From 37a8cec6386d57e76b3067af48c78867aa9922ed Mon Sep 17 00:00:00 2001
From: Vladyslav Katasonov <cpud47@gmail.com>
Date: Sat, 13 Feb 2021 21:46:21 +0300
Subject: [PATCH] expose hir::Type::type_paramters

Used to get E parameter from `Result<T, E>`
---
 .../assists/src/handlers/extract_function.rs  | 29 ++++---------------
 crates/hir/src/code_model.rs                  | 12 ++++++++
 2 files changed, 17 insertions(+), 24 deletions(-)

diff --git a/crates/assists/src/handlers/extract_function.rs b/crates/assists/src/handlers/extract_function.rs
index 74fa2013cc4..9f34cc7250f 100644
--- a/crates/assists/src/handlers/extract_function.rs
+++ b/crates/assists/src/handlers/extract_function.rs
@@ -1131,8 +1131,11 @@ fn make_ret_ty(ctx: &AssistContext, module: hir::Module, fun: &Function) -> Opti
             make::ty_generic(make::name_ref("Option"), iter::once(fun_ty.make_ty(ctx, module)))
         }
         FlowHandler::Try { kind: TryKind::Result { ty: parent_ret_ty } } => {
-            let handler_ty =
-                result_err_ty(parent_ret_ty, ctx, module).unwrap_or_else(make::ty_unit);
+            let handler_ty = parent_ret_ty
+                .type_parameters()
+                .nth(1)
+                .map(|ty| make_ty(&ty, ctx, module))
+                .unwrap_or_else(make::ty_unit);
             make::ty_generic(
                 make::name_ref("Result"),
                 vec![fun_ty.make_ty(ctx, module), handler_ty],
@@ -1161,28 +1164,6 @@ fn make_ret_ty(ctx: &AssistContext, module: hir::Module, fun: &Function) -> Opti
     Some(make::ret_type(ret_ty))
 }
 
-/// Extract `E` type from `Result<T, E>`
-fn result_err_ty(
-    parent_ret_ty: &hir::Type,
-    ctx: &AssistContext,
-    module: hir::Module,
-) -> Option<ast::Type> {
-    // FIXME: use hir to extract argument information
-    //        currently we use `format -> take part -> parse`
-    let path_ty = match make_ty(&parent_ret_ty, ctx, module) {
-        ast::Type::PathType(path_ty) => path_ty,
-        _ => return None,
-    };
-    let arg_list = path_ty.path()?.segment()?.generic_arg_list()?;
-    let err_arg = arg_list.generic_args().nth(1)?;
-    let type_arg = match err_arg {
-        ast::GenericArg::TypeArg(type_arg) => type_arg,
-        _ => return None,
-    };
-
-    type_arg.ty()
-}
-
 fn make_body(
     ctx: &AssistContext,
     old_indent: IndentLevel,
diff --git a/crates/hir/src/code_model.rs b/crates/hir/src/code_model.rs
index 35b53260216..7d43d409719 100644
--- a/crates/hir/src/code_model.rs
+++ b/crates/hir/src/code_model.rs
@@ -1802,6 +1802,18 @@ impl Type {
         None
     }
 
+    pub fn type_parameters(&self) -> impl Iterator<Item = Type> + '_ {
+        let ty = self.ty.value.strip_references();
+        let substs = match ty {
+            Ty::Apply(apply_ty) => &apply_ty.parameters,
+            Ty::Opaque(opaque_ty) => &opaque_ty.parameters,
+            _ => return Either::Left(iter::empty()),
+        };
+
+        let iter = substs.iter().map(move |ty| self.derived(ty.clone()));
+        Either::Right(iter)
+    }
+
     pub fn iterate_method_candidates<T>(
         &self,
         db: &dyn HirDatabase,