Auto merge of #13494 - lowr:feat/gats, r=flodiebold

feat: type inference for generic associated types

This PR implements type inference for generic associated types. Basically, this PR lowers generic arguments for associated types in valid places and creates `Substitution`s for them.

I focused on the inference for correct Rust programs, so there are cases where we *accidentally* manage to infer things that are actually invalid (which would then be reported by flycheck so I deem them non-fatal). See the following tests and FIXME notes on them: `gats_with_dyn`, `gats_with_impl_trait`.

The added tests are rather arbitrary. Let me know if there are cases I'm missing or I should add.

Closes #9673

Author: bors

Diff for: crates/hir-def/src/item_tree/lower.rs

@@ -662,8 +662,12 @@ fn desugar_future_path(orig: TypeRef) -> Path {
     let mut generic_args: Vec<_> =
         std::iter::repeat(None).take(path.segments().len() - 1).collect();
     let mut last = GenericArgs::empty();
-    let binding =
-        AssociatedTypeBinding { name: name![Output], type_ref: Some(orig), bounds: Vec::new() };
+    let binding = AssociatedTypeBinding {
+        name: name![Output],
+        args: None,
+        type_ref: Some(orig),
+        bounds: Vec::new(),
+    };
     last.bindings.push(binding);
     generic_args.push(Some(Interned::new(last)));
 

Diff for: crates/hir-def/src/path.rs

@@ -68,6 +68,9 @@ pub struct GenericArgs {
 pub struct AssociatedTypeBinding {
     /// The name of the associated type.
     pub name: Name,
+    /// The generic arguments to the associated type. e.g. For `Trait<Assoc<'a, T> = &'a T>`, this
+    /// would be `['a, T]`.
+    pub args: Option<Interned<GenericArgs>>,
     /// The type bound to this associated type (in `Item = T`, this would be the
     /// `T`). This can be `None` if there are bounds instead.
     pub type_ref: Option<TypeRef>,

Diff for: crates/hir-def/src/path/lower.rs

@@ -163,6 +163,10 @@ pub(super) fn lower_generic_args(
             ast::GenericArg::AssocTypeArg(assoc_type_arg) => {
                 if let Some(name_ref) = assoc_type_arg.name_ref() {
                     let name = name_ref.as_name();
+                    let args = assoc_type_arg
+                        .generic_arg_list()
+                        .and_then(|args| lower_generic_args(lower_ctx, args))
+                        .map(Interned::new);
                     let type_ref = assoc_type_arg.ty().map(|it| TypeRef::from_ast(lower_ctx, it));
                     let bounds = if let Some(l) = assoc_type_arg.type_bound_list() {
                         l.bounds()
@@ -171,7 +175,7 @@ pub(super) fn lower_generic_args(
                     } else {
                         Vec::new()
                     };
-                    bindings.push(AssociatedTypeBinding { name, type_ref, bounds });
+                    bindings.push(AssociatedTypeBinding { name, args, type_ref, bounds });
                 }
             }
             ast::GenericArg::LifetimeArg(lifetime_arg) => {
@@ -214,6 +218,7 @@ fn lower_generic_args_from_fn_path(
         let type_ref = TypeRef::from_ast_opt(ctx, ret_type.ty());
         bindings.push(AssociatedTypeBinding {
             name: name![Output],
+            args: None,
             type_ref: Some(type_ref),
             bounds: Vec::new(),
         });
@@ -222,6 +227,7 @@ fn lower_generic_args_from_fn_path(
         let type_ref = TypeRef::Tuple(Vec::new());
         bindings.push(AssociatedTypeBinding {
             name: name![Output],
+            args: None,
             type_ref: Some(type_ref),
             bounds: Vec::new(),
         });

Diff for: crates/hir-ty/src/chalk_ext.rs

@@ -11,9 +11,9 @@ use syntax::SmolStr;
 
 use crate::{
     db::HirDatabase, from_assoc_type_id, from_chalk_trait_id, from_foreign_def_id,
-    from_placeholder_idx, to_chalk_trait_id, AdtId, AliasEq, AliasTy, Binders, CallableDefId,
-    CallableSig, FnPointer, ImplTraitId, Interner, Lifetime, ProjectionTy, QuantifiedWhereClause,
-    Substitution, TraitRef, Ty, TyBuilder, TyKind, WhereClause,
+    from_placeholder_idx, to_chalk_trait_id, utils::generics, AdtId, AliasEq, AliasTy, Binders,
+    CallableDefId, CallableSig, FnPointer, ImplTraitId, Interner, Lifetime, ProjectionTy,
+    QuantifiedWhereClause, Substitution, TraitRef, Ty, TyBuilder, TyKind, WhereClause,
 };
 
