fix: Always overwrite with tables, even if empty

While the last commit restored behavior to mostly where it was in v0.15.2, this
commit introduces a behavior change with the goal of treating empty tables the
same as non-empty ones (in particular, `int_to_empty` is now treated the same
as `int_to_non_empty`, both of them overwriting the integer with the table).

Treating them the same makes a lot of sense logically, and the fact that we
weren't doing so is probably a bug in the old implementation. But it is a
notable behavior change, and one worth considering carefully.

src/path/mod.rs

@@ -150,8 +150,9 @@ impl Expression {
         let parent = self.get_mut_forcibly(root);
         match value.kind {
             ValueKind::Table(ref incoming_map) => {
-                // If the parent is nil, treat it as an empty table
-                if matches!(parent.kind, ValueKind::Nil) {
+                // If the parent is not a table, overwrite it, treating it as a
+                // table
+                if !matches!(parent.kind, ValueKind::Table(_)) {
                     *parent = Map::<String, Value>::new().into();
                 }
 

tests/testsuite/merge.rs

@@ -87,13 +87,11 @@ fn test_merge_empty_maps() {
     use std::collections::BTreeMap;
 
     #[derive(Debug, Deserialize)]
-    #[allow(dead_code)] // temporary while this test is broken
     struct Settings {
         profile: BTreeMap<String, Profile>,
     }
 
     #[derive(Debug, Default, Deserialize)]
-    #[allow(dead_code)] // temporary while this test is broken
     struct Profile {
         name: Option<String>,
     }
@@ -149,10 +147,35 @@ fn test_merge_empty_maps() {
         .build()
         .unwrap();
 
-    // This is currently broken -- the next commit fixes it.
-    let error = cfg.try_deserialize::<Settings>().unwrap_err();
+    let settings = cfg.try_deserialize::<Settings>().unwrap();
+    assert_eq!(settings.profile.len(), 10);
+
+    assert_eq!(settings.profile["missing_to_empty"].name, None);
+    assert_eq!(
+        settings.profile["missing_to_non_empty"].name,
+        Some("bar".to_owned())
+    );
+    assert_eq!(settings.profile["empty_to_empty"].name, None);
+    assert_eq!(
+        settings.profile["empty_to_non_empty"].name,
+        Some("bar".to_owned())
+    );
+    assert_eq!(
+        settings.profile["non_empty_to_empty"].name,
+        Some("foo".to_owned())
+    );
+    assert_eq!(
+        settings.profile["non_empty_to_non_empty"].name,
+        Some("bar".to_owned())
+    );
+    assert_eq!(settings.profile["null_to_empty"].name, None);
+    assert_eq!(
+        settings.profile["null_to_non_empty"].name,
+        Some("bar".to_owned())
+    );
+    assert_eq!(settings.profile["int_to_empty"].name, None);
     assert_eq!(
-        error.to_string(),
-        "invalid type: unit value, expected struct 42 for key `profile.int_to_empty`"
+        settings.profile["int_to_non_empty"].name,
+        Some("bar".to_owned())
     );
 }