use hir::HirDisplay;
use ide_db::{base_db::FileId, helpers::SnippetCap};
use rustc_hash::{FxHashMap, FxHashSet};
use stdx::to_lower_snake_case;
use syntax::{
    ast::{
        self,
        edit::{AstNodeEdit, IndentLevel},
        make, ArgListOwner, AstNode, ModuleItemOwner,
    },
    SyntaxKind, SyntaxNode, TextSize,
};

use crate::{
    utils::{render_snippet, Cursor},
    AssistContext, AssistId, AssistKind, Assists,
};

// Assist: generate_function
//
// Adds a stub function with a signature matching the function under the cursor.
//
// ```
// struct Baz;
// fn baz() -> Baz { Baz }
// fn foo() {
//     bar$0("", baz());
// }
//
// ```
// ->
// ```
// struct Baz;
// fn baz() -> Baz { Baz }
// fn foo() {
//     bar("", baz());
// }
//
// fn bar(arg: &str, baz: Baz) ${0:-> ()} {
//     todo!()
// }
//
// ```
pub(crate) fn generate_function(acc: &mut Assists, ctx: &AssistContext) -> Option<()> {
    let path_expr: ast::PathExpr = ctx.find_node_at_offset()?;
    let call = path_expr.syntax().parent().and_then(ast::CallExpr::cast)?;
    let path = path_expr.path()?;

    if ctx.sema.resolve_path(&path).is_some() {
        // The function call already resolves, no need to add a function
        return None;
    }

    let target_module = match path.qualifier() {
        Some(qualifier) => match ctx.sema.resolve_path(&qualifier) {
            Some(hir::PathResolution::Def(hir::ModuleDef::Module(module))) => Some(module),
            _ => return None,
        },
        None => None,
    };

    let function_builder = FunctionBuilder::from_call(ctx, &call, &path, target_module)?;

    let target = call.syntax().text_range();
    acc.add(
        AssistId("generate_function", AssistKind::Generate),
        format!("Generate `{}` function", function_builder.fn_name),
        target,
        |builder| {
            let function_template = function_builder.render();
            builder.edit_file(function_template.file);
            let new_fn = function_template.to_string(ctx.config.snippet_cap);
            match ctx.config.snippet_cap {
                Some(cap) => builder.insert_snippet(cap, function_template.insert_offset, new_fn),
                None => builder.insert(function_template.insert_offset, new_fn),
            }
        },
    )
}

struct FunctionTemplate {
    insert_offset: TextSize,
    leading_ws: String,
    fn_def: ast::Fn,
    ret_type: ast::RetType,
    should_render_snippet: bool,
    trailing_ws: String,
    file: FileId,
}

impl FunctionTemplate {
    fn to_string(&self, cap: Option<SnippetCap>) -> String {
        let f = match (cap, self.should_render_snippet) {
            (Some(cap), true) => {
                render_snippet(cap, self.fn_def.syntax(), Cursor::Replace(self.ret_type.syntax()))
            }
            _ => self.fn_def.to_string(),
        };
        format!("{}{}{}", self.leading_ws, f, self.trailing_ws)
    }
}

struct FunctionBuilder {
    target: GeneratedFunctionTarget,
    fn_name: ast::Name,
    type_params: Option<ast::GenericParamList>,
    params: ast::ParamList,
    ret_type: ast::RetType,
    should_render_snippet: bool,
    file: FileId,
    needs_pub: bool,
}

impl FunctionBuilder {
    /// Prepares a generated function that matches `call`.
    /// The function is generated in `target_module` or next to `call`
    fn from_call(
        ctx: &AssistContext,
        call: &ast::CallExpr,
        path: &ast::Path,
        target_module: Option<hir::Module>,
    ) -> Option<Self> {
        let mut file = ctx.frange.file_id;
        let target = match &target_module {
            Some(target_module) => {
                let module_source = target_module.definition_source(ctx.db());
                let (in_file, target) = next_space_for_fn_in_module(ctx.sema.db, &module_source)?;
                file = in_file;
                target
            }
            None => next_space_for_fn_after_call_site(call)?,
        };
        let needs_pub = target_module.is_some();
        let target_module = target_module.or_else(|| ctx.sema.scope(target.syntax()).module())?;
        let fn_name = fn_name(path)?;
        let (type_params, params) = fn_args(ctx, target_module, call)?;

