visit.rs

//! Traits for visiting bits of IR.
use std::fmt::Debug;

use crate::{
    BoundVar, Const, ConstValue, DebruijnIndex, DomainGoal, Goal, InferenceVar, Interner, Lifetime,
    LifetimeData, PlaceholderIndex, ProgramClause, Ty, TyKind, WhereClause,
};

mod binder_impls;
mod boring_impls;
pub mod visitors;

pub use visitors::VisitExt;

/// An copy of the unstable `std::ops::ControlFlow` for use in Chalk visitors.
pub enum ControlFlow<B, C = ()> {
    /// Continue in the loop, using the given value for the next iteration
    Continue(C),
    /// Exit the loop, yielding the given value
    Break(B),
}

impl<B, C> ControlFlow<B, C> {
    /// Returns `true` if this is a `Break` variant.
    #[inline]
    pub fn is_break(&self) -> bool {
        matches!(*self, ControlFlow::Break(_))
    }

    /// Returns `true` if this is a `Continue` variant.
    #[inline]
    pub fn is_continue(&self) -> bool {
        matches!(*self, ControlFlow::Continue(_))
    }

    /// Converts the `ControlFlow` into an `Option` which is `Some`
    /// if the `ControlFlow` was `Break` and `None` otherwise.
    #[inline]
    pub fn break_value(self) -> Option<B> {
        match self {
            ControlFlow::Continue(..) => None,
            ControlFlow::Break(x) => Some(x),
        }
    }
}

impl<B> ControlFlow<B, ()> {
    /// It's frequently the case that there's no value needed with `Continue`,
    /// so this provides a way to avoid typing `(())`, if you prefer it.
    pub const CONTINUE: Self = ControlFlow::Continue(());
}

impl<C> ControlFlow<(), C> {
    /// APIs like `try_for_each` don't need values with `Break`,
    /// so this provides a way to avoid typing `(())`, if you prefer it.
    pub const BREAK: Self = ControlFlow::Break(());
}

/// Unwraps a `ControlFlow` or propagates its `Break` value.
/// This replaces the `Try` implementation that would be used
/// with `std::ops::ControlFlow`.
#[macro_export]
macro_rules! try_break {
    ($expr:expr) => {
        match $expr {
            $crate::visit::ControlFlow::Continue(c) => c,
            $crate::visit::ControlFlow::Break(b) => return $crate::visit::ControlFlow::Break(b),
        }
    };
}

/// A "visitor" recursively folds some term -- that is, some bit of IR,
/// such as a `Goal`, and computes a value as a result.
///
///
/// To **apply** a visitor, use the `Visit::visit_with` method, like so
///
/// ```rust,ignore
/// let result = x.visit_with(&mut visitor, 0);
/// ```
pub trait Visitor<'i, I: Interner>
where
    I: 'i,
{
    /// The "break type" of the visitor, often `()`. It represents the result
    /// the visitor yields when it stops visiting.
    type BreakTy;

    /// Creates a `dyn` value from this visitor. Unfortunately, this
    /// must be added manually to each impl of visitor; it permits the
    /// default implements below to create a `&mut dyn Visitor` from
    /// `Self` without knowing what `Self` is (by invoking this
    /// method). Effectively, this limits impls of `visitor` to types
    /// for which we are able to create a dyn value (i.e., not `[T]`
    /// types).
    fn as_dyn(&mut self) -> &mut dyn Visitor<'i, I, BreakTy = Self::BreakTy>;

    /// Top-level callback: invoked for each `Ty<I>` that is
    /// encountered when visiting. By default, invokes
    /// `super_visit_with`, which will in turn invoke the more
    /// specialized visiting methods below, like `visit_free_var`.
    fn visit_ty(&mut self, ty: &Ty<I>, outer_binder: DebruijnIndex) -> ControlFlow<Self::BreakTy> {
        ty.super_visit_with(self.as_dyn(), outer_binder)
    }

    /// Top-level callback: invoked for each `Lifetime<I>` that is
    /// encountered when visiting. By default, invokes
    /// `super_visit_with`, which will in turn invoke the more
    /// specialized visiting methods below, like `visit_free_var`.
    fn visit_lifetime(
        &mut self,
        lifetime: &Lifetime<I>,
        outer_binder: DebruijnIndex,
    ) -> ControlFlow<Self::BreakTy> {
        lifetime.super_visit_with(self.as_dyn(), outer_binder)
    }

    /// Top-level callback: invoked for each `Const<I>` that is
    /// encountered when visiting. By default, invokes
    /// `super_visit_with`, which will in turn invoke the more
    /// specialized visiting methods below, like `visit_free_var`.
    fn visit_const(
        &mut self,
        constant: &Const<I>,
        outer_binder: DebruijnIndex,
    ) -> ControlFlow<Self::BreakTy> {
        constant.super_visit_with(self.as_dyn(), outer_binder)
    }

