Fix applicable on underscore for bind_unused_param

Fixes:
- applicable on underscore prefix parameter
- using binding mode as an expression

Examples
---
```rust
fn foo($0_x: i32, y: i32) {}
```

**Before this PR**:

```rust
fn foo(_x: i32, y: i32) {
    let _ = _x;
}
```

**After this PR**:

Assist not applicable

---

```rust
fn foo(ref $0y: i32) {}
```

**Before this PR**:

```rust
fn foo(ref y: i32) {
    let _ = ref y;
}
```

**After this PR**:

```rust
fn foo(ref y: i32) {
    let _ = y;
}
```

Author: A4-Tacks

src/tools/rust-analyzer/crates/ide-assists/src/handlers/bind_unused_param.rs

@@ -2,7 +2,7 @@ use crate::assist_context::{AssistContext, Assists};
 use ide_db::{LineIndexDatabase, assists::AssistId, defs::Definition};
 use syntax::{
     AstNode,
-    ast::{self, edit_in_place::Indent},
+    ast::{self, HasName, edit_in_place::Indent},
 };
 
 // Assist: bind_unused_param
@@ -22,6 +22,7 @@ pub(crate) fn bind_unused_param(acc: &mut Assists, ctx: &AssistContext<'_>) -> O
     let param: ast::Param = ctx.find_node_at_offset()?;
 
     let Some(ast::Pat::IdentPat(ident_pat)) = param.pat() else { return None };
+    let name = ident_pat.name().filter(|n| !n.text().starts_with('_'))?;
 
     let param_def = {
         let local = ctx.sema.to_def(&ident_pat)?;
@@ -39,14 +40,14 @@ pub(crate) fn bind_unused_param(acc: &mut Assists, ctx: &AssistContext<'_>) -> O
 
     acc.add(
         AssistId::quick_fix("bind_unused_param"),
-        format!("Bind as `let _ = {ident_pat};`"),
+        format!("Bind as `let _ = {name};`"),
         param.syntax().text_range(),
         |builder| {
             let line_index = ctx.db().line_index(ctx.vfs_file_id());
 
             let indent = func.indent_level();
             let text_indent = indent + 1;
-            let mut text = format!("\n{text_indent}let _ = {ident_pat};");
+            let mut text = format!("\n{text_indent}let _ = {name};");
 
             let left_line = line_index.line_col(l_curly_range.end()).line;
             let right_line = line_index.line_col(r_curly_range.start()).line;
@@ -83,6 +84,22 @@ fn foo(y: i32) {
         );
     }
 
+    #[test]
+    fn bind_unused_ref_ident_pat() {
+        cov_mark::check!(single_line);
+        check_assist(
+            bind_unused_param,
+            r#"
+fn foo(ref $0y: i32) {}
+"#,
+            r#"
+fn foo(ref y: i32) {
+    let _ = y;
+}
+"#,
+        );
+    }
+
     #[test]
     fn bind_unused_empty_block_with_newline() {
         check_assist(
@@ -149,6 +166,16 @@ impl Trait for () {
             bind_unused_param,
             r#"
 fn foo(x: i32, $0y: i32) { y; }
+"#,
+        );
+    }
+
+    #[test]
+    fn keep_underscore_used() {
+        check_assist_not_applicable(
+            bind_unused_param,
+            r#"
+fn foo($0_x: i32, y: i32) {}
 "#,
         );
     }