checker: prevent usage of imported module name prefix as identifier names, to avoid cgen collisions (#25280)

vlib/v/checker/assign.v

@@ -444,6 +444,7 @@ fn (mut c Checker) assign_stmt(mut node ast.AssignStmt) {
 						if c.check_import_sym_conflict(left.name) {
 							c.error('duplicate of an import symbol `${left.name}`', left.pos)
 						}
+						c.check_module_name_conflict(left.name, left.pos)
 					}
 					if node.op == .assign && left_type.has_flag(.option) && right is ast.UnsafeExpr
 						&& right.expr.is_nil() {

vlib/v/checker/checker.v

@@ -145,6 +145,7 @@ mut:
 	variant_data_type  ast.Type
 	fn_return_type     ast.Type
 	orm_table_fields   map[string][]ast.StructField // known table structs
+	short_module_names []string                     // to check for function names colliding with module functions
 
 	v_current_commit_hash string // same as old C.V_CURRENT_COMMIT_HASH
 	assign_stmt_attr      string // for `x := [1,2,3] @[freed]`
@@ -331,6 +332,14 @@ pub fn (mut c Checker) change_current_file(file &ast.File) {
 	c.vmod_file_content = ''
 	c.mod = file.mod.name
 	c.is_generated = file.is_generated
+	c.short_module_names = ['builtin']
+	for import_sym in c.file.imports {
+		c.short_module_names << if import_sym.alias == '' {
+			import_sym.mod.all_after_last('.')
+		} else {
+			import_sym.alias
+		}
+	}
 }
 
 pub fn (mut c Checker) check_files(ast_files []&ast.File) {
@@ -5790,6 +5799,16 @@ fn (c &Checker) check_import_sym_conflict(ident string) bool {
 	return false
 }
 
+fn (mut c Checker) check_module_name_conflict(ident string, pos token.Pos) {
+	if ident.contains('__') {
+		prefix := ident.all_before('__')
+		if prefix in c.short_module_names {
+			c.error('identifier cannot use prefix `${prefix}__` of imported module `${prefix}`',
+				pos)
+		}
+	}
+}
+
 // update_unresolved_fixed_sizes updates the unresolved type symbols for array fixed return type and alias type.
 pub fn (mut c Checker) update_unresolved_fixed_sizes() {
 	for mut stmt in c.unresolved_fixed_sizes {

vlib/v/checker/fn.v

@@ -104,6 +104,7 @@ fn (mut c Checker) fn_decl(mut node ast.FnDecl) {
 		if !node.is_method && node.mod == 'main' && node.short_name in c.table.builtin_pub_fns {
 			c.error('cannot redefine builtin public function `${node.short_name}`', node.pos)
 		}
+		c.check_module_name_conflict(node.short_name, node.pos)
 	}
 	if node.name == 'main.main' {
 		c.main_fn_decl_node = *node

vlib/v/checker/tests/clash_ident_module_name_prefix.out

@@ -0,0 +1,28 @@
+vlib/v/checker/tests/clash_ident_module_name_prefix.vv:7:1: error: identifier cannot use prefix `builtin__` of imported module `builtin`
+    5 | // os is not imported so the os__ prefix should not produce an error
+    6 |
+    7 | fn builtin__string_str() {
+      | ~~~~~~~~~~~~~~~~~~~~~~~~
+    8 | }
+    9 |
+vlib/v/checker/tests/clash_ident_module_name_prefix.vv:10:1: error: identifier cannot use prefix `time__` of imported module `time`
+    8 | }
+    9 |
+   10 | fn time__utc() {
+      | ~~~~~~~~~~~~~~
+   11 | }
+   12 |
+vlib/v/checker/tests/clash_ident_module_name_prefix.vv:17:2: error: identifier cannot use prefix `builtin__` of imported module `builtin`
+   15 |
+   16 | fn main() {
+   17 |     builtin__string_str := 'Hello V!'.str()
+      |     ~~~~~~~~~~~~~~~~~~~
+   18 |     time__now := time.now()
+   19 |     os__log := 'Hello V!'
+vlib/v/checker/tests/clash_ident_module_name_prefix.vv:18:2: error: identifier cannot use prefix `time__` of imported module `time`
+   16 | fn main() {
+   17 |     builtin__string_str := 'Hello V!'.str()
+   18 |     time__now := time.now()
+      |     ~~~~~~~~~
+   19 |     os__log := 'Hello V!'
+   20 |     println(builtin__string_str)

vlib/v/checker/tests/clash_ident_module_name_prefix.vv

@@ -0,0 +1,23 @@
+import time
+
+// builtin__ prefix should always produce an error
+// time is imported so the time__ prefix should always produce an error
+// os is not imported so the os__ prefix should not produce an error
+
+fn builtin__string_str() {
+}
+
+fn time__utc() {
+}
+
+fn os__getwd() {
+}
+
+fn main() {
+	builtin__string_str := 'Hello V!'.str()
+	time__now := time.now()
+	os__log := 'Hello V!'
+	println(builtin__string_str)
+	println(time__now)
+	println(os__log)
+}