disable rule when grid enabled, and print warning

src/bin/bat/app.rs

@@ -288,8 +288,8 @@ impl App {
 
     fn style_components(&self) -> Result<StyleComponents> {
         let matches = &self.matches;
-        Ok(StyleComponents(
-            if matches.value_of("decorations") == Some("never") {
+        let mut styled_components =
+            StyleComponents(if matches.value_of("decorations") == Some("never") {
                 HashSet::new()
             } else if matches.is_present("number") {
                 [StyleComponent::LineNumbers].iter().cloned().collect()
@@ -323,7 +323,17 @@ impl App {
                         acc.extend(components.iter().cloned());
                         acc
                     })
-            },
-        ))
+            });
+
+        // If `grid` is set, remove `rule` as it is a subset of `grid`, and print a warning.
+        if styled_components.grid() && styled_components.0.remove(&StyleComponent::Rule) {
+            use ansi_term::Colour::Yellow;
+            eprintln!(
+                "{}: Style 'rule' is a subset of style 'grid', 'rule' will not be visible.",
+                Yellow.paint("[bat warning]"),
+            );
+        }
+
+        Ok(styled_components)
     }
 }