Show ? for optional entries when displaying CellPaths (#14042)

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# Description
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This PR makes the `Display` implementation for `CellPath` show a `?`
suffix on every optional entry, which makes the output consistent with
the language syntax.

Before this PR, the printing of cell paths was confusing, e.g. `$.x` and
`$.x?` were both printed as `x`. Now, the second one is printed as `x?`.

# User-Facing Changes
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helps us keep track of breaking changes. -->

The formatting of cell paths now matches the syntax used to create them,
reducing confusion.

# Tests + Formatting
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All tests pass, including `stdlib` tests.

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crates/nu-command/tests/commands/select.rs

@@ -166,7 +166,7 @@ fn select_ignores_errors_successfully1() {
 fn select_ignores_errors_successfully2() {
     let actual = nu!("[{a: 1} {a: 2} {a: 3}] | select b? | to nuon");
 
-    assert_eq!(actual.out, "[[b]; [null], [null], [null]]".to_string());
+    assert_eq!(actual.out, "[[b?]; [null], [null], [null]]".to_string());
     assert!(actual.err.is_empty());
 }
 
@@ -174,7 +174,7 @@ fn select_ignores_errors_successfully2() {
 fn select_ignores_errors_successfully3() {
     let actual = nu!("{foo: bar} | select invalid_key? | to nuon");
 
-    assert_eq!(actual.out, "{invalid_key: null}".to_string());
+    assert_eq!(actual.out, "{invalid_key?: null}".to_string());
     assert!(actual.err.is_empty());
 }
 
@@ -184,7 +184,10 @@ fn select_ignores_errors_successfully4() {
         r#""key val\na 1\nb 2\n" | lines | split column --collapse-empty " " | select foo? | to nuon"#
     );
 
-    assert_eq!(actual.out, r#"[[foo]; [null], [null], [null]]"#.to_string());
+    assert_eq!(
+        actual.out,
+        r#"[[foo?]; [null], [null], [null]]"#.to_string()
+    );
     assert!(actual.err.is_empty());
 }
 
@@ -234,7 +237,7 @@ fn ignore_errors_works() {
         [{}] | select -i $path | to nuon
         "#);
 
-    assert_eq!(actual.out, "[[foo]; [null]]");
+    assert_eq!(actual.out, "[[foo?]; [null]]");
 }
 
 #[test]

crates/nu-protocol/src/ast/cell_path.rs

@@ -182,8 +182,14 @@ impl Display for CellPath {
                 write!(f, ".")?;
             }
             match elem {
-                PathMember::Int { val, .. } => write!(f, "{val}")?,
-                PathMember::String { val, .. } => write!(f, "{val}")?,
+                PathMember::Int { val, optional, .. } => {
+                    let question_mark = if *optional { "?" } else { "" };
+                    write!(f, "{val}{question_mark}")?
+                }
+                PathMember::String { val, optional, .. } => {
+                    let question_mark = if *optional { "?" } else { "" };
+                    write!(f, "{val}{question_mark}")?
+                }
             }
         }
         Ok(())