allow view-source to read rest arguments (#9247)

# Description
Fixes #8896. Also went back and cleaned up the code slightly.

# User-Facing Changes
`view-source` now is more comprehensive when viewing definitions. 

# Tests + Formatting
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---------

Co-authored-by: Darren Schroeder <343840+fdncred@users.noreply.github.com>

crates/nu-command/src/debug/view_source.rs

@@ -5,6 +5,7 @@ use nu_protocol::{
     Category, Example, IntoPipelineData, PipelineData, ShellError, Signature, SyntaxShape, Type,
     Value,
 };
+use std::fmt::Write;
 
 #[derive(Clone)]
 pub struct ViewSource;
@@ -54,47 +55,43 @@ impl Command for ViewSource {
                     let sig = decl.signature();
                     let vec_of_required = &sig.required_positional;
                     let vec_of_optional = &sig.optional_positional;
+                    let rest = &sig.rest_positional;
                     let vec_of_flags = &sig.named;
                     // gets vector of positionals.
                     if let Some(block_id) = decl.get_block_id() {
                         let block = engine_state.get_block(block_id);
                         if let Some(block_span) = block.span {
                             let contents = engine_state.get_span_contents(&block_span);
-                            let mut final_contents = String::from("def ");
-                            final_contents.push_str(&val);
-                            // The name of the function...
-                            final_contents.push_str(" [ ");
+                            // name of function
+                            let mut final_contents = format!("def {val} [ ");
                             for n in vec_of_required {
-                                final_contents.push_str(&n.name);
-                                // name of positional arg
-                                final_contents.push(':');
-                                final_contents.push_str(&n.shape.to_string());
-                                final_contents.push(' ');
+                                let _ = write!(&mut final_contents, "{}: {} ", n.name, n.shape);
+                                // positional argu,emts
                             }
                             for n in vec_of_optional {
-                                final_contents.push_str(&n.name);
-                                // name of positional arg
-                                final_contents.push_str("?:");
-                                final_contents.push_str(&n.shape.to_string());
-                                final_contents.push(' ');
+                                let _ = write!(&mut final_contents, "{}?: {} ", n.name, n.shape);
                             }
                             for n in vec_of_flags {
-                                final_contents.push_str("--");
-                                final_contents.push_str(&n.long);
-                                final_contents.push(' ');
-                                if n.short.is_some() {
-                                    final_contents.push_str("(-");
-                                    final_contents.push(n.short.expect("this cannot trigger."));
-                                    final_contents.push(')');
+                                // skip adding the help flag
+                                if n.long == "help" {
+                                    continue;
                                 }
-                                if n.arg.is_some() {
-                                    final_contents.push_str(": ");
-                                    final_contents.push_str(
-                                        &n.arg.as_ref().expect("this cannot trigger.").to_string(),
-                                    );
+                                let _ = write!(&mut final_contents, "--{}", n.long);
+                                if let Some(short) = n.short {
+                                    let _ = write!(&mut final_contents, "(-{})", short);
+                                }
+                                if let Some(arg) = &n.arg {
+                                    let _ = write!(&mut final_contents, ": {}", arg);
                                 }
                                 final_contents.push(' ');
                             }
+                            if let Some(rest_arg) = rest {
+                                let _ = write!(
+                                    &mut final_contents,
+                                    "...{}:{}",
+                                    rest_arg.name, rest_arg.shape
+                                );
+                            }
                             final_contents.push_str("] ");
                             final_contents.push_str(&String::from_utf8_lossy(contents));
                             Ok(Value::string(final_contents, call.head).into_pipeline_data())
@@ -157,17 +154,22 @@ impl Command for ViewSource {
             Example {
                 description: "View the source of a code block",
                 example: r#"let abc = {|| echo 'hi' }; view source $abc"#,
-                result: Some(Value::test_string("{ echo 'hi' }")),
+                result: Some(Value::test_string("{|| echo 'hi' }")),
             },
             Example {
                 description: "View the source of a custom command",
                 example: r#"def hi [] { echo 'Hi!' }; view source hi"#,
-                result: Some(Value::test_string("{ echo 'Hi!' }")),
+                result: Some(Value::test_string("def hi [] { echo 'Hi!' }")),
             },
             Example {
                 description: "View the source of a custom command, which participates in the caller environment",
                 example: r#"def-env foo [] { let-env BAR = 'BAZ' }; view source foo"#,
-                result: Some(Value::test_string("{ let-env BAR = 'BAZ' }")),
+                result: Some(Value::test_string("def foo [] { let-env BAR = 'BAZ' }")),
+            },
+            Example {
+                description: "View the source of a custom command with flags and arguments",
+                example: r#"def test [a?:any --b:int ...rest:string] { echo 'test' }; view source test"#,
+                result: Some(Value::test_string("def test [ a?: any --b: int ...rest: string] { echo 'test' }")),
             },
             Example {
                 description: "View the source of a module",