transform: keep escaped ${ literal in interpolated strings

[medvednikov]

Problem

v ~/.vmodules/ui2/ide fails in C:

src.c: In function 'companion_main_source':
error: 'err' undeclared (first use in this function)
error: 'event' undeclared (first use in this function)

companion_main_source builds V source text with a string like
"... ${vml_escape(vml_name)}: \${err}') ...". The transform re-parsed the text of every
string literal inside a string interpolation as ${...} code. The parser has already
unescaped that text, so an escaped \${name} was expanded as a real interpolation.

Undeclared names fail in C. Declared names are silently substituted:

x := 'X'
println('${x} \${x}')             // printed `X X`, expected `X ${x}`
println('${x} \x24{x}')           // printed `X X`
println('${id('\${x}')}')         // printed `X`
println('${r'${x}'}')             // printed `X`
println('${tags.map('--\${x}')}') // printed `--X`

A string literal on its own ('\${x}') was fine. The bug needed a real interpolation in the
same string, or the literal nested inside one.

Fix

The re-parse was a workaround from before the scanner split nested interpolations itself
(#28331). A string literal the parser produces can no longer contain a real interpolation, so
transform_expr now passes string literals through unchanged. The recovery helpers this
leaves unused are removed, along with the in_string_interp_part flag. The ORM path still
uses simple_nested_string_interpolation for SQL tokens and is unchanged.

#28416 fixed the it case of this same bug on its own ('--\${it}' inside a map). That
test still passes.

Still open: transform_nested_if_string_interp_node / ..._match_... (and the matching
helper in if.v) apply the same kind of text heuristic to a 5-part interpolation, so
'\${if ok { ${a} } else { ${b} }}' still prints A. That needs an escaped \${if or
\${match followed by branch-shaped real interpolations. It is left out to keep this
change focused.

Tests

• New vlib/v/tests/builtin_strings_and_interpolation/string_interpolation_escaped_dollar_test.v:
  escaped \${, \x24{, \044{, raw literals and escaped text inside interpolation
  expressions stay literal, and real nested interpolation still expands. Fails on master:
  C error for the undeclared-name case, assertion failures (X X vs X ${x}) without it.
• v test vlib/v/tests/builtin_strings_and_interpolation/: 51/51 pass.
• vlib/v/compiler_tests/nested_escaped_map_it_interp_test.v: passes.
• ui2/ide now builds.

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-24T16:00:31.109875Z	bf5542f	PR opened

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review" or "@codex security review".

Codex reacts with 👀 while any review is running, comments if it has suggestions, and reacts with 👍 once all reviews finish with no findings.