fix(ser): Render empty tables when using to_string_pretty

I considered making it so we always showed empty implicit tables but I
figured that in the editing case, we could show tables that were
originally hidden just by deleting entries.  This seems surprising for a
user and they shouldn't have to account for it.

So I limited the fix to `to_string_pretty`.

This was found by rust-lang/cargo#10349.

src/encode.rs

@@ -201,6 +201,16 @@ fn visit_table(
     is_array_of_tables: bool,
 ) -> Result {
     let children = table.get_values();
+    // We are intentionally hiding implicit tables without any tables nested under them (ie
+    // `table.is_empty()` which is in contrast to `table.get_values().is_empty()`).  We are
+    // trusting the user that an empty implicit table is not semantically meaningful
+    //
+    // This allows a user to delete all tables under this implicit table and the implicit table
+    // will disappear.
+    //
+    // However, this means that users need to take care in deciding what tables get marked as
+    // implicit.
+    let is_visible_std_table = !(table.implicit && children.is_empty());
 
     if path.is_empty() {
         // don't print header for the root node
@@ -216,7 +226,7 @@ fn visit_table(
             "]]{}",
             table.decor.suffix().unwrap_or(DEFAULT_TABLE_DECOR.1)
         )?;
-    } else if !(table.implicit && children.is_empty()) {
+    } else if is_visible_std_table {
         write!(
             buf,
             "{}[",

src/ser/pretty.rs

@@ -16,7 +16,11 @@ impl crate::visit_mut::VisitMut for Pretty {
 
     fn visit_table_mut(&mut self, node: &mut crate::Table) {
         node.decor_mut().clear();
-        node.set_implicit(true);
+
+        // Empty tables could be semantically meaningful, so make sure they are not implicit
+        if !node.is_empty() {
+            node.set_implicit(true);
+        }
 
         crate::visit_mut::visit_table_mut(self, node);
     }

tests/pretty.rs

@@ -109,6 +109,19 @@ fn table_array() {
     assert_eq!(toml, &result);
 }
 
+const PRETTY_EMPTY_TABLE: &str = r#"[example]
+"#;
+
+#[test]
+fn pretty_empty_table() {
+    let toml = PRETTY_EMPTY_TABLE;
+    let value: toml_edit::easy::Value = toml_edit::easy::from_str(toml).unwrap();
+    let result = toml_edit::easy::to_string_pretty(&value).unwrap();
+    println!("EXPECTED:\n{}", toml);
+    println!("\nRESULT:\n{}", result);
+    assert_eq!(toml, &result);
+}
+
 #[test]
 fn error_includes_key() {
     #[derive(Debug, serde::Serialize, serde::Deserialize)]