Apply nightly clippy fixes (#11083)

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# Description
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Clippy fixes for rust 1.76.0-nightly

# User-Facing Changes
<!-- List of all changes that impact the user experience here. This
helps us keep track of breaking changes. -->

N/A
# Tests + Formatting
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- `cargo fmt --all -- --check` to check standard code formatting (`cargo
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crates/nu-cli/src/completions/completer.rs

@@ -67,7 +67,7 @@ impl NuCompleter {
         let mut callee_stack = stack.gather_captures(&self.engine_state, &block.captures);
 
         // Line
-        if let Some(pos_arg) = block.signature.required_positional.get(0) {
+        if let Some(pos_arg) = block.signature.required_positional.first() {
             if let Some(var_id) = pos_arg.var_id {
                 callee_stack.add_var(
                     var_id,

crates/nu-cli/src/util.rs

@@ -111,7 +111,7 @@ fn gather_env_vars(
             let name = if let Some(Token {
                 contents: TokenContents::Item,
                 span,
-            }) = parts.get(0)
+            }) = parts.first()
             {
                 let mut working_set = StateWorkingSet::new(engine_state);
                 let bytes = working_set.get_span_contents(*span);

crates/nu-cli/tests/completions.rs

@@ -122,14 +122,14 @@ fn dotnu_completions() {
     let suggestions = completer.complete(&completion_str, completion_str.len());
 
     assert_eq!(1, suggestions.len());
-    assert_eq!("custom_completion.nu", suggestions.get(0).unwrap().value);
+    assert_eq!("custom_completion.nu", suggestions.first().unwrap().value);
 
     // Test use completion
     let completion_str = "use ".to_string();
     let suggestions = completer.complete(&completion_str, completion_str.len());
 
     assert_eq!(1, suggestions.len());
-    assert_eq!("custom_completion.nu", suggestions.get(0).unwrap().value);
+    assert_eq!("custom_completion.nu", suggestions.first().unwrap().value);
 }
 
 #[test]
@@ -141,7 +141,7 @@ fn external_completer_trailing_space() {
 
     let suggestions = run_external_completion(block, &input);
     assert_eq!(3, suggestions.len());
-    assert_eq!("gh", suggestions.get(0).unwrap().value);
+    assert_eq!("gh", suggestions.first().unwrap().value);
     assert_eq!("alias", suggestions.get(1).unwrap().value);
     assert_eq!("", suggestions.get(2).unwrap().value);
 }
@@ -153,7 +153,7 @@ fn external_completer_no_trailing_space() {
 
     let suggestions = run_external_completion(block, &input);
     assert_eq!(2, suggestions.len());
-    assert_eq!("gh", suggestions.get(0).unwrap().value);
+    assert_eq!("gh", suggestions.first().unwrap().value);
     assert_eq!("alias", suggestions.get(1).unwrap().value);
 }
 
@@ -164,7 +164,7 @@ fn external_completer_pass_flags() {
 
     let suggestions = run_external_completion(block, &input);
     assert_eq!(3, suggestions.len());
-    assert_eq!("gh", suggestions.get(0).unwrap().value);
+    assert_eq!("gh", suggestions.first().unwrap().value);
     assert_eq!("api", suggestions.get(1).unwrap().value);
     assert_eq!("--", suggestions.get(2).unwrap().value);
 }