truly flexible csv/tsv parsing (nushell#14399)

- fixes nushell#14398

I will properly fill out this PR and fix any tests that might break when
I have the time, this was a quick fix.

# Description

This PR makes `from csv` and `from tsv`, with the `--flexible` flag,
stop dropping extra/unexpected columns.

# User-Facing Changes

`$text`'s contents
```csv
value
1,aaa
2,bbb
3
4,ddd
5,eee,extra
```

Old behavior
```nushell
> $text | from csv --flexible --noheaders 
╭─#─┬─column0─╮
│ 0 │ value   │
│ 1 │       1 │
│ 2 │       2 │
│ 3 │       3 │
│ 4 │       4 │
│ 5 │       5 │
╰─#─┴─column0─╯
```

New behavior
```nushell
> $text | from csv --flexible --noheaders 
╭─#─┬─column0─┬─column1─┬─column2─╮
│ 0 │ value   │   ❎    │   ❎    │
│ 1 │       1 │ aaa     │   ❎    │
│ 2 │       2 │ bbb     │   ❎    │
│ 3 │       3 │   ❎    │   ❎    │
│ 4 │       4 │ ddd     │   ❎    │
│ 5 │       5 │ eee     │ extra   │
╰─#─┴─column0─┴─column1─┴─column2─╯
```

- The first line in a csv (or tsv) document no longer limits the number
of columns
- Missing values in columns are longer automatically filled with `null`
with this change, as a later row can introduce new columns. **BREAKING
CHANGE**

Because missing columns are different from empty columns, operations on
possibly missing columns will have to use optional access syntax e.g.
`get foo` => `get foo?`
  
# Tests + Formatting
Added examples that run as tests and adjusted existing tests to confirm
the new behavior.

# After Submitting

Update the workaround with fish completer mentioned
[here](https://www.nushell.sh/cookbook/external_completers.html#fish-completer)

crates/nu-command/src/formats/from/csv.rs

@@ -11,7 +11,10 @@ impl Command for FromCsv {
 
     fn signature(&self) -> Signature {
         Signature::build("from csv")
-            .input_output_types(vec![(Type::String, Type::table())])
+            .input_output_types(vec![
+                (Type::String, Type::table()),
+                (Type::String, Type::list(Type::Any)),
+            ])
             .named(
                 "separator",
                 SyntaxShape::String,
@@ -82,6 +85,26 @@ impl Command for FromCsv {
                     })],
                 ))
             },
+            Example {
+                description: "Convert comma-separated data to a table, allowing variable number of columns per row",
+                example: "\"ColA,ColB\n1,2\n3,4,5\n6\" | from csv --flexible",
+                result: Some(Value::test_list (
+                    vec![
+                        Value::test_record(record! {
+                            "ColA" => Value::test_int(1),
+                            "ColB" => Value::test_int(2),
+                        }),
+                        Value::test_record(record! {
+                            "ColA" => Value::test_int(3),
+                            "ColB" => Value::test_int(4),
+                            "column2" => Value::test_int(5),
+                        }),
+                        Value::test_record(record! {
+                            "ColA" => Value::test_int(6),
+                        }),
+                    ],
+                ))
+            },
             Example {
                 description: "Convert comma-separated data to a table, ignoring headers",
                 example: "open data.txt | from csv --noheaders",

crates/nu-command/src/formats/from/delimited.rs

@@ -39,12 +39,7 @@ fn from_delimited_stream(
         .from_reader(input_reader);
 
     let headers = if noheaders {
-        (0..reader
-            .headers()
-            .map_err(|err| from_csv_error(err, span))?
-            .len())
-            .map(|i| format!("column{i}"))
-            .collect::<Vec<String>>()
+        vec![]
     } else {
         reader
             .headers()
@@ -54,32 +49,28 @@ fn from_delimited_stream(
             .collect()
     };
 
+    let n = headers.len();
+    let columns = headers
+        .into_iter()
+        .chain((n..).map(|i| format!("column{i}")));
     let iter = reader.into_records().map(move |row| {
         let row = match row {
             Ok(row) => row,
             Err(err) => return Value::error(from_csv_error(err, span), span),
         };
-        let columns = headers.iter().cloned();
-        let values = row
-            .into_iter()
-            .map(|s| {
-                if no_infer {
-                    Value::string(s, span)
-                } else if let Ok(i) = s.parse() {
-                    Value::int(i, span)
-                } else if let Ok(f) = s.parse() {
-                    Value::float(f, span)
-                } else {
-                    Value::string(s, span)
-                }
-            })
-            .chain(std::iter::repeat(Value::nothing(span)));
+        let columns = columns.clone();
+        let values = row.into_iter().map(|s| {
+            if no_infer {
+                Value::string(s, span)
+            } else if let Ok(i) = s.parse() {
+                Value::int(i, span)
+            } else if let Ok(f) = s.parse() {
+                Value::float(f, span)
+            } else {
+                Value::string(s, span)
+            }
+        });
 
-        // If there are more values than the number of headers,
-        // then the remaining values are ignored.
-        //
-        // Otherwise, if there are less values than headers,
-        // then `Value::nothing(span)` is used to fill the remaining columns.
         Value::record(columns.zip(values).collect(), span)
     });
 

crates/nu-command/src/formats/from/tsv.rs

@@ -11,7 +11,10 @@ impl Command for FromTsv {
 
     fn signature(&self) -> Signature {
         Signature::build("from tsv")
-            .input_output_types(vec![(Type::String, Type::table())])
+            .input_output_types(vec![
+                (Type::String, Type::table()),
+                (Type::String, Type::list(Type::Any)),
+            ])
             .named(
                 "comment",
                 SyntaxShape::String,
@@ -76,6 +79,21 @@ impl Command for FromTsv {
                     })],
                 ))
             },
+            Example {
+                description: "Convert comma-separated data to a table, allowing variable number of columns per row and ignoring headers",
+                example: "\"value 1\nvalue 2\tdescription 2\" | from tsv --flexible --noheaders",
+                result: Some(Value::test_list (
+                    vec![
+                        Value::test_record(record! {
+                            "column0" => Value::test_string("value 1"),
+                        }),
+                        Value::test_record(record! {
+                            "column0" => Value::test_string("value 2"),
+                            "column1" => Value::test_string("description 2"),
+                        }),
+                    ],
+                ))
+            },
             Example {
                 description: "Create a tsv file with header columns and open it",
                 example: r#"$'c1(char tab)c2(char tab)c3(char nl)1(char tab)2(char tab)3' | save tsv-data | open tsv-data | from tsv"#,

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

@@ -469,7 +469,7 @@ fn from_csv_test_flexible_extra_vals() {
           echo "a,b\n1,2,3" | from csv --flexible | first | values | to nuon
         "#
     ));
-    assert_eq!(actual.out, "[1, 2]");
+    assert_eq!(actual.out, "[1, 2, 3]");
 }
 
 #[test]
@@ -479,5 +479,5 @@ fn from_csv_test_flexible_missing_vals() {
           echo "a,b\n1" | from csv --flexible | first | values | to nuon
         "#
     ));
-    assert_eq!(actual.out, "[1, null]");
+    assert_eq!(actual.out, "[1]");
 }