0.17.1 nightlies remove #"…"# hash string literals (E0098) with no render-preserving migration path — the backtick replacement dedents indent-bearing prompt fragments #4591

Summary

0.17.1-nightly.20260824.b removes #"…"# hash string literals (error[E0098] removed language feature) with no migration path that preserves the rendered prompt:

• baml fmt under the nightly does not migrate them — it exits 4 ("formatted 27 of 47 file(s); 20 failed") and leaves every hash literal in place (while reformatting the other 27 files, which is separate churn). There is no baml migrate command.
• The obvious mechanical replacement — a multi-line backtick string — is not render-equivalent: backtick strings are dedented by their common indent, so an indent-bearing hash literal (used as a prompt fragment) renders differently after the rewrite.

Measured on a real prompt corpus (47 .baml files)

• Census: 358 hash literals across 20 files; 36 of them multi-line (indent is part of the render); 127 contain a backtick (needs escaping in the backtick form); 17 contain " (needs \" in the quoted form); 0 contain ${ or $ or \.
• Naive #"…"# → backtick migration: baml check green, but 11 of 33 rendered prompts change — proven exactly: same line counts, 28 differing lines, all of the form old == " " + new (a 4-space common-indent dedent of shared number-format fragments).
• A render-neutral migration exists: every hash literal becomes ONE double-quoted string with \\, \", \n escaped. baml check exit 0 on both 0.17.0 and the nightly, and 0/33 render diffs. Cost: a ~2.3 kB prompt block becomes one enormous single line — prompt review as prose becomes materially harder. No indent-preserving multi-line form survives common-indent dedent unless a line sits at column 0, which changes the render.

Why this matters

For a prompt-corpus consumer, "render-identical" is the difference between a mechanical toolchain upgrade and a re-verification of every prompt against a live model (recorded-eval re-record). We just paid the 0.15→0.17 migration with a per-function byte-level render differential to avoid exactly that; this removal forces the same exercise again, and the only render-safe target form is much less reviewable than what was removed.

Asks (either would resolve it)

1. A migration mode — baml fmt --fix-removed-features or similar — that rewrites hash literals to a render-identical form automatically; or
2. A raw multi-line string form that preserves indentation (no common-indent dedent), so indent-bearing prompt fragments have a faithful home.

Environment

baml wrapper 0.2.0; 0.17.0 (accepts hash literals) vs 0.17.1-nightly.20260824.b (E0098); Linux x86_64. Related context from this consumer: #4588 (fixed on this nightly — thank you), #4589/#4590 (still reproduce on it), #4370.