Add auto trait name for generate_trait_from_impl

Auto generate trait name from first method.

Input:

```rust
impl S$0 {
    fn foo() {}
}
```

Old output:

```rust
trait NewTrait {
    fn foo();
}

impl NewTrait for S {
    fn foo() {}
}
```

This PR output:

```rust
trait Foo {
    fn foo();
}

impl Foo for S {
    fn foo() {}
}
```

Author: A4-Tacks

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

@@ -49,7 +49,7 @@ use syntax::{
 //     };
 // }
 //
-// trait ${0:NewTrait}<const N: usize> {
+// trait ${0:Create}<const N: usize> {
 //     // Used as an associated constant.
 //     const CONST_ASSOC: usize = N * 4;
 //
@@ -58,7 +58,7 @@ use syntax::{
 //     const_maker! {i32, 7}
 // }
 //
-// impl<const N: usize> ${0:NewTrait}<N> for Foo<N> {
+// impl<const N: usize> ${0:Create}<N> for Foo<N> {
 //     // Used as an associated constant.
 //     const CONST_ASSOC: usize = N * 4;
 //
@@ -115,7 +115,7 @@ pub(crate) fn generate_trait_from_impl(acc: &mut Assists, ctx: &AssistContext<'_
 
             let trait_ast = make::trait_(
                 false,
-                "NewTrait",
+                &trait_name(&impl_items).text(),
                 impl_ast.generic_param_list(),
                 impl_ast.where_clause(),
                 trait_items,
@@ -161,6 +161,14 @@ pub(crate) fn generate_trait_from_impl(acc: &mut Assists, ctx: &AssistContext<'_
     Some(())
 }
 
+fn trait_name(items: &ast::AssocItemList) -> ast::Name {
+    items
+        .assoc_items()
+        .find_map(|x| if let ast::AssocItem::Fn(f) = x { f.name() } else { None })
+        .map(|name| make::name(&stdx::to_camel_case(&name.text())))
+        .unwrap_or_else(|| make::name("NewTrait"))
+}
+
 /// `E0449` Trait items always share the visibility of their trait
 fn remove_items_visibility(item: &ast::AssocItem) {
     if let Some(has_vis) = ast::AnyHasVisibility::cast(item.syntax().clone()) {
@@ -219,11 +227,11 @@ impl F$0oo {
             r#"
 struct Foo(f64);
 
-trait NewTrait {
+trait Add {
     fn add(&mut self, x: f64);
 }
 
-impl NewTrait for Foo {
+impl Add for Foo {
     fn add(&mut self, x: f64) {
         self.0 += x;
     }
@@ -332,11 +340,11 @@ impl F$0oo {
             r#"
 struct Foo;
 
-trait NewTrait {
+trait AFunc {
      fn a_func() -> Option<()>;
 }
 
-impl NewTrait for Foo {
+impl AFunc for Foo {
      fn a_func() -> Option<()> {
         Some(())
     }
@@ -366,11 +374,11 @@ mod a {
 }"#,
             r#"
 mod a {
-    trait NewTrait {
+    trait Foo {
         fn foo();
     }
 
-    impl NewTrait for S {
+    impl Foo for S {
         fn foo() {}
     }
 }"#,

crates/ide-assists/src/tests/generated.rs

@@ -2055,7 +2055,7 @@ macro_rules! const_maker {
     };
 }
 
-trait ${0:NewTrait}<const N: usize> {
+trait ${0:Create}<const N: usize> {
     // Used as an associated constant.
     const CONST_ASSOC: usize = N * 4;
 
@@ -2064,7 +2064,7 @@ trait ${0:NewTrait}<const N: usize> {
     const_maker! {i32, 7}
 }
 
-impl<const N: usize> ${0:NewTrait}<N> for Foo<N> {
+impl<const N: usize> ${0:Create}<N> for Foo<N> {
     // Used as an associated constant.
     const CONST_ASSOC: usize = N * 4;