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.

8 files changed, 141 insertions, 80 deletions

diff --git a/mesonbuild/coredata.py b/mesonbuild/coredata.py
index 34adad3..b8f0578 100644
--- a/mesonbuild/coredata.py
+++ b/mesonbuild/coredata.py
@@ -243,7 +243,7 @@ class CoreData:
         self.init_builtins()
         self.backend_options = {}
         self.user_options = {}
-        self.compiler_options = PerMachine({}, {}, {})
+        self.compiler_options = PerMachine({}, {})
         self.base_options = {}
         self.cross_files = self.__load_config_files(options.cross_file, 'cross')
         self.compilers = OrderedDict()
diff --git a/mesonbuild/dependencies/base.py b/mesonbuild/dependencies/base.py
index 8f38d6c..034f6df 100644
--- a/mesonbuild/dependencies/base.py
+++ b/mesonbuild/dependencies/base.py
@@ -522,7 +522,7 @@ class ConfigToolDependency(ExternalDependency):
 class PkgConfigDependency(ExternalDependency):
     # The class's copy of the pkg-config path. Avoids having to search for it
     # multiple times in the same Meson invocation.
-    class_pkgbin = PerMachine(None, None, None)
+    class_pkgbin = PerMachine(None, None)
     # We cache all pkg-config subprocess invocations to avoid redundant calls
     pkgbin_cache = {}
 
@@ -957,9 +957,9 @@ class CMakeTarget:
 class CMakeDependency(ExternalDependency):
     # The class's copy of the CMake path. Avoids having to search for it
     # multiple times in the same Meson invocation.
-    class_cmakebin = PerMachine(None, None, None)
-    class_cmakevers = PerMachine(None, None, None)
-    class_cmakeinfo = PerMachine(None, None, None)
+    class_cmakebin = PerMachine(None, None)
+    class_cmakevers = PerMachine(None, None)
+    class_cmakeinfo = PerMachine(None, None)
     # We cache all pkg-config subprocess invocations to avoid redundant calls
     cmake_cache = {}
     # Version string for the minimum CMake version
diff --git a/mesonbuild/envconfig.py b/mesonbuild/envconfig.py
index 70f964e..0cdb4c4 100644
--- a/mesonbuild/envconfig.py
+++ b/mesonbuild/envconfig.py
@@ -16,7 +16,7 @@ import configparser, os, shlex, subprocess
 import typing
 
 from . import mesonlib
-from .mesonlib import EnvironmentException, MachineChoice, PerMachine
+from .mesonlib import EnvironmentException
 from . import mlog
 
 _T = typing.TypeVar('_T')
@@ -255,40 +255,6 @@ class MachineInfo:
     def libdir_layout_is_win(self) -> bool:
         return self.is_windows() or self.is_cygwin()
 
-class PerMachineDefaultable(PerMachine[typing.Optional[_T]]):
-    """Extends `PerMachine` with the ability to default from `None`s.
-    """
-    def __init__(self) -> None:
-        super().__init__(None, None, None)
-
-    def default_missing(self) -> None:
-        """Default host to buid and target to host.
-
-        This allows just specifying nothing in the native case, just host in the
-        cross non-compiler case, and just target in the native-built
-        cross-compiler case.
-        """
-        if self.host is None:
-            self.host = self.build
-        if self.target is None:
-            self.target = self.host
-
-    def miss_defaulting(self) -> None:
-        """Unset definition duplicated from their previous to None
-
-        This is the inverse of ''default_missing''. By removing defaulted
-        machines, we can elaborate the original and then redefault them and thus
-        avoid repeating the elaboration explicitly.
-        """
-        if self.target == self.host:
-            self.target = None
-        if self.host == self.build:
-            self.host = None
-
-class MachineInfos(PerMachineDefaultable[MachineInfo]):
-    def matches_build_machine(self, machine: MachineChoice) -> bool:
-        return self.build == self[machine]
-
 class BinaryTable(HasEnvVarFallback):
     def __init__(
             self,
diff --git a/mesonbuild/environment.py b/mesonbuild/environment.py
index 9620639..f86b613 100644
--- a/mesonbuild/environment.py
+++ b/mesonbuild/environment.py
@@ -19,12 +19,13 @@ from .linkers import ArLinker, ArmarLinker, VisualStudioLinker, DLinker, CcrxLin
 from . import mesonlib
 from .mesonlib import (
     MesonException, EnvironmentException, MachineChoice, Popen_safe,
+    PerMachineDefaultable, PerThreeMachineDefaultable
 )
 from . import mlog
 
 from .envconfig import (
-    BinaryTable, Directories, MachineInfo, MachineInfos, MesonConfigFile,
-    PerMachineDefaultable, Properties, known_cpu_families,
+    BinaryTable, Directories, MachineInfo, MesonConfigFile,
+    Properties, known_cpu_families,
 )
 from . import compilers
 from .compilers import (
@@ -403,48 +404,62 @@ class Environment:
             # Just create a fresh coredata in this case
             self.create_new_coredata(options)
 
