interpreter: add a feature.enable_auto_if

It's always been strange to me we don't have an opposite method of the
`disable_auto_if` method, but I've been pressed to find a case where we
_need_ one, because `disable_auto_if` can't be logically contorted to
work. I finally found the case where they're not equivalent: when you
don't want to convert to a boolean:

```meson
f = get_option('feat').disable_auto_if(not foo)
g = get_option('feat').enable_auto_if(foo)

dep1 = dependency('foo', required : f)
dep2 = dependency('foo', required : g)
```

4 files changed, 39 insertions, 1 deletions

diff --git a/docs/markdown/snippets/feature_enable_auto_if.md b/docs/markdown/snippets/feature_enable_auto_if.md
new file mode 100644
index 0000000..5461004
--- /dev/null
+++ b/docs/markdown/snippets/feature_enable_auto_if.md
@@ -0,0 +1,4 @@
+## Feature objects now have an enable_auto_if method
+
+This performs the opposite task of the disable_auto_if method, enabling the
+feature if the condition is true.
diff --git a/docs/yaml/objects/feature.yaml b/docs/yaml/objects/feature.yaml
index b6a754b..01209eb 100644
--- a/docs/yaml/objects/feature.yaml
+++ b/docs/yaml/objects/feature.yaml
@@ -38,6 +38,23 @@ methods:
       type: bool
       description: See the table above
 
+- name: enable_auto_if
+  since: 1.1.0
+  returns: feature
+  description: |
+    Returns the feature, with `'auto'` converted to `'enabled'` if value is true.
+
+    | Feature / Condition | `value = true` | `value = false` |
+    | ------------------- | -------------- | --------------- |
+    | Enabled             | Enabled        | Enabled         |
+    | Disabled            | Disabled       | Disabled        |
+    | Auto                | Enabled        | Auto            |
+
+  posargs:
+    value:
+      type: bool
+      description: See the table above
+
 - name: require
   returns: feature
   since: 0.59.0
diff --git a/mesonbuild/interpreter/interpreterobjects.py b/mesonbuild/interpreter/interpreterobjects.py
index 538d134..a750da9 100644
--- a/mesonbuild/interpreter/interpreterobjects.py
+++ b/mesonbuild/interpreter/interpreterobjects.py
@@ -41,7 +41,6 @@ if T.TYPE_CHECKING:
 
         separator: str
 
-
 def extract_required_kwarg(kwargs: 'kwargs.ExtractRequired',
                            subproject: 'SubProject',
                            feature_check: T.Optional[FeatureCheckBase] = None,
@@ -97,6 +96,7 @@ class FeatureOptionHolder(ObjectHolder[coredata.UserFeatureOption]):
                              'auto': self.auto_method,
                              'require': self.require_method,
                              'disable_auto_if': self.disable_auto_if_method,
+                             'enable_auto_if': self.enable_auto_if_method,
                              })
 
     @property
@@ -108,6 +108,11 @@ class FeatureOptionHolder(ObjectHolder[coredata.UserFeatureOption]):
         disabled.value = 'disabled'
         return disabled
 
+    def as_enabled(self) -> coredata.UserFeatureOption:
+        enabled = copy.deepcopy(self.held_object)
+        enabled.value = 'enabled'
+        return enabled
+
     @noPosargs
     @noKwargs
     def enabled_method(self, args: T.List[TYPE_var], kwargs: TYPE_kwargs) -> bool:
@@ -152,6 +157,12 @@ class FeatureOptionHolder(ObjectHolder[coredata.UserFeatureOption]):
     def disable_auto_if_method(self, args: T.Tuple[bool], kwargs: TYPE_kwargs) -> coredata.UserFeatureOption:
         return copy.deepcopy(self.held_object) if self.value != 'auto' or not args[0] else self.as_disabled()
 
+    @FeatureNew('feature_option.enable_auto_if()', '1.1.0')
+    @noKwargs
+    @typed_pos_args('feature_option.enable_auto_if', bool)
+    def enable_auto_if_method(self, args: T.Tuple[bool], kwargs: TYPE_kwargs) -> coredata.UserFeatureOption:
+        return self.as_enabled() if self.value == 'auto' and args[0] else copy.deepcopy(self.held_object)
+
 
 class RunProcess(MesonInterpreterObject):
 
diff --git a/test cases/common/192 feature option/meson.build b/test cases/common/192 feature option/meson.build
index b5e26fa..d6de744 100644
--- a/test cases/common/192 feature option/meson.build
+++ b/test cases/common/192 feature option/meson.build
@@ -17,6 +17,8 @@ assert(required_opt.allowed(), 'Should be enabled option')
 assert(required_opt.require(true, error_message: 'xyz').enabled(), 'Should be enabled option')
 assert(required_opt.disable_auto_if(true).enabled(), 'Should be enabled option')
 assert(required_opt.disable_auto_if(false).enabled(), 'Should be enabled option')
+assert(required_opt.enable_auto_if(true).enabled(), 'Should be enabled option')
+assert(required_opt.enable_auto_if(false).enabled(), 'Should be enabled option')
 
 assert(not optional_opt.enabled(), 'Should be auto option')
 assert(not optional_opt.disabled(), 'Should be auto option')
@@ -26,6 +28,8 @@ assert(optional_opt.require(true).auto(), 'Should be auto option')
 assert(optional_opt.require(false, error_message: 'xyz').disabled(), 'Should be disabled auto option')
 assert(optional_opt.disable_auto_if(true).disabled(), 'Should be disabled auto option')
 assert(optional_opt.disable_auto_if(false).auto(), 'Should be auto option')
+assert(optional_opt.enable_auto_if(true).enabled(), 'Should be disabled auto option')
+assert(optional_opt.enable_auto_if(false).auto(), 'Should be auto option')
 
 assert(not disabled_opt.enabled(), 'Should be disabled option')
 assert(disabled_opt.disabled(), 'Should be disabled option')
@@ -35,6 +39,8 @@ assert(disabled_opt.require(true).disabled(), 'Should be disabled option')
 assert(disabled_opt.require(false, error_message: 'xyz').disabled(), 'Should be disabled option')
 assert(disabled_opt.disable_auto_if(true).disabled(), 'Should be disabled option')
 assert(disabled_opt.disable_auto_if(false).disabled(), 'Should be disabled option')
+assert(disabled_opt.enable_auto_if(true).disabled(), 'Should be disabled option')
+assert(disabled_opt.enable_auto_if(false).disabled(), 'Should be disabled option')
 
 dep = dependency('threads', required : required_opt)
 assert(dep.found(), 'Should find required "threads" dep')