[Docs] Difference between buildtype=plain and buildtype=custom

[Neumann-A]

Can somebody explain what exactly the difference here is?

for plain the explanation is:

no extra build flags are used, even for compiler warnings, useful for distro packagers and other cases where you need to specify all arguments by yourself

but from my point of view i also sometimes want to add the flag -Ddebug=true which changes the buildtype to custom?
Are custom and plain basically the same buildtypes just with different names?

[Bassem-Ramzy]

I understand that:

1. plain is false debug and 0 optimization, while custom should be different in either or both of them.
2. You can select plain as a build type in flag -Dbuildtype=plain, but doing -Dbuildtype=custom has no meaning if you didn't set different flags values, and has no meaning to be passed if you set different flags values as it will be concluded by meson automatically.
3. Build type, if not defined using -Dbuildtype, will be concluded by meson based on other flags values , as per meson documentation:

For setting optimization levels and toggling debug, you can either set the buildtype option, or you can set the optimization and debug options which give finer control over the same. Whichever you decide to use, the other will be deduced from it. For example, -Dbuildtype=debugoptimized is the same as -Ddebug=true -Doptimization=2 and vice-versa.

4. As a conclusion, custom build type is set automatically by meson when you select combination of flags that don't comply with one of plain, debug, debugoptimized, release, minsize build types as per the "two-way mapping" table.

[mpontillo]

Similarly, I'm confused about the following:

buildtype	debug	optimization
plain	false	plain
debug	true	0

I was debugging a binary built with --buildmode=plain and gdb kept telling me that values had been <optimized out>. So I thought to turn off optimization, but the difference between plain and 0 isn't clear to me.