1 files changed, 272 insertions, 0 deletions

diff --git a/crates/hir_ty/src/traits.rs b/crates/hir_ty/src/traits.rs
new file mode 100644
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+++ b/crates/hir_ty/src/traits.rs
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+//! Trait solving using Chalk.
+use std::sync::Arc;
+
+use base_db::CrateId;
+use chalk_ir::cast::Cast;
+use chalk_solve::Solver;
+use hir_def::{lang_item::LangItemTarget, TraitId};
+
+use crate::{db::HirDatabase, DebruijnIndex, Substs};
+
+use super::{Canonical, GenericPredicate, HirDisplay, ProjectionTy, TraitRef, Ty, TypeWalk};
+
+use self::chalk::{from_chalk, Interner, ToChalk};
+
+pub(crate) mod chalk;
+
+// This controls the maximum size of types Chalk considers. If we set this too
+// high, we can run into slow edge cases; if we set it too low, Chalk won't
+// find some solutions.
+// FIXME this is currently hardcoded in the recursive solver
+// const CHALK_SOLVER_MAX_SIZE: usize = 10;
+
+/// This controls how much 'time' we give the Chalk solver before giving up.
+const CHALK_SOLVER_FUEL: i32 = 100;
+
+#[derive(Debug, Copy, Clone)]
+struct ChalkContext<'a> {
+    db: &'a dyn HirDatabase,
+    krate: CrateId,
+}
+
+fn create_chalk_solver() -> chalk_recursive::RecursiveSolver<Interner> {
+    let overflow_depth = 100;
+    let caching_enabled = true;
+    chalk_recursive::RecursiveSolver::new(overflow_depth, caching_enabled)
+}
+
+/// A set of clauses that we assume to be true. E.g. if we are inside this function:
+/// ```rust
+/// fn foo<T: Default>(t: T) {}
+/// ```
+/// we assume that `T: Default`.
+#[derive(Clone, Debug, PartialEq, Eq, Hash)]
+pub struct TraitEnvironment {
+    pub predicates: Vec<GenericPredicate>,
+}
+
+impl TraitEnvironment {
+    /// Returns trait refs with the given self type which are supposed to hold
+    /// in this trait env. E.g. if we are in `foo<T: SomeTrait>()`, this will
+    /// find that `T: SomeTrait` if we call it for `T`.
+    pub(crate) fn trait_predicates_for_self_ty<'a>(
+        &'a self,
+        ty: &'a Ty,
+    ) -> impl Iterator<Item = &'a TraitRef> + 'a {
+        self.predicates.iter().filter_map(move |pred| match pred {
+            GenericPredicate::Implemented(tr) if tr.self_ty() == ty => Some(tr),
+            _ => None,
+        })
+    }
+}
+
+/// Something (usually a goal), along with an environment.
+#[derive(Clone, Debug, PartialEq, Eq, Hash)]
+pub struct InEnvironment<T> {
+    pub environment: Arc<TraitEnvironment>,
+    pub value: T,
+}
+
+impl<T> InEnvironment<T> {
+    pub fn new(environment: Arc<TraitEnvironment>, value: T) -> InEnvironment<T> {
+        InEnvironment { environment, value }
+    }
+}
+
+/// Something that needs to be proven (by Chalk) during type checking, e.g. that
+/// a certain type implements a certain trait. Proving the Obligation might
+/// result in additional information about inference variables.
+#[derive(Clone, Debug, PartialEq, Eq, Hash)]
+pub enum Obligation {
+    /// Prove that a certain type implements a trait (the type is the `Self` type
+    /// parameter to the `TraitRef`).
+    Trait(TraitRef),
+    Projection(ProjectionPredicate),
+}
+
+impl Obligation {
+    pub fn from_predicate(predicate: GenericPredicate) -> Option<Obligation> {
+        match predicate {
+            GenericPredicate::Implemented(trait_ref) => Some(Obligation::Trait(trait_ref)),
+            GenericPredicate::Projection(projection_pred) => {
+                Some(Obligation::Projection(projection_pred))
+            }
+            GenericPredicate::Error => None,
+        }
+    }
+}
+
+#[derive(Clone, Debug, PartialEq, Eq, Hash)]
+pub struct ProjectionPredicate {
+    pub projection_ty: ProjectionTy,
+    pub ty: Ty,
+}
+
+impl TypeWalk for ProjectionPredicate {
+    fn walk(&self, f: &mut impl FnMut(&Ty)) {
+        self.projection_ty.walk(f);
+        self.ty.walk(f);
+    }
+
+    fn walk_mut_binders(
+        &mut self,
+        f: &mut impl FnMut(&mut Ty, DebruijnIndex),
+        binders: DebruijnIndex,
+    ) {
+        self.projection_ty.walk_mut_binders(f, binders);
+        self.ty.walk_mut_binders(f, binders);
+    }
+}
+
+/// Solve a trait goal using Chalk.
