Raw string assists

[matklad]

Add the following assists:

• make raw string: "hello" -> r#"hello"#
• add hash: r#"hello"# -> r##"hello"##
• remove hash: r#"hello"# -> r"hello"
• make usual string: r"hello" -> "hello"

Note that, ideally, we should apply escape/unescape for the first and last assist, but it's ok to not do it for the first pass.

Example assits are here: https://github.com/rust-analyzer/rust-analyzer/blob/e055cfacdfe3b3451484dae5d6ed08aefba133ca/crates/ra_assists/src/lib.rs

Each familiy lives in a separate mode, which contains the tests as well. See #1281 for an example of PR which adds an assist.

[bjorn3]

-make usual string: r"hello" -> r"hello"
+make usual string: r"hello" -> "hello"

[bjorn3]

Note that, ideally, we should apply escape/unescape for the first and last assist, but it's ok to not do it for the first pass.

Maybe add two variants of the first assist. One which doesn't unescape and one which does. You may have the invalid "C:\Users" (\U is not a valid escape) and want to turn it into r"C:\Users", or you may want to make a valid normal string into a raw string to prevent having to escape stuff.

[matklad]

The unescaping one is better as a quick fix for lexer error, and not as a separe assits, but yeah, this sounds right (but out of scope for this particular issue).

[Geobert]

I'm going to try this one :)

[Geobert]

What should be the result of applying make_raw_string to "aaa\nbbb"?
Same question with make_usual_string and different cases.

[matklad]

What should be the result of applying make_raw_string to "aaa\nbbb"?

Just r#"aaa\nbbb"# for the first pass. Ideally, it shoould be

r#"aaa
bbb"#

You can use rustc_lexer::unescape module to deal with escape sequqneces properly.

[Geobert]

Quick notice: I'm still on this, holidays are gone so less time, but I'll complete that PR :)