Revert "Clarify mutable objects usage"

This reverts commit 9f02d0a3e5a5ffc82256391c244b1af38e41ef78.

It turns out that this does introduce a behavioral change in existing
users of ConfigurationData, which it wasn't supposed to (it was supposed
to preserve behavior there, and add a new *warning* for
EnvironmentVariables).

This breaks projects such as pulseaudio, libvirt, and probably more.
Roll back the change and try again after 1.5.0 is released.

Fixes: #13372

11 files changed, 36 insertions, 102 deletions

diff --git a/docs/markdown/Configuration.md b/docs/markdown/Configuration.md
index b5875e55e..48f071e6c 100644
--- a/docs/markdown/Configuration.md
+++ b/docs/markdown/Configuration.md
@@ -39,7 +39,7 @@ use a single `configuration_data` object as many times as you like,
 but it becomes immutable after being passed to the `configure_file`
 function. That is, after it has been used once to generate output the
 `set` function becomes unusable and trying to call it causes an error.
-*Since 1.5.0* Copy of immutable `configuration_data` is however mutable.
+Copy of immutable `configuration_data` is still immutable.
 
 For more complex configuration file generation Meson provides a second
 form. To use it, put a line like this in your configuration file.
diff --git a/docs/yaml/objects/cfg_data.yaml b/docs/yaml/objects/cfg_data.yaml
index 069cadbf6..03abb1709 100644
--- a/docs/yaml/objects/cfg_data.yaml
+++ b/docs/yaml/objects/cfg_data.yaml
@@ -1,14 +1,10 @@
 name: cfg_data
 long_name: Configuration data object
 description: |
-  This object encapsulates configuration values to be used for generating
-  configuration files. A more in-depth description can be found in the
-  [the configuration page](Configuration.md).
-
-  This object becomes immutable after first use. This means that
-  calling set() or merge_from() will cause an error if this object has
-  already been used in any function arguments. However, assignment creates a
-  mutable copy.
+  This object encapsulates
+  configuration values to be used for generating configuration files. A
+  more in-depth description can be found in the [the configuration wiki
+  page](Configuration.md).
 
 methods:
 - name: set
diff --git a/docs/yaml/objects/env.yaml b/docs/yaml/objects/env.yaml
index 3b2e2a851..714da4fe4 100644
--- a/docs/yaml/objects/env.yaml
+++ b/docs/yaml/objects/env.yaml
@@ -9,11 +9,6 @@ description: |
   on the same `varname`. Earlier Meson versions would warn and only the last
   operation took effect.
 
-  *Since 1.5.0* This object becomes immutable after first use. This means that
-  calling append(), prepend() or set() will cause a deprecation warning if this
-  object has already been used in any function arguments. However, assignment
-  creates a mutable copy.
-
 example: |
   ```meson
   env = environment()
@@ -23,14 +18,6 @@ example: |
   env.append('MY_PATH', '2')
   env.append('MY_PATH', '3')
   env.prepend('MY_PATH', '0')
-
-  # Deprecated since 1.5.0
-  run_command('script.py', env: env)
-  env.append('MY_PATH', '4')
-
-  # Allowed and only env2 is modified
-  env2 = env
-  env2.append('MY_PATH', '4')
   ```
 
 methods:
diff --git a/mesonbuild/interpreter/interpreterobjects.py b/mesonbuild/interpreter/interpreterobjects.py
index 32f05bafb..d5c1efaa8 100644
--- a/mesonbuild/interpreter/interpreterobjects.py
+++ b/mesonbuild/interpreter/interpreterobjects.py
@@ -284,7 +284,6 @@ class EnvironmentVariablesHolder(ObjectHolder[mesonlib.EnvironmentVariables], Mu
 
     def __init__(self, obj: mesonlib.EnvironmentVariables, interpreter: 'Interpreter'):
         super().__init__(obj, interpreter)
-        MutableInterpreterObject.__init__(self)
         self.methods.update({'set': self.set_method,
                              'unset': self.unset_method,
                              'append': self.append_method,
@@ -309,14 +308,12 @@ class EnvironmentVariablesHolder(ObjectHolder[mesonlib.EnvironmentVariables], Mu
     @typed_kwargs('environment.set', ENV_SEPARATOR_KW)
     def set_method(self, args: T.Tuple[str, T.List[str]], kwargs: 'EnvironmentSeparatorKW') -> None:
         name, values = args
-        self.check_used(self.interpreter, fatal=False)
         self.held_object.set(name, values, kwargs['separator'])
 
