murarth
Permit trailing commas in macro invocations
This commit uses the ? operator to support optional trailing commas in
macro calls without requiring the call to use the panic-message arms.
The ? macro operator was stabilized in 1.32 for the 2018 edition,
and 1.37 for the 2015 edition. Applying this commit requires raising
the crate MSRV to one of these two versions; for the lower, the crate
must be explicitly set to the edition = "2018" key in its manifest.
I wrote this patch because I noticed that my rustfmt settings were causing macro calls to break. Insertion of the trailing commas forced the macro expansion into the arms expecting a panic message.
As noted, it forces an MSRV. Please feel completely free to reject the PR if this is unacceptable.
I have to say, I'm not persuaded that there's a good reason to accept this proposed change and I'm inclined to reject it.
It seems to me that rustfmt, as a rule, should not change the tokens within a macro invocation. Indeed, it seems that the default settings of rustfmt do not change macro tokens. Further, the matches macro recently added to std does not accept a trailing comma (although that could change between now and the time of its stabilization).
My motivation for submitting this change was predicated solely on the fact that all the std macros started out without trailing comma support, and then added it in 1.26. I elected to use the ? operator instead of duplicate arms only for simplicity. If you're opposed to the requisite MSRV that imposes, I'm happy to rewrite to the std style; if you're opposed syntactically, I have absolutely no hard feelings about closing this.
I've stumbled upon this issue recently. I have to notice that the matches! std macro already accepts a trailing comma. Moreover, I do not see any reason not to accept an optional trailing comma here, I suppose that it should be the user's choice which code style to use.