        // should_render_snippet intends to express a rough level of confidence about
        // the correctness of the return type.
        //
        // If we are able to infer some return type, and that return type is not unit, we
        // don't want to render the snippet. The assumption here is in this situation the
        // return type is just as likely to be correct as any other part of the generated
        // function.
        //
        // In the case where the return type is inferred as unit it is likely that the
        // user does in fact intend for this generated function to return some non unit
        // type, but that the current state of their code doesn't allow that return type
        // to be accurately inferred.
        let (ret_ty, should_render_snippet) = {
            match ctx.sema.type_of_expr(&ast::Expr::CallExpr(call.clone())) {
                Some(ty) if ty.is_unknown() || ty.is_unit() => (make::ty_unit(), true),
                Some(ty) => {
                    let rendered = ty.display_source_code(ctx.db(), target_module.into());
                    match rendered {
                        Ok(rendered) => (make::ty(&rendered), false),
                        Err(_) => (make::ty_unit(), true),
                    }
                }
                None => (make::ty_unit(), true),
            }
        };
        let ret_type = make::ret_type(ret_ty);

        Some(Self {
            target,
            fn_name,
            type_params,
            params,
            ret_type,
            should_render_snippet,
            file,
            needs_pub,
        })
    }

    fn render(self) -> FunctionTemplate {
        let placeholder_expr = make::ext::expr_todo();
        let fn_body = make::block_expr(vec![], Some(placeholder_expr));
        let visibility = if self.needs_pub { Some(make::visibility_pub_crate()) } else { None };
        let mut fn_def = make::fn_(
            visibility,
            self.fn_name,
            self.type_params,
            self.params,
            fn_body,
            Some(self.ret_type),
        );
        let leading_ws;
        let trailing_ws;

        let insert_offset = match self.target {
            GeneratedFunctionTarget::BehindItem(it) => {
                let indent = IndentLevel::from_node(&it);
                leading_ws = format!("\n\n{}", indent);
                fn_def = fn_def.indent(indent);
                trailing_ws = String::new();
                it.text_range().end()
            }
            GeneratedFunctionTarget::InEmptyItemList(it) => {
                let indent = IndentLevel::from_node(&it);
                leading_ws = format!("\n{}", indent + 1);
                fn_def = fn_def.indent(indent + 1);
                trailing_ws = format!("\n{}", indent);
                it.text_range().start() + TextSize::of('{')
            }
        };

        FunctionTemplate {
            insert_offset,
            leading_ws,
            ret_type: fn_def.ret_type().unwrap(),
            should_render_snippet: self.should_render_snippet,
            fn_def,
            trailing_ws,
            file: self.file,
        }
    }
}

enum GeneratedFunctionTarget {
    BehindItem(SyntaxNode),
    InEmptyItemList(SyntaxNode),
}

impl GeneratedFunctionTarget {
    fn syntax(&self) -> &SyntaxNode {
        match self {
            GeneratedFunctionTarget::BehindItem(it) => it,
            GeneratedFunctionTarget::InEmptyItemList(it) => it,
        }
    }
}

fn fn_name(call: &ast::Path) -> Option<ast::Name> {
    let name = call.segment()?.syntax().to_string();
    Some(make::name(&name))
}

/// Computes the type variables and arguments required for the generated function
fn fn_args(
    ctx: &AssistContext,
    target_module: hir::Module,
    call: &ast::CallExpr,
) -> Option<(Option<ast::GenericParamList>, ast::ParamList)> {
    let mut arg_names = Vec::new();
    let mut arg_types = Vec::new();
    for arg in call.arg_list()?.args() {
        arg_names.push(match fn_arg_name(&arg) {
            Some(name) => name,
            None => String::from("arg"),
        });
        arg_types.push(match fn_arg_type(ctx, target_module, &arg) {
            Some(ty) => ty,
            None => String::from("()"),
        });
    }
    deduplicate_arg_names(&mut arg_names);
    let params = arg_names
        .into_iter()
        .zip(arg_types)
        .map(|(name, ty)| make::param(make::ident_pat(make::name(&name)).into(), make::ty(&ty)));
    Some((None, make::param_list(None, params)))
}

