Lint nested and independent `if let` clauses

[Luro02]

This code

if let Some(start) = (1_usize).checked_add(5) {
    if let Some(end) = (2_usize).checked_add(5) {
        // ...
    }
}

could be written like this

if let (Some(start), Some(end)) = ((1_usize).checked_add(5), (2_usize).checked_add(5)) {
    // ...
}

which would remove a layer of nesting.

---

The lint should only lint nested if let clauses, that are independent of each other.

For example this must be ignored (or handled like in #2521):

if let Some(start) = (1_usize).checked_add(5) {
    if let Some(end) = call_fn(start) {
        // ...
    }
}

but it should not matter, if there is code in between the if let clauses, as long as it does not influence the other if let clause(s):

if let Some(start) = (1_usize).checked_add(5) {
    let x = 13_usize;
    if let Some(end) = (2_usize).checked_add(5) {
        // ...
    }
    let y = 15_usize;
}

I think that there should not be a limit on how many nested if let clauses should be linted, because it would improve the readability either way.

Another thing to consider would be else clauses. I think this lint should only trigger, if the nested else clauses have the same body:

if let Some(start) = (1_usize).checked_add(5) {
    if let Some(end) = (2_usize).checked_add(5) {
        // ...
    } else {
        call_x();
    }
} else {
    call_x();
}

would become

if let (Some(start), Some(end)) = ((1_usize).checked_add(5), (2_usize).checked_add(5)) {
    // ...
} else {
    call_x();
}

---

It might be too difficult (for clippy), but it would be possible to refactor nested if let-clauses with different else-clauses too

if let Some(start) = (1_usize).checked_add(5) {
    if let Some(end) = (2_usize).checked_add(5) {
        // ...
    } else {
        call_z(start);
    }
} else {
    call_y();
}

->

match ((1_usize).checked_add(5), (2_usize).checked_add(5)) {
    (Some(start), Some(end)) => {
        // ...
    }

    (Some(start), None) => {
        call_z(start);
    }

    (None, None) => {
        call_y();
    }

    _ => {
        // this was not reachable in the original code (end would only be matched if Some(start))
    }
}

Should I move this issue in to several smaller ones?

[flip1995]

which would remove a layer of nesting.

While this is true, this could increase the cognitive complexity, since the if-condition is longer. So we have to be careful here.

For example this must be ignored (or handled like in #2521):

This has to be ignored, except for the special in #2521, where the x in Some(x) is directly used and not in a parent expression.

as long as it does not influence the other if let clause(s):

I don't think we should lint this, since detecting side-effects is really hard. In the collapsible_if lint, the lint isn't even triggered when there is a comment in between. We should also use this heuristic here.

I think that there should not be a limit on how many nested if let clauses should be linted, because it would improve the readability either way.

As stated above, I'm not sure about that, we may need some other opinions about this.

It might be too difficult (for clippy), but it would be possible to refactor nested if let-clauses with different else-clauses too

This seems hard for more complex enums, than Option.

Should I move this issue in to several smaller ones?

We can track this in this issue.

[Luro02]

which would remove a layer of nesting.

While this is true, this could increase the cognitive complexity, since the if-condition is longer. So we have to be careful here.

I think that there should not be a limit on how many nested if let clauses should be linted, because it would improve the readability either way.

As stated above, I'm not sure about that, we may need some other opinions about this.

let variable_01 = Some(1_usize);
let variable_02 = Some(2_usize);
let variable_03 = Some(3_usize);
let variable_04 = Some(4_usize);
let variable_05 = Some(5_usize);
let variable_06 = Some(6_usize);
let variable_07 = Some(7_usize);
let variable_08 = Some(8_usize);

if let Some(variable_01) = variable_01 {
    if let Some(variable_02) = variable_02 {
        if let Some(variable_03) = variable_03 {
            if let Some(variable_04) = variable_04 {
                if let Some(variable_05) = variable_05 {
                    if let Some(variable_06) = variable_06 {
                        if let Some(variable_07) = variable_07 {
                            if let Some(variable_08) = variable_08 {
                                println!("{}", variable_01);
                                println!("{}", variable_02);
                                println!("{}", variable_03);
                                println!("{}", variable_04);
                                println!("{}", variable_05);
                                println!("{}", variable_06);
                                println!("{}", variable_07);
                                println!("{}", variable_08);
                            }
                        }
                    }
                }
            }
        }
    }
}

if let (
    Some(variable_01),
    Some(variable_02),
    Some(variable_03),
    Some(variable_04),
    Some(variable_05),
    Some(variable_06),
    Some(variable_07),
    Some(variable_08),
) = (
    variable_01,
    variable_02,
    variable_03,
    variable_04,
    variable_05,
    variable_06,
    variable_07,
    variable_08,
) {
    println!("{}", variable_01);
    println!("{}", variable_02);
    println!("{}", variable_03);
    println!("{}", variable_04);
    println!("{}", variable_05);
    println!("{}", variable_06);
    println!("{}", variable_07);
    println!("{}", variable_08);
}

