Allow enabling config-include feature in config

Prior to this change, it is not possible to enable the unstable
`config-include` flag from within a config file itself. This is due to
the fact that, while cargo does reload configuration files if this
flag is set, it does not parse the top-level configuration file's
unstable flags before checking whether this flag is present.

This commit forces cargo to load unstable features before this check,
and if the flag is present, re-loads configuration files with
`config-include` enabled.

Author: lschuermann

src/cargo/util/context/mod.rs

@@ -1035,6 +1035,11 @@ impl GlobalContext {
             self.cli_config = Some(cli_config.iter().map(|s| s.to_string()).collect());
             self.merge_cli_args()?;
         }
+
+        // Load the unstable flags from config file here first, as the config
+        // file itself may enable inclusion of other configs. In that case, we
+        // want to re-load configs with includes enabled:
+        self.load_unstable_flags_from_config()?;
         if self.unstable_flags.config_include {
             // If the config was already loaded (like when fetching the
             // `[alias]` table), it was loaded with includes disabled because

tests/testsuite/config_include.rs

@@ -48,6 +48,35 @@ fn simple() {
     assert_eq!(gctx.get::<i32>("key3").unwrap(), 4);
 }
 
+#[cargo_test]
+fn enable_in_unstable_config() {
+    // config-include enabled in the unstable config table:
+    write_config_at(
+        ".cargo/config.toml",
+        "
+        include = 'other.toml'
+        key1 = 1
+        key2 = 2
+
+        [unstable]
+        config-include = true
+        ",
+    );
+    write_config_at(
+        ".cargo/other.toml",
+        "
+        key2 = 3
+        key3 = 4
+        ",
+    );
+    let gctx = GlobalContextBuilder::new()
+        .nightly_features_allowed(true)
+        .build();
+    assert_eq!(gctx.get::<i32>("key1").unwrap(), 1);
+    assert_eq!(gctx.get::<i32>("key2").unwrap(), 2);
+    assert_eq!(gctx.get::<i32>("key3").unwrap(), 4);
+}
+
 #[cargo_test]
 fn works_with_cli() {
     write_config_at(