Do not check the Flow parser's AST for dead code

Without the AST mapper, which read every field to rebuild nodes, most
fields of flow_ast.ml are only written: make dead-code reported 435
record labels and 54 modules of them, and nothing else in the Flow
parser. The parser fills in the whole AST and the compiler reads only
what it needs to classify %raw code, so that file is suppressed like
the frozen parsetree0.ml; the rest of the Flow parser stays checked.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Kbf3gLDo8mBJKpNE25WAwT
Signed-off-by: Christoph Knittel <ck@cca.io>

Author: cknitt

compiler/flow_parser/README.md

@@ -3,7 +3,8 @@
 This directory contains the OCaml Flow parser used by the ReScript compiler.
 It is a pruned fork: Flow's parser has moved to Rust, so there are no further
 OCaml releases to sync with, and code ReScript does not use is removed like
-dead code anywhere else in the repository (`make dead-code` covers it).
+dead code anywhere else in the repository (`make dead-code` covers it, except
+the AST in `parser/flow_ast.ml`).
 
 - Upstream repository: https://github.com/facebook/flow
 - Flow parser version: `0.320.0`
@@ -40,7 +41,12 @@ printers (`[@@deriving show]`), the file-key variants (ReScript parses only
 strings), error variants that are never raised, and AST fields that the parser
 fills in but nothing reads (template literal quasi values, tolerated-syntax
 records, JSX text and fragment locations, comment `on_newline`, accessibility
-comments).
+comments). The AST mapper classes that moved trailing comments between nodes
+(`Flow_ast_mapper` and its subclasses in `Comment_attachment`) are gone too:
+ReScript never reads trailing comments, and creating the classes was most of
+the compiler's startup work. They were also what read every AST field, so most
+fields of `flow_ast.ml` are now only written; `make dead-code` does not check
+that file.
 
 Vendored sources are excluded from the repository-wide OCamlformat check so
 that they keep Flow's formatting.

scripts/dead_code.sh

@@ -24,8 +24,12 @@ trap 'rm -rf "$root"' EXIT
 ln -s "$PWD/_build/default" "$root/native"
 ln -s "$PWD/_build_playground/default" "$root/playground"
 
-# Not reported: compiler/ml/parsetree0.ml is the frozen AST of PPXs, which
-# build the constructors the compiler itself never builds; it must not change.
+# Not reported:
+# - compiler/ml/parsetree0.ml is the frozen AST of PPXs, which build the
+#   constructors the compiler itself never builds; it must not change.
+# - compiler/flow_parser/parser/flow_ast.ml is the vendored Flow parser's AST:
+#   the parser fills in every field, the compiler reads only the few it needs
+#   to classify %raw code.
 "$reanalyze" -dce-cmt "$root" \
-  -suppress compiler/ml/parsetree0.ml \
+  -suppress compiler/ml/parsetree0.ml,compiler/flow_parser/parser/flow_ast.ml \
   -set-exit-code