Added a hint for wrong escaping of curly braces in format strings.

[contradictioned]

As i proposed in #24262 (and found acceptance in the IRC channel), added a compiler warning for wrong escaping of curly braces.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. The way Github handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see CONTRIBUTING.md for more information.

[alexcrichton]

Unfortunately this may not be the best place for this error. For example I think that this will trigger the error message as well:

let foo = "\{";

[alexcrichton]

Er sorry, posted too soon! Perhaps this could issue a span_help after the normal span_err is emitted? It may also want to mention something along the lines of "if you're in a formatting string, use ..."

[contradictioned]

@alexcrichton thanks, I will look into this tomorrow!

[contradictioned]

When you forget to put a self before an attribute attr in a method, the compiler gives the following warning:

unresolved name `attr`. Did you mean `self.attr`?

So the information is all in one error. I wanted to emulate this behaviour. I have followed your suggestion and added a span_help, such that the compiler output looks like:

msg_test.rs:12:21: 12:22 error: unknown character escape
msg_test.rs:12   let foo = "hello \{";
                                   ^
msg_test.rs:12:21: 12:22 help: If used in a formatting string, curly braces are escaped with '{{' and '}}'.
msg_test.rs:12   let foo = "hello \{";
                                   ^

(PS: this also confirms that this can also happen in a non-format string :))

[rprichard]

Oh sorry, I didn't read your comment very closely.

[contradictioned]

I did the changes suggested by @rprichard, thanks for those!

[alexcrichton]

Thanks! Could you squash the commits together and add a test as well?

[contradictioned]

Sorry for the confusion-if any-but I somewhat destroyed my git history whilst trying to squash, and then Github automatically closed the pr. Sorry for your lost comments @rprichard.

I have added the test, and put it together with the changes into one commit.

[alexcrichton]

@bors: r+ 4abade5

Thanks!

[bors]

⌛ Testing commit 4abade5 with merge a4eb5a6...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-32-nopt-t, auto-win-32-opt, auto-win-64-nopt-t, auto-win-64-opt