Make `PerMachine` and `MachineChoice` have just `build` and `host`

Meson itself *almost* only cares about the build and host platforms. The
exception is it takes a `target_machine` in the cross file and exposes
it to the user; but it doesn't do anything else with it. It's therefore
overkill to put target in `PerMachine` and `MachineChoice`. Instead, we
make a `PerThreeMachine` only for the machine infos.

Additionally fix a few other things that were bugging me in the process:

 - Get rid of `MachineInfos` class. Since `envconfig.py` was created, it
   has no methods that couldn't just got on `PerMachine`

 - Make `default_missing` and `miss_defaulting` work functionally. That
   means we can just locally bind rather than bind as class vars the
   "unfrozen" configuration. This helps prevent bugs where one forgets
   to freeze a configuration.

1 files changed, 4 insertions, 4 deletions

diff --git a/mesonbuild/interpreter.py b/mesonbuild/interpreter.py
index 1e8f972..0afbb10 100644
--- a/mesonbuild/interpreter.py
+++ b/mesonbuild/interpreter.py
@@ -2047,12 +2047,12 @@ class Interpreter(InterpreterBase):
         if not mock:
             self.parse_project()
 
-        # Initialize machine descriptions. We can do a better job now because we
+        # Re-initialize machine descriptions. We can do a better job now because we
         # have the compilers needed to gain more knowledge, so wipe out old
         # inferrence and start over.
-        self.build.environment.machines.miss_defaulting()
-        self.build.environment.detect_build_machine(self.coredata.compilers)
-        self.build.environment.machines.default_missing()
+        machines = self.build.environment.machines.miss_defaulting()
+        machines.build = environment.detect_machine_info(self.coredata.compilers)
+        self.build.environment.machines = machines.default_missing()
         assert self.build.environment.machines.build.cpu is not None
         assert self.build.environment.machines.host.cpu is not None
         assert self.build.environment.machines.target.cpu is not None