Treat all CliUnstable values as default-false

This is a workaround in case y'all would like to land this change
before I'm able to finish up a Deserializer refactor.

Author: bearcage

src/cargo/core/features.rs

@@ -334,32 +334,68 @@ impl Features {
 ///
 /// If you have any trouble with this, please let us know!
 #[derive(Default, Debug, Deserialize)]
-#[serde(default, deny_unknown_fields, rename_all="kebab-case")]
+#[serde(rename_all="kebab-case")]
 pub struct CliUnstable {
+    #[serde(deserialize_with="default_false")]
     pub print_im_a_teapot: bool,
+    #[serde(deserialize_with="default_false")]
     pub unstable_options: bool,
+    #[serde(deserialize_with="default_false")]
     pub no_index_update: bool,
+    #[serde(deserialize_with="default_false")]
     pub avoid_dev_deps: bool,
+    #[serde(deserialize_with="default_false")]
     pub minimal_versions: bool,
+    #[serde(deserialize_with="default_false")]
     pub package_features: bool,
+    #[serde(deserialize_with="default_false")]
     pub advanced_env: bool,
+    #[serde(deserialize_with="default_false")]
     pub config_include: bool,
+    #[serde(deserialize_with="default_false")]
     pub dual_proc_macros: bool,
+    #[serde(deserialize_with="default_false")]
     pub mtime_on_use: bool,
+    #[serde(deserialize_with="default_false")]
     pub named_profiles: bool,
+    #[serde(deserialize_with="default_false")]
     pub binary_dep_depinfo: bool,
     pub build_std: Option<Vec<String>>,
     pub timings: Option<Vec<String>>,
+    #[serde(deserialize_with="default_false")]
     pub doctest_xcompile: bool,
+    #[serde(deserialize_with="default_false")]
     pub panic_abort_tests: bool,
+    #[serde(deserialize_with="default_false")]
     pub jobserver_per_rustc: bool,
     pub features: Option<Vec<String>>,
+    #[serde(deserialize_with="default_false")]
     pub crate_versions: bool,
+    #[serde(deserialize_with="default_false")]
     pub separate_nightlies: bool,
+    #[serde(deserialize_with="default_false")]
     pub multitarget: bool,
+    #[serde(deserialize_with="default_false")]
     pub rustdoc_map: bool,
 }
 
+/// Treat boolean Zflags as optionals for deserializing them.
+/// This allows missing settings to default to disabled (effectively recreating
+/// the serde `default` behavior). Our Deserializer merges multiple sources
+/// inline, so the serde facilities for handling missing or additional fields
+/// don't quite fit.
+///
+/// TODO: This limitation can likely be removed by refactoring 
+///       `de::ConfigMapAccess` to iterate the union of the config and env sets.
+fn default_false<'de, D, T>(deserializer: D) -> Result<T, D::Error>
+where
+    D: serde::Deserializer<'de>,
+    T: Deserialize<'de> + Default,
+{
+    let option = Option::deserialize(deserializer)?;
+    Ok(option.unwrap_or_default())
+}
+
 impl CliUnstable {
     pub fn parse(&mut self, flags: &[String]) -> CargoResult<()> {
         if !flags.is_empty() && !nightly_features_allowed() {