env.unset method

1 files changed, 17 insertions, 0 deletions

diff --git a/mesonbuild/utils/core.py b/mesonbuild/utils/core.py
index 1b005cf95..92f9d2c70 100644
--- a/mesonbuild/utils/core.py
+++ b/mesonbuild/utils/core.py
@@ -63,6 +63,7 @@ class EnvironmentVariables(HoldableObject):
         self.envvars: T.List[T.Tuple[T.Callable[[T.Dict[str, str], str, T.List[str], str, T.Optional[str]], str], str, T.List[str], str]] = []
         # The set of all env vars we have operations for. Only used for self.has_name()
         self.varnames: T.Set[str] = set()
+        self.unset_vars: T.Set[str] = set()
 
         if values:
             init_func = getattr(self, init_method)
@@ -92,16 +93,30 @@ class EnvironmentVariables(HoldableObject):
         for method, name, values, separator in other.envvars:
             self.varnames.add(name)
             self.envvars.append((method, name, values, separator))
+            if name in self.unset_vars:
+                self.unset_vars.remove(name)
+        self.unset_vars.update(other.unset_vars)
 
     def set(self, name: str, values: T.List[str], separator: str = os.pathsep) -> None:
+        if name in self.unset_vars:
+            raise MesonException(f'You cannot set the already unset variable {name!r}')
         self.varnames.add(name)
         self.envvars.append((self._set, name, values, separator))
 
+    def unset(self, name: str) -> None:
+        if name in self.varnames:
+            raise MesonException(f'You cannot unset the {name!r} variable because it is already set')
+        self.unset_vars.add(name)
+
     def append(self, name: str, values: T.List[str], separator: str = os.pathsep) -> None:
+        if name in self.unset_vars:
+            raise MesonException(f'You cannot append to unset variable {name!r}')
         self.varnames.add(name)
         self.envvars.append((self._append, name, values, separator))
 
     def prepend(self, name: str, values: T.List[str], separator: str = os.pathsep) -> None:
+        if name in self.unset_vars:
+            raise MesonException(f'You cannot prepend to unset variable {name!r}')
         self.varnames.add(name)
         self.envvars.append((self._prepend, name, values, separator))
 
@@ -124,6 +139,8 @@ class EnvironmentVariables(HoldableObject):
         for method, name, values, separator in self.envvars:
             default_value = default_fmt.format(name) if default_fmt else None
             env[name] = method(env, name, values, separator, default_value)
+        for name in self.unset_vars:
+            env.pop(name, None)
         return env