PSA: Gecko now uses C++20

[Emilio Cobos Álvarez]

Hi,

As of bug 176811, we now use C++20 for all our C++ code.

That means you should be able to do things like initializing your bitfields inline now, along with a lot of other language features :)

There are probably also some follow-up opportunities:

• I think some of the `FMT_STRING` clunkiness could be cleaned up.
• MOZ_LIKELY / MOZ_UNLIKELY could be moved to the standard `[[likely]]` and `[[unlikely]]` macros.
• Tons of constexpr stuff could be improved.
• Maybe std::span could be used instead of mozilla::Span.

And probably tons of others I'm missing.

Anyways, this took a surprising amount of work, see the bug for details. Chris Peterson did most of it (thanks!), I just pushed it over the finish line. Thanks to everyone else who helped too (and to Mike for all the reviews as usual :P).

Cheers,

 -- Emilio

[Emilio Cobos Álvarez]

On Thu, Nov 13, 2025 at 3:46 AM Emilio Cobos Álvarez <emi...@mozilla.com> wrote:

Hi,

As of bug 176811, we now use C++20 for all our C++ code.

Err, of course that should've said bug 1768116, sorry about that.

[Andrew McCreight]

On Wed, Nov 12, 2025 at 10:47 AM 'Emilio Cobos Álvarez' via dev-pl...@mozilla.org <dev-pl...@mozilla.org> wrote:

Hi,

As of bug 176811, we now use C++20 for all our C++ code.

That means you should be able to do things like initializing your bitfields inline now, along with a lot of other language features :)

There are probably also some follow-up opportunities:

• I think some of the `FMT_STRING` clunkiness could be cleaned up.
• MOZ_LIKELY / MOZ_UNLIKELY could be moved to the standard `[[likely]]` and `[[unlikely]]` macros.
• Tons of constexpr stuff could be improved.
• Maybe std::span could be used instead of mozilla::Span.

Keep in mind that std::span currently does not do bounds checking while mozilla:Span does, so you are potentially introducing security problems if you make this substitution. (If you'd like to help change that, please take a look at some of the bugs blocking bug 1986488!)

Andrew

 

And probably tons of others I'm missing.

Anyways, this took a surprising amount of work, see the bug for details. Chris Peterson did most of it (thanks!), I just pushed it over the finish line. Thanks to everyone else who helped too (and to Mike for all the reviews as usual :P).

Cheers,

 -- Emilio

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