compilers: Add optimization=plain option

https://github.com/mesonbuild/meson/pull/9287 changed the `optimization=0`
to pass `-O0` to the compiler. This change is reasonable by itself
but unfortunately, it breaks `buildtype=plain`, which promises
that “no extra build flags are used”.

`buildtype=plain` is important for distros like NixOS,
which manage compiler flags for optimization and hardening
themselves.

Let’s introduce a new optimization level that does nothing
and set it as the default for `buildtype=plain`.

1 files changed, 1 insertions, 0 deletions

diff --git a/mesonbuild/compilers/swift.py b/mesonbuild/compilers/swift.py
index 2e427f1..6d434ae 100644
--- a/mesonbuild/compilers/swift.py
+++ b/mesonbuild/compilers/swift.py
@@ -27,6 +27,7 @@ if T.TYPE_CHECKING:
     from ..mesonlib import MachineChoice
 
 swift_optimization_args = {
+    'plain': [],
     '0': [],
     'g': [],
     '1': ['-O'],