4 files changed, 25 insertions, 17 deletions

diff --git a/crates/ide_assists/src/handlers/introduce_named_lifetime.rs b/crates/ide_assists/src/handlers/introduce_named_lifetime.rs
index 9f4f71d6c..cb71ca8bd 100644
--- a/crates/ide_assists/src/handlers/introduce_named_lifetime.rs
+++ b/crates/ide_assists/src/handlers/introduce_named_lifetime.rs
@@ -139,12 +139,12 @@ fn generate_unique_lifetime_param_name(
 
 fn add_lifetime_param(type_params: ast::GenericParamList, new_lifetime_param: char) {
     let generic_param =
-        make::generic_param(format!("'{}", new_lifetime_param), None).clone_for_update();
+        make::generic_param(&format!("'{}", new_lifetime_param), None).clone_for_update();
     type_params.add_generic_param(generic_param);
 }
 
 fn make_ast_lifetime(new_lifetime_param: char) -> ast::Lifetime {
-    make::generic_param(format!("'{}", new_lifetime_param), None)
+    make::generic_param(&format!("'{}", new_lifetime_param), None)
         .syntax()
         .descendants()
         .find_map(ast::Lifetime::cast)
diff --git a/crates/ide_assists/src/handlers/replace_impl_trait_with_generic.rs b/crates/ide_assists/src/handlers/replace_impl_trait_with_generic.rs
index 8509a5e11..26a0e81f0 100644
--- a/crates/ide_assists/src/handlers/replace_impl_trait_with_generic.rs
+++ b/crates/ide_assists/src/handlers/replace_impl_trait_with_generic.rs
@@ -1,6 +1,6 @@
 use syntax::ast::{self, edit::AstNodeEdit, make, AstNode, GenericParamsOwner};
 
-use crate::{AssistContext, AssistId, AssistKind, Assists};
+use crate::{utils::suggest_name, AssistContext, AssistId, AssistKind, Assists};
 
 // Assist: replace_impl_trait_with_generic
 //
@@ -17,30 +17,30 @@ pub(crate) fn replace_impl_trait_with_generic(
     acc: &mut Assists,
     ctx: &AssistContext,
 ) -> Option<()> {
-    let type_impl_trait = ctx.find_node_at_offset::<ast::ImplTraitType>()?;
-    let type_param = type_impl_trait.syntax().parent().and_then(ast::Param::cast)?;
-    let type_fn = type_param.syntax().ancestors().find_map(ast::Fn::cast)?;
+    let impl_trait_type = ctx.find_node_at_offset::<ast::ImplTraitType>()?;
+    let param = impl_trait_type.syntax().parent().and_then(ast::Param::cast)?;
+    let fn_ = param.syntax().ancestors().find_map(ast::Fn::cast)?;
 
-    let impl_trait_ty = type_impl_trait.type_bound_list()?;
+    let type_bound_list = impl_trait_type.type_bound_list()?;
 
-    let target = type_fn.syntax().text_range();
+    let target = fn_.syntax().text_range();
     acc.add(
         AssistId("replace_impl_trait_with_generic", AssistKind::RefactorRewrite),
         "Replace impl trait with generic",
         target,
         |edit| {
-            let generic_letter = impl_trait_ty.to_string().chars().next().unwrap().to_string();
+            let type_param_name = suggest_name::generic_parameter(&impl_trait_type);
 
-            let generic_param_list = type_fn
+            let generic_param_list = fn_
                 .generic_param_list()
                 .unwrap_or_else(|| make::generic_param_list(None))
-                .append_param(make::generic_param(generic_letter.clone(), Some(impl_trait_ty)));
+                .append_param(make::generic_param(&type_param_name, Some(type_bound_list)));
 
-            let new_type_fn = type_fn
-                .replace_descendant::<ast::Type>(type_impl_trait.into(), make::ty(&generic_letter))
+            let new_type_fn = fn_
+                .replace_descendant::<ast::Type>(impl_trait_type.into(), make::ty(&type_param_name))
                 .with_generic_param_list(generic_param_list);
 
-            edit.replace_ast(type_fn.clone(), new_type_fn);
+            edit.replace_ast(fn_.clone(), new_type_fn);
         },
     )
 }
diff --git a/crates/ide_assists/src/utils/suggest_name.rs b/crates/ide_assists/src/utils/suggest_name.rs
index deafcd630..c8487846d 100644
--- a/crates/ide_assists/src/utils/suggest_name.rs
+++ b/crates/ide_assists/src/utils/suggest_name.rs
@@ -6,7 +6,7 @@ use itertools::Itertools;
 use stdx::to_lower_snake_case;
 use syntax::{
     ast::{self, NameOwner},
-    match_ast, AstNode,
+    match_ast, AstNode, SmolStr,
 };
 
 /// Trait names, that will be ignored when in `impl Trait` and `dyn Trait`
@@ -57,6 +57,14 @@ const USELESS_METHODS: &[&str] = &[
     "iter_mut",
 ];
 
+pub(crate) fn generic_parameter(ty: &ast::ImplTraitType) -> SmolStr {
+    let c = ty
+        .type_bound_list()
+        .and_then(|bounds| bounds.syntax().text().char_at(0.into()))
+        .unwrap_or('T');
+    c.encode_utf8(&mut [0; 4]).into()
+}
+
 /// Suggest name of variable for given expression
 ///
 /// **NOTE**: it is caller's responsibility to guarantee uniqueness of the name.
diff --git a/crates/syntax/src/ast/make.rs b/crates/syntax/src/ast/make.rs
index 5a6687397..2289d8f3e 100644
--- a/crates/syntax/src/ast/make.rs
+++ b/crates/syntax/src/ast/make.rs
@@ -475,8 +475,8 @@ pub fn param_list(
     };
     ast_from_text(&list)
 }
-
-pub fn generic_param(name: String, ty: Option<ast::TypeBoundList>) -> ast::GenericParam {
+// FIXME: s/&str/ast:Name
+pub fn generic_param(name: &str, ty: Option<ast::TypeBoundList>) -> ast::GenericParam {
     let bound = match ty {
         Some(it) => format!(": {}", it),
         None => String::new(),