/// Makes duplicate argument names unique by appending incrementing numbers.
///
/// ```
/// let mut names: Vec<String> =
///     vec!["foo".into(), "foo".into(), "bar".into(), "baz".into(), "bar".into()];
/// deduplicate_arg_names(&mut names);
/// let expected: Vec<String> =
///     vec!["foo_1".into(), "foo_2".into(), "bar_1".into(), "baz".into(), "bar_2".into()];
/// assert_eq!(names, expected);
/// ```
fn deduplicate_arg_names(arg_names: &mut Vec<String>) {
    let arg_name_counts = arg_names.iter().fold(FxHashMap::default(), |mut m, name| {
        *m.entry(name).or_insert(0) += 1;
        m
    });
    let duplicate_arg_names: FxHashSet<String> = arg_name_counts
        .into_iter()
        .filter(|(_, count)| *count >= 2)
        .map(|(name, _)| name.clone())
        .collect();

    let mut counter_per_name = FxHashMap::default();
    for arg_name in arg_names.iter_mut() {
        if duplicate_arg_names.contains(arg_name) {
            let counter = counter_per_name.entry(arg_name.clone()).or_insert(1);
            arg_name.push('_');
            arg_name.push_str(&counter.to_string());
            *counter += 1;
        }
    }
}

fn fn_arg_name(fn_arg: &ast::Expr) -> Option<String> {
    match fn_arg {
        ast::Expr::CastExpr(cast_expr) => fn_arg_name(&cast_expr.expr()?),
        _ => {
            let s = fn_arg
                .syntax()
                .descendants()
                .filter(|d| ast::NameRef::can_cast(d.kind()))
                .last()?
                .to_string();
            Some(to_lower_snake_case(&s))
        }
    }
}

fn fn_arg_type(
    ctx: &AssistContext,
    target_module: hir::Module,
    fn_arg: &ast::Expr,
) -> Option<String> {
    let ty = ctx.sema.type_of_expr(fn_arg)?;
    if ty.is_unknown() {
        return None;
    }

    if let Ok(rendered) = ty.display_source_code(ctx.db(), target_module.into()) {
        Some(rendered)
    } else {
        None
    }
}

/// Returns the position inside the current mod or file
/// directly after the current block
/// We want to write the generated function directly after
/// fns, impls or macro calls, but inside mods
fn next_space_for_fn_after_call_site(expr: &ast::CallExpr) -> Option<GeneratedFunctionTarget> {
    let mut ancestors = expr.syntax().ancestors().peekable();
    let mut last_ancestor: Option<SyntaxNode> = None;
    while let Some(next_ancestor) = ancestors.next() {
        match next_ancestor.kind() {
            SyntaxKind::SOURCE_FILE => {
                break;
            }
            SyntaxKind::ITEM_LIST => {
                if ancestors.peek().map(|a| a.kind()) == Some(SyntaxKind::MODULE) {
                    break;
                }
            }
            _ => {}
        }
        last_ancestor = Some(next_ancestor);
    }
    last_ancestor.map(GeneratedFunctionTarget::BehindItem)
}

fn next_space_for_fn_in_module(
    db: &dyn hir::db::AstDatabase,
    module_source: &hir::InFile<hir::ModuleSource>,
) -> Option<(FileId, GeneratedFunctionTarget)> {
    let file = module_source.file_id.original_file(db);
    let assist_item = match &module_source.value {
        hir::ModuleSource::SourceFile(it) => {
            if let Some(last_item) = it.items().last() {
                GeneratedFunctionTarget::BehindItem(last_item.syntax().clone())
            } else {
                GeneratedFunctionTarget::BehindItem(it.syntax().clone())
            }
        }
        hir::ModuleSource::Module(it) => {
            if let Some(last_item) = it.item_list().and_then(|it| it.items().last()) {
                GeneratedFunctionTarget::BehindItem(last_item.syntax().clone())
            } else {
                GeneratedFunctionTarget::InEmptyItemList(it.item_list()?.syntax().clone())
            }
        }
        hir::ModuleSource::BlockExpr(it) => {
            if let Some(last_item) =
                it.statements().take_while(|stmt| matches!(stmt, ast::Stmt::Item(_))).last()
            {
                GeneratedFunctionTarget::BehindItem(last_item.syntax().clone())
            } else {
                GeneratedFunctionTarget::InEmptyItemList(it.syntax().clone())
            }
        }
    };
    Some((file, assist_item))
}

#[cfg(test)]
mod tests {
    use crate::tests::{check_assist, check_assist_not_applicable};

    use super::*;

