Reuse existing small allocations if possible (#12335)

Those allocations are all small and insignificant in the grand scheme of
things and the optimizer may be able to resolve some of those but better
to be nice anyways.

Primarily inspired by the new
[`clippy::assigning_clones`](https://rust-lang.github.io/rust-clippy/master/index.html#/assigning_clones)

- **Avoid reallocs with `clone_from` in `nu-parser`**
- **Avoid realloc on assignment in `Stack`**
- **Fix `clippy::assigning_clones` in `nu-cli`**
- **Reuse allocations in `nu-explore` if possible**

crates/nu-explore/src/commands/help.rs

@@ -104,7 +104,7 @@ impl ViewCommand for HelpCmd {
     }
 
     fn parse(&mut self, args: &str) -> Result<()> {
-        self.input_command = args.trim().to_owned();
+        args.trim().clone_into(&mut self.input_command);
 
         Ok(())
     }

crates/nu-explore/src/commands/nu.rs

@@ -62,7 +62,7 @@ impl ViewCommand for NuCmd {
     }
 
     fn parse(&mut self, args: &str) -> Result<()> {
-        self.command = args.trim().to_owned();
+        args.trim().clone_into(&mut self.command);
 
         Ok(())
     }

crates/nu-explore/src/commands/try.rs

@@ -64,7 +64,7 @@ impl ViewCommand for TryCmd {
     }
 
     fn parse(&mut self, args: &str) -> Result<()> {
-        self.command = args.trim().to_owned();
+        args.trim().clone_into(&mut self.command);
 
         Ok(())
     }

crates/nu-explore/src/pager/mod.rs

@@ -295,7 +295,7 @@ fn render_ui(
         if pager.cmd_buf.run_cmd {
             let args = pager.cmd_buf.buf_cmd2.clone();
             pager.cmd_buf.run_cmd = false;
-            pager.cmd_buf.buf_cmd2 = String::new();
+            pager.cmd_buf.buf_cmd2.clear();
 
             let out =
                 pager_run_command(engine_state, stack, pager, &mut view_stack, &commands, args);
@@ -814,21 +814,21 @@ fn handle_general_key_events2<V>(
 {
     match key.code {
         KeyCode::Char('?') => {
-            search.buf_cmd_input = String::new();
+            search.buf_cmd_input.clear();
             search.is_search_input = true;
             search.is_reversed = true;
 
             info.report = None;
         }
         KeyCode::Char('/') => {
-            search.buf_cmd_input = String::new();
+            search.buf_cmd_input.clear();
             search.is_search_input = true;
             search.is_reversed = false;
 
             info.report = None;
         }
         KeyCode::Char(':') => {
-            command.buf_cmd2 = String::new();
+            command.buf_cmd2.clear();
             command.is_cmd_input = true;
             command.cmd_exec_info = None;
 
@@ -837,7 +837,7 @@ fn handle_general_key_events2<V>(
         KeyCode::Char('n') => {
             if !search.search_results.is_empty() {
                 if search.buf_cmd_input.is_empty() {
-                    search.buf_cmd_input = search.buf_cmd.clone();
+                    search.buf_cmd_input.clone_from(&search.buf_cmd);
                 }
 
                 if search.search_index + 1 == search.search_results.len() {
@@ -863,7 +863,7 @@ fn search_input_key_event(
 ) -> bool {
     match &key.code {
         KeyCode::Esc => {
-            buf.buf_cmd_input = String::new();
+            buf.buf_cmd_input.clear();
 
             if let Some(view) = view {
                 if !buf.buf_cmd.is_empty() {
@@ -878,7 +878,7 @@ fn search_input_key_event(
             true
         }
         KeyCode::Enter => {
-            buf.buf_cmd = buf.buf_cmd_input.clone();
+            buf.buf_cmd.clone_from(&buf.buf_cmd_input);
             buf.is_search_input = false;
 
             true
@@ -982,7 +982,8 @@ fn cmd_input_key_event(buf: &mut CommandBuf, key: &KeyEvent) -> bool {
                     buf.cmd_history_pos + 1,
                     buf.cmd_history.len().saturating_sub(1),
                 );
-                buf.buf_cmd2 = buf.cmd_history[buf.cmd_history_pos].clone();
+                buf.buf_cmd2
+                    .clone_from(&buf.cmd_history[buf.cmd_history_pos]);
             }
 
             true
@@ -991,7 +992,8 @@ fn cmd_input_key_event(buf: &mut CommandBuf, key: &KeyEvent) -> bool {
             if !buf.cmd_history.is_empty() {
                 buf.cmd_history_allow = true;
                 buf.cmd_history_pos = buf.cmd_history_pos.saturating_sub(1);
-                buf.buf_cmd2 = buf.cmd_history[buf.cmd_history_pos].clone();
+                buf.buf_cmd2
+                    .clone_from(&buf.cmd_history[buf.cmd_history_pos]);
             }
 
             true

crates/nu-explore/src/views/record/mod.rs

@@ -812,7 +812,7 @@ fn _transpose_table(
     let mut data = vec![vec![Value::default(); count_rows]; count_columns];
     for (row, values) in values.iter().enumerate() {
         for (column, value) in values.iter().enumerate() {
-            data[column][row] = value.to_owned();
+            data[column][row].clone_from(value);
         }
     }