Switch unstable config parsing to serde

Tests are currently failing, looks like there's something in the
Deserializer impl that's forcing field-missing errors even when
the serde `default` annotation is applied.

src/cargo/core/features.rs

@@ -333,7 +333,10 @@ impl Features {
 ///    and then test for your flag or your value and act accordingly.
 ///
 /// If you have any trouble with this, please let us know!
-#[derive(Default, Debug)]
+#[derive(Default, Debug, Deserialize)]
+#[serde(default)]
+#[serde(deny_unknown_fields)]
+#[serde(rename_all="kebab-case")]
 pub struct CliUnstable {
     pub print_im_a_teapot: bool,
     pub unstable_options: bool,

src/cargo/util/config/mod.rs

@@ -63,7 +63,7 @@ use std::str::FromStr;
 use std::sync::Once;
 use std::time::Instant;
 
-use anyhow::{anyhow, bail, Context};
+use anyhow::{anyhow, bail};
 use curl::easy::Easy;
 use lazycell::LazyCell;
 use serde::Deserialize;
@@ -745,18 +745,13 @@ impl Config {
         // If nightly features are enabled, allow setting Z-flags from config
         // using the `unstable` table. Ignore that block otherwise.
         if nightly_features_allowed() {
-            if let Some(unstable_configs) =
-                self.get::<Option<HashMap<String, String>>>("unstable")?
-            {
-                self.unstable_flags
-                    .update_with_table(&unstable_configs)
-                    .with_context(|| "Invalid [unstable] entry in Cargo config")?;
-
-                // NB. It sucks to parse these twice, but doing it again here
-                //     allows the CLI to override config files for both enabling
-                //     and disabling.
-                self.unstable_flags.parse(unstable_flags)?;
+            if let Some(unstable_flags) = self.get::<Option<CliUnstable>>("unstable")? {
+                self.unstable_flags = unstable_flags;
             }
+            // NB. It sucks to parse these twice, but doing it again here
+            //     allows the CLI to override config files for both enabling
+            //     and disabling.
+            self.unstable_flags.parse(unstable_flags)?;
         }
 
         Ok(())

tests/testsuite/config.rs

@@ -1097,12 +1097,12 @@ Caused by:
 #[cargo_test]
 /// Assert that unstable options can be configured with the `unstable` table in
 /// cargo config files
-fn config_unstable_table() {
+fn unstable_table_notation() {
     cargo::core::enable_nightly_features();
     write_config(
         "\
 [unstable]
-print-im-a-teapot = 'yes'
+print-im-a-teapot = true
 ",
     );
     let config = ConfigBuilder::new().build();
@@ -1111,11 +1111,11 @@ print-im-a-teapot = 'yes'
 
 #[cargo_test]
 /// Assert that dotted notation works for configuring unstable options
-fn config_unstable_dotted() {
+fn unstable_dotted_notation() {
     cargo::core::enable_nightly_features();
     write_config(
         "\
-unstable.print-im-a-teapot = 'yes'
+unstable.print-im-a-teapot = true
 ",
     );
     let config = ConfigBuilder::new().build();
@@ -1124,11 +1124,11 @@ unstable.print-im-a-teapot = 'yes'
 
 #[cargo_test]
 /// Assert that Zflags on the CLI take precedence over those from config
-fn config_unstable_cli_wins() {
+fn unstable_cli_precedence() {
     cargo::core::enable_nightly_features();
     write_config(
         "\
-unstable.print-im-a-teapot = 'yes'
+unstable.print-im-a-teapot = true
 ",
     );
     let config = ConfigBuilder::new().build();
@@ -1142,8 +1142,8 @@ unstable.print-im-a-teapot = 'yes'
 
 #[cargo_test]
 /// Assert that atempting to set an unstable flag that doesn't exist via config
-/// errors out the same as it would on the command line
-fn config_unstable_invalid_flag() {
+/// errors out the same as it would on the command line (nightly only)
+fn unstable_invalid_flag_errors_on_nightly() {
     cargo::core::enable_nightly_features();
     write_config(
         "\
@@ -1160,6 +1160,32 @@ Caused by:
     );
 }
 
+#[cargo_test]
+/// Assert that atempting to set an unstable flag that doesn't exist via config
+/// is ignored on stable
+fn unstable_invalid_flag_ignored_on_stable() {
+    write_config(
+        "\
+unstable.an-invalid-flag = 'yes'
+",
+    );
+    assert!(ConfigBuilder::new().build_err().is_ok());
+}
+
+#[cargo_test]
+/// Assert that unstable options can be configured with the `unstable` table in
+/// cargo config files
+fn unstable_flags_ignored_on_stable() {
+    write_config(
+        "\
+[unstable]
+print-im-a-teapot = true
+",
+    );
+    let config = ConfigBuilder::new().build();
+    assert_eq!(config.cli_unstable().print_im_a_teapot, false);
+}
+
 #[cargo_test]
 fn table_merge_failure() {
     // Config::merge fails to merge entries in two tables.