 pub trait TyExt {
@@ -338,10 +338,13 @@ pub trait ProjectionTyExt {
 
 impl ProjectionTyExt for ProjectionTy {
     fn trait_ref(&self, db: &dyn HirDatabase) -> TraitRef {
-        TraitRef {
-            trait_id: to_chalk_trait_id(self.trait_(db)),
-            substitution: self.substitution.clone(),
-        }
+        // FIXME: something like `Split` trait from chalk-solve might be nice.
+        let generics = generics(db.upcast(), from_assoc_type_id(self.associated_ty_id).into());
+        let substitution = Substitution::from_iter(
+            Interner,
+            self.substitution.iter(Interner).skip(generics.len_self()),
+        );
+        TraitRef { trait_id: to_chalk_trait_id(self.trait_(db)), substitution }
     }
 
     fn trait_(&self, db: &dyn HirDatabase) -> TraitId {

Diff for: crates/hir-ty/src/display.rs

@@ -289,16 +289,18 @@ impl HirDisplay for ProjectionTy {
             return write!(f, "{}", TYPE_HINT_TRUNCATION);
         }
 
-        let trait_ = f.db.trait_data(self.trait_(f.db));
+        let trait_ref = self.trait_ref(f.db);
         write!(f, "<")?;
-        self.self_type_parameter(f.db).hir_fmt(f)?;
-        write!(f, " as {}", trait_.name)?;
-        if self.substitution.len(Interner) > 1 {
+        fmt_trait_ref(&trait_ref, f, true)?;
+        write!(f, ">::{}", f.db.type_alias_data(from_assoc_type_id(self.associated_ty_id)).name)?;
+        let proj_params_count =
+            self.substitution.len(Interner) - trait_ref.substitution.len(Interner);
+        let proj_params = &self.substitution.as_slice(Interner)[..proj_params_count];
+        if !proj_params.is_empty() {
             write!(f, "<")?;
-            f.write_joined(&self.substitution.as_slice(Interner)[1..], ", ")?;
+            f.write_joined(proj_params, ", ")?;
             write!(f, ">")?;
         }
-        write!(f, ">::{}", f.db.type_alias_data(from_assoc_type_id(self.associated_ty_id)).name)?;
         Ok(())
     }
 }
@@ -641,9 +643,12 @@ impl HirDisplay for Ty {
                 // Use placeholder associated types when the target is test (https://rust-lang.github.io/chalk/book/clauses/type_equality.html#placeholder-associated-types)
                 if f.display_target.is_test() {
                     write!(f, "{}::{}", trait_.name, type_alias_data.name)?;
+                    // Note that the generic args for the associated type come before those for the
+                    // trait (including the self type).
+                    // FIXME: reconsider the generic args order upon formatting?
                     if parameters.len(Interner) > 0 {
                         write!(f, "<")?;
-                        f.write_joined(&*parameters.as_slice(Interner), ", ")?;
+                        f.write_joined(parameters.as_slice(Interner), ", ")?;
                         write!(f, ">")?;
                     }
                 } else {
@@ -972,9 +977,20 @@ fn write_bounds_like_dyn_trait(
                     angle_open = true;
                 }
                 if let AliasTy::Projection(proj) = alias {
-                    let type_alias =
-                        f.db.type_alias_data(from_assoc_type_id(proj.associated_ty_id));
-                    write!(f, "{} = ", type_alias.name)?;
+                    let assoc_ty_id = from_assoc_type_id(proj.associated_ty_id);
+                    let type_alias = f.db.type_alias_data(assoc_ty_id);
+                    write!(f, "{}", type_alias.name)?;
+
+                    let proj_arg_count = generics(f.db.upcast(), assoc_ty_id.into()).len_self();
+                    if proj_arg_count > 0 {
+                        write!(f, "<")?;
+                        f.write_joined(
+                            &proj.substitution.as_slice(Interner)[..proj_arg_count],
+                            ", ",
+                        )?;
+                        write!(f, ">")?;
+                    }
+                    write!(f, " = ")?;
                 }
                 ty.hir_fmt(f)?;
             }

Diff for: crates/hir-ty/src/infer/path.rs

@@ -157,7 +157,7 @@ impl<'a> InferenceContext<'a> {
                     remaining_segments_for_ty,
                     true,
                 );
-                if let TyKind::Error = ty.kind(Interner) {
+                if ty.is_unknown() {
                     return None;
                 }
 

