Represent %raw, %ffi and %%raw with parsetree nodes

[cknitt]

Part of #8624 ("extension-keyed language features → nodes"). Builds on #8875 (merged).

Summary

%raw, %ffi and %%raw get dedicated parsetree nodes instead of being extensions with a string payload, and Pconst_raw_source is removed.

| Pexp_raw of {kind: raw_kind; name_loc: Location.t; source: raw_source}
| Pstr_raw of {name_loc: Location.t; source: raw_source; attrs: attributes}

and raw_kind = Raw_expression (* %raw *) | Raw_function (* %ffi *)

and raw_source =
  | Raw_string of string loc (* %raw("..."): the JavaScript source *)
  | Raw_template of string loc (* %raw(`...`): the template source as written *)

• %raw and %ffi share one node: both lower to the same raw JavaScript, and %ffi only adds the function check and an arity-based type. The kind keeps them apart for printing, typing and the v0 bridge.
• The source is kept as written, so the formatter output is unchanged. Ast_raw.js_source gives the JavaScript.
• When a node is turned back into an extension (printer, comments table, v0 bridge), a string is printed as a template if its source contains an unescaped ", as the printer did before. Unlike before, the template escapes backquotes, ${ and backslashes where needed, so it parses back to the same JavaScript.
• name_loc is the location of %raw / %ffi / %%raw, which cannot be derived from the node's location when attributes precede it.
• Payloads the node can't represent (braced or attributed payloads, non-strings) stay extensions and are processed as before.
• PPXs still see %raw/%ffi/%%raw extensions with the same v0 encoding, and Ast_mapper_from0 reads them back into the nodes.
• The .cmi, .cmt and AST magic numbers are bumped.

Fix

Through a PPX, a template payload such as %raw(`"a\${b}"`) lost its template marker in the v0 bridge. Its escapes were then not decoded, and locations in error messages shifted by one character. The node keeps the template, so compiling with and without a PPX now gives the same result.

Braced string payloads such as %raw({"\"a\""}) were taken as JavaScript source through a PPX, and came back with a @res.braces attribute instead of braces. They are now mapped like any other expression, so they are decoded as ordinary strings with and without a PPX.

Tests

• ast-mapping/RawExtensions.res covers strings, templates with escapes, braced strings, %ffi, attributes, doc comments and %%raw.
• raw_output_test.res compiles braced %raw and %%raw strings.
• ounit_ast_mapper0_tests: raw sources printed as templates parse back to the same JavaScript, and attributed templates survive the v0 bridge.
• ounit_ast_mapper0_tests: raw nodes reach v0 as the extensions they are written as and come back unchanged, including a %%raw with attributes.

Compared with master (#8875), the formatter output and the v0 tree a PPX receives are identical for every .res/.resi file in the repository. Compiled output and errors are identical for about 200 %raw/%ffi/%%raw forms in many positions, including types, patterns and modules where they stay extensions. Through a PPX they differ only in the fixes above.

🤖 Generated with Claude Code

https://claude.ai/code/session_01NP47qMfjMbjUTKZDsLthDo

[codecov]

Codecov Report

❌ Patch coverage is 73.83513% with 73 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.98%. Comparing base (735a485) to head (36f11f8).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
compiler/ml/printast.ml	0.00%	9 Missing ⚠️
compiler/syntax/src/res_parens.ml	0.00%	8 Missing ⚠️
tests/ounit_tests/ounit_ast_mapper0_tests.ml	87.50%	8 Missing ⚠️
compiler/frontend/ast_exp_handle_external.ml	86.11%	5 Missing ⚠️
compiler/ml/ast_iterator.ml	0.00%	5 Missing ⚠️
compiler/ml/typecore.ml	0.00%	5 Missing ⚠️
compiler/syntax/src/res_comments_table.ml	0.00%	5 Missing ⚠️
compiler/ml/ast_mapper_from0.ml	82.60%	4 Missing ⚠️
compiler/ml/ast_payload.ml	55.55%	4 Missing ⚠️
compiler/ml/ast_raw.ml	91.66%	3 Missing ⚠️
... and 11 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8879      +/-   ##
==========================================
- Coverage   81.01%   80.98%   -0.03%     
==========================================
  Files         454      455       +1     
  Lines       63187    63390     +203     
==========================================
+ Hits        51193    51339     +146     
- Misses      11994    12051      +57     

Files with missing lines	Coverage Δ
analysis/src/dump_ast.ml	44.01% <ø> (+0.18%)	⬆️
compiler/ext/config.ml	100.00% <ø> (ø)
compiler/frontend/bs_builtin_ppx.ml	90.61% <100.00%> (+0.12%)	⬆️
compiler/ml/ast_mapper.ml	98.45% <100.00%> (+0.03%)	⬆️
compiler/ml/ast_mapper_to0.ml	77.75% <100.00%> (+0.46%)	⬆️
compiler/syntax/src/res_core.ml	91.99% <100.00%> (+<0.01%)	⬆️
compiler/syntax/src/res_parsetree_viewer.ml	94.42% <ø> (-0.05%)	⬇️
compiler/syntax/src/res_printer.ml	94.03% <100.00%> (+0.10%)	⬆️
analysis/reanalyze/src/annotation.ml	52.63% <0.00%> (ø)
analysis/src/document_symbol.ml	56.09% <0.00%> (ø)
... and 19 more

... and 1 file with indirect coverage changes

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[cknitt]

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[cknitt]

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Semantic diff: https://cdn.rescript-lang.org/semantic-diff/pr-8879/index.html?v=36f11f847

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Reviewed commit: 36f11f847a

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