fix: set warnings for upstream compiler libs too

jscomp/core/melc_warnings.ml

@@ -70,11 +70,16 @@
   - 102 Mel_polymorphic_comparison
 *)
 
-open Import
-
 let defaults_w = "+a-4-29-40-41-42-44-45-48-58-59-60-61-63..70-102"
 let defaults_warn_error = "-a+5+6+101+109"
 (*TODO: add +10*)
 
 let parse_warnings ~warn_error w =
-  Option.iter Location.(prerr_alert none) (Warnings.parse_options warn_error w)
+  (* Set warnings for the OCaml lexer / parser, which also emit
+     `Illegal_backslash` (warning 14) *)
+  Option.iter Location.(prerr_alert none) (Warnings.parse_options warn_error w);
+
+  (* Now set the warnings for Melange_compiler_libs *)
+  Option.iter
+    Location.(prerr_alert none)
+    (Melange_compiler_libs.Warnings.parse_options warn_error w)

test/blackbox-tests/flags.t

@@ -0,0 +1,141 @@
+Test flags and compile_flags fields on melange.emit stanza
+
+  $ . ./setup.sh
+  $ cat > dune-project <<EOF
+  > (lang dune 3.8)
+  > (using melange 0.1)
+  > EOF
+
+Using flags field in melange.emit stanzas is not supported
+
+  $ cat > dune <<EOF
+  > (melange.emit
+  >  (target output)
+  >  (emit_stdlib false)
+  >  (modules main)
+  >  (flags -w -14-26))
+  > EOF
+
+  $ dune build @mel
+  File "dune", line 5, characters 2-7:
+  5 |  (flags -w -14-26))
+        ^^^^^
+  Error: Unknown field flags
+  [1]
+
+Adds a module that contains unused var (warning 26) and illegal backlash (warning 14)
+
+  $ cat > main.ml <<EOF
+  > let t = "\e\n" in
+  > print_endline "hello"
+  > EOF
+
+  $ cat > dune <<EOF
+  > (melange.emit
+  >  (target output)
+  >  (emit_stdlib false)
+  >  (modules main)
+  >  (alias mel))
+  > EOF
+
+Trying to build triggers both warnings
+
+  $ dune build @mel
+  File "main.ml", line 1, characters 9-11:
+  1 | let t = "\e\n" in
+               ^^
+  Error (warning 14 [illegal-backslash]): illegal backslash escape in string.
+  Hint: Single backslashes \ are reserved for escape sequences
+  (\n, \r, ...). Did you check the list of OCaml escape sequences?
+  To get a backslash character, escape it with a second backslash: \\.
+  File "main.ml", line 1, characters 4-5:
+  1 | let t = "\e\n" in
+          ^
+  Error (warning 26 [unused-var]): unused variable t.
+  [1]
+
+Let's ignore them using compile_flags
+
+  $ cat > dune <<EOF
+  > (melange.emit
+  >  (target output)
+  >  (modules main)
+  >  (emit_stdlib false)
+  >  (alias mel)
+  >  (compile_flags -w -14-26))
+  > EOF
+
+  $ dune build @mel
+  $ node _build/default/output/main.js
+  hello
+
+Can also pass flags from the env stanza. Let's go back to failing state:
+
+  $ cat > dune <<EOF
+  > (melange.emit
+  >  (target output)
+  >  (emit_stdlib false)
+  >  (modules main)
+  >  (alias mel))
+  > EOF
+
+  $ dune build @mel
+  File "main.ml", line 1, characters 9-11:
+  1 | let t = "\e\n" in
+               ^^
+  Error (warning 14 [illegal-backslash]): illegal backslash escape in string.
+  Hint: Single backslashes \ are reserved for escape sequences
+  (\n, \r, ...). Did you check the list of OCaml escape sequences?
+  To get a backslash character, escape it with a second backslash: \\.
+  File "main.ml", line 1, characters 4-5:
+  1 | let t = "\e\n" in
+          ^
+  Error (warning 26 [unused-var]): unused variable t.
+  [1]
+
+Adding env stanza with both warnings silenced allows the build to pass successfully
+
+  $ cat > dune <<EOF
+  > (env
+  >  (_
+  >   (melange.compile_flags -w -14-26)))
+  > (melange.emit
+  >  (alias mel)
+  >  (target output)
+  >  (emit_stdlib false)
+  >  (modules main))
+  > EOF
+
+  $ dune build @mel
+  $ node _build/default/output/main.js
+  hello
+
+Warning 102 (Melange only) is available if explicitly set
+
+  $ cat > main.ml <<EOF
+  > let compare a b = compare a b
+  > EOF
+
+  $ cat > dune <<EOF
+  > (melange.emit
+  >  (target output)
+  >  (modules main)
+  >  (compile_flags -w +a-70))
+  > EOF
+
+  $ dune build output/main.js
+  File "main.ml", line 1, characters 18-29:
+  1 | let compare a b = compare a b
+                        ^^^^^^^^^^^
+  Warning 102 [polymorphic-comparison-introduced]: Polymorphic comparison introduced (maybe unsafe)
+
+But it is disabled by default
+
+  $ cat > dune <<EOF
+  > (melange.emit
+  >  (target output)
+  >  (emit_stdlib false)
+  >  (modules main))
+  > EOF
+
+  $ dune build output/main.js