parser: fix anonymous function name collisions across files (#26642)

Author: quaesitor-scientiam

vlib/v/ast/str.v

@@ -4,6 +4,7 @@
 @[has_globals]
 module ast
 
+import v.token
 import v.util
 import strings
 import sync.stdatomic
@@ -18,8 +19,8 @@ pub fn (f &FnDecl) get_name() string {
 }
 
 // get_anon_fn_name returns the unique anonymous function name, based on the prefix, the func signature and its position in the source code
-pub fn (table &Table) get_anon_fn_name(prefix string, func &Fn, pos int) string {
-	return 'anon_fn_${prefix}_${table.fn_type_signature(func)}_${pos}'
+pub fn (table &Table) get_anon_fn_name(prefix string, func &Fn, pos token.Pos) string {
+	return 'anon_fn_${prefix}_${pos.file_idx}_${table.fn_type_signature(func)}_${pos.pos}'
 }
 
 // get_name returns the real name for the function calling

vlib/v/checker/lambda_expr.v

@@ -83,7 +83,7 @@ pub fn (mut c Checker) lambda_expr(mut node ast.LambdaExpr, exp_typ ast.Type) as
 			return_type: return_type
 			is_method:   false
 		}
-		name := c.table.get_anon_fn_name(c.file.unique_prefix, func, node.pos.pos)
+		name := c.table.get_anon_fn_name(c.file.unique_prefix, func, node.pos)
 		func.name = name
 		idx := c.table.find_or_register_fn_type(func, true, false)
 		typ := ast.new_type(idx)

vlib/v/checker/tests/modules/anon_fn_name_across_files.out

@@ -0,0 +1,2 @@
+1
+2

vlib/v/checker/tests/modules/anon_fn_name_across_files/a.v

@@ -0,0 +1,11 @@
+module main
+
+// Both a.v and b.v define an anonymous function with the same signature (fn() int)
+// at the same byte offset from the start of their respective files.
+// Before the fix in get_anon_fn_name(), both would produce the same C symbol name,
+// causing a linker collision or silent wrong behaviour.
+fn get_fn_a() fn () int {
+	return fn () int {
+		return 1
+	}
+}

vlib/v/checker/tests/modules/anon_fn_name_across_files/b.v

@@ -0,0 +1,11 @@
+module main
+
+// This file intentionally pads its content so that the anonymous function below
+// starts at the same byte offset as the one in a.v. Before the fix in
+// get_anon_fn_name() (which now includes pos.file_idx in the C symbol name),
+// both would produce the same name and trigger a linker error.
+fn get_fn_b() fn () int {
+	return fn () int {
+		return 2
+	}
+}

vlib/v/checker/tests/modules/anon_fn_name_across_files/main.v

@@ -0,0 +1,8 @@
+module main
+
+fn main() {
+	fa := get_fn_a()
+	fb := get_fn_b()
+	println(fa())
+	println(fb())
+}

vlib/v/parser/fn.v

@@ -1218,7 +1218,7 @@ fn (mut p Parser) anon_fn() ast.AnonFn {
 		return_type: return_type
 		is_method:   false
 	}
-	name := p.table.get_anon_fn_name(p.unique_prefix, func, p.tok.pos)
+	name := p.table.get_anon_fn_name(p.unique_prefix, func, p.tok.pos())
 	keep_fn_name := p.cur_fn_name
 	p.cur_fn_name = name
 	if p.tok.kind == .lcbr {