checker/cgen: add checks and fix cgen for [typedef] for C structs (#8169)

vlib/math/big/big.v

@@ -4,7 +4,6 @@ module big
 #flag -I @VROOT/thirdparty/bignum
 #flag @VROOT/thirdparty/bignum/bn.o
 #include "bn.h"
-[typedef]
 struct C.bn {
 mut:
 	array [32]u32

vlib/v/checker/checker.v

@@ -387,6 +387,11 @@ pub fn (mut c Checker) struct_decl(mut decl ast.StructDecl) {
 				c.error('`$embed_sym.name` is not a struct', embed.pos)
 			}
 		}
+		for attr in decl.attrs {
+			if attr.name == 'typedef' && decl.language != .c {
+				c.error('`typedef` attribute can only be used with C structs', decl.pos)
+			}
+		}
 		for i, field in decl.fields {
 			if decl.language == .v {
 				c.check_valid_snake_case(field.name, 'field name', field.pos)

vlib/v/checker/tests/typedef_attr_v_struct_err.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/typedef_attr_v_struct_err.vv:2:1: error: `typedef` attribute can only be used with C structs
+    1 | [typedef]
+    2 | struct Point{}
+      | ~~~~~~~~~~~~
+    3 | 
+    4 | fn main() {

vlib/v/checker/tests/typedef_attr_v_struct_err.vv

@@ -0,0 +1,7 @@
+[typedef]
+struct Point{}
+
+fn main() {
+    p := Point{}
+    println(p)
+}

vlib/v/gen/cgen.v

@@ -660,7 +660,15 @@ typedef struct {
 			.alias {
 				parent := unsafe { &g.table.types[typ.parent_idx] }
 				is_c_parent := parent.name.len > 2 && parent.name[0] == `C` && parent.name[1] == `.`
-				parent_styp := if is_c_parent { 'struct ' + parent.cname[3..] } else { parent.cname }
+				mut is_typedef := false
+				if parent.info is table.Struct {
+					is_typedef = parent.info.is_typedef
+				}
+				parent_styp := if is_c_parent {
+					if !is_typedef { 'struct ' + parent.cname[3..] } else { parent.cname[3..] }
+				} else {
+					parent.cname
+				}
 				g.type_definitions.writeln('typedef $parent_styp $typ.cname;')
 			}
 			.array {