Potential false positive for let_and_return

[Keats]

I have the following macro:

#[macro_export]
macro_rules! try_get_value {
    ($filter_name:expr, $var_name:expr, $ty:ty, $val:expr) => {{
        let v: $ty = match ::serde_json::value::from_value($val.clone()) {
            Ok(s) => s,
            Err(_) => {
                return Err(::errors::TeraError::FilterIncorrectArgType(
                    $filter_name.to_string(),
                    $var_name.to_string(),
                    $val,
                    stringify!($ty).to_string())
                );
            }
        };
        v
    }};
}

And running clippy, it complains that I can directly return the match but doing so fails since it doesn't know the type ($ty can f32, String, etc).
Afaik there is no other way to cast the s variable other than that syntax (as $ty doesn't work). Am I missing a way to do it or is it a real false positive?

[oli-obk]

it's a false positive, but you can actually improve it in this case, by annotating the from_value function call

#[macro_export]
macro_rules! try_get_value {
    ($filter_name:expr, $var_name:expr, $ty:ty, $val:expr) => {{
        match ::serde_json::value::from_value::<$ty>($val.clone()) {
            Ok(s) => s,
            Err(_) => {
                return Err(::errors::TeraError::FilterIncorrectArgType(
                    $filter_name.to_string(),
                    $var_name.to_string(),
                    $val,
                    stringify!($ty).to_string())
                );
            }
        }
    }};
}

[Keats]

Embarrassing that I forgot to try that >_>

Thanks!
Do I leave the issue open for the false positive or close it?

[oli-obk]

leave it open, we need to improve the message

[ebroto]

I think this issue can be closed because let_and_return currently does not lint in the presence of type annotations (since 81b35d1)

rust-clippy/clippy_lints/src/returns.rs

Lines 200 to 201 in a8957ac

	// don't lint in the presence of type inference
	if local.ty.is_none();

and it also avoids linting inside macros (since #5008)

rust-clippy/clippy_lints/src/returns.rs

Line 210 in a8957ac

if !in_macro(local.span);