     @FeatureNew('environment.unset', '1.4.0')
     @typed_pos_args('environment.unset', str)
     @noKwargs
     def unset_method(self, args: T.Tuple[str], kwargs: TYPE_kwargs) -> None:
-        self.check_used(self.interpreter, fatal=False)
         self.held_object.unset(args[0])
 
     @typed_pos_args('environment.append', str, varargs=str, min_varargs=1)
@@ -324,7 +321,6 @@ class EnvironmentVariablesHolder(ObjectHolder[mesonlib.EnvironmentVariables], Mu
     def append_method(self, args: T.Tuple[str, T.List[str]], kwargs: 'EnvironmentSeparatorKW') -> None:
         name, values = args
         self.warn_if_has_name(name)
-        self.check_used(self.interpreter, fatal=False)
         self.held_object.append(name, values, kwargs['separator'])
 
     @typed_pos_args('environment.prepend', str, varargs=str, min_varargs=1)
@@ -332,7 +328,6 @@ class EnvironmentVariablesHolder(ObjectHolder[mesonlib.EnvironmentVariables], Mu
     def prepend_method(self, args: T.Tuple[str, T.List[str]], kwargs: 'EnvironmentSeparatorKW') -> None:
         name, values = args
         self.warn_if_has_name(name)
-        self.check_used(self.interpreter, fatal=False)
         self.held_object.prepend(name, values, kwargs['separator'])
 
 
@@ -343,7 +338,6 @@ class ConfigurationDataHolder(ObjectHolder[build.ConfigurationData], MutableInte
 
     def __init__(self, obj: build.ConfigurationData, interpreter: 'Interpreter'):
         super().__init__(obj, interpreter)
-        MutableInterpreterObject.__init__(self)
         self.methods.update({'set': self.set_method,
                              'set10': self.set10_method,
                              'set_quoted': self.set_quoted_method,
@@ -355,31 +349,32 @@ class ConfigurationDataHolder(ObjectHolder[build.ConfigurationData], MutableInte
                              })
 
     def __deepcopy__(self, memo: T.Dict) -> 'ConfigurationDataHolder':
-        obj = ConfigurationDataHolder(copy.deepcopy(self.held_object), self.interpreter)
+        return ConfigurationDataHolder(copy.deepcopy(self.held_object), self.interpreter)
+
+    def is_used(self) -> bool:
+        return self.held_object.used
+
+    def __check_used(self) -> None:
         if self.is_used():
-            # Copies of used ConfigurationData used to be immutable. It is now
-            # allowed but we need this flag to print a FeatureNew warning if
-            # that happens.
-            obj.mutable_feature_new = True
-        return obj
+            raise InterpreterException("Can not set values on configuration object that has been used.")
 
     @typed_pos_args('configuration_data.set', str, (str, int, bool))
     @typed_kwargs('configuration_data.set', _CONF_DATA_SET_KWS)
     def set_method(self, args: T.Tuple[str, T.Union[str, int, bool]], kwargs: 'kwargs.ConfigurationDataSet') -> None:
-        self.check_used(self.interpreter)
+        self.__check_used()
         self.held_object.values[args[0]] = (args[1], kwargs['description'])
 
