, partition the where clause into its separate components.
  26     //
  27     // We use `FxIndexMap` so that the insertion order is preserved to prevent messing up to
  28     // the order of the generated bounds.
  29     let mut tybounds = FxIndexMap::default();
  30     let mut lifetimes = Vec::new();
  31     let mut equalities = Vec::new();
  32
  33     for clause in clauses {
  34         match clause {
  35             WP::BoundPredicate { ty, bounds, bound_params } => {
  36                 let (b, p): &mut (Vec<_>, Vec<_>) = tybounds.entry(ty).or_default();
  37                 b.extend(bounds);
  38                 p.extend(bound_params);
  39             }
  40             WP::RegionPredicate { lifetime, bounds } => {
  41                 lifetimes.push((lifetime, bounds));
  42             }
  43             WP::EqPredicate { lhs, rhs, bound_params } => equalities.push((lhs, rhs, bound_params)),
  44         }
  45     }
  46
  47     // Look for equality predicates on associated types that can be merged into
  48     // general bound predicates.
  49     equalities.retain(|&(ref lhs, ref rhs, ref bound_params)| {
  50         let Some((ty, trait_did, name)) = lhs.projection() else { return true; };
  51         let Some((bounds, _)) = tybounds.get_mut(ty) else { return true };
  52         let bound_params = bound_params
  53             .into_iter()
  54             .map(|param| clean::GenericParamDef::lifetime(param.0))
  55             .collect();
  56         merge_bounds(cx, bounds, bound_params, trait_did, name, rhs)
  57     });
  58
  59     // And finally, let's reassemble everything
  60     let mut clauses = ThinVec::with_capacity(lifetimes.len() + tybounds.len() + equalities.len());
  61     clauses.extend(
  62         lifetimes.into_iter().map(|(lt, bounds)| WP::RegionPredicate { lifetime: lt, bounds }),
  63     );
  64     clauses.extend(tybounds.into_iter().map(|(ty, (bounds, bound_params))| WP::BoundPredicate {
  65         ty,
  66         bounds,
  67         bound_params,
  68     }));
  69     clauses.extend(equalities.into_iter().map(|(lhs, rhs, bound_params)| WP::EqPredicate {
  70         lhs,
  71         rhs,
  72         bound_params,
  73     }));
  74     clauses
  75 }
  76
  77 pub(crate) fn merge_bounds(
  78     cx: &clean::DocContext<'_>,
  79     bounds: &mut Vec<clean::GenericBound>,
  80     mut bound_params: Vec<clean::GenericParamDef>,
  81     trait_did: DefId,
  82     assoc: clean::PathSegment,
  83     rhs: &clean::Term,
  84 ) -> bool {
  85     !bounds.iter_mut().any(|b| {
  86         let trait_ref = match *b {
  87             clean::GenericBound::TraitBound(ref mut tr, _) => tr,
  88             clean::GenericBound::Outlives(..) => return false,
  89         };
  90         // If this QPath's trait `trait_did` is the same as, or a supertrait
  91         // of, the bound's trait `did` then we can keep going, otherwise
  92         // this is just a plain old equality bound.
  93         if !trait_is_same_or_supertrait(cx, trait_ref.trait_.def_id(), trait_did) {
  94             return false;
  95         }
  96         let last = trait_ref.trait_.segments.last_mut().expect("segments were empty");
  97
  98         trait_ref.generic_params.append(&mut bound_params);
  99         // Sort parameters (likely) originating from a hashset alphabetically to
 100         // produce predictable output (and to allow for full deduplication).
 101         trait_ref.generic_params.sort_unstable_by(|p, q| p.name.as_str().cmp(q.name.as_str()));
 102         trait_ref.generic_params.dedup_by_key(|p| p.name);
 103
 104         match last.args {
 105             PP::AngleBracketed { ref mut bindings, .. } => {
 106                 bindings.push(clean::TypeBinding {
 107                     assoc: assoc.clone(),
 108                     kind: clean::TypeBindingKind::Equality { term: rhs.clone() },
 109                 });
 110             }
 111             PP::Parenthesized { ref mut output, .. } => match output {
 112                 Some(o) => assert_eq!(&clean::Term::Type(o.as_ref().clone()), rhs),
 113                 None => {
 114                     if *rhs != clean::Term::Type(clean::Type::Tuple(Vec::new())) {
 115                         *output = Some(Box::new(rhs.ty().unwrap().clone()));
 116                     }
 117                 }
 118             },
 119         };
 120         true
 121     })
 122 }
 123
 124 fn trait_is_same_or_supertrait(cx: &DocContext<'_>, child: DefId, trait_: DefId) -> bool {
 125     if child == trait_ {
 126         return true;
 127     }
 128     let predicates = cx.tcx.super_predicates_of(child);
 129     debug_assert!(cx.tcx.generics_of(child).has_self);
 130     let self_ty = cx.tcx.types.self_param;
 131     predicates
 132         .predicates
 133         .iter()
 134         .filter_map(|(pred, _)| {
 135             if let ty::PredicateKind::Clause(ty::Clause::Trait(pred)) = pred.kind().skip_binder() {
 136                 if pred.trait_ref.self_ty() == self_ty { Some(pred.def_id()) } else { None }
 137             } else {
 138                 None
 139             }
 140         })
 141         .any(|did| trait_is_same_or_supertrait(cx, did, trait_))
 142 }