Add clippy::manual_let_else at warn level to lints

[Kanabenki]

Objective

Related to #10612.

Enable the clippy::manual_let_else lint as a warning. The let else form seems more idiomatic to me than a match/if else that either match a pattern or diverge, and from the clippy doc, the lint doesn't seem to have any possible false positive.

Solution

Add the lint as warning in Cargo.toml, refactor places where the lint triggers.

[rlidwka]

Having this lint is good, and it did simplify a lot of code. In particular, let Ok(...) are all nice changes.

I'm concerned about few cases where variable being assigned is hidden deep inside the destructuring, so it is not obvious from the first glance what it is. uuid_str being a very egregious example of this. Maybe keep old code in there, disable lint for the statement, and (if you want) open clippy bugreport for emitting bad advice.

[rlidwka]

This expression became much less readable imho.

uuid_str is hidden inside a lot of syntax, and it is not immediately clear that it is the value being assigned.

Maybe disable lint just for this one line, or rewrite it to let uuid_str = if (...) {} else {} statement.

[Kanabenki]

Reverted this one, but for some reason the allow attribute only works when applied on the upper block level, not on the expression itself 😕

[rlidwka]

same here, kind of

[Kanabenki]

While I agree for the other since matched value was quite deep in the pattern, here I think that while it introduce a bit of indentation to read the binded variable, it is still clearer than the match version ?

[rlidwka]

not sure about this one, do as you like

[rlidwka]

I guess my problem is with one overly complex statement:

let VertexAttributeValues::Float32x3(positions) =
    mesh.attribute(Mesh::ATTRIBUTE_POSITION).ok_or(
        GenerateTangentsError::MissingVertexAttribute(Mesh::ATTRIBUTE_POSITION.name),
    )?
else {
    return Err(GenerateTangentsError::InvalidVertexAttributeFormat(
        Mesh::ATTRIBUTE_POSITION.name,
        VertexFormat::Float32x3,
    ));
};

Breaking it into multiple simpler statements might be more readable (here, I'm creating attribute variable for this purpose):

let attribute = mesh.attribute(Mesh::ATTRIBUTE_POSITION).ok_or(
    GenerateTangentsError::MissingVertexAttribute(Mesh::ATTRIBUTE_POSITION.name),
)?;
let VertexAttributeValues::Float32x3(positions) = attribute else {
    return Err(GenerateTangentsError::InvalidVertexAttributeFormat(
        Mesh::ATTRIBUTE_POSITION.name,
        VertexFormat::Float32x3,
    ));
};

edit: that's called "expression complexity", I've just found a nice article about it:
https://grugbrain.dev/#grug-on-expression-complexity

[Kanabenki]

Good suggestion, done 🙂

[JoJoJet]

I think you should follow rlidwka's suggestion for the vertex attributes, but otherwise LGTM.