Use inferred type in SourceAnalyzer::type_of_type

This uses the new InferenceResult::type_of_type_placeholder data to
turn type references into completely resolved types instead of just
returning the lexical type.

Author: nicolas-guichard

crates/hir-ty/src/infer.rs

@@ -612,6 +612,9 @@ impl<'db> InferenceResult<'db> {
     pub fn placeholder_types(&self) -> impl Iterator<Item = (TypeRefId, &Ty<'db>)> {
         self.type_of_type_placeholder.iter()
     }
+    pub fn type_of_type_placeholder(&self, type_ref: TypeRefId) -> Option<Ty<'db>> {
+        self.type_of_type_placeholder.get(type_ref).copied()
+    }
     pub fn closure_info(&self, closure: InternedClosureId) -> &(Vec<CapturedItem<'db>>, FnTrait) {
         self.closure_info.get(&closure).unwrap()
     }

crates/hir/src/source_analyzer.rs

@@ -21,7 +21,7 @@ use hir_def::{
     lang_item::LangItem,
     nameres::MacroSubNs,
     resolver::{HasResolver, Resolver, TypeNs, ValueNs, resolver_for_scope},
-    type_ref::{Mutability, TypeRefId},
+    type_ref::{Mutability, TypeRef, TypeRefId},
 };
 use hir_expand::{
     HirFileId, InFile,
@@ -267,8 +267,11 @@ impl<'db> SourceAnalyzer<'db> {
         db: &'db dyn HirDatabase,
         ty: &ast::Type,
     ) -> Option<Type<'db>> {
+        let interner = DbInterner::new_with(db, None, None);
+
         let type_ref = self.type_id(ty)?;
-        let ty = TyLoweringContext::new(
+
+        let mut ty = TyLoweringContext::new(
             db,
             &self.resolver,
             self.store()?,
@@ -279,6 +282,31 @@ impl<'db> SourceAnalyzer<'db> {
             LifetimeElisionKind::Infer,
         )
         .lower_ty(type_ref);
+
+        // Try and substitute unknown types using InferenceResult
+        if let Some(infer) = self.infer()
+            && let Some(store) = self.store()
+        {
+            let mut inferred_types = vec![];
+            TypeRef::walk(type_ref, store, &mut |type_ref_id, type_ref| {
+                if matches!(type_ref, TypeRef::Placeholder) {
+                    inferred_types.push(infer.type_of_type_placeholder(type_ref_id));
+                }
+            });
+            let mut inferred_types = inferred_types.into_iter();
+
+            let substituted_ty = hir_ty::next_solver::fold::fold_tys(interner, ty, |ty| {
+                if ty.is_ty_error() { inferred_types.next().flatten().unwrap_or(ty) } else { ty }
+            });
+
+            // Only used the result if the placeholder and unknown type counts matched
+            let success =
+                inferred_types.next().is_none() && !substituted_ty.references_non_lt_error();
+            if success {
+                ty = substituted_ty;
+            }
+        }
+
         Some(Type::new_with_resolver(db, &self.resolver, ty))
     }