     @typed_pos_args('configuration_data.set_quoted', str, str)
     @typed_kwargs('configuration_data.set_quoted', _CONF_DATA_SET_KWS)
     def set_quoted_method(self, args: T.Tuple[str, str], kwargs: 'kwargs.ConfigurationDataSet') -> None:
-        self.check_used(self.interpreter)
+        self.__check_used()
         escaped_val = '\\"'.join(args[1].split('"'))
         self.held_object.values[args[0]] = (f'"{escaped_val}"', kwargs['description'])
 
     @typed_pos_args('configuration_data.set10', str, (int, bool))
     @typed_kwargs('configuration_data.set10', _CONF_DATA_SET_KWS)
     def set10_method(self, args: T.Tuple[str, T.Union[int, bool]], kwargs: 'kwargs.ConfigurationDataSet') -> None:
-        self.check_used(self.interpreter)
+        self.__check_used()
         # bool is a subclass of int, so we need to check for bool explicitly.
         # We already have typed_pos_args checking that this is either a bool or
         # an int.
@@ -442,7 +437,6 @@ class ConfigurationDataHolder(ObjectHolder[build.ConfigurationData], MutableInte
     @noKwargs
     def merge_from_method(self, args: T.Tuple[build.ConfigurationData], kwargs: TYPE_kwargs) -> None:
         from_object = args[0]
-        self.check_used(self.interpreter)
         self.held_object.values.update(from_object.values)
 
 
diff --git a/mesonbuild/interpreterbase/_unholder.py b/mesonbuild/interpreterbase/_unholder.py
index e32b77fc7..c62aafe8e 100644
--- a/mesonbuild/interpreterbase/_unholder.py
+++ b/mesonbuild/interpreterbase/_unholder.py
@@ -5,7 +5,7 @@ from __future__ import annotations
 
 import typing as T
 
-from .baseobjects import InterpreterObject, MesonInterpreterObject, ObjectHolder, HoldableTypes, MutableInterpreterObject
+from .baseobjects import InterpreterObject, MesonInterpreterObject, ObjectHolder, HoldableTypes
 from .exceptions import InvalidArguments
 from ..mesonlib import HoldableObject, MesonBugException
 
@@ -13,8 +13,6 @@ if T.TYPE_CHECKING:
     from .baseobjects import TYPE_var
 
 def _unholder(obj: InterpreterObject) -> TYPE_var:
-    if isinstance(obj, MutableInterpreterObject):
-        obj.mark_used()
     if isinstance(obj, ObjectHolder):
         assert isinstance(obj.held_object, HoldableTypes)
         return obj.held_object
diff --git a/mesonbuild/interpreterbase/baseobjects.py b/mesonbuild/interpreterbase/baseobjects.py
index c6864a478..9a119a98a 100644
--- a/mesonbuild/interpreterbase/baseobjects.py
+++ b/mesonbuild/interpreterbase/baseobjects.py
@@ -120,27 +120,6 @@ class MesonInterpreterObject(InterpreterObject):
 
 class MutableInterpreterObject:
     ''' Dummy class to mark the object type as mutable '''
-    def __init__(self) -> None:
-        self.used = False
-        self.mutable_feature_new = False
-
-    def mark_used(self) -> None:
-        self.used = True
-
-    def is_used(self) -> bool:
-        return self.used
-
-    def check_used(self, interpreter: Interpreter, fatal: bool = True) -> None:
-        from .decorators import FeatureDeprecated, FeatureNew
-        if self.is_used():
-            if fatal:
-                raise InvalidArguments('Can not modify object after it has been used.')
-            FeatureDeprecated.single_use('Modify object after it has been used', '1.5.0',
-                                         interpreter.subproject, location=interpreter.current_node)
-        elif self.mutable_feature_new:
-            FeatureNew.single_use('Modify a copy of an immutable object', '1.5.0',
-                                  interpreter.subproject, location=interpreter.current_node)
-            self.mutable_feature_new = False
 
