Auto merge of rust-lang#15524 - vsrs:bind_unused_param, r=Veykril

Bind unused parameter assistant

This PR introduces a new **Bind unused parameter assistant**.

While we do have a QuickFix from `rustc` (prefixing the parameter with an underscore), it's sometimes more convenient to suppress the warning using the following approach:
```rust
 fn some_function(unused: i32) {}
```
->
```rust
fn some_function(unused: i32) {
    let _ = unused;
}
```

Author: bors

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

@@ -0,0 +1,159 @@
+use crate::assist_context::{AssistContext, Assists};
+use ide_db::{
+    assists::{AssistId, AssistKind},
+    defs::Definition,
+    LineIndexDatabase,
+};
+use syntax::{
+    ast::{self, edit_in_place::Indent},
+    AstNode,
+};
+
+// Assist: bind_unused_param
+//
+// Binds unused function parameter to an underscore.
+//
+// ```
+// fn some_function(x: i32$0) {}
+// ```
+// ->
+// ```
+// fn some_function(x: i32) {
+//     let _ = x;
+// }
+// ```
+pub(crate) fn bind_unused_param(acc: &mut Assists, ctx: &AssistContext<'_>) -> Option<()> {
+    let param: ast::Param = ctx.find_node_at_offset()?;
+
+    let Some(ast::Pat::IdentPat(ident_pat)) = param.pat() else { return None };
+
+    let param_def = {
+        let local = ctx.sema.to_def(&ident_pat)?;
+        Definition::Local(local)
+    };
+    if param_def.usages(&ctx.sema).at_least_one() {
+        cov_mark::hit!(keep_used);
+        return None;
+    }
+
+    let func = param.syntax().ancestors().find_map(ast::Fn::cast)?;
+    let stmt_list = func.body()?.stmt_list()?;
+    let l_curly_range = stmt_list.l_curly_token()?.text_range();
+    let r_curly_range = stmt_list.r_curly_token()?.text_range();
+
+    acc.add(
+        AssistId("bind_unused_param", AssistKind::QuickFix),
+        &format!("Bind as `let _ = {};`", &ident_pat),
+        param.syntax().text_range(),
+        |builder| {
+            let line_index = ctx.db().line_index(ctx.file_id());
+
+            let indent = func.indent_level();
+            let text_indent = indent + 1;
+            let mut text = format!("\n{text_indent}let _ = {ident_pat};");
+
+            let left_line = line_index.line_col(l_curly_range.end()).line;
+            let right_line = line_index.line_col(r_curly_range.start()).line;
+
+            if left_line == right_line {
+                cov_mark::hit!(single_line);
+                text.push_str(&format!("\n{indent}"));
+            }
+
+            builder.insert(l_curly_range.end(), text);
+        },
+    )
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::tests::{check_assist, check_assist_not_applicable};
+
+    use super::*;
+
+    #[test]
+    fn bind_unused_empty_block() {
+        cov_mark::check!(single_line);
+        check_assist(
+            bind_unused_param,
+            r#"
+fn foo($0y: i32) {}
+"#,
+            r#"
+fn foo(y: i32) {
+    let _ = y;
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn bind_unused_empty_block_with_newline() {
+        check_assist(
+            bind_unused_param,
+            r#"
+fn foo($0y: i32) {
+}
+"#,
+            r#"
+fn foo(y: i32) {
+    let _ = y;
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn bind_unused_generic() {
+        check_assist(
+            bind_unused_param,
+            r#"
+fn foo<T>($0y: T)
+where T : Default {
+}
+"#,
+            r#"
+fn foo<T>(y: T)
+where T : Default {
+    let _ = y;
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn trait_impl() {
+        check_assist(
+            bind_unused_param,
+            r#"
+trait Trait {
+    fn foo(x: i32);
+}
+impl Trait for () {
+    fn foo($0x: i32) {}
+}
+"#,
+            r#"
+trait Trait {
+    fn foo(x: i32);
+}
+impl Trait for () {
+    fn foo(x: i32) {
+        let _ = x;
+    }
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn keep_used() {
+        cov_mark::check!(keep_used);
+        check_assist_not_applicable(
+            bind_unused_param,
+            r#"
+fn foo(x: i32, $0y: i32) { y; }
+"#,
+        );
+    }
+}

crates/ide-assists/src/lib.rs

@@ -114,6 +114,7 @@ mod handlers {
     mod add_turbo_fish;
     mod apply_demorgan;
     mod auto_import;
+    mod bind_unused_param;
     mod change_visibility;
     mod convert_bool_then;
     mod convert_comment_block;
@@ -225,6 +226,7 @@ mod handlers {
             add_turbo_fish::add_turbo_fish,
             apply_demorgan::apply_demorgan,
             auto_import::auto_import,
+            bind_unused_param::bind_unused_param,
             change_visibility::change_visibility,
             convert_bool_then::convert_bool_then_to_if,
             convert_bool_then::convert_if_to_bool_then,

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

@@ -265,6 +265,21 @@ pub mod std { pub mod collections { pub struct HashMap { } } }
     )
 }
 
+#[test]
+fn doctest_bind_unused_param() {
+    check_doc_test(
+        "bind_unused_param",
+        r#####"
+fn some_function(x: i32$0) {}
+"#####,
+        r#####"
+fn some_function(x: i32) {
+    let _ = x;
+}
+"#####,
+    )
+}
+
 #[test]
 fn doctest_change_visibility() {
     check_doc_test(