Unify closure serializing logic for to nuon, to msgpack, and to json (#15285)

# Description
Before this PR, `to msgpack`/`to msgpackz` and `to json` serialize
closures as `nil`/`null` respectively, when the `--serialize` option
isn't passed. This PR makes it an error to serialize closures to msgpack
or JSON without the `--serialize` flag, which is the behavior of `to
nuon`.

This PR also adds the `--serialize` flag to `to msgpack`.

This PR also changes `to nuon` and `to json` to return an error if they
cannot find the block contents of a closure, rather than serializing an
empty string or an error string, respectively. This behavior is
replicated for `to msgpack`.

It also changes `to nuon`'s error message for serializing closures
without `--serialize` to be the same as the new errors for `to json` and
`to msgpack`.

# User-Facing Changes

* Add `--serialize` flag to `to msgpack`, similar to the `--serialize`
flag for `to nuon` and `to json`.
* Serializing closures to JSON or msgpack without `--serialize`

Partially fixes #11738

crates/nu-protocol/src/engine/capture_block.rs

@@ -1,9 +0,0 @@
-use crate::{BlockId, Value, VarId};
-
-use serde::{Deserialize, Serialize};
-
-#[derive(Clone, Debug, Serialize, Deserialize)]
-pub struct Closure {
-    pub block_id: BlockId,
-    pub captures: Vec<(VarId, Value)>,
-}

crates/nu-protocol/src/engine/closure.rs

@@ -0,0 +1,35 @@
+use std::borrow::Cow;
+
+use crate::{engine::EngineState, BlockId, ShellError, Span, Value, VarId};
+
+use serde::{Deserialize, Serialize};
+
+#[derive(Clone, Debug, Serialize, Deserialize)]
+pub struct Closure {
+    pub block_id: BlockId,
+    pub captures: Vec<(VarId, Value)>,
+}
+
+impl Closure {
+    pub fn coerce_into_string<'a>(
+        &self,
+        engine_state: &'a EngineState,
+        span: Span,
+    ) -> Result<Cow<'a, str>, ShellError> {
+        let block = engine_state.get_block(self.block_id);
+        if let Some(span) = block.span {
+            let contents_bytes = engine_state.get_span_contents(span);
+            Ok(String::from_utf8_lossy(contents_bytes))
+        } else {
+            Err(ShellError::CantConvert {
+                to_type: "string".into(),
+                from_type: "closure".into(),
+                span,
+                help: Some(format!(
+                    "unable to retrieve block contents for closure with id {}",
+                    self.block_id.get()
+                )),
+            })
+        }
+    }
+}

crates/nu-protocol/src/engine/mod.rs

@@ -3,7 +3,7 @@ mod argument;
 mod cached_file;
 mod call;
 mod call_info;
-mod capture_block;
+mod closure;
 mod command;
 mod description;
 mod engine_state;
@@ -23,7 +23,7 @@ pub use cached_file::CachedFile;
 pub use argument::*;
 pub use call::*;
 pub use call_info::*;
-pub use capture_block::*;
+pub use closure::*;
 pub use command::*;
 pub use engine_state::*;
 pub use error_handler::*;