Enable lint clippy::string_slice

[EdJoPaTo]

Problem

Slicing of strings can panic on multi width characters. See for example #1032

Solution

Enable the following lint to find all cases where this might happen (now and in the future).

https://rust-lang.github.io/rust-clippy/master/index.html#string_slice

This lint does not catch #1032 for example. But it catches other places. It is probably a good idea to suggest false negatives like #1032 to clippy to be integrated in this lint.

$ cargo clippy -- -W clippy::string_slice
warning: indexing into a string may panic if the index is within a UTF-8 character
   --> src/style/color.rs:237:49
    |
237 | ...                   u8::from_str_radix(&s[1..3], 16),
    |                                           ^^^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#string_slice
    = note: requested on the command line with `-W clippy::string-slice`

warning: indexing into a string may panic if the index is within a UTF-8 character
   --> src/style/color.rs:238:49
    |
238 | ...                   u8::from_str_radix(&s[3..5], 16),
    |                                           ^^^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#string_slice

warning: indexing into a string may panic if the index is within a UTF-8 character
   --> src/style/color.rs:239:49
    |
239 | ...                   u8::from_str_radix(&s[5..7], 16),
    |                                           ^^^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#string_slice

warning: indexing into a string may panic if the index is within a UTF-8 character
   --> src/widgets/reflow.rs:338:6
    |
338 |     &src[start..]
    |      ^^^^^^^^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#string_slice

warning: `ratatui` (lib) generated 4 warnings
    Finished dev [unoptimized + debuginfo] target(s) in 6.70s

[joshka]

Sounds like a good plan. Just to clarify, this entails actually fixing those places identified by the lint, right?

[EdJoPaTo]

Enabling the lint and fixing the places yeah. But also finding cases where this lint is not noticing this bad behavior to report this and improve the lint (for us and everyone else).

Created rust-lang/rust-clippy#12708 for the missing hint on Cow, which ratatui uses heavily. Not sure whether there are other false negatives of the lint.