add the ast command to peek at the internals of nushell (#6423)

* add the ast command to peak at the internals of nushell

* fixed a bug in an example

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

@@ -0,0 +1,77 @@
+use nu_engine::CallExt;
+use nu_parser::parse;
+use nu_protocol::{
+    ast::Call,
+    engine::{Command, EngineState, Stack, StateWorkingSet},
+    Category, Example, PipelineData, ShellError, Signature, Spanned, SyntaxShape,
+};
+
+#[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")
+            .required(
+                "pipeline",
+                SyntaxShape::String,
+                "the pipeline to print the ast for",
+            )
+            .category(Category::Core)
+    }
+
+    fn run(
+        &self,
+        engine_state: &EngineState,
+        stack: &mut Stack,
+        call: &Call,
+        _input: PipelineData,
+    ) -> Result<PipelineData, ShellError> {
+        let head = call.head;
+        let pipeline: Spanned<String> = call.req(engine_state, stack, 0)?;
+        let mut working_set = StateWorkingSet::new(engine_state);
+
+        let (output, err) = parse(&mut working_set, None, pipeline.item.as_bytes(), false, &[]);
+        eprintln!("output: {:#?}\nerror: {:#?}", output, err);
+
+        Ok(PipelineData::new(head))
+    }
+
+    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/core_commands/mod.rs

@@ -1,4 +1,5 @@
 mod alias;
+mod ast;
 mod debug;
 mod def;
 mod def_env;
@@ -29,6 +30,7 @@ mod use_;
 mod version;
 
 pub use alias::Alias;
+pub use ast::Ast;
 pub use debug::Debug;
 pub use def::Def;
 pub use def_env::DefEnv;

crates/nu-command/src/default_context.rs

@@ -29,6 +29,7 @@ pub fn create_default_context() -> EngineState {
         // Core
         bind_command! {
             Alias,
+            Ast,
             Debug,
             Def,
             DefEnv,