    #[test]
    fn add_function_with_no_args() {
        check_assist(
            generate_function,
            r"
fn foo() {
    bar$0();
}
",
            r"
fn foo() {
    bar();
}

fn bar() ${0:-> ()} {
    todo!()
}
",
        )
    }

    #[test]
    fn add_function_from_method() {
        // This ensures that the function is correctly generated
        // in the next outer mod or file
        check_assist(
            generate_function,
            r"
impl Foo {
    fn foo() {
        bar$0();
    }
}
",
            r"
impl Foo {
    fn foo() {
        bar();
    }
}

fn bar() ${0:-> ()} {
    todo!()
}
",
        )
    }

    #[test]
    fn add_function_directly_after_current_block() {
        // The new fn should not be created at the end of the file or module
        check_assist(
            generate_function,
            r"
fn foo1() {
    bar$0();
}

fn foo2() {}
",
            r"
fn foo1() {
    bar();
}

fn bar() ${0:-> ()} {
    todo!()
}

fn foo2() {}
",
        )
    }

    #[test]
    fn add_function_with_no_args_in_same_module() {
        check_assist(
            generate_function,
            r"
mod baz {
    fn foo() {
        bar$0();
    }
}
",
            r"
mod baz {
    fn foo() {
        bar();
    }

    fn bar() ${0:-> ()} {
        todo!()
    }
}
",
        )
    }

    #[test]
    fn add_function_with_upper_camel_case_arg() {
        check_assist(
            generate_function,
            r"
struct BazBaz;
fn foo() {
    bar$0(BazBaz);
}
",
            r"
struct BazBaz;
fn foo() {
    bar(BazBaz);
}

fn bar(baz_baz: BazBaz) ${0:-> ()} {
    todo!()
}
",
        );
    }

    #[test]
    fn add_function_with_upper_camel_case_arg_as_cast() {
        check_assist(
            generate_function,
            r"
struct BazBaz;
fn foo() {
    bar$0(&BazBaz as *const BazBaz);
}
",
            r"
struct BazBaz;
fn foo() {
    bar(&BazBaz as *const BazBaz);
}

fn bar(baz_baz: *const BazBaz) ${0:-> ()} {
    todo!()
}
",
        );
    }

    #[test]
    fn add_function_with_function_call_arg() {
        check_assist(
            generate_function,
            r"
struct Baz;
fn baz() -> Baz { todo!() }
fn foo() {
    bar$0(baz());
}
",
            r"
struct Baz;
fn baz() -> Baz { todo!() }
fn foo() {
    bar(baz());
}

fn bar(baz: Baz) ${0:-> ()} {
    todo!()
}
",
        );
    }

    #[test]
    fn add_function_with_method_call_arg() {
        check_assist(
            generate_function,
            r"
struct Baz;
impl Baz {
    fn foo(&self) -> Baz {
        ba$0r(self.baz())
    }
    fn baz(&self) -> Baz {
        Baz
    }
}
",
            r"
struct Baz;
impl Baz {
    fn foo(&self) -> Baz {
        bar(self.baz())
    }
    fn baz(&self) -> Baz {
        Baz
    }
}

fn bar(baz: Baz) -> Baz {
    todo!()
}
",
        )
    }

    #[test]
    fn add_function_with_string_literal_arg() {
        check_assist(
            generate_function,
            r#"
fn foo() {
    $0bar("bar")
}
"#,
            r#"
fn foo() {
    bar("bar")
}

fn bar(arg: &str) ${0:-> ()} {
    todo!()
}
"#,
        )
    }

    #[test]
    fn add_function_with_char_literal_arg() {
        check_assist(
            generate_function,
            r#"
fn foo() {
    $0bar('x')
}
"#,
            r#"
fn foo() {
    bar('x')
}

fn bar(arg: char) ${0:-> ()} {
    todo!()
}
"#,
        )
    }

    #[test]
    fn add_function_with_int_literal_arg() {
        check_assist(
            generate_function,
            r"
fn foo() {
    $0bar(42)
}
",
            r"
fn foo() {
    bar(42)
}

fn bar(arg: i32) ${0:-> ()} {
    todo!()
}
",
        )
    }

    #[test]
    fn add_function_with_cast_int_literal_arg() {
        check_assist(
            generate_function,
            r"
fn foo() {
    $0bar(42 as u8)
}
",
            r"
fn foo() {
    bar(42 as u8)
}

