Rust 1.78 (#11314)

Notable things I've had to fix due to 1.78:
- Better detection of unused items
- New clippy lint (`assigning_clones`) that points out places where assignment operations with clone rhs could be replaced with more performant `clone_into`
Release Notes:

- N/A

crates/theme/src/settings.rs

@@ -441,7 +441,7 @@ impl settings::Settings for ThemeSettings {
                 }
             }
 
-            this.theme_overrides = value.theme_overrides.clone();
+            this.theme_overrides.clone_from(&value.theme_overrides);
             this.apply_theme_overrides();
 
             merge(&mut this.ui_font_size, value.ui_font_size.map(Into::into));