    /// Invoked for every program clause. By default, recursively visits the goals contents.
    fn visit_program_clause(
        &mut self,
        clause: &ProgramClause<I>,
        outer_binder: DebruijnIndex,
    ) -> ControlFlow<Self::BreakTy> {
        clause.super_visit_with(self.as_dyn(), outer_binder)
    }

    /// Invoked for every goal. By default, recursively visits the goals contents.
    fn visit_goal(
        &mut self,
        goal: &Goal<I>,
        outer_binder: DebruijnIndex,
    ) -> ControlFlow<Self::BreakTy> {
        goal.super_visit_with(self.as_dyn(), outer_binder)
    }

    /// Invoked for each domain goal.
    fn visit_domain_goal(
        &mut self,
        domain_goal: &DomainGoal<I>,
        outer_binder: DebruijnIndex,
    ) -> ControlFlow<Self::BreakTy> {
        domain_goal.super_visit_with(self.as_dyn(), outer_binder)
    }

    /// If overridden to return true, then visiting will panic if a
    /// free variable is encountered. This should be done if free
    /// type/lifetime/const variables are not expected.
    fn forbid_free_vars(&self) -> bool {
        false
    }

    /// Invoked for `BoundVar` instances that are not bound
    /// within the type being visited over:
    fn visit_free_var(
        &mut self,
        bound_var: BoundVar,
        outer_binder: DebruijnIndex,
    ) -> ControlFlow<Self::BreakTy> {
        if self.forbid_free_vars() {
            panic!(
                "unexpected free variable `{:?}` with outer binder {:?}",
                bound_var, outer_binder
            )
        } else {
            ControlFlow::CONTINUE
        }
    }

    /// If overridden to return true, we will panic when a free
    /// placeholder type/lifetime is encountered.
    fn forbid_free_placeholders(&self) -> bool {
        false
    }

    /// Invoked for each occurrence of a placeholder type; these are
    /// used when we instantiate binders universally.
    fn visit_free_placeholder(
        &mut self,
        universe: PlaceholderIndex,
        _outer_binder: DebruijnIndex,
    ) -> ControlFlow<Self::BreakTy> {
        if self.forbid_free_placeholders() {
            panic!("unexpected placeholder type `{:?}`", universe)
        } else {
            ControlFlow::CONTINUE
        }
    }

    /// Invoked for each where clause.
    fn visit_where_clause(
        &mut self,
        where_clause: &WhereClause<I>,
        outer_binder: DebruijnIndex,
    ) -> ControlFlow<Self::BreakTy> {
        where_clause.super_visit_with(self.as_dyn(), outer_binder)
    }

    /// If overridden to return true, inference variables will trigger
    /// panics when visited. Used when inference variables are
    /// unexpected.
    fn forbid_inference_vars(&self) -> bool {
        false
    }

    /// Invoked for each occurrence of a inference type; these are
    /// used when we instantiate binders universally.
    fn visit_inference_var(
        &mut self,
        var: InferenceVar,
        _outer_binder: DebruijnIndex,
    ) -> ControlFlow<Self::BreakTy> {
        if self.forbid_inference_vars() {
            panic!("unexpected inference type `{:?}`", var)
        } else {
            ControlFlow::CONTINUE
        }
    }

    /// Invoked for each occurence of a `Binders`, before visiting them.
    fn before_binders(&mut self, _kinds: &crate::VariableKinds<I>) {}
    /// Invoked for each occurence of a `Canonical`, before visiting them.
    fn before_canonical(&mut self, _kinds: &crate::CanonicalVarKinds<I>) {}
    /// Invoked for each occurence of a `FnPointer`, before visiting them.
    fn before_fn_pointer_substs(&mut self, _number: usize) {}
    /// Invoked for each occurence of a type introducing binders, after visiting them.
    fn after_any_binders(&mut self) {}

    /// Gets the visitor's interner.
    fn interner(&self) -> &'i I;
}

/// Applies the given `visitor` to a value, producing a visited result
/// of type `Visitor::Result`.
pub trait Visit<I: Interner>: Debug {
    /// Apply the given visitor `visitor` to `self`; `binders` is the
    /// number of binders that are in scope when beginning the
    /// visitor. Typically `binders` starts as 0, but is adjusted when
    /// we encounter `Binders<T>` in the IR or other similar
    /// constructs.
    fn visit_with<'i, B>(
        &self,
        visitor: &mut dyn Visitor<'i, I, BreakTy = B>,
        outer_binder: DebruijnIndex,
    ) -> ControlFlow<B>
    where
        I: 'i;
}

