move ast command to the debug group (#8077)

# Description

This PR tweaks how `ast` works a tiny bit by outputting values in stead
of eprintln!'s. It also moves the `ast` command into the folder with the
rest of the debug commands and changes the category to debug.

I started adding some tests but couldn't figure out a good way to do it
since every `ast` command contains spans that will be different on each
invocation.

# User-Facing Changes



# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

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

@@ -0,0 +1,91 @@
+use nu_engine::CallExt;
+use nu_parser::parse;
+use nu_protocol::{
+    ast::Call,
+    engine::{Command, EngineState, Stack, StateWorkingSet},
+    Category, Example, IntoPipelineData, PipelineData, ShellError, Signature, Spanned, SyntaxShape,
+    Type, Value,
+};
+
+#[derive(Clone)]
+pub struct Ast;
+
+impl Command for Ast {
+    fn name(&self) -> &str {
+        "ast"
+    }
+
+    fn usage(&self) -> &str {
+        "Print the abstract syntax tree (ast) for a pipeline."
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::build("ast")
+            .input_output_types(vec![(Type::String, Type::Record(vec![]))])
+            .required(
+                "pipeline",
+                SyntaxShape::String,
+                "the pipeline to print the ast for",
+            )
+            .allow_variants_without_examples(true)
+            .category(Category::Debug)
+    }
+
+    fn run(
+        &self,
+        engine_state: &EngineState,
+        stack: &mut Stack,
+        call: &Call,
+        _input: PipelineData,
+    ) -> Result<PipelineData, ShellError> {
+        let pipeline: Spanned<String> = call.req(engine_state, stack, 0)?;
+        let mut working_set = StateWorkingSet::new(engine_state);
+
+        let (block_output, error_output) =
+            parse(&mut working_set, None, pipeline.item.as_bytes(), false, &[]);
+        let block_value = Value::String {
+            val: format!("{block_output:#?}"),
+            span: pipeline.span,
+        };
+        let error_value = Value::String {
+            val: format!("{error_output:#?}"),
+            span: pipeline.span,
+        };
+        let output_record = Value::Record {
+            cols: vec!["block".to_string(), "error".to_string()],
+            vals: vec![block_value, error_value],
+            span: pipeline.span,
+        };
+        Ok(output_record.into_pipeline_data())
+    }
+
+    fn examples(&self) -> Vec<Example> {
+        vec![
+            Example {
+                description: "Print the ast of a string",
+                example: "ast 'hello'",
+                result: None,
+            },
+            Example {
+                description: "Print the ast of a pipeline",
+                example: "ast 'ls | where name =~ README'",
+                result: None,
+            },
+            Example {
+                description: "Print the ast of a pipeline with an error",
+                example: "ast 'for x in 1..10 { echo $x '",
+                result: None,
+            },
+        ]
+    }
+}
+
+#[cfg(test)]
+mod test {
+    #[test]
+    fn test_examples() {
+        use super::Ast;
+        use crate::test_examples;
+        test_examples(Ast {})
+    }
+}

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

@@ -1,3 +1,4 @@
+mod ast;
 mod debug_;
 mod explain;
 mod inspect;
@@ -10,6 +11,7 @@ mod view_files;
 mod view_source;
 mod view_span;
 
+pub use ast::Ast;
 pub use debug_::Debug;
 pub use explain::Explain;
 pub use inspect::Inspect;