or with a more complex enum

enum Example {
    Variant01 {
        string: String,
        number: usize,
        character: char,
    },
    Variant02 {
        string: String,
        number: usize,
        character: char,
    },
    Variant03 {
        string: String,
        number: usize,
        character: char,
    },
    Variant04 {
        string: String,
        number: usize,
        character: char,
    },
    Variant05 {
        string: String,
        number: usize,
        character: char,
    },
    Variant06 {
        string: String,
        number: usize,
        character: char,
    },
    Variant07 {
        string: String,
        number: usize,
        character: char,
    },
    Variant08 {
        string: String,
        number: usize,
        character: char,
    },
}

let value_01 = Example::Variant01 {
    "example".into(),
    2_usize,
    'b',
};

let value_02 = Example::Variant02 {
    "example".into(),
    2_usize,
    'b',
};

let value_03 = Example::Variant03 {
    "example".into(),
    2_usize,
    'b',
};

let value_04 = Example::Variant04 {
    "example".into(),
    2_usize,
    'b',
};

let value_05 = Example::Variant05 {
    "example".into(),
    2_usize,
    'b',
};

let value_06 = Example::Variant06 {
    "example".into(),
    2_usize,
    'b',
};

let value_07 = Example::Variant07 {
    "example".into(),
    2_usize,
    'b',
};

let value_08 = Example::Variant08 {
    "example".into(),
    2_usize,
    'b',
};


if let Example::Variant01 { string_01, number_01, character_01 } = value_01 {
    println!("{}: {},{}", string_01, number_01, character_01);

    if let Example::Variant02 { string_02, number_02, character_02 } = value_02 {
        println!("{}: {},{}", string_02, number_02, character_02);

        if let Example::Variant03 { string_03, number_03, character_03 } = value_03 {
            println!("{}: {},{}", string_03, number_03, character_03);

            if let Example::Variant04 { string_04, number_04, character_04 } = value_04 {
                println!("{}: {},{}", string_04, number_04, character_04);

                if let Example::Variant05 { string_05, number_05, character_05 } = value_05 {
                    println!("{}: {},{}", string_05, number_05, character_05);

                    if let Example::Variant06 { string_06, number_06, character_06 } = value_06 {
                        println!("{}: {},{}", string_06, number_06, character_06);

                        if let Example::Variant07 { string_07, number_07, character_07 } = value_07 {
                            println!("{}: {},{}", string_07, number_07, character_07);

                            if let Example::Variant08 { string_08, number_08, character_08 } = value_08 {
                                println!("{}: {},{}", string_08, number_08, character_08);

                            }
                        }
                    }
                }
            }
        }
    }
}

if let (
    Example::Variant01 { string_01, number_01, character_01 },
    Example::Variant02 { string_02, number_02, character_02 },
    Example::Variant03 { string_03, number_03, character_03 },
    Example::Variant04 { string_04, number_04, character_04 },
    Example::Variant05 { string_05, number_05, character_05 },
    Example::Variant06 { string_06, number_06, character_06 },
    Example::Variant07 { string_07, number_07, character_07 },
    Example::Variant08 { string_08, number_08, character_08 },
) = (
    value_01,
    value_02,
    value_03,
    value_04,
    value_05,
    value_06,
    value_07,
    value_08,
) {
    println!("{}: {},{}", string_01, number_01, character_01);
    println!("{}: {},{}", string_02, number_02, character_02);
    println!("{}: {},{}", string_03, number_03, character_03);
    println!("{}: {},{}", string_04, number_04, character_04);
    println!("{}: {},{}", string_05, number_05, character_05);
    println!("{}: {},{}", string_06, number_06, character_06);
    println!("{}: {},{}", string_07, number_07, character_07);
    println!("{}: {},{}", string_08, number_08, character_08);
}

both of the above examples look cleaner with the tuple version.

---

We would have to look out for name shadowing: (maybe suggest to change the variable names to something unique?)

if let Example::Variant01 { string, number, character } = value_01 {
    println!("{}: {},{}", string, number, character);

    if let Example::Variant02 { string, number, character } = value_02 {
        println!("{}: {},{}", string, number, character);
    }
}

if let (
    Example::Variant01 { string, number, character },
    Example::Variant02 { string, number, character },
) = (value_01, value_02) {
    println!("{}: {},{}", string, number, character);
    println!("{}: {},{}", string, number, character);
}

[Luro02]

This lint might be changed with
rust-lang/rfcs#2497

[camsteffen]

Wouldn't this remove short-circuiting?

if let (Some(a), Some(b)) = (expensive(), expensive()) { .. }