Auto merge of rust-lang#16068 - roife:fix-introduce-named-generic-name-conflicts, r=Veykril

fix: pick up new names when the name conflicts in 'introduce_named_generic'

Improve generation of names for generic parameters in `introduce_named_generics`.

fix rust-lang#15731.

### Changes

- Modified `for_generic_parameter` function in `suggest_name.rs` to handle conflicts with existing generic parameters and generate unique names accordingly.
- Update `introduce_named_generic` function and pass existing params to `for_generic_parameter`, enabling the detection and handling of name collisions.

Author: bors

crates/ide-assists/src/handlers/introduce_named_generic.rs

@@ -31,15 +31,16 @@ pub(crate) fn introduce_named_generic(acc: &mut Assists, ctx: &AssistContext<'_>
         |edit| {
             let impl_trait_type = edit.make_mut(impl_trait_type);
             let fn_ = edit.make_mut(fn_);
-
-            let type_param_name = suggest_name::for_generic_parameter(&impl_trait_type);
+            let fn_generic_param_list = fn_.get_or_create_generic_param_list();
+            let type_param_name =
+                suggest_name::for_impl_trait_as_generic(&impl_trait_type, &fn_generic_param_list);
 
             let type_param = make::type_param(make::name(&type_param_name), Some(type_bound_list))
                 .clone_for_update();
             let new_ty = make::ty(&type_param_name).clone_for_update();
 
             ted::replace(impl_trait_type.syntax(), new_ty.syntax());
-            fn_.get_or_create_generic_param_list().add_generic_param(type_param.into());
+            fn_generic_param_list.add_generic_param(type_param.into());
 
             if let Some(cap) = ctx.config.snippet_cap {
                 if let Some(generic_param) =
@@ -111,12 +112,19 @@ fn foo<$0B: Bar
 
     #[test]
     fn replace_impl_trait_with_exist_generic_letter() {
-        // FIXME: This is wrong, we should pick a different name if the one we
-        // want is already bound.
         check_assist(
             introduce_named_generic,
             r#"fn foo<B>(bar: $0impl Bar) {}"#,
-            r#"fn foo<B, $0B: Bar>(bar: B) {}"#,
+            r#"fn foo<B, $0B0: Bar>(bar: B0) {}"#,
+        );
+    }
+
+    #[test]
+    fn replace_impl_trait_with_more_exist_generic_letter() {
+        check_assist(
+            introduce_named_generic,
+            r#"fn foo<B, B0, B1, B3>(bar: $0impl Bar) {}"#,
+            r#"fn foo<B, B0, B1, B3, $0B2: Bar>(bar: B2) {}"#,
         );
     }
 

crates/ide-assists/src/utils/suggest_name.rs

@@ -1,5 +1,7 @@
 //! This module contains functions to suggest names for expressions, functions and other items
 
+use std::collections::HashSet;
+
 use hir::Semantics;
 use ide_db::RootDatabase;
 use itertools::Itertools;
@@ -58,6 +60,14 @@ const USELESS_METHODS: &[&str] = &[
     "into_future",
 ];
 
+/// Suggest a unique name for generic parameter.
+///
+/// `existing_params` is used to check if the name conflicts with existing
+/// generic parameters.
+///
+/// The function checks if the name conflicts with existing generic parameters.
+/// If so, it will try to resolve the conflict by adding a number suffix, e.g.
+/// `T`, `T0`, `T1`, ...
 pub(crate) fn for_unique_generic_name(
     name: &str,
     existing_params: &ast::GenericParamList,
@@ -68,12 +78,9 @@ pub(crate) fn for_unique_generic_name(
             ast::GenericParam::TypeParam(t) => t.name().unwrap().to_string(),
             p => p.to_string(),
         })
-        .collect_vec();
+        .collect::<HashSet<_>>();
     let mut name = name.to_string();
     let base_len = name.len();
-    // 4*len bytes for base, and 2 bytes for 2 digits
-    name.reserve(4 * base_len + 2);
-
     let mut count = 0;
     while param_names.contains(&name) {
         name.truncate(base_len);
@@ -84,12 +91,28 @@ pub(crate) fn for_unique_generic_name(
     name.into()
 }
 
-pub(crate) fn for_generic_parameter(ty: &ast::ImplTraitType) -> SmolStr {
+/// Suggest name of impl trait type
+///
+/// `existing_params` is used to check if the name conflicts with existing
+/// generic parameters.
+///
+/// # Current implementation
+///
+/// In current implementation, the function tries to get the name from the first
+/// character of the name for the first type bound.
+///
+/// If the name conflicts with existing generic parameters, it will try to
+/// resolve the conflict with `for_unique_generic_name`.
+pub(crate) fn for_impl_trait_as_generic(
+    ty: &ast::ImplTraitType,
+    existing_params: &ast::GenericParamList,
+) -> 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()
+
+    for_unique_generic_name(c.encode_utf8(&mut [0; 4]), existing_params)
 }
 
 /// Suggest name of variable for given expression