rustdoc/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
14use rustc_data_structures::fx::FxIndexMap;
15use rustc_data_structures::unord::UnordSet;
16use rustc_hir::def_id::DefId;
17use thin_vec::ThinVec;
18
19use crate::clean;
20use crate::clean::{GenericArgs as PP, WherePredicate as WP};
21use crate::core::DocContext;
22
23pub(crate) fn where_clauses(cx: &DocContext<'_>, clauses: ThinVec<WP>) -> ThinVec<WP> {
24    // First, partition the where clause into its separate components.
25    //
26    // We use `FxIndexMap` so that the insertion order is preserved to prevent messing up to
27    // the order of the generated bounds.
28    let mut tybounds = FxIndexMap::default();
29    let mut lifetimes = Vec::new();
30    let mut equalities = Vec::new();
31
32    for clause in clauses {
33        match clause {
34            WP::BoundPredicate { ty, bounds, bound_params } => {
35                let (b, p): &mut (Vec<_>, Vec<_>) = tybounds.entry(ty).or_default();
36                b.extend(bounds);
37                p.extend(bound_params);
38            }
39            WP::RegionPredicate { lifetime, bounds } => {
40                lifetimes.push((lifetime, bounds));
41            }
42            WP::EqPredicate { lhs, rhs } => equalities.push((lhs, rhs)),
43        }
44    }
45
46    // Look for equality predicates on associated types that can be merged into
47    // general bound predicates.
48    equalities.retain(|(lhs, rhs)| {
49        let Some((ty, trait_did, name)) = lhs.projection() else {
50            return true;
51        };
52        let Some((bounds, _)) = tybounds.get_mut(ty) else { return true };
53        merge_bounds(cx, bounds, trait_did, name, rhs)
54    });
55
56    // And finally, let's reassemble everything
57    let mut clauses = ThinVec::with_capacity(lifetimes.len() + tybounds.len() + equalities.len());
58    clauses.extend(
59        lifetimes.into_iter().map(|(lt, bounds)| WP::RegionPredicate { lifetime: lt, bounds }),
60    );
61    clauses.extend(tybounds.into_iter().map(|(ty, (bounds, bound_params))| WP::BoundPredicate {
62        ty,
63        bounds,
64        bound_params,
65    }));
66    clauses.extend(equalities.into_iter().map(|(lhs, rhs)| WP::EqPredicate { lhs, rhs }));
67    clauses
68}
69
70pub(crate) fn merge_bounds(
71    cx: &clean::DocContext<'_>,
72    bounds: &mut [clean::GenericBound],
73    trait_did: DefId,
74    assoc: clean::PathSegment,
75    rhs: &clean::Term,
76) -> bool {
77    !bounds.iter_mut().any(|b| {
78        let trait_ref = match *b {
79            clean::GenericBound::TraitBound(ref mut tr, _) => tr,
80            clean::GenericBound::Outlives(..) | clean::GenericBound::Use(_) => return false,
81        };
82        // If this QPath's trait `trait_did` is the same as, or a supertrait
83        // of, the bound's trait `did` then we can keep going, otherwise
84        // this is just a plain old equality bound.
85        if !trait_is_same_or_supertrait(cx, trait_ref.trait_.def_id(), trait_did) {
86            return false;
87        }
88        let last = trait_ref.trait_.segments.last_mut().expect("segments were empty");
89
90        match last.args {
91            PP::AngleBracketed { ref mut constraints, .. } => {
92                constraints.push(clean::AssocItemConstraint {
93                    assoc: assoc.clone(),
94                    kind: clean::AssocItemConstraintKind::Equality { term: rhs.clone() },
95                });
96            }
97            PP::Parenthesized { ref mut output, .. } => match output {
98                Some(o) => assert_eq!(&clean::Term::Type(o.as_ref().clone()), rhs),
99                None => {
100                    if *rhs != clean::Term::Type(clean::Type::Tuple(Vec::new())) {
101                        *output = Some(Box::new(rhs.ty().unwrap().clone()));
102                    }
103                }
104            },
105            PP::ReturnTypeNotation => {
106                // Cannot merge bounds with RTN.
