From 27a427d17fc35ea774f2d881efee4686f2d0180d Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Tue, 21 Apr 2026 17:03:34 +0300
Subject: [PATCH] builder: fix import vsl.la failing (fixes #17389)

---
 vlib/v/builder/rebuilding.v                   |  6 +++
 vlib/v/slow_tests/repl/enum.repl              | 21 +-------
 .../repl/import_vmodules_submodule_test.v     | 52 +++++++++++++++++++
 3 files changed, 60 insertions(+), 19 deletions(-)

diff --git a/vlib/v/builder/rebuilding.v b/vlib/v/builder/rebuilding.v
index 3bdbedf9d..6246dae14 100644
--- a/vlib/v/builder/rebuilding.v
+++ b/vlib/v/builder/rebuilding.v
@@ -17,6 +17,12 @@ pub fn (mut b Builder) rebuild_modules() {
 	if !b.pref.use_cache || b.pref.build_mode == .build_module {
 		return
 	}
+	if b.pref.check_only || b.pref.only_check_syntax {
+		// Check-only flows should not trigger side-effecting cache rebuilds.
+		// In the REPL import path that can compile imported `.c.v` modules
+		// and surface irrelevant missing-header errors for declaration-only checks.
+		return
+	}
 	all_files := b.parsed_files.map(it.path)
 	$if trace_invalidations ? {
 		eprintln('> rebuild_modules all_files: ${all_files}')
diff --git a/vlib/v/slow_tests/repl/enum.repl b/vlib/v/slow_tests/repl/enum.repl
index d34a0d8b2..659e2cbf5 100644
--- a/vlib/v/slow_tests/repl/enum.repl
+++ b/vlib/v/slow_tests/repl/enum.repl
@@ -2,22 +2,5 @@ enum Info { young old }
 fn get_info(age int) Info { if age < 25 { return .young } else { return .old } }
 mut age := 20; println(get_info(age)); age = 45; println(get_info(age))
 ===output===
-warning: module 'os' is imported but never used. Use `import os as _`, to silence this warning, or just remove the unused import line
-    1 | import os
-      |        ~~
-    2 | 
-    3 | import time
-warning: module 'time' is imported but never used. Use `import time as _`, to silence this warning, or just remove the unused import line
-    1 | import os
-    2 | 
-    3 | import time
-      |        ~~~~
-    4 | 
-    5 | import math
-warning: module 'math' is imported but never used. Use `import math as _`, to silence this warning, or just remove the unused import line
-    3 | import time
-    4 | 
-    5 | import math
-      |        ~~~~
-    6 | 
-    7 | enum Info {
+young
+old
diff --git a/vlib/v/slow_tests/repl/import_vmodules_submodule_test.v b/vlib/v/slow_tests/repl/import_vmodules_submodule_test.v
index ff0f0a8db..0d5ade41c 100644
--- a/vlib/v/slow_tests/repl/import_vmodules_submodule_test.v
+++ b/vlib/v/slow_tests/repl/import_vmodules_submodule_test.v
@@ -48,3 +48,55 @@ pub fn testme() int {
 	assert p.code == 0, 'stdout: ${output}\nstderr: ${errors}'
 	assert output.trim_space() == '1', 'stdout: ${output}\nstderr: ${errors}'
 }
+
+fn test_repl_import_does_not_rebuild_c_headers_during_check_only() {
+	old_vmodules := os.getenv('VMODULES')
+	temp_dir := os.join_path(os.vtmp_dir(), 'v_repl_import_vmodules_c_header_${os.getpid()}')
+	vmodules_dir := os.join_path(temp_dir, '.vmodules')
+	module_root := os.join_path(vmodules_dir, 'importc')
+	submodule_dir := os.join_path(module_root, 'la')
+	repl_dir := os.join_path(temp_dir, 'repl')
+	defer {
+		if old_vmodules == '' {
+			os.unsetenv('VMODULES')
+		} else {
+			os.setenv('VMODULES', old_vmodules, true)
+		}
+		os.rmdir_all(temp_dir) or {}
+	}
+	os.rmdir_all(temp_dir) or {}
+	os.mkdir_all(submodule_dir) or { panic(err) }
+	os.mkdir_all(repl_dir) or { panic(err) }
+	os.write_file(os.join_path(submodule_dir, 'la.v'), 'module la
+
+pub fn ping() int {
+	return 1
+}
+') or {
+		panic(err)
+	}
+	os.write_file(os.join_path(submodule_dir, 'ffi.c.v'), 'module la
+
+#include "missing_header_17389.h"
+
+fn C.importc_test_missing_header() int
+') or {
+		panic(err)
+	}
+	os.setenv('VMODULES', vmodules_dir, true)
+	vexec := runner.full_path_to_v(5)
+	mut p := os.new_process(vexec)
+	p.set_args(['repl', '-replfolder', repl_dir, '-replprefix', 'import_vmodules_c_header.'])
+	p.set_work_folder(module_root)
+	p.set_redirect_stdio()
+	p.run()
+	p.stdin_write('import importc.la\nexit\n')
+	p.wait()
+	output := p.stdout_slurp()
+	errors := p.stderr_slurp()
+	p.close()
+	combined := output + errors
+	assert p.code == 0, 'stdout: ${output}\nstderr: ${errors}'
+	assert !combined.contains('builder error:'), 'stdout: ${output}\nstderr: ${errors}'
+	assert !combined.contains('missing_header_17389.h'), 'stdout: ${output}\nstderr: ${errors}'
+}
-- 
2.39.5