GNU bug report logs - #75964
Switching the Emacs build to -Wswitch-enum in src/

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From: Paul Eggert <eggert <at> cs.ucla.edu>
To: Pip Cet <pipcet <at> protonmail.com>
Cc: Eli Zaretskii <eliz <at> gnu.org>, 75964 <at> debbugs.gnu.org, stefankangas <at> gmail.com
Subject: bug#75964: Switching the Emacs build to -Wswitch-enum in src/
Date: Sun, 2 Feb 2025 10:27:02 -0800

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On 2025-02-02 03:48, Pip Cet wrote:
> I've also stayed away from lib-src for now.

Although you did look at lib-src (I see some changes there) you 
understandably did not tackle lib-src/etags.c which has many enum 
switches and gimmicks to pacify GCC. So I took a crack at etags.c and 
came up with an alternative proposal consisting of two suggestions to 
support the following goals:

* Cleanly compile with -Wswitch-enum to catch potential errors.
* A style that is easy to explain and understand.
* No changes to GCC needed for now.
* No new C macros in the Emacs source code.
* Source code that is shorter and easier to maintain.

Here's the proposal:

Suggestion 1: Don't treat all enum switch statements the same. Some enum 
switch statements merely want to treat a couple of enum values 
specially, with all other enum values being default. For these, it's 
unlikely that forcing the programmer to list a case for every enum value 
will catch many bugs; it's even possible that this would cause more 
problems than it'll cure, and it's certainly an annoyance.

For these switch statements, use "switch (+E)" instead of "switch (E)". 
This pacifies GCC and clearly signals to the reader that the switch's 
cases are not intended to exhaust the enum. A "switch (E)" must list all 
the enum values; a "switch (+E)" need not do so. A reasonable guideline 
is that if a switch statement has more than three "case X: break;"s then 
it may be a good idea to use "switch (+E)" instead of "switch (E)".

Suggestion 2: Omit "default: break;"s present only to tell GCC and/or 
the reader that the switch is otherwise not exhaustive. "switch (+expr)" 
already does this more concisely and more usefully.


Combine these two suggestions, and I made etags.c shorter (by 23 lines) 
and easier to read once you know the style. See attached patch, which 
compiles cleanly with -Wswitch-enum.

I assume similar results would apply elsewhere in Emacs.

[0001-Pacify-gcc-Wswitch-enum-in-etags.c.patch (text/x-patch, attachment)]

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