107                return false;
108            }
109        };
110        true
111    })
112}
113
114fn trait_is_same_or_supertrait(cx: &DocContext<'_>, child: DefId, trait_: DefId) -> bool {
115    if child == trait_ {
116        return true;
117    }
118    let predicates = cx.tcx.explicit_super_predicates_of(child);
119    predicates
120        .iter_identity_copied()
121        .filter_map(|(pred, _)| Some(pred.as_trait_clause()?.def_id()))
122        .any(|did| trait_is_same_or_supertrait(cx, did, trait_))
123}
124
125pub(crate) fn sized_bounds(cx: &mut DocContext<'_>, generics: &mut clean::Generics) {
126    let mut sized_params = UnordSet::new();
127
128    // In the surface language, all type parameters except `Self` have an
129    // implicit `Sized` bound unless removed with `?Sized`.
130    // However, in the list of where-predicates below, `Sized` appears like a
131    // normal bound: It's either present (the type is sized) or
132    // absent (the type might be unsized) but never *maybe* (i.e. `?Sized`).
133    //
134    // This is unsuitable for rendering.
135    // Thus, as a first step remove all `Sized` bounds that should be implicit.
136    //
137    // Note that associated types also have an implicit `Sized` bound but we
138    // don't actually know the set of associated types right here so that
139    // should be handled when cleaning associated types.
140    generics.where_predicates.retain(|pred| {
141        if let WP::BoundPredicate { ty: clean::Generic(param), bounds, .. } = pred
142            && bounds.iter().any(|b| b.is_sized_bound(cx))
143        {
144            sized_params.insert(*param);
145            false
146        } else {
147            true
148        }
149    });
150
151    // As a final step, go through the type parameters again and insert a
152    // `?Sized` bound for each one we didn't find to be `Sized`.
153    for param in &generics.params {
154        if let clean::GenericParamDefKind::Type { .. } = param.kind
155            && !sized_params.contains(&param.name)
156        {
157            generics.where_predicates.push(WP::BoundPredicate {
158                ty: clean::Type::Generic(param.name),
159                bounds: vec![clean::GenericBound::maybe_sized(cx)],
160                bound_params: Vec::new(),
161            })
162        }
163    }
164}
165
166/// Move bounds that are (likely) directly attached to generic parameters from the where-clause to
167/// the respective parameter.
168///
169/// There is no guarantee that this is what the user actually wrote but we have no way of knowing.
170// FIXME(fmease): It'd make a lot of sense to just incorporate this logic into `clean_ty_generics`
171// making every of its users benefit from it.
172pub(crate) fn move_bounds_to_generic_parameters(generics: &mut clean::Generics) {
173    use clean::types::*;
174
175    let mut where_predicates = ThinVec::new();
176    for mut pred in generics.where_predicates.drain(..) {
177        if let WherePredicate::BoundPredicate { ty: Generic(arg), bounds, .. } = &mut pred
178            && let Some(GenericParamDef {
179                kind: GenericParamDefKind::Type { bounds: param_bounds, .. },
180                ..
181            }) = generics.params.iter_mut().find(|param| &param.name == arg)
182        {
183            param_bounds.extend(bounds.drain(..));
184        } else if let WherePredicate::RegionPredicate { lifetime: Lifetime(arg), bounds } =
185            &mut pred
186            && let Some(GenericParamDef {
187                kind: GenericParamDefKind::Lifetime { outlives: param_bounds },
188                ..
189            }) = generics.params.iter_mut().find(|param| &param.name == arg)
190        {
191            param_bounds.extend(bounds.drain(..).map(|bound| match bound {
192                GenericBound::Outlives(lifetime) => lifetime,
193                _ => unreachable!(),
194            }));
195        } else {
196            where_predicates.push(pred);
197        }
198    }
199    generics.where_predicates = where_predicates;
200}
rustdoc/clean/simplify.rs

rustdoc/clean/
simplify.rs
