v.parser: forbid unsupported language functions/types in specific backends (#12655)

vlib/v/parser/fn.v

@@ -207,6 +207,7 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 	p.open_scope()
 	// C. || JS.
 	mut language := ast.Language.v
+	language_tok_pos := p.tok.position()
 	if p.tok.kind == .name && p.tok.lit == 'C' {
 		is_unsafe = !is_trusted
 		language = .c
@@ -224,12 +225,12 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 	}
 	if is_keep_alive && language != .c {
 		p.error_with_pos('attribute [keep_args_alive] is only supported for C functions',
-			p.tok.position())
+			language_tok_pos)
 	}
 	if language != .v {
 		p.next()
 		p.check(.dot)
-		p.check_for_impure_v(language, p.tok.position())
+		p.check_for_impure_v(language, language_tok_pos)
 	}
 	// Receiver?
 	mut rec := ReceiverParsingInfo{

vlib/v/parser/parser.v

@@ -1627,24 +1627,46 @@ fn (mut p Parser) parse_attr() ast.Attr {
 	}
 }
 
+pub fn (mut p Parser) language_not_allowed_error(language ast.Language, pos token.Position) {
+	upcase_language := language.str().to_upper()
+	p.error_with_pos('$upcase_language code is not allowed in .${p.file_backend_mode}.v files, please move it to a .${language}.v file',
+		pos)
+}
+
+pub fn (mut p Parser) language_not_allowed_warning(language ast.Language, pos token.Position) {
+	upcase_language := language.str().to_upper()
+	p.warn_with_pos('$upcase_language code will not be allowed in pure .v files, please move it to a .${language}.v file instead',
+		pos)
+}
+
 pub fn (mut p Parser) check_for_impure_v(language ast.Language, pos token.Position) {
 	if language == .v {
 		// pure V code is always allowed everywhere
 		return
+	} else {
+		match p.file_backend_mode {
+			.c {
+				if language != .c {
+					p.language_not_allowed_error(language, pos)
+					return
+				}
+			}
+			.js {
+				if language != .js {
+					p.language_not_allowed_error(language, pos)
+					return
+				}
+			}
+			else {}
+		}
 	}
 	if !p.pref.warn_impure_v {
 		// the stricter mode is not ON yet => allow everything for now
 		return
 	}
 	if p.file_backend_mode != language {
-		upcase_language := language.str().to_upper()
 		if p.file_backend_mode == .v {
-			p.warn_with_pos('$upcase_language code will not be allowed in pure .v files, please move it to a .${language}.v file instead',
+			p.language_not_allowed_warning(language, pos)
-				pos)
-			return
-		} else {
-			p.warn_with_pos('$upcase_language code is not allowed in .${p.file_backend_mode}.v files, please move it to a .${language}.v file',
-				pos)
 			return
 		}
 	}

vlib/v/parser/tests/forbidden_language_support_c.c.out

@@ -0,0 +1,3 @@
+vlib/v/parser/tests/forbidden_language_support_c.c.vv:1:4: error: JS code is not allowed in .c.v files, please move it to a .js.v file
+    1 | fn JS.a() int
+      |    ~~

vlib/v/parser/tests/forbidden_language_support_c.c.vv

@@ -0,0 +1 @@
+fn JS.a() int

vlib/v/parser/tests/forbidden_language_support_js.js.out

@@ -0,0 +1,3 @@
+vlib/v/parser/tests/forbidden_language_support_js.js.vv:1:4: error: C code is not allowed in .js.v files, please move it to a .c.v file
+    1 | fn C.a() int
+      |    ^

vlib/v/parser/tests/forbidden_language_support_js.js.vv

@@ -0,0 +1 @@
+fn C.a() int

vlib/v/parser/tests/forbidden_language_support_js.out

@@ -0,0 +1,3 @@
+v\vlib\v\tests\inout\forbidden_language_support_js.js.vv:1:4: error: C code is not allowed in .js.v files, please move it to a .c.v file
+    1 | fn C.a() int
+      |