Fix missing note of escaping { for braces including whitespaces

[Kokoro2336]

Fixes #156868

Add note of escaping { for braces including whitespaces

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[mejrs]

r? me

[mejrs]

white_space_only_arg_spaces is a vec but you never access any of its contents, you only ever check whether it is empty or not. So this feels like it is the wrong approach. You also risk emitting it for {} if the format string is something like { } {}.

We track the spans of all arguments already, so you shouldn't need to make any changes to rustc_parse_format. The argument spans are tracked in the position_span fields/variables.

In short, I suggest the following approach:

• Drop all changes to rustc_parse_format
• in format.rs, if an implicit argument is missing check position_span and if it is non-empty, suggest escaping the braces.

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[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[Kokoro2336]

@mejrs Fixed @rustbot ready