fn bar(arg: u8) ${0:-> ()} {
    todo!()
}
",
        )
    }

    #[test]
    fn name_of_cast_variable_is_used() {
        // Ensures that the name of the cast type isn't used
        // in the generated function signature.
        check_assist(
            generate_function,
            r"
fn foo() {
    let x = 42;
    bar$0(x as u8)
}
",
            r"
fn foo() {
    let x = 42;
    bar(x as u8)
}

fn bar(x: u8) ${0:-> ()} {
    todo!()
}
",
        )
    }

    #[test]
    fn add_function_with_variable_arg() {
        check_assist(
            generate_function,
            r"
fn foo() {
    let worble = ();
    $0bar(worble)
}
",
            r"
fn foo() {
    let worble = ();
    bar(worble)
}

fn bar(worble: ()) ${0:-> ()} {
    todo!()
}
",
        )
    }

    #[test]
    fn add_function_with_impl_trait_arg() {
        check_assist(
            generate_function,
            r"
trait Foo {}
fn foo() -> impl Foo {
    todo!()
}
fn baz() {
    $0bar(foo())
}
",
            r"
trait Foo {}
fn foo() -> impl Foo {
    todo!()
}
fn baz() {
    bar(foo())
}

fn bar(foo: impl Foo) ${0:-> ()} {
    todo!()
}
",
        )
    }

    #[test]
    fn borrowed_arg() {
        check_assist(
            generate_function,
            r"
struct Baz;
fn baz() -> Baz { todo!() }

fn foo() {
    bar$0(&baz())
}
",
            r"
struct Baz;
fn baz() -> Baz { todo!() }

fn foo() {
    bar(&baz())
}

fn bar(baz: &Baz) ${0:-> ()} {
    todo!()
}
",
        )
    }

    #[test]
    fn add_function_with_qualified_path_arg() {
        check_assist(
            generate_function,
            r"
mod Baz {
    pub struct Bof;
    pub fn baz() -> Bof { Bof }
}
fn foo() {
    $0bar(Baz::baz())
}
",
            r"
mod Baz {
    pub struct Bof;
    pub fn baz() -> Bof { Bof }
}
fn foo() {
    bar(Baz::baz())
}

fn bar(baz: Baz::Bof) ${0:-> ()} {
    todo!()
}
",
        )
    }

    #[test]
    #[ignore]
    // FIXME fix printing the generics of a `Ty` to make this test pass
    fn add_function_with_generic_arg() {
        check_assist(
            generate_function,
            r"
fn foo<T>(t: T) {
    $0bar(t)
}
",
            r"
fn foo<T>(t: T) {
    bar(t)
}

fn bar<T>(t: T) ${0:-> ()} {
    todo!()
}
",
        )
    }

    #[test]
    #[ignore]
    // FIXME Fix function type printing to make this test pass
    fn add_function_with_fn_arg() {
        check_assist(
            generate_function,
            r"
struct Baz;
impl Baz {
    fn new() -> Self { Baz }
}
fn foo() {
    $0bar(Baz::new);
}
",
            r"
struct Baz;
impl Baz {
    fn new() -> Self { Baz }
}
fn foo() {
    bar(Baz::new);
}

fn bar(arg: fn() -> Baz) ${0:-> ()} {
    todo!()
}
",
        )
    }

    #[test]
    #[ignore]
    // FIXME Fix closure type printing to make this test pass
    fn add_function_with_closure_arg() {
        check_assist(
            generate_function,
            r"
fn foo() {
    let closure = |x: i64| x - 1;
    $0bar(closure)
}
",
            r"
fn foo() {
    let closure = |x: i64| x - 1;
    bar(closure)
}

fn bar(closure: impl Fn(i64) -> i64) ${0:-> ()} {
    todo!()
}
",
        )
    }