Diff for: crates/hir-ty/src/lib.rs

@@ -124,14 +124,6 @@ pub type ConstrainedSubst = chalk_ir::ConstrainedSubst<Interner>;
 pub type Guidance = chalk_solve::Guidance<Interner>;
 pub type WhereClause = chalk_ir::WhereClause<Interner>;
 
-// FIXME: get rid of this
-pub fn subst_prefix(s: &Substitution, n: usize) -> Substitution {
-    Substitution::from_iter(
-        Interner,
-        s.as_slice(Interner)[..std::cmp::min(s.len(Interner), n)].iter().cloned(),
-    )
-}
-
 /// Return an index of a parameter in the generic type parameter list by it's id.
 pub fn param_idx(db: &dyn HirDatabase, id: TypeOrConstParamId) -> Option<usize> {
     generics(db.upcast(), id.parent).param_idx(id)
@@ -382,7 +374,6 @@ pub(crate) fn fold_tys_and_consts<T: HasInterner<Interner = Interner> + TypeFold
 pub fn replace_errors_with_variables<T>(t: &T) -> Canonical<T>
 where
     T: HasInterner<Interner = Interner> + TypeFoldable<Interner> + Clone,
-    T: HasInterner<Interner = Interner>,
 {
     use chalk_ir::{
         fold::{FallibleTypeFolder, TypeSuperFoldable},

Diff for: crates/hir-ty/src/lower.rs

@@ -447,12 +447,31 @@ impl<'a> TyLoweringContext<'a> {
                             .db
                             .trait_data(trait_ref.hir_trait_id())
                             .associated_type_by_name(segment.name);
+
                         match found {
                             Some(associated_ty) => {
-                                // FIXME handle type parameters on the segment
+                                // FIXME: `substs_from_path_segment()` pushes `TyKind::Error` for every parent
+                                // generic params. It's inefficient to splice the `Substitution`s, so we may want
+                                // that method to optionally take parent `Substitution` as we already know them at
+                                // this point (`trait_ref.substitution`).
+                                let substitution = self.substs_from_path_segment(
+                                    segment,
+                                    Some(associated_ty.into()),
+                                    false,
+                                    None,
+                                );
+                                let len_self =
+                                    generics(self.db.upcast(), associated_ty.into()).len_self();
+                                let substitution = Substitution::from_iter(
+                                    Interner,
+                                    substitution
+                                        .iter(Interner)
+                                        .take(len_self)
+                                        .chain(trait_ref.substitution.iter(Interner)),
+                                );
                                 TyKind::Alias(AliasTy::Projection(ProjectionTy {
                                     associated_ty_id: to_assoc_type_id(associated_ty),
-                                    substitution: trait_ref.substitution,
+                                    substitution,
                                 }))
                                 .intern(Interner)
                             }
@@ -590,36 +609,48 @@ impl<'a> TyLoweringContext<'a> {
             res,
             Some(segment.name.clone()),
             move |name, t, associated_ty| {
-                if name == segment.name {
-                    let substs = match self.type_param_mode {
-                        ParamLoweringMode::Placeholder => {
-                            // if we're lowering to placeholders, we have to put
-                            // them in now
-                            let generics = generics(
-                                self.db.upcast(),
-                                self.resolver
-                                    .generic_def()
-                                    .expect("there should be generics if there's a generic param"),
-                            );
-                            let s = generics.placeholder_subst(self.db);
-                            s.apply(t.substitution.clone(), Interner)
-                        }
-                        ParamLoweringMode::Variable => t.substitution.clone(),
-                    };
-                    // We need to shift in the bound vars, since
-                    // associated_type_shorthand_candidates does not do that
-                    let substs = substs.shifted_in_from(Interner, self.in_binders);
-                    // FIXME handle type parameters on the segment
-                    Some(
-                        TyKind::Alias(AliasTy::Projection(ProjectionTy {
-                            associated_ty_id: to_assoc_type_id(associated_ty),
-                            substitution: substs,
-                        }))
-                        .intern(Interner),
-                    )
-                } else {
-                    None
+                if name != segment.name {
+                    return None;
                 }
+
+                // FIXME: `substs_from_path_segment()` pushes `TyKind::Error` for every parent
+                // generic params. It's inefficient to splice the `Substitution`s, so we may want
+                // that method to optionally take parent `Substitution` as we already know them at
+                // this point (`t.substitution`).
+                let substs = self.substs_from_path_segment(
+                    segment.clone(),
+                    Some(associated_ty.into()),
+                    false,
+                    None,
+                );
+
+                let len_self = generics(self.db.upcast(), associated_ty.into()).len_self();
+
+                let substs = Substitution::from_iter(
+                    Interner,
+                    substs.iter(Interner).take(len_self).chain(t.substitution.iter(Interner)),
+                );
+
+                let substs = match self.type_param_mode {
+                    ParamLoweringMode::Placeholder => {
+                        // if we're lowering to placeholders, we have to put
+                        // them in now
+                        let generics = generics(self.db.upcast(), def);
+                        let s = generics.placeholder_subst(self.db);
+                        s.apply(substs, Interner)
+                    }
+                    ParamLoweringMode::Variable => substs,
+                };
+                // We need to shift in the bound vars, since
+                // associated_type_shorthand_candidates does not do that
+                let substs = substs.shifted_in_from(Interner, self.in_binders);
+                Some(
+                    TyKind::Alias(AliasTy::Projection(ProjectionTy {
+                        associated_ty_id: to_assoc_type_id(associated_ty),
+                        substitution: substs,
+                    }))
+                    .intern(Interner),
+                )
             },
         );
 
