1 files changed, 131 insertions, 0 deletions

diff --git a/crates/hir_ty/src/autoderef.rs b/crates/hir_ty/src/autoderef.rs
new file mode 100644
index 000000000..ece68183e
--- /dev/null
+++ b/crates/hir_ty/src/autoderef.rs
@@ -0,0 +1,131 @@
+//! In certain situations, rust automatically inserts derefs as necessary: for
+//! example, field accesses `foo.bar` still work when `foo` is actually a
+//! reference to a type with the field `bar`. This is an approximation of the
+//! logic in rustc (which lives in librustc_typeck/check/autoderef.rs).
+
+use std::iter::successors;
+
+use base_db::CrateId;
+use hir_def::lang_item::LangItemTarget;
+use hir_expand::name::name;
+use log::{info, warn};
+
+use crate::{
+    db::HirDatabase,
+    traits::{InEnvironment, Solution},
+    utils::generics,
+    BoundVar, Canonical, DebruijnIndex, Obligation, Substs, TraitRef, Ty,
+};
+
+const AUTODEREF_RECURSION_LIMIT: usize = 10;
+
+pub fn autoderef<'a>(
+    db: &'a dyn HirDatabase,
+    krate: Option<CrateId>,
+    ty: InEnvironment<Canonical<Ty>>,
+) -> impl Iterator<Item = Canonical<Ty>> + 'a {
+    let InEnvironment { value: ty, environment } = ty;
+    successors(Some(ty), move |ty| {
+        deref(db, krate?, InEnvironment { value: ty, environment: environment.clone() })
+    })
+    .take(AUTODEREF_RECURSION_LIMIT)
+}
+
+pub(crate) fn deref(
+    db: &dyn HirDatabase,
+    krate: CrateId,
+    ty: InEnvironment<&Canonical<Ty>>,
+) -> Option<Canonical<Ty>> {
+    if let Some(derefed) = ty.value.value.builtin_deref() {
+        Some(Canonical { value: derefed, kinds: ty.value.kinds.clone() })
+    } else {
+        deref_by_trait(db, krate, ty)
+    }
+}
+
+fn deref_by_trait(
+    db: &dyn HirDatabase,
+    krate: CrateId,
+    ty: InEnvironment<&Canonical<Ty>>,
+) -> Option<Canonical<Ty>> {
+    let deref_trait = match db.lang_item(krate, "deref".into())? {
+        LangItemTarget::TraitId(it) => it,
+        _ => return None,
+    };
+    let target = db.trait_data(deref_trait).associated_type_by_name(&name![Target])?;
+
+    let generic_params = generics(db.upcast(), target.into());
+    if generic_params.len() != 1 {
+        // the Target type + Deref trait should only have one generic parameter,
+        // namely Deref's Self type
+        return None;
+    }
+
+    // FIXME make the Canonical / bound var handling nicer
+
+    let parameters =
+        Substs::build_for_generics(&generic_params).push(ty.value.value.clone()).build();
+
+    // Check that the type implements Deref at all
+    let trait_ref = TraitRef { trait_: deref_trait, substs: parameters.clone() };
+    let implements_goal = Canonical {
+        kinds: ty.value.kinds.clone(),
+        value: InEnvironment {
+            value: Obligation::Trait(trait_ref),
+            environment: ty.environment.clone(),
+        },
+    };
+    if db.trait_solve(krate, implements_goal).is_none() {
+        return None;
+    }
+
+    // Now do the assoc type projection
+    let projection = super::traits::ProjectionPredicate {
+        ty: Ty::Bound(BoundVar::new(DebruijnIndex::INNERMOST, ty.value.kinds.len())),
+        projection_ty: super::ProjectionTy { associated_ty: target, parameters },
+    };
+
+    let obligation = super::Obligation::Projection(projection);
+
+    let in_env = InEnvironment { value: obligation, environment: ty.environment };
+
+    let canonical =
+        Canonical::new(in_env, ty.value.kinds.iter().copied().chain(Some(super::TyKind::General)));
+
+    let solution = db.trait_solve(krate, canonical)?;
+
+    match &solution {
+        Solution::Unique(vars) => {
+            // FIXME: vars may contain solutions for any inference variables
+            // that happened to be inside ty. To correctly handle these, we
+            // would have to pass the solution up to the inference context, but
+            // that requires a larger refactoring (especially if the deref
+            // happens during method resolution). So for the moment, we just
+            // check that we're not in the situation we're we would actually
+            // need to handle the values of the additional variables, i.e.
+            // they're just being 'passed through'. In the 'standard' case where
+            // we have `impl<T> Deref for Foo<T> { Target = T }`, that should be
+            // the case.
+
+            // FIXME: if the trait solver decides to truncate the type, these
+            // assumptions will be broken. We would need to properly introduce
+            // new variables in that case
+
+            for i in 1..vars.0.kinds.len() {
+                if vars.0.value[i - 1] != Ty::Bound(BoundVar::new(DebruijnIndex::INNERMOST, i - 1))
+                {
+                    warn!("complex solution for derefing {:?}: {:?}, ignoring", ty.value, solution);
+                    return None;
+                }
+            }
+            Some(Canonical {
+                value: vars.0.value[vars.0.value.len() - 1].clone(),
+                kinds: vars.0.kinds.clone(),
+            })
+        }
+        Solution::Ambig(_) => {
+            info!("Ambiguous solution for derefing {:?}: {:?}", ty.value, solution);
+            None
+        }
+    }
+}