fix: non-mergeable list from config cli merge the same way (#16219)

### What does this PR try to resolve?

Before we iterated key values directly for config cli values,
so top level table name of config key is never tracked.
And that resulted in
`registries.example.credential-provider` becoming
`example.credential-provider`,
making `is_nonmergable_list` return false.

This fix replaces the manual traversal
with a direct `ConfigValue::merge` call.

Now we unify how config values merge, regardless theird sources.

Fixes #16209

### How to test and review this PR?

There are some slight diagnostic regressions[^1].
I may try addressing them once later.

[^1]: Well, in `cli_merge_failed` test, the old one wasn't much better.
It knew the failure was from `--config` CLI but didn't say anything
about the actual `include`'d file. You need to look at the deeper cause
and associate them together.

Author: epage

src/cargo/util/context/mod.rs

@@ -1286,9 +1286,9 @@ impl GlobalContext {
 
     /// Start a config file discovery from a path and merges all config values found.
     fn load_values_from(&self, path: &Path) -> CargoResult<HashMap<String, ConfigValue>> {
-        // This definition path is ignored, this is just a temporary container
-        // representing the entire file.
-        let mut cfg = CV::Table(HashMap::new(), Definition::Path(PathBuf::from(".")));
+        // The root config value container isn't from any external source,
+        // so its definition should be built-in.
+        let mut cfg = CV::Table(HashMap::new(), Definition::BuiltIn);
         let home = self.home_path.clone().into_path_unlocked();
 
         self.walk_tree(path, &home, |path| {
@@ -1546,24 +1546,18 @@ impl GlobalContext {
 
     /// Add config arguments passed on the command line.
     fn merge_cli_args(&mut self) -> CargoResult<()> {
-        let CV::Table(loaded_map, _def) = self.cli_args_as_table()? else {
-            unreachable!()
-        };
-        let values = self.values_mut()?;
-        for (key, value) in loaded_map.into_iter() {
-            match values.entry(key) {
-                Vacant(entry) => {
-                    entry.insert(value);
-                }
-                Occupied(mut entry) => entry.get_mut().merge(value, true).with_context(|| {
-                    format!(
-                        "failed to merge --config key `{}` into `{}`",
-                        entry.key(),
-                        entry.get().definition(),
-                    )
-                })?,
-            };
-        }
+        let cv_from_cli = self.cli_args_as_table()?;
+        assert!(cv_from_cli.is_table(), "cv from CLI must be a table");
+
+        let root_cv = mem::take(self.values_mut()?);
+        // The root config value container isn't from any external source,
+        // so its definition should be built-in.
+        let mut root_cv = CV::Table(root_cv, Definition::BuiltIn);
+        root_cv.merge(cv_from_cli, true)?;
+
+        // Put it back to gctx
+        mem::swap(self.values_mut()?, root_cv.table_mut("<root>")?.0);
+
         Ok(())
     }
 
@@ -1710,9 +1704,7 @@ impl GlobalContext {
         let mut value = self.load_file(&credentials)?;
         // Backwards compatibility for old `.cargo/credentials` layout.
         {
-            let CV::Table(ref mut value_map, ref def) = value else {
-                unreachable!();
-            };
+            let (value_map, def) = value.table_mut("<root>")?;
 
             if let Some(token) = value_map.remove("token") {
                 if let Vacant(entry) = value_map.entry("registry".into()) {
@@ -2299,6 +2291,20 @@ impl ConfigValue {
         }
     }
 
+    pub fn table_mut(
+        &mut self,
+        key: &str,
+    ) -> CargoResult<(&mut HashMap<String, ConfigValue>, &mut Definition)> {
+        match self {
+            CV::Table(table, def) => Ok((table, def)),
+            _ => self.expected("table", key),
+        }
+    }
+
+    pub fn is_table(&self) -> bool {
+        matches!(self, CV::Table(_table, _def))
+    }
+
     pub fn string_list(&self, key: &str) -> CargoResult<Vec<(String, Definition)>> {
         match self {
             CV::List(list, _) => list

tests/testsuite/config.rs

@@ -652,7 +652,7 @@ Caused by:
     assert_error(
         gctx.unwrap_err(),
         str![[r#"
-failed to merge --config key `a` into `[ROOT]/.cargo/config.toml`
+failed to merge key `a` between [ROOT]/.cargo/config.toml and --config cli option
 
 Caused by:
   failed to merge config value from `--config cli option` into `[ROOT]/.cargo/config.toml`: expected boolean, but found array
@@ -2209,6 +2209,21 @@ credential-provider = ['c', 'd']
         .unwrap();
     assert_eq!(provider.path.raw_value(), "c");
     assert_eq!(provider.args, ["d"]);
+
+    let cli_arg = "registries.example.credential-provider=['cli', 'cli-arg']";
+    let gctx = GlobalContextBuilder::new()
+        .config_arg(cli_arg)
+        .cwd("foo")
+        .build();
+    let provider = gctx
+        .get::<Option<RegistryConfig>>(&format!("registries.example"))
+        .unwrap()
+        .unwrap()
+        .credential_provider
+        .unwrap();
+    // expect: no merge happens; config CLI takes precedence
+    assert_eq!(provider.path.raw_value(), "cli");
+    assert_eq!(provider.args, ["cli-arg"]);
 }
 
 #[cargo_test]

tests/testsuite/config_include.rs

@@ -459,7 +459,7 @@ fn cli_merge_failed() {
     assert_error(
         gctx.unwrap_err(),
         str![[r#"
-failed to merge --config key `foo` into `[ROOT]/.cargo/config.toml`
+failed to merge key `foo` between [ROOT]/.cargo/config.toml and [ROOT]/.cargo/other.toml
 
 Caused by:
   failed to merge config value from `[ROOT]/.cargo/other.toml` into `[ROOT]/.cargo/config.toml`: expected array, but found string