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, 4 insertions, 2 deletions

diff --git a/mesonbuild/compilers/mixins/intel.py b/mesonbuild/compilers/mixins/intel.py
index 2698b39..9877a54 100644
--- a/mesonbuild/compilers/mixins/intel.py
+++ b/mesonbuild/compilers/mixins/intel.py
@@ -58,7 +58,8 @@ class IntelGnuLikeCompiler(GnuLikeCompiler):
         'custom': [],
     }  # type: T.Dict[str, T.List[str]]
 
-    OPTIM_ARGS = {
+    OPTIM_ARGS: T.Dict[str, T.List[str]] = {
+        'plain': [],
         '0': ['-O0'],
         'g': ['-O0'],
         '1': ['-O1'],
@@ -136,7 +137,8 @@ class IntelVisualStudioLikeCompiler(VisualStudioLikeCompiler):
         'custom': [],
     }  # type: T.Dict[str, T.List[str]]
 
-    OPTIM_ARGS = {
+    OPTIM_ARGS: T.Dict[str, T.List[str]] = {
+        'plain': [],
         '0': ['/Od'],
         'g': ['/Od'],
         '1': ['/O1'],