 HoldableTypes = (HoldableObject, int, bool, str, list, dict)
 TYPE_HoldableTypes = T.Union[TYPE_elementary, HoldableObject]
diff --git a/test cases/common/113 interpreter copy mutable var on assignment/check_env.py b/test cases/common/113 interpreter copy mutable var on assignment/check_env.py
deleted file mode 100755
index 034d29243..000000000
--- a/test cases/common/113 interpreter copy mutable var on assignment/check_env.py
+++ /dev/null
@@ -1,7 +0,0 @@
-#!/usr/bin/env python3
-
-import os
-import sys
-
-if sys.argv[1] not in os.environ:
-    exit(42)
diff --git a/test cases/common/113 interpreter copy mutable var on assignment/meson.build b/test cases/common/113 interpreter copy mutable var on assignment/meson.build
index 3d4f3b039..d414bfc74 100644
--- a/test cases/common/113 interpreter copy mutable var on assignment/meson.build
+++ b/test cases/common/113 interpreter copy mutable var on assignment/meson.build
@@ -1,4 +1,4 @@
-project('foo', meson_version: '>=1.5')
+project('foo')
 
 a = configuration_data()
 a.set('HELLO', 1)
@@ -10,15 +10,6 @@ assert(b.has('HELLO'), 'Original config data should be set on copy')
 
 configure_file(output : 'b.h', configuration : b)
 
-testcase expect_error('Can not modify object after it has been used.')
-  b.set('WORLD', 1)
-endtestcase
-
-# A copy of immutable object is mutable. This should print FeatureNew warning
-# if meson_version is lower than 1.5.
-c = b
-c.set('WORLD', 1)
-
 # This should still work, as we didn't use the original above but a copy!
 a.set('WORLD', 1)
 
@@ -26,26 +17,3 @@ assert(a.has('WORLD'), 'New config data should have been set')
 assert(not b.has('WORLD'), 'New config data set should not affect var copied earlier')
 
 configure_file(output : 'a.h', configuration : a)
-
-env1 = environment()
-env1.set('FOO', '1')
-env2 = env1
-env1.set('BAR', '1')
-
-# FOO should be in env1 and env2
-run_command('check_env.py', 'FOO', env: env1, check: true)
-run_command('check_env.py', 'FOO', env: env2, check: true)
-
-# BAR should be only in env1
-run_command('check_env.py', 'BAR', env: env1, check: true)
-assert(run_command('check_env.py', 'BAR', env: env2, check: false).returncode() == 42)
-
-# This should print deprecation warning but still work
-env1.set('PLOP', '1')
-run_command('check_env.py', 'PLOP', env: env1, check: true)
-
-# A copy of used env should be mutable and not print warning
-env3 = env1
-env3.set('BAZ', '1')
-run_command('check_env.py', 'PLOP', env: env3, check: true)
-run_command('check_env.py', 'BAZ', env: env3, check: true)
diff --git a/test cases/failing/70 configuration immutable/input b/test cases/failing/70 configuration immutable/input
new file mode 100644
index 000000000..e69de29bb
--- /dev/null
+++ b/test cases/failing/70 configuration immutable/input
diff --git a/test cases/failing/70 configuration immutable/meson.build b/test cases/failing/70 configuration immutable/meson.build
new file mode 100644
index 000000000..b6cac4126
--- /dev/null
+++ b/test cases/failing/70 configuration immutable/meson.build
@@ -0,0 +1,12 @@
+project('configuration_data is immutable')
+
+a = configuration_data()
+
+configure_file(
+    configuration : a,
+    input : 'input',
+    output : 'output',
+)
+
+still_immutable = a
+still_immutable.set('hello', 'world')
diff --git a/test cases/failing/70 configuration immutable/test.json b/test cases/failing/70 configuration immutable/test.json
new file mode 100644
index 000000000..fc735fa38
--- /dev/null
+++ b/test cases/failing/70 configuration immutable/test.json
@@ -0,0 +1,7 @@
+{
+  "stdout": [
+    {
+      "line": "test cases/failing/70 configuration immutable/meson.build:12:16: ERROR: Can not set values on configuration object that has been used."
+    }
+  ]
+}