src/librustdoc/clean/simplify.rs

//! Simplification of where-clauses and parameter bounds into a prettier and
//! more canonical form.
//!
//! Currently all cross-crate-inlined function use `rustc_middle::ty` to reconstruct
//! the AST (e.g., see all of `clean::inline`), but this is not always a
//! non-lossy transformation. The current format of storage for where-clauses
//! for functions and such is simply a list of predicates. One example of this
//! is that the AST predicate of: `where T: Trait<Foo = Bar>` is encoded as:
//! `where T: Trait, <T as Trait>::Foo = Bar`.
//!
//! This module attempts to reconstruct the original where and/or parameter
//! bounds by special casing scenarios such as these. Fun!

use rustc_data_structures::fx::FxIndexMap;
use rustc_hir::def_id::DefId;
use rustc_middle::ty;
use thin_vec::ThinVec;

use crate::clean;
use crate::clean::GenericArgs as PP;
use crate::clean::WherePredicate as WP;
use crate::core::DocContext;

pub(crate) fn where_clauses(cx: &DocContext<'_>, clauses: Vec<WP>) -> ThinVec<WP> {
    // First, partition the where clause into its separate components.
    //
    // We use `FxIndexMap` so that the insertion order is preserved to prevent messing up to
    // the order of the generated bounds.
    let mut tybounds = FxIndexMap::default();
    let mut lifetimes = Vec::new();
    let mut equalities = Vec::new();

    for clause in clauses {
        match clause {
            WP::BoundPredicate { ty, bounds, bound_params } => {
                let (b, p): &mut (Vec<_>, Vec<_>) = tybounds.entry(ty).or_default();
                b.extend(bounds);
                p.extend(bound_params);
            }
            WP::RegionPredicate { lifetime, bounds } => {
                lifetimes.push((lifetime, bounds));
            }
            WP::EqPredicate { lhs, rhs } => equalities.push((lhs, rhs)),
        }
    }

    // Look for equality predicates on associated types that can be merged into
    // general bound predicates.
    equalities.retain(|(lhs, rhs)| {
        let Some((ty, trait_did, name)) = lhs.projection() else {
            return true;
        };
        let Some((bounds, _)) = tybounds.get_mut(ty) else { return true };
        merge_bounds(cx, bounds, trait_did, name, rhs)
    });

    // And finally, let's reassemble everything
    let mut clauses = ThinVec::with_capacity(lifetimes.len() + tybounds.len() + equalities.len());
    clauses.extend(
        lifetimes.into_iter().map(|(lt, bounds)| WP::RegionPredicate { lifetime: lt, bounds }),
    );
    clauses.extend(tybounds.into_iter().map(|(ty, (bounds, bound_params))| WP::BoundPredicate {
        ty,
        bounds,
        bound_params,
    }));
    clauses.extend(equalities.into_iter().map(|(lhs, rhs)| WP::EqPredicate { lhs, rhs }));
    clauses
}

pub(crate) fn merge_bounds(
    cx: &clean::DocContext<'_>,
    bounds: &mut Vec<clean::GenericBound>,
    trait_did: DefId,
    assoc: clean::PathSegment,
    rhs: &clean::Term,
) -> bool {
    !bounds.iter_mut().any(|b| {
        let trait_ref = match *b {
            clean::GenericBound::TraitBound(ref mut tr, _) => tr,
            clean::GenericBound::Outlives(..) => return false,
        };
        // If this QPath's trait `trait_did` is the same as, or a supertrait
        // of, the bound's trait `did` then we can keep going, otherwise
        // this is just a plain old equality bound.
        if !trait_is_same_or_supertrait(cx, trait_ref.trait_.def_id(), trait_did) {
            return false;
        }
        let last = trait_ref.trait_.segments.last_mut().expect("segments were empty");

        match last.args {
            PP::AngleBracketed { ref mut bindings, .. } => {
                bindings.push(clean::TypeBinding {
                    assoc: assoc.clone(),
                    kind: clean::TypeBindingKind::Equality { term: rhs.clone() },
                });
            }
            PP::Parenthesized { ref mut output, .. } => match output {
                Some(o) => assert_eq!(&clean::Term::Type(o.as_ref().clone()), rhs),
                None => {
                    if *rhs != clean::Term::Type(clean::Type::Tuple(Vec::new())) {
                        *output = Some(Box::new(rhs.ty().unwrap().clone()));
                    }
                }
            },
        };
        true
    })
}

fn trait_is_same_or_supertrait(cx: &DocContext<'_>, child: DefId, trait_: DefId) -> bool {
    if child == trait_ {
        return true;
    }
    let predicates = cx.tcx.super_predicates_of(child);
    debug_assert!(cx.tcx.generics_of(child).has_self);
    let self_ty = cx.tcx.types.self_param;
    predicates
        .predicates
        .iter()
        .filter_map(|(pred, _)| {
            if let ty::ClauseKind::Trait(pred) = pred.kind().skip_binder() {
                if pred.trait_ref.self_ty() == self_ty { Some(pred.def_id()) } else { None }
            } else {
                None
            }
        })
        .any(|did| trait_is_same_or_supertrait(cx, did, trait_))
}

/// Move bounds that are (likely) directly attached to generic parameters from the where-clause to
/// the respective parameter.
///
/// There is no guarantee that this is what the user actually wrote but we have no way of knowing.
// FIXME(fmease): It'd make a lot of sense to just incorporate this logic into `clean_ty_generics`
// making every of its users benefit from it.
pub(crate) fn move_bounds_to_generic_parameters(generics: &mut clean::Generics) {
    use clean::types::*;

    let mut where_predicates = ThinVec::new();
    for mut pred in generics.where_predicates.drain(..) {
        if let WherePredicate::BoundPredicate { ty: Generic(arg), bounds, .. } = &mut pred
            && let Some(GenericParamDef {
                kind: GenericParamDefKind::Type { bounds: param_bounds, .. },
                ..
            }) = generics.params.iter_mut().find(|param| &param.name == arg)
        {
            param_bounds.extend(bounds.drain(..));
        } else if let WherePredicate::RegionPredicate { lifetime: Lifetime(arg), bounds } = &mut pred
            && let Some(GenericParamDef {
                kind: GenericParamDefKind::Lifetime { outlives: param_bounds },
                ..
            }) = generics.params.iter_mut().find(|param| &param.name == arg)
        {
            param_bounds.extend(bounds.drain(..).map(|bound| match bound {
                GenericBound::Outlives(lifetime) => lifetime,
                _ => unreachable!(),
            }));
        } else {
            where_predicates.push(pred);
        }
    }
    generics.where_predicates = where_predicates;
}