@@ -777,7 +808,15 @@ impl<'a> TyLoweringContext<'a> {
         // handle defaults. In expression or pattern path segments without
         // explicitly specified type arguments, missing type arguments are inferred
         // (i.e. defaults aren't used).
-        if !infer_args || had_explicit_args {
+        // Generic parameters for associated types are not supposed to have defaults, so we just
+        // ignore them.
+        let is_assoc_ty = if let GenericDefId::TypeAliasId(id) = def {
+            let container = id.lookup(self.db.upcast()).container;
+            matches!(container, ItemContainerId::TraitId(_))
+        } else {
+            false
+        };
+        if !is_assoc_ty && (!infer_args || had_explicit_args) {
             let defaults = self.db.generic_defaults(def);
             assert_eq!(total_len, defaults.len());
             let parent_from = item_len - substs.len();
@@ -966,9 +1005,28 @@ impl<'a> TyLoweringContext<'a> {
                     None => return SmallVec::new(),
                     Some(t) => t,
                 };
+                // FIXME: `substs_from_path_segment()` pushes `TyKind::Error` for every parent
+                // generic params. It's inefficient to splice the `Substitution`s, so we may want
+                // that method to optionally take parent `Substitution` as we already know them at
+                // this point (`super_trait_ref.substitution`).
+                let substitution = self.substs_from_path_segment(
+                    // FIXME: This is hack. We shouldn't really build `PathSegment` directly.
+                    PathSegment { name: &binding.name, args_and_bindings: binding.args.as_deref() },
+                    Some(associated_ty.into()),
+                    false, // this is not relevant
+                    Some(super_trait_ref.self_type_parameter(Interner)),
+                );
+                let self_params = generics(self.db.upcast(), associated_ty.into()).len_self();
+                let substitution = Substitution::from_iter(
+                    Interner,
+                    substitution
+                        .iter(Interner)
+                        .take(self_params)
+                        .chain(super_trait_ref.substitution.iter(Interner)),
+                );
                 let projection_ty = ProjectionTy {
                     associated_ty_id: to_assoc_type_id(associated_ty),
-                    substitution: super_trait_ref.substitution,
+                    substitution,
                 };
                 let mut preds: SmallVec<[_; 1]> = SmallVec::with_capacity(
                     binding.type_ref.as_ref().map_or(0, |_| 1) + binding.bounds.len(),

Diff for: crates/hir-ty/src/tests/display_source_code.rs

@@ -196,3 +196,34 @@ fn test(
 "#,
     );
 }
+
+#[test]
+fn projection_type_correct_arguments_order() {
+    check_types_source_code(
+        r#"
+trait Foo<T> {
+    type Assoc<U>;
+}
+fn f<T: Foo<i32>>(a: T::Assoc<usize>) {
+    a;
+  //^ <T as Foo<i32>>::Assoc<usize>
+}
+"#,
+    );
+}
+
+#[test]
+fn generic_associated_type_binding_in_impl_trait() {
+    check_types_source_code(
+        r#"
+//- minicore: sized
+trait Foo<T> {
+    type Assoc<U>;
+}
+fn f(a: impl Foo<i8, Assoc<i16> = i32>) {
+    a;
+  //^ impl Foo<i8, Assoc<i16> = i32>
+}
+        "#,
+    );
+}