To toml fix (#7597)

# Description

Fixes #7510 .
Remove support for tables from `to toml` command and update description.
Previously, as indicated in #7510 , a table could be converted to toml
and would result in this invalid toml:


![image](https://user-images.githubusercontent.com/17511668/209443930-c3dd3a3f-5ffd-4273-9c10-acbb345c788e.png)

This commit removes functionality of serializing tables and now `to
toml` produces an error:


![image](https://user-images.githubusercontent.com/17511668/209443975-be119465-8946-4644-8994-489ca94f6006.png)

The `from toml` command already acknowledges the fact that toml can
contain only records as indicated in its signature


![image](https://user-images.githubusercontent.com/17511668/209443995-1590d044-a790-4be3-a967-b26292a6e70c.png)

Now help of `to toml` reflects this feature of format as well:


![image](https://user-images.githubusercontent.com/17511668/209444014-7cfe8f8e-ad8a-4845-a151-24df6b99a1a2.png)

Additionally new tests were created for `to toml` command. See
`crates\nu-command\tests\format_conversions\toml.rs`.

Also removed undocumented behavior that would accept and validate a
string as toml:


![image](https://user-images.githubusercontent.com/17511668/209449482-5d876074-fc5b-4b21-b8a5-64e643a50083.png)


# User-Facing Changes

- Serializing tables to toml now produces error instead of invalid toml
- Updated `to toml` help
- Remove undocumented "validation" (not really user-facing)

# Tests + Formatting

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

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

- [x] `cargo fmt --all -- --check` to check standard code formatting
(`cargo fmt --all` applies these changes)
- [x] `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- [x] `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/formats/to/toml.rs

@@ -14,19 +14,19 @@ impl Command for ToToml {
 
     fn signature(&self) -> Signature {
         Signature::build("to toml")
-            .input_output_types(vec![(Type::Any, Type::String)])
+            .input_output_types(vec![(Type::Record(vec![]), Type::String)])
             .category(Category::Formats)
     }
 
     fn usage(&self) -> &str {
-        "Convert table into .toml text"
+        "Convert record into .toml text"
     }
 
     fn examples(&self) -> Vec<Example> {
         vec![Example {
-            description: "Outputs an TOML string representing the contents of this table",
-            example: r#"[[foo bar]; ["1" "2"]] | to toml"#,
-            result: Some(Value::test_string("bar = \"2\"\nfoo = \"1\"\n")),
+            description: "Outputs an TOML string representing the contents of this record",
+            example: r#"{foo: 1 bar: 'qwe'} | to toml"#,
+            result: Some(Value::test_string("bar = \"qwe\"\nfoo = 1\n")),
         }]
     }
 
@@ -127,26 +127,6 @@ fn value_to_toml_value(
 ) -> Result<toml::Value, ShellError> {
     match v {
         Value::Record { .. } => helper(engine_state, v),
-        Value::List { ref vals, span } => match &vals[..] {
-            [Value::Record { .. }, _end @ ..] => helper(engine_state, v),
-            _ => Err(ShellError::UnsupportedInput(
-                "Expected a table with TOML-compatible structure".to_string(),
-                "value originates from here".into(),
-                head,
-                *span,
-            )),
-        },
-        Value::String { val, span } => {
-            // Attempt to de-serialize the String
-            toml::de::from_str(val).map_err(|_| {
-                ShellError::UnsupportedInput(
-                    "unable to de-serialize string to TOML".into(),
-                    format!("input: '{:?}'", val),
-                    head,
-                    *span,
-                )
-            })
-        }
         // Propagate existing errors
         Value::Error { error } => Err(error.clone()),
         _ => Err(ShellError::UnsupportedInput(
@@ -225,30 +205,6 @@ mod tests {
                 toml::Value::String("array".to_owned())
             ]))
         );
-        // TOML string
-        let tv = value_to_toml_value(
-            &engine_state,
-            &Value::test_string(
-                r#"
-            title = "TOML Example"
-
-            [owner]
-            name = "Tom Preston-Werner"
-            dob = 1979-05-27T07:32:00-08:00 # First class dates
-
-            [dependencies]
-            rustyline = "4.1.0"
-            sysinfo = "0.8.4"
-            chrono = { version = "0.4.23", features = ["serde"] }
-            "#,
-            ),
-            Span::test_data(),
-        )
-        .expect("Expected Ok from valid TOML string");
-        assert_eq!(
-            tv.get("title").unwrap(),
-            &toml::Value::String("TOML Example".to_owned())
-        );
         //
         // Negative Tests
         //

crates/nu-command/tests/format_conversions/toml.rs

@@ -1,7 +1,93 @@
 use nu_test_support::{nu, pipeline};
 
 #[test]
-fn table_to_toml_text_and_from_toml_text_back_into_table() {
+fn record_map_to_toml() {
+    let actual = nu!(
+    cwd: "tests/fixtures/formats", pipeline(
+        r#"
+            {a: 1 b: 2 c: 'qwe'} 
+            | to toml
+            | from toml
+            | $in == {a: 1 b: 2 c: 'qwe'}
+        "#
+    ));
+
+    assert_eq!(actual.out, "true");
+}
+
+#[test]
+fn nested_records_to_toml() {
+    let actual = nu!(
+    cwd: "tests/fixtures/formats", pipeline(
+        r#"
+            {a: {a: a b: b} c: 1} 
+            | to toml
+            | from toml
+            | $in == {a: {a: a b: b} c: 1}
+        "#
+    ));
+
+    assert_eq!(actual.out, "true");
+}
+
+#[test]
+fn records_with_tables_to_toml() {
+    let actual = nu!(
+    cwd: "tests/fixtures/formats", pipeline(
+        r#"
+            {a: [[a b]; [1 2] [3 4]] b: [[c d e]; [1 2 3]]}
+            | to toml
+            | from toml
+            | $in == {a: [[a b]; [1 2] [3 4]] b: [[c d e]; [1 2 3]]}
+        "#
+    ));
+
+    assert_eq!(actual.out, "true");
+}
+
+#[test]
+fn nested_tables_to_toml() {
+    let actual = nu!(
+    cwd: "tests/fixtures/formats", pipeline(
+        r#"
+            {c: [[f g]; [[[h k]; [1 2] [3 4]] 1]]}
+            | to toml
+            | from toml
+            | $in == {c: [[f g]; [[[h k]; [1 2] [3 4]] 1]]}
+        "#
+    ));
+
+    assert_eq!(actual.out, "true");
+}
+
+#[test]
+fn table_to_toml_fails() {
+    // Tables cant be represented in toml
+    let actual = nu!(
+    cwd: "tests/fixtures/formats", pipeline(
+        r#"
+        try { [[a b]; [1 2] [5 6]] | to toml | false } catch { true }
+        "#
+    ));
+
+    assert_eq!(actual.out, "true");
+}
+
+#[test]
+fn string_to_toml_fails() {
+    // Strings are not a top-level toml structure
+    let actual = nu!(
+    cwd: "tests/fixtures/formats", pipeline(
+        r#"
+        try { 'not a valid toml' | to toml | false } catch { true }
+        "#
+    ));
+
+    assert_eq!(actual.out, "true");
+}
+
+#[test]
+fn big_record_to_toml_text_and_from_toml_text_back_into_record() {
     let actual = nu!(
         cwd: "tests/fixtures/formats", pipeline(
         r#"