Resolve assigning_clones clippy lint

    warning: assigning the result of `Clone::clone()` may be inefficient
      --> impl/src/ast.rs:85:21
       |
    85 |                     *display = attrs.display.clone();
       |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `clone_from()`: `display.clone_from(&attrs.display)`
       |
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#assigning_clones
       = note: `#[warn(clippy::assigning_clones)]` on by default

    warning: assigning the result of `Clone::clone()` may be inefficient
       --> impl/src/expand.rs:158:9
        |
    158 |         display_implied_bounds = display.implied_bounds.clone();
        |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `clone_from()`: `display_implied_bounds.clone_from(&display.implied_bounds)`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#assigning_clones

    warning: assigning the result of `Clone::clone()` may be inefficient
       --> impl/src/expand.rs:402:21
        |
    402 |                     display_implied_bounds = display.implied_bounds.clone();
        |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `clone_from()`: `display_implied_bounds.clone_from(&display.implied_bounds)`
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#assigning_clones

Author: dtolnay

impl/src/ast.rs

@@ -82,7 +82,7 @@ impl<'a> Enum<'a> {
             .map(|node| {
                 let mut variant = Variant::from_syn(node, &scope, span)?;
                 if let display @ None = &mut variant.attrs.display {
-                    *display = attrs.display.clone();
+                    display.clone_from(&attrs.display);
                 }
                 if let Some(display) = &mut variant.attrs.display {
                     display.expand_shorthand(&variant.fields);

impl/src/expand.rs

@@ -155,7 +155,7 @@ fn impl_struct(input: Struct) -> TokenStream {
             ::core::fmt::Display::fmt(&self.#only_field, __formatter)
         })
     } else if let Some(display) = &input.attrs.display {
-        display_implied_bounds = display.implied_bounds.clone();
+        display_implied_bounds.clone_from(&display.implied_bounds);
         let use_as_display = use_as_display(display.has_bonus_display);
         let pat = fields_pat(&input.fields);
         Some(quote! {
@@ -399,7 +399,7 @@ fn impl_enum(input: Enum) -> TokenStream {
             let mut display_implied_bounds = Set::new();
             let display = match &variant.attrs.display {
                 Some(display) => {
-                    display_implied_bounds = display.implied_bounds.clone();
+                    display_implied_bounds.clone_from(&display.implied_bounds);
                     display.to_token_stream()
                 }
                 None => {