fix(tvix/eval): quote keys which are not valid identifiers

The impl Display for NixAttrs needs to wrap double quotes around any
keys which are not valid Nix identifiers.  This commit does that,
and adds a test (which fails prior to this commit and passes after
this commit).

Change-Id: Ie31ce91e8637cb27073f23f115db81feefdc6424
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7084
Autosubmit: Adam Joseph <adam@westernsemico.com>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/src/tests/tvix_tests/eval-okay-escapify-integer-keys.exp

@@ -0,0 +1 @@
+{ "3" = 3; }

tvix/eval/src/tests/tvix_tests/eval-okay-escapify-integer-keys.nix

@@ -0,0 +1 @@
+{ "3" = 3; }

tvix/eval/src/value/string.rs

@@ -118,7 +118,13 @@ impl NixString {
         match escaped {
             // A borrowed string is unchanged and can be returned as
             // is.
-            Cow::Borrowed(_) => escaped,
+            Cow::Borrowed(_) => {
+                if is_valid_nix_identifier(&escaped) {
+                    escaped
+                } else {
+                    Cow::Owned(format!("\"{}\"", escaped))
+                }
+            }
 
             // An owned string has escapes, and needs the outer quotes
             // for display.
@@ -152,6 +158,23 @@ fn nix_escape_char(ch: char, next: Option<&char>) -> Option<&'static str> {
     }
 }
 
+/// Return true if this string can be used as an identifier in Nix.
+fn is_valid_nix_identifier(s: &str) -> bool {
+    // adapted from rnix-parser's tokenizer.rs
+    let mut chars = s.chars();
+    match chars.next() {
+        Some('a'..='z' | 'A'..='Z' | '_') => (),
+        _ => return false,
+    }
+    for c in chars {
+        match c {
+            'a'..='z' | 'A'..='Z' | '0'..='9' | '_' | '-' => (),
+            _ => return false,
+        }
+    }
+    return true;
+}
+
 /// Escape a Nix string for display, as most user-visible representation
 /// are escaped strings.
 ///