interpreter: add FeatureOption.enable_if and .disable_if

This adds two new methods, that are conceptually related in the same way
that `enable_auto_if` and `disable_auto_if` are. They are different
however, in that they will always replace an `auto` value with an
`enabled` or `disabled` value, or error if the feature is in the
opposite state (calling `feature(disabled).enable_if(true)`, for
example). This matters when the feature will be passed to
dependency(required : …)`, which has different behavior when passed an
enabled feature than an auto one.

The `disable_if` method will be controversial, I'm sure, since it
can be expressed via `feature.require()` (`feature.require(not
condition) == feature.disable_if(condition)`). I have two defences of
this:

1) `feature.require` is difficult to reason about, I would expect
   require to be equivalent to `feature.enable_if(condition)`, not to
   `feature.disable_if(not condition)`.
2) mixing `enable_if` and `disable_if` in the same call chain is much
   clearer than mixing `require` and `enable_if`:
   ```meson
   get_option('feat') \
     .enable_if(foo) \
     .disable_if(bar) \
     .enable_if(opt)
   ```
   vs
   ```meson
   get_option('feat') \
     .enable_if(foo) \
     .require(not bar) \
     .enable_if(opt)
   ```
   In the first chain it's immediately obvious what is happening, in the
   second, not so much, especially if you're not familiar with what
   `require` means.

1 files changed, 60 insertions, 0 deletions

diff --git a/docs/yaml/objects/feature.yaml b/docs/yaml/objects/feature.yaml
index 01209eb53..9cb597be8 100644
--- a/docs/yaml/objects/feature.yaml
+++ b/docs/yaml/objects/feature.yaml
@@ -85,3 +85,63 @@ methods:
       type: str
       default: "''"
       description: The error Message to print if the check fails
+
+- name: enable_if
+  returns: feature
+  since: 1.1.0
+  description: |
+    Returns the object itself if the value is false; an error if the object is
+    `'disabled'` and the value is true; an enabled feature if the object
+    is `'auto'` or `'enabled'` and the value is true.
+
+  example: |
+    `enable_if` is useful to restrict the applicability of `'auto'` features,
+    particularly when passing them to [[dependency]]:
+
+    ```
+    use_llvm = get_option('llvm').enable_if(with_clang).enable_if(with_llvm_libs)
+    dep_llvm = dependency('llvm', require : use_llvm)
+    ```
+
+  posargs:
+    value:
+      type: bool
+      description: The value to check
+
+  kwargs:
+    error_message:
+      type: str
+      default: "''"
+      description: The error Message to print if the check fails
+
+- name: disable_if
+  returns: feature
+  since: 1.1.0
+  description: |
+    Returns the object itself if the value is false; an error if the object is
+    `'enabled'` and the value is true; a disabled feature if the object
+    is `'auto'` or `'disabled'` and the value is true.
+
+    This is equivalent to `feature_opt.require(not condition)`, but may make
+    code easier to reason about, especially when mixed with `enable_if`
+
+  example: |
+    `disable_if` is useful to restrict the applicability of `'auto'` features,
+    particularly when passing them to [[dependency]]:
+
+    ```
+    use_os_feature = get_option('foo') \
+      .disable_if(host_machine.system() == 'darwin', error_message : 'os feature not supported on MacOS')
+    dep_os_feature = dependency('os_feature', require : use_os_feature)
+    ```
+
+  posargs:
+    value:
+      type: bool
+      description: The value to check
+
+  kwargs:
+    error_message:
+      type: str
+      default: "''"
+      description: The error Message to print if the check fails