New lint: replace `x.get(x.len()-1)` with `x.last()`

[detrumi]

The following can be replaced with x.last():

let x = vec![1,2,3];
let _ = x.get(x.len() - 1);

[matthiaskrgr]

For a vector, x.last() returns Option<T> while x[idx] returns just T so it's not quite the same?

[detrumi]

Oh right, so it doesn't work for x[x.len() - 1] then.

[HarrisonMc555]

I think I could try to do this one (if nobody else is working on it). I'll try to be following the instructions in CONTRIBUTING.md, but I assume I can ask here if I need more help?

[oli-obk]

I assume I can ask here if I need more help?

Yes always. Or on irc, or on discord.

[HarrisonMc555]

Ok so I think I have something working, but I can't manage to get the UI tests to run. Every time I try, it says error: unknown clippy lint: clippy::use_last. I ran utils/dev update_lints and I can see the changes in clippy_lints/src/lib.rs, but it still doesn't seem to be working.

I've tried looking around and haven't found anything. Any idea what's going on?

Here is the branch:

https://github.com/HarrisonMc555/rust-clippy/tree/use_last

Here is the UI test (link):

#![warn(clippy::use_last)] // This gives an error

fn dont_use_last() -> Option<i32> {
    let x = vec![2, 3, 5];
    let last_element = x.get(x.len() - 1); // ~ERROR Use _.last()
    last_element.map(|val| val + 1) // To avoid warnings
}

fn index_into_last() -> i32 {
    let x = vec![2, 3, 5];
    let last_element = x[x.len() - 1];
    last_element + 1 // To avoid warnings
}

fn main() {
    let expected_value: i32 = 5;
    assert_eq!(dont_use_last(), Some(expected_value));
    assert_eq!(index_into_last(), expected_value);
}

[flip1995]

You forgot to add

reg.register_early_lint_pass(box foo_functions::FooFunctionsPass);

to clippy_lints/src/lib.rs.

After registering the lint it should produce output.

Currently in the making, but might be useful: #3824

[HarrisonMc555]

Awesome, thanks for your help! I used the automatic update script which added it to the complexity lists, etc., but didn't do this. Whoops.

I have one more question: how do I check to make sure that the vector that they're calling get on is the same as the one they're calling len on? I tried comparing the hir_id values and that didn't work. That makes sense, since that's an Expr, and the expressions are in different places.

However, when I matched on my_struct_expr.node, it was a ExprKind::Path. Can I compare these directly to see if they are the same things? I don't know how to check for identical "objects".

Thanks again for the help!