    #[test]
    fn unresolveable_types_default_to_unit() {
        check_assist(
            generate_function,
            r"
fn foo() {
    $0bar(baz)
}
",
            r"
fn foo() {
    bar(baz)
}

fn bar(baz: ()) ${0:-> ()} {
    todo!()
}
",
        )
    }

    #[test]
    fn arg_names_dont_overlap() {
        check_assist(
            generate_function,
            r"
struct Baz;
fn baz() -> Baz { Baz }
fn foo() {
    $0bar(baz(), baz())
}
",
            r"
struct Baz;
fn baz() -> Baz { Baz }
fn foo() {
    bar(baz(), baz())
}

fn bar(baz_1: Baz, baz_2: Baz) ${0:-> ()} {
    todo!()
}
",
        )
    }

    #[test]
    fn arg_name_counters_start_at_1_per_name() {
        check_assist(
            generate_function,
            r#"
struct Baz;
fn baz() -> Baz { Baz }
fn foo() {
    $0bar(baz(), baz(), "foo", "bar")
}
"#,
            r#"
struct Baz;
fn baz() -> Baz { Baz }
fn foo() {
    bar(baz(), baz(), "foo", "bar")
}

fn bar(baz_1: Baz, baz_2: Baz, arg_1: &str, arg_2: &str) ${0:-> ()} {
    todo!()
}
"#,
        )
    }

    #[test]
    fn add_function_in_module() {
        check_assist(
            generate_function,
            r"
mod bar {}

fn foo() {
    bar::my_fn$0()
}
",
            r"
mod bar {
    pub(crate) fn my_fn() ${0:-> ()} {
        todo!()
    }
}

fn foo() {
    bar::my_fn()
}
",
        )
    }

    #[test]
    #[ignore]
    // Ignored until local imports are supported.
    // See https://github.com/rust-analyzer/rust-analyzer/issues/1165
    fn qualified_path_uses_correct_scope() {
        check_assist(
            generate_function,
            "
mod foo {
    pub struct Foo;
}
fn bar() {
    use foo::Foo;
    let foo = Foo;
    baz$0(foo)
}
",
            "
mod foo {
    pub struct Foo;
}
fn bar() {
    use foo::Foo;
    let foo = Foo;
    baz(foo)
}

fn baz(foo: foo::Foo) ${0:-> ()} {
    todo!()
}
",
        )
    }

    #[test]
    fn add_function_in_module_containing_other_items() {
        check_assist(
            generate_function,
            r"
mod bar {
    fn something_else() {}
}

fn foo() {
    bar::my_fn$0()
}
",
            r"
mod bar {
    fn something_else() {}

    pub(crate) fn my_fn() ${0:-> ()} {
        todo!()
    }
}

fn foo() {
    bar::my_fn()
}
",
        )
    }

    #[test]
    fn add_function_in_nested_module() {
        check_assist(
            generate_function,
            r"
mod bar {
    mod baz {}
}

fn foo() {
    bar::baz::my_fn$0()
}
",
            r"
mod bar {
    mod baz {
        pub(crate) fn my_fn() ${0:-> ()} {
            todo!()
        }
    }
}

fn foo() {
    bar::baz::my_fn()
}
",
        )
    }

    #[test]
    fn add_function_in_another_file() {
        check_assist(
            generate_function,
            r"
//- /main.rs
mod foo;

fn main() {
    foo::bar$0()
}
//- /foo.rs
",
            r"


pub(crate) fn bar() ${0:-> ()} {
    todo!()
}",
        )
    }

    #[test]
    fn add_function_with_return_type() {
        check_assist(
            generate_function,
            r"
fn main() {
    let x: u32 = foo$0();
}
",
            r"
fn main() {
    let x: u32 = foo();
}

fn foo() -> u32 {
    todo!()
}
",
        )
    }

    #[test]
    fn add_function_not_applicable_if_function_already_exists() {
        check_assist_not_applicable(
            generate_function,
            r"
fn foo() {
    bar$0();
}

fn bar() {}
",
        )
    }

    #[test]
    fn add_function_not_applicable_if_unresolved_variable_in_call_is_selected() {
        check_assist_not_applicable(
            // bar is resolved, but baz isn't.
            // The assist is only active if the cursor is on an unresolved path,
            // but the assist should only be offered if the path is a function call.
            generate_function,
            r"
fn foo() {
    bar(b$0az);
}

fn bar(baz: ()) {}
",
        )
    }

    #[test]
    #[ignore]
    fn create_method_with_no_args() {
        check_assist(
            generate_function,
            r"
struct Foo;
impl Foo {
    fn foo(&self) {
        self.bar()$0;
    }
}
        ",
            r"
struct Foo;
impl Foo {
    fn foo(&self) {
        self.bar();
    }
    fn bar(&self) {
        todo!();
    }
}
        ",
        )
    }
}