Use is_empty instead of comparing to empty slice/str

[Urcra]

What it does

Suggest using is_empty() instead of checking for equality with an empty slice == [] or == "" to be more consistent with similar lints such as len_zero

Categories (optional)

• Kind: clippy::perf or clippy::style

What is the advantage of the recommended code over the original code

• Consistency with similar lints such as len_zero so that we suggest lints for both str.len() == 0 and str == ""
• Performance wise the equality check will also usually take longer to compute compared to is_empty()

Drawbacks

None.

Example

if s == "" {
  ...
}

if arr == [] {
  ...
}

Could be written as:

if s.is_empty() {
  ...
}

if arr.is_empty() {
  ...
}

[Urcra]

Bit unsure about with category it should be in. I would probably lean mostly towards style. Since it's a bit hard to evaluate the performance after optimization. Though the performance hit is quite evident in debug mode.

Also not sure if this lint should deal with the case of arr != [] and suggest !arr.is_empty()?

I would like to try and hack on this lint if it seems like something that's useful