/// For types where "visit" invokes a callback on the `visitor`, the
/// `SuperVisit` trait captures the recursive behavior that visits all
/// the contents of the type.
pub trait SuperVisit<I: Interner>: Visit<I> {
    /// Recursively visits the type contents.
    fn super_visit_with<'i, B>(
        &self,
        visitor: &mut dyn Visitor<'i, I, BreakTy = B>,
        outer_binder: DebruijnIndex,
    ) -> ControlFlow<B>
    where
        I: 'i;
}

/// "visiting" a type invokes the `visit_ty` method on the visitor; this
/// usually (in turn) invokes `super_visit_ty` to visit the individual
/// parts.
impl<I: Interner> Visit<I> for Ty<I> {
    fn visit_with<'i, B>(
        &self,
        visitor: &mut dyn Visitor<'i, I, BreakTy = B>,
        outer_binder: DebruijnIndex,
    ) -> ControlFlow<B>
    where
        I: 'i,
    {
        visitor.visit_ty(self, outer_binder)
    }
}

/// "Super visit" for a type invokes the more detailed callbacks on the type
impl<I> SuperVisit<I> for Ty<I>
where
    I: Interner,
{
    fn super_visit_with<'i, B>(
        &self,
        visitor: &mut dyn Visitor<'i, I, BreakTy = B>,
        outer_binder: DebruijnIndex,
    ) -> ControlFlow<B>
    where
        I: 'i,
    {
        let interner = visitor.interner();
        match self.kind(interner) {
            TyKind::BoundVar(bound_var) => {
                if let Some(_) = bound_var.shifted_out_to(outer_binder) {
                    visitor.visit_free_var(*bound_var, outer_binder)
                } else {
                    ControlFlow::CONTINUE
                }
            }
            TyKind::Dyn(clauses) => clauses.visit_with(visitor, outer_binder),
            TyKind::InferenceVar(var, _) => visitor.visit_inference_var(*var, outer_binder),
            TyKind::Placeholder(ui) => visitor.visit_free_placeholder(*ui, outer_binder),
            TyKind::Alias(proj) => proj.visit_with(visitor, outer_binder),
            TyKind::Function(fun) => fun.visit_with(visitor, outer_binder),
            TyKind::Adt(_id, substitution) => substitution.visit_with(visitor, outer_binder),
            TyKind::AssociatedType(_assoc_ty, substitution) => {
                substitution.visit_with(visitor, outer_binder)
            }
            TyKind::Scalar(scalar) => scalar.visit_with(visitor, outer_binder),
            TyKind::Str => ControlFlow::CONTINUE,
            TyKind::Tuple(arity, substitution) => {
                try_break!(arity.visit_with(visitor, outer_binder));
                substitution.visit_with(visitor, outer_binder)
            }
            TyKind::OpaqueType(opaque_ty, substitution) => {
                try_break!(opaque_ty.visit_with(visitor, outer_binder));
                substitution.visit_with(visitor, outer_binder)
            }
            TyKind::Slice(substitution) => substitution.visit_with(visitor, outer_binder),
            TyKind::FnDef(fn_def, substitution) => {
                try_break!(fn_def.visit_with(visitor, outer_binder));
                substitution.visit_with(visitor, outer_binder)
            }
            TyKind::Ref(mutability, lifetime, ty) => {
                try_break!(mutability.visit_with(visitor, outer_binder));
                try_break!(lifetime.visit_with(visitor, outer_binder));
                ty.visit_with(visitor, outer_binder)
            }
            TyKind::Raw(mutability, ty) => {
                try_break!(mutability.visit_with(visitor, outer_binder));
                ty.visit_with(visitor, outer_binder)
            }
            TyKind::Never => ControlFlow::CONTINUE,
            TyKind::Array(ty, const_) => {
                try_break!(ty.visit_with(visitor, outer_binder));
                const_.visit_with(visitor, outer_binder)
            }
            TyKind::Closure(id, substitution) => {
                try_break!(id.visit_with(visitor, outer_binder));
                substitution.visit_with(visitor, outer_binder)
            }
            TyKind::Generator(generator, substitution) => {
                try_break!(generator.visit_with(visitor, outer_binder));
                substitution.visit_with(visitor, outer_binder)
            }
            TyKind::GeneratorWitness(witness, substitution) => {
                try_break!(witness.visit_with(visitor, outer_binder));
                substitution.visit_with(visitor, outer_binder)
            }
            TyKind::Foreign(foreign_ty) => foreign_ty.visit_with(visitor, outer_binder),
            TyKind::Error => ControlFlow::CONTINUE,
        }
    }
}