-        self.machines = MachineInfos()
-        # Will be fully initialized later using compilers later.
-        self.detect_build_machine()
+        ## locally bind some unfrozen configuration
+
+        # Stores machine infos, the only *three* machine one because we have a
+        # target machine info on for the user (Meson never cares about the
+        # target machine.)
+        machines = PerThreeMachineDefaultable()
 
         # Similar to coredata.compilers and build.compilers, but lower level in
         # that there is no meta data, only names/paths.
-        self.binaries = PerMachineDefaultable()
+        binaries = PerMachineDefaultable()
 
         # Misc other properties about each machine.
-        self.properties = PerMachineDefaultable()
-
-        # Just uses hard-coded defaults and environment variables. Might be
-        # overwritten by a native file.
-        self.binaries.build = BinaryTable()
-        self.properties.build = Properties()
+        properties = PerMachineDefaultable()
 
         # Store paths for native and cross build files. There is no target
         # machine information here because nothing is installed for the target
         # architecture, just the build and host architectures
-        self.paths = PerMachineDefaultable()
+        paths = PerMachineDefaultable()
+
+        ## Setup build machine defaults
+
+        # Will be fully initialized later using compilers later.
+        machines.build = detect_machine_info()
+
+        # Just uses hard-coded defaults and environment variables. Might be
+        # overwritten by a native file.
+        binaries.build = BinaryTable()
+        properties.build = Properties()
+
+        ## Read in native file(s) to override build machine configuration
 
         if self.coredata.config_files is not None:
             config = MesonConfigFile.from_config_parser(
                 coredata.load_configs(self.coredata.config_files))
-            self.binaries.build = BinaryTable(config.get('binaries', {}))
-            self.paths.build = Directories(**config.get('paths', {}))
+            binaries.build = BinaryTable(config.get('binaries', {}))
+            paths.build = Directories(**config.get('paths', {}))
+
+        ## Read in cross file(s) to override host machine configuration
 
         if self.coredata.cross_files:
             config = MesonConfigFile.from_config_parser(
                 coredata.load_configs(self.coredata.cross_files))
-            self.properties.host = Properties(config.get('properties', {}), False)
-            self.binaries.host = BinaryTable(config.get('binaries', {}), False)
+            properties.host = Properties(config.get('properties', {}), False)
+            binaries.host = BinaryTable(config.get('binaries', {}), False)
             if 'host_machine' in config:
-                self.machines.host = MachineInfo.from_literal(config['host_machine'])
+                machines.host = MachineInfo.from_literal(config['host_machine'])
             if 'target_machine' in config:
-                self.machines.target = MachineInfo.from_literal(config['target_machine'])
-            self.paths.host = Directories(**config.get('paths', {}))
+                machines.target = MachineInfo.from_literal(config['target_machine'])
+            paths.host = Directories(**config.get('paths', {}))
+
+        ## "freeze" now initialized configuration, and "save" to the class.
 
-        self.machines.default_missing()
-        self.binaries.default_missing()
-        self.properties.default_missing()
-        self.paths.default_missing()
+        self.machines = machines.default_missing()
+        self.binaries = binaries.default_missing()
+        self.properties = properties.default_missing()
+        self.paths = paths.default_missing()
 
         exe_wrapper = self.binaries.host.lookup_entry('exe_wrapper')
         if exe_wrapper is not None:
@@ -1233,9 +1248,6 @@ class Environment:
         self._handle_exceptions(popen_exceptions, linkers, 'linker')
         raise EnvironmentException('Unknown static linker "%s"' % ' '.join(linkers))
 
-    def detect_build_machine(self, compilers = None):
-        self.machines.build = detect_machine_info(compilers)
-
     def get_source_dir(self):
         return self.source_dir
 
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
diff --git a/mesonbuild/mesonlib.py b/mesonbuild/mesonlib.py
index 07beb69..f53197b 100644
--- a/mesonbuild/mesonlib.py
+++ b/mesonbuild/mesonlib.py
@@ -313,37 +313,120 @@ class OrderedEnum(Enum):
             return self.value < other.value
         return NotImplemented
 
+
 class MachineChoice(OrderedEnum):
 
-    """Enum class representing one of the three possible values for binaries,
-    the build, host, and target machines.
+    """Enum class representing one of the two abstract machine names used in
+    most places: the build, and host, machines.
     """
 
     BUILD = 0
     HOST = 1
-    TARGET = 2
+
 
 class PerMachine(typing.Generic[_T]):
-    def __init__(self, build: _T, host: _T, target: _T):
+    def __init__(self, build: _T, host: _T):
         self.build = build
         self.host = host
-        self.target = target
 
     def __getitem__(self, machine: MachineChoice) -> _T:
         return {
             MachineChoice.BUILD:  self.build,
             MachineChoice.HOST:   self.host,
-            MachineChoice.TARGET: self.target
         }[machine]
 
     def __setitem__(self, machine: MachineChoice, val: _T) -> None:
         key = {
             MachineChoice.BUILD:  'build',
             MachineChoice.HOST:   'host',
-            MachineChoice.TARGET: 'target'
         }[machine]
         setattr(self, key, val)
 