+pub(crate) fn trait_solve_query(
+    db: &dyn HirDatabase,
+    krate: CrateId,
+    goal: Canonical<InEnvironment<Obligation>>,
+) -> Option<Solution> {
+    let _p = profile::span("trait_solve_query").detail(|| match &goal.value.value {
+        Obligation::Trait(it) => db.trait_data(it.trait_).name.to_string(),
+        Obligation::Projection(_) => "projection".to_string(),
+    });
+    log::info!("trait_solve_query({})", goal.value.value.display(db));
+
+    if let Obligation::Projection(pred) = &goal.value.value {
+        if let Ty::Bound(_) = &pred.projection_ty.parameters[0] {
+            // Hack: don't ask Chalk to normalize with an unknown self type, it'll say that's impossible
+            return Some(Solution::Ambig(Guidance::Unknown));
+        }
+    }
+
+    let canonical = goal.to_chalk(db).cast(&Interner);
+
+    // We currently don't deal with universes (I think / hope they're not yet
+    // relevant for our use cases?)
+    let u_canonical = chalk_ir::UCanonical { canonical, universes: 1 };
+    let solution = solve(db, krate, &u_canonical);
+    solution.map(|solution| solution_from_chalk(db, solution))
+}
+
+fn solve(
+    db: &dyn HirDatabase,
+    krate: CrateId,
+    goal: &chalk_ir::UCanonical<chalk_ir::InEnvironment<chalk_ir::Goal<Interner>>>,
+) -> Option<chalk_solve::Solution<Interner>> {
+    let context = ChalkContext { db, krate };
+    log::debug!("solve goal: {:?}", goal);
+    let mut solver = create_chalk_solver();
+
+    let fuel = std::cell::Cell::new(CHALK_SOLVER_FUEL);
+
+    let should_continue = || {
+        context.db.check_canceled();
+        let remaining = fuel.get();
+        fuel.set(remaining - 1);
+        if remaining == 0 {
+            log::debug!("fuel exhausted");
+        }
+        remaining > 0
+    };
+    let mut solve = || {
+        let solution = solver.solve_limited(&context, goal, should_continue);
+        log::debug!("solve({:?}) => {:?}", goal, solution);
+        solution
+    };
+    // don't set the TLS for Chalk unless Chalk debugging is active, to make
+    // extra sure we only use it for debugging
+    let solution =
+        if is_chalk_debug() { chalk::tls::set_current_program(db, solve) } else { solve() };
+
+    solution
+}
+
+fn is_chalk_debug() -> bool {
+    std::env::var("CHALK_DEBUG").is_ok()
+}
+
+fn solution_from_chalk(
+    db: &dyn HirDatabase,
+    solution: chalk_solve::Solution<Interner>,
+) -> Solution {
+    let convert_subst = |subst: chalk_ir::Canonical<chalk_ir::Substitution<Interner>>| {
+        let result = from_chalk(db, subst);
+        SolutionVariables(result)
+    };
+    match solution {
+        chalk_solve::Solution::Unique(constr_subst) => {
+            let subst = chalk_ir::Canonical {
+                value: constr_subst.value.subst,
+                binders: constr_subst.binders,
+            };
+            Solution::Unique(convert_subst(subst))
+        }
+        chalk_solve::Solution::Ambig(chalk_solve::Guidance::Definite(subst)) => {
+            Solution::Ambig(Guidance::Definite(convert_subst(subst)))
+        }
+        chalk_solve::Solution::Ambig(chalk_solve::Guidance::Suggested(subst)) => {
+            Solution::Ambig(Guidance::Suggested(convert_subst(subst)))
+        }
+        chalk_solve::Solution::Ambig(chalk_solve::Guidance::Unknown) => {
+            Solution::Ambig(Guidance::Unknown)
+        }
+    }
+}
+
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub struct SolutionVariables(pub Canonical<Substs>);
+
+#[derive(Clone, Debug, PartialEq, Eq)]
+/// A (possible) solution for a proposed goal.
+pub enum Solution {
+    /// The goal indeed holds, and there is a unique value for all existential
+    /// variables.
+    Unique(SolutionVariables),
+
+    /// The goal may be provable in multiple ways, but regardless we may have some guidance
+    /// for type inference. In this case, we don't return any lifetime
+    /// constraints, since we have not "committed" to any particular solution
+    /// yet.
+    Ambig(Guidance),
+}
+
+#[derive(Clone, Debug, PartialEq, Eq)]
+/// When a goal holds ambiguously (e.g., because there are multiple possible
+/// solutions), we issue a set of *guidance* back to type inference.
+pub enum Guidance {
+    /// The existential variables *must* have the given values if the goal is
+    /// ever to hold, but that alone isn't enough to guarantee the goal will
+    /// actually hold.
+    Definite(SolutionVariables),
+
+    /// There are multiple plausible values for the existentials, but the ones
+    /// here are suggested as the preferred choice heuristically. These should
+    /// be used for inference fallback only.
+    Suggested(SolutionVariables),
+
+    /// There's no useful information to feed back to type inference
+    Unknown,
+}
+
+#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
+pub enum FnTrait {
+    FnOnce,
+    FnMut,
+    Fn,
+}
+
+impl FnTrait {
+    fn lang_item_name(self) -> &'static str {
+        match self {
+            FnTrait::FnOnce => "fn_once",
+            FnTrait::FnMut => "fn_mut",
+            FnTrait::Fn => "fn",
+        }
+    }
+
+    pub fn get_id(&self, db: &dyn HirDatabase, krate: CrateId) -> Option<TraitId> {
+        let target = db.lang_item(krate, self.lang_item_name().into())?;
+        match target {
+            LangItemTarget::TraitId(t) => Some(t),
+            _ => None,
+        }
+    }
+}