feat: adds an assist to remove underscores from used variables

adds an assist that suggests removing any underscore prefixes from used
variables.

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

@@ -0,0 +1,191 @@
+use ide_db::{
+    assists::AssistId,
+    defs::{Definition, NameClass, NameRefClass},
+};
+use syntax::{AstNode, ast};
+
+use crate::{AssistContext, Assists};
+
+// Assist: remove_underscore_from_used_variables
+//
+// Removes underscore from used variables.
+//
+// ```
+// fn main() {
+//     let mut _$0foo = 1;
+//     _foo = 2;
+// }
+// ```
+// ->
+// ```
+// fn main() {
+//     let mut foo = 1;
+//     foo = 2;
+// }
+// ```
+pub(crate) fn remove_underscore(acc: &mut Assists, ctx: &AssistContext<'_>) -> Option<()> {
+    let (text, text_range, def) = if let Some(name_ref) = ctx.find_node_at_offset::<ast::Name>() {
+        let text = name_ref.text();
+        if !text.starts_with('_') {
+            return None;
+        }
+
+        let def = match NameClass::classify(&ctx.sema, &name_ref)? {
+            NameClass::Definition(def @ Definition::Local(_)) => def,
+            NameClass::PatFieldShorthand { local_def, .. } => Definition::Local(local_def),
+            _ => return None,
+        };
+        (text.to_owned(), name_ref.syntax().text_range(), def)
+    } else if let Some(name_ref) = ctx.find_node_at_offset::<ast::NameRef>() {
+        let text = name_ref.text();
+        if !text.starts_with('_') {
+            return None;
+        }
+        let def = match NameRefClass::classify(&ctx.sema, &name_ref)? {
+            NameRefClass::Definition(def @ Definition::Local(_), _) => def,
+            NameRefClass::FieldShorthand { local_ref, .. } => Definition::Local(local_ref),
+            _ => return None,
+        };
+        (text.to_owned(), name_ref.syntax().text_range(), def)
+    } else {
+        return None;
+    };
+
+    if !def.usages(&ctx.sema).at_least_one() {
+        return None;
+    }
+
+    let new_name = text.trim_start_matches('_');
+    acc.add(
+        AssistId::refactor("remove_underscore_from_used_variables"),
+        "Remove underscore from a used variable",
+        text_range,
+        |builder| {
+            let changes = def.rename(&ctx.sema, new_name).unwrap();
+            builder.source_change = changes;
+        },
+    )
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::tests::{check_assist, check_assist_not_applicable};
+
+    use super::*;
+
+    #[test]
+    fn remove_underscore_from_used_variable() {
+        check_assist(
+            remove_underscore,
+            r#"
+fn main() {
+    let mut _$0foo = 1;
+    _foo = 2;
+}
+"#,
+            r#"
+fn main() {
+    let mut foo = 1;
+    foo = 2;
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn not_applicable_for_unused() {
+        check_assist_not_applicable(
+            remove_underscore,
+            r#"
+fn main() {
+    let _$0unused = 1;
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn not_applicable_for_no_underscore() {
+        check_assist_not_applicable(
+            remove_underscore,
+            r#"
+fn main() {
+    let f$0oo = 1;
+    foo = 2;
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn remove_multiple_underscores() {
+        check_assist(
+            remove_underscore,
+            r#"
+fn main() {
+    let mut _$0_foo = 1;
+    __foo = 2;
+}
+"#,
+            r#"
+fn main() {
+    let mut foo = 1;
+    foo = 2;
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn remove_underscore_on_usage() {
+        check_assist(
+            remove_underscore,
+            r#"
+fn main() {
+    let mut _foo = 1;
+    _$0foo = 2;
+}
+"#,
+            r#"
+fn main() {
+    let mut foo = 1;
+    foo = 2;
+}
+"#,
+        );
+    }
+
+    #[test]
+    fn remove_underscore_in_function_parameter_usage() {
+        check_assist(
+            remove_underscore,
+            r#"
+fn foo(_foo: i32) {
+    let bar = _$0foo + 1;
+}
+"#,
+            r#"
+fn foo(foo: i32) {
+    let bar = foo + 1;
+}
+"#,
+        )
+    }
+
+    #[test]
+    fn remove_underscore_in_function_parameter() {
+        check_assist(
+            remove_underscore,
+            r#"
+fn foo(_$0foo: i32) {
+    let bar = _foo + 1;
+}
+"#,
+            r#"
+fn foo(foo: i32) {
+    let bar = foo + 1;
+}
+"#,
+        )
+    }
+}

crates/ide-assists/src/lib.rs

@@ -200,6 +200,7 @@ mod handlers {
     mod remove_dbg;
     mod remove_mut;
     mod remove_parentheses;
+    mod remove_underscore;
     mod remove_unused_imports;
     mod remove_unused_param;
     mod reorder_fields;
@@ -335,6 +336,7 @@ mod handlers {
             remove_dbg::remove_dbg,
             remove_mut::remove_mut,
             remove_parentheses::remove_parentheses,
+            remove_underscore::remove_underscore,
             remove_unused_imports::remove_unused_imports,
             remove_unused_param::remove_unused_param,
             reorder_fields::reorder_fields,

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

@@ -2748,6 +2748,25 @@ fn main() {
     )
 }
 
+#[test]
+fn doctest_remove_underscore_from_used_variables() {
+    check_doc_test(
+        "remove_underscore_from_used_variables",
+        r#####"
+fn main() {
+    let mut _$0foo = 1;
+    _foo = 2;
+}
+"#####,
+        r#####"
+fn main() {
+    let mut foo = 1;
+    foo = 2;
+}
+"#####,
+    )
+}
+
 #[test]
 fn doctest_remove_unused_imports() {
     check_doc_test(