+    def miss_defaulting(self) -> "PerMachineDefaultable[typing.Optional[_T]]":
+        """Unset definition duplicated from their previous to None
+
+        This is the inverse of ''default_missing''. By removing defaulted
+        machines, we can elaborate the original and then redefault them and thus
+        avoid repeating the elaboration explicitly.
+        """
+        unfreeze = PerMachineDefaultable() # type: PerMachineDefaultable[typing.Optional[_T]]
+        unfreeze.build = self.build
+        unfreeze.host = self.host
+        if unfreeze.host == unfreeze.build:
+            unfreeze.host = None
+        return unfreeze
+
+
+class PerThreeMachine(PerMachine[_T]):
+    """Like `PerMachine` but includes `target` too.
+
+    It turns out just one thing do we need track the target machine. There's no
+    need to computer the `target` field so we don't bother overriding the
+    `__getitem__`/`__setitem__` methods.
+    """
+    def __init__(self, build: _T, host: _T, target: _T):
+        super().__init__(build, host)
+        self.target = target
+
+    def miss_defaulting(self) -> "PerThreeMachineDefaultable[typing.Optional[_T]]":
+        """Unset definition duplicated from their previous to None
+
+        This is the inverse of ''default_missing''. By removing defaulted
+        machines, we can elaborate the original and then redefault them and thus
+        avoid repeating the elaboration explicitly.
+        """
+        unfreeze = PerThreeMachineDefaultable() # type: PerThreeMachineDefaultable[typing.Optional[_T]]
+        unfreeze.build = self.build
+        unfreeze.host = self.host
+        unfreeze.target = self.target
+        if unfreeze.target == unfreeze.host:
+            unfreeze.target = None
+        if unfreeze.host == unfreeze.build:
+            unfreeze.host = None
+        return unfreeze
+
+    def matches_build_machine(self, machine: MachineChoice) -> bool:
+        return self.build == self[machine]
+
+class PerMachineDefaultable(PerMachine[typing.Optional[_T]]):
+    """Extends `PerMachine` with the ability to default from `None`s.
+    """
+    def __init__(self) -> None:
+        super().__init__(None, None)
+
+    def default_missing(self) -> "PerMachine[typing.Optional[_T]]":
+        """Default host to buid
+
+        This allows just specifying nothing in the native case, and just host in the
+        cross non-compiler case.
+        """
+        freeze = PerMachine(self.build, self.host)
+        if freeze.host is None:
+            freeze.host = freeze.build
+        return freeze
+
+
+class PerThreeMachineDefaultable(PerMachineDefaultable, PerThreeMachine[typing.Optional[_T]]):
+    """Extends `PerThreeMachine` with the ability to default from `None`s.
+    """
+    def __init__(self) -> None:
+        PerThreeMachine.__init__(self, None, None, None)
+
+    def default_missing(self) -> "PerThreeMachine[typing.Optional[_T]]":
+        """Default host to buid and target to host.
+
+        This allows just specifying nothing in the native case, just host in the
+        cross non-compiler case, and just target in the native-built
+        cross-compiler case.
+        """
+        freeze = PerThreeMachine(self.build, self.host, self.target)
+        if freeze.host is None:
+            freeze.host = freeze.build
+        if freeze.target is None:
+            freeze.target = freeze.host
+        return freeze
+
+
 def is_sunos() -> bool:
     return platform.system().lower() == 'sunos'
 
diff --git a/run_tests.py b/run_tests.py
index a4b0fa2..f427736 100755
--- a/run_tests.py
+++ b/run_tests.py
@@ -220,7 +220,7 @@ def clear_meson_configure_class_caches():
     mesonbuild.compilers.CCompiler.find_library_cache = {}
     mesonbuild.compilers.CCompiler.find_framework_cache = {}
     mesonbuild.dependencies.PkgConfigDependency.pkgbin_cache = {}
-    mesonbuild.dependencies.PkgConfigDependency.class_pkgbin = mesonlib.PerMachine(None, None, None)
+    mesonbuild.dependencies.PkgConfigDependency.class_pkgbin = mesonlib.PerMachine(None, None)
 
 def run_configure_inprocess(commandlist):
     old_stdout = sys.stdout
diff --git a/run_unittests.py b/run_unittests.py
index f4d969c..7242a5f 100755
--- a/run_unittests.py
+++ b/run_unittests.py
@@ -848,7 +848,7 @@ class InternalTests(unittest.TestCase):
                 PkgConfigDependency.check_pkgconfig = old_check
                 # Reset dependency class to ensure that in-process configure doesn't mess up
                 PkgConfigDependency.pkgbin_cache = {}
-                PkgConfigDependency.class_pkgbin = PerMachine(None, None, None)
+                PkgConfigDependency.class_pkgbin = PerMachine(None, None)
 
     def test_version_compare(self):
         comparefunc = mesonbuild.mesonlib.version_compare_many