fix(tvix/eval): use byte, not codepoint index for slicing in escape

This fixes a subtle issue which would occasionally lead to a crash (e.g.
when evaluating (pkgs.systemd.outPath with --trace-runtime): With each
character in the string that has a multi byte representation in UTF-8,
the actual byte position and what tvix thought it was would get out of
sync. This could either lead to

* Tvix swallowing characters or jumbling characters if multi byte
  characters would cause the tracked index to become out of sync with
  the byte position before the first character to be escaped, or

* Tvix crashing if (in the same situation) the out of sync index would
  be within a UTF-8 byte sequence.

Luckily, std's `char_indices()` iterator implements exactly what
`nix_escape_char()`'s original author had in mind with
`.chars().enumerate()`. Using `i + 1` for continuing is safe, since all
characters that need (in fact, can) to be escaped in Nix are represented
as a single byte in UTF-8.

Change-Id: I1c836f70cde3d72db1c644e9112852f0d824715e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8952
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
Autosubmit: sterni <sternenseemann@systemli.org>

tvix/eval/src/tests/tvix_tests/eval-okay-escape-string-correct-char-boundaries.exp

@@ -0,0 +1 @@
+"💭(\":thonking:\")"

tvix/eval/src/tests/tvix_tests/eval-okay-escape-string-correct-char-boundaries.nix

@@ -0,0 +1,6 @@
+# Regression test for a bug where tvix would crash in nix_escape_string
+# because it counted the string position by unicode code point count,
+# but then used it as a byte index for slicing. Consequently, it would
+# try slicing 💭 in half, thinking the first element to be escaped was
+# at byte index 2 (i.e. the quote).
+"💭(\":thonking:\")"

tvix/eval/src/value/string.rs

@@ -193,7 +193,7 @@ fn is_valid_nix_identifier(s: &str) -> bool {
 ///
 /// Note that this does not add the outer pair of surrounding quotes.
 fn nix_escape_string(input: &str) -> Cow<str> {
-    let mut iter = input.chars().enumerate().peekable();
+    let mut iter = input.char_indices().peekable();
 
     while let Some((i, c)) = iter.next() {
         if let Some(esc) = nix_escape_char(c, iter.peek().map(|(_, c)| c)) {
@@ -201,6 +201,10 @@ fn nix_escape_string(input: &str) -> Cow<str> {
             escaped.push_str(&input[..i]);
             escaped.push_str(esc);
 
+            // In theory we calculate how many bytes it takes to represent `esc`
+            // in UTF-8 and use that for the offset. It is, however, safe to
+            // assume that to be 1, as all characters that can be escaped in a
+            // Nix string are ASCII.
             let mut inner_iter = input[i + 1..].chars().peekable();
             while let Some(c) = inner_iter.next() {
                 match nix_escape_char(c, inner_iter.peek()) {