impl<I: Interner> Visit<I> for Lifetime<I> {
    fn visit_with<'i, B>(
        &self,
        visitor: &mut dyn Visitor<'i, I, BreakTy = B>,
        outer_binder: DebruijnIndex,
    ) -> ControlFlow<B>
    where
        I: 'i,
    {
        visitor.visit_lifetime(self, outer_binder)
    }
}

impl<I: Interner> SuperVisit<I> for Lifetime<I> {
    fn super_visit_with<'i, B>(
        &self,
        visitor: &mut dyn Visitor<'i, I, BreakTy = B>,
        outer_binder: DebruijnIndex,
    ) -> ControlFlow<B>
    where
        I: 'i,
    {
        let interner = visitor.interner();
        match self.data(interner) {
            LifetimeData::BoundVar(bound_var) => {
                if let Some(_) = bound_var.shifted_out_to(outer_binder) {
                    visitor.visit_free_var(*bound_var, outer_binder)
                } else {
                    ControlFlow::CONTINUE
                }
            }
            LifetimeData::InferenceVar(var) => visitor.visit_inference_var(*var, outer_binder),
            LifetimeData::Placeholder(universe) => {
                visitor.visit_free_placeholder(*universe, outer_binder)
            }
            LifetimeData::Static | LifetimeData::Empty(_) | LifetimeData::Erased => {
                ControlFlow::CONTINUE
            }
            LifetimeData::Phantom(void, ..) => match *void {},
        }
    }
}

impl<I: Interner> Visit<I> for Const<I> {
    fn visit_with<'i, B>(
        &self,
        visitor: &mut dyn Visitor<'i, I, BreakTy = B>,
        outer_binder: DebruijnIndex,
    ) -> ControlFlow<B>
    where
        I: 'i,
    {
        visitor.visit_const(self, outer_binder)
    }
}

impl<I: Interner> SuperVisit<I> for Const<I> {
    fn super_visit_with<'i, B>(
        &self,
        visitor: &mut dyn Visitor<'i, I, BreakTy = B>,
        outer_binder: DebruijnIndex,
    ) -> ControlFlow<B>
    where
        I: 'i,
    {
        let interner = visitor.interner();
        match &self.data(interner).value {
            ConstValue::BoundVar(bound_var) => {
                if let Some(_) = bound_var.shifted_out_to(outer_binder) {
                    visitor.visit_free_var(*bound_var, outer_binder)
                } else {
                    ControlFlow::CONTINUE
                }
            }
            ConstValue::InferenceVar(var) => visitor.visit_inference_var(*var, outer_binder),
            ConstValue::Placeholder(universe) => {
                visitor.visit_free_placeholder(*universe, outer_binder)
            }
            ConstValue::Concrete(_) => ControlFlow::CONTINUE,
        }
    }
}

impl<I: Interner> Visit<I> for Goal<I> {
    fn visit_with<'i, B>(
        &self,
        visitor: &mut dyn Visitor<'i, I, BreakTy = B>,
        outer_binder: DebruijnIndex,
    ) -> ControlFlow<B>
    where
        I: 'i,
    {
        visitor.visit_goal(self, outer_binder)
    }
}

impl<I: Interner> SuperVisit<I> for Goal<I> {
    fn super_visit_with<'i, B>(
        &self,
        visitor: &mut dyn Visitor<'i, I, BreakTy = B>,
        outer_binder: DebruijnIndex,
    ) -> ControlFlow<B>
    where
        I: 'i,
    {
        let interner = visitor.interner();
        self.data(interner).visit_with(visitor, outer_binder)
    }
}

impl<I: Interner> Visit<I> for ProgramClause<I> {
    fn visit_with<'i, B>(
        &self,
        visitor: &mut dyn Visitor<'i, I, BreakTy = B>,
        outer_binder: DebruijnIndex,
    ) -> ControlFlow<B>
    where
        I: 'i,
    {
        visitor.visit_program_clause(self, outer_binder)
    }
}

impl<I: Interner> Visit<I> for WhereClause<I> {
    fn visit_with<'i, B>(
        &self,
        visitor: &mut dyn Visitor<'i, I, BreakTy = B>,
        outer_binder: DebruijnIndex,
    ) -> ControlFlow<B>
    where
        I: 'i,
    {
        visitor.visit_where_clause(self, outer_binder)
    }
}

impl<I: Interner> Visit<I> for DomainGoal<I> {
    fn visit_with<'i, B>(
        &self,
        visitor: &mut dyn Visitor<'i, I, BreakTy = B>,
        outer_binder: DebruijnIndex,
    ) -> ControlFlow<B>
    where
        I: 'i,
    {
        visitor.visit_domain_goal(self, outer_binder)
    }
}