From 9ef51ee714b185f2c0bc0ff057ace918f7d64f63 Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Sat, 19 Jul 2025 18:47:44 +0300
Subject: [PATCH] v: virtual C consts with custom types (const C.MY_CONST u8)

---
 vlib/v/ast/ast.v               | 13 ++++++------
 vlib/v/checker/checker.v       |  4 ++++
 vlib/v/fmt/fmt.v               | 12 ++++++++++--
 vlib/v/parser/parser.v         | 36 ++++++++++++++++++++++++++--------
 vlib/v/tests/c_const_u8_test.v |  9 +++++++++
 5 files changed, 58 insertions(+), 16 deletions(-)
 create mode 100644 vlib/v/tests/c_const_u8_test.v

diff --git a/vlib/v/ast/ast.v b/vlib/v/ast/ast.v
index ae01e099a4..eb1e7989a8 100644
--- a/vlib/v/ast/ast.v
+++ b/vlib/v/ast/ast.v
@@ -398,12 +398,13 @@ pub fn (f &StructField) equals(o &StructField) bool {
 // const field in const declaration group
 pub struct ConstField {
 pub:
-	mod         string
-	name        string
-	is_pub      bool
-	is_markused bool // an explicit `@[markused]` tag; the const will NOT be removed by `-skip-unused`, no matter what
-	pos         token.Pos
-	attrs       []Attr // same value as `attrs` of the ConstDecl to which it belongs
+	mod          string
+	name         string
+	is_pub       bool
+	is_markused  bool // an explicit `@[markused]` tag; the const will NOT be removed by `-skip-unused`, no matter what
+	pos          token.Pos
+	attrs        []Attr // same value as `attrs` of the ConstDecl to which it belongs
+	is_virtual_c bool   // `const C.MY_CONST u8`
 pub mut:
 	expr         Expr      // the value expr of field; everything after `=`
 	typ          Type      // the type of the const field, it can be any type in V
diff --git a/vlib/v/checker/checker.v b/vlib/v/checker/checker.v
index df1e98c28b..f4dd3adac6 100644
--- a/vlib/v/checker/checker.v
+++ b/vlib/v/checker/checker.v
@@ -4297,6 +4297,10 @@ fn (mut c Checker) ident(mut node ast.Ident) ast.Type {
 		if node.name == 'C.NULL' {
 			return ast.voidptr_type
 		}
+		// TODO remove main. to avoid extra concats
+		if x := c.table.global_scope.find_const('main.' + node.name) {
+			return x.typ
+		}
 		return ast.int_type
 	}
 	if c.inside_sql {
diff --git a/vlib/v/fmt/fmt.v b/vlib/v/fmt/fmt.v
index 7029ee7c98..b6811980f2 100644
--- a/vlib/v/fmt/fmt.v
+++ b/vlib/v/fmt/fmt.v
@@ -968,9 +968,17 @@ pub fn (mut f Fmt) const_decl(node ast.ConstDecl) {
 			}
 			f.write('const ')
 		}
+		if field.is_virtual_c {
+			f.write('C.')
+		}
 		f.write('${name} ')
-		f.write('= ')
-		f.expr(field.expr)
+		if field.is_virtual_c {
+			// f.typ(field.typ)
+			f.write(f.table.type_to_str(field.typ))
+		} else {
+			f.write('= ')
+			f.expr(field.expr)
+		}
 		f.comments(field.end_comments, same_line: true)
 		if node.is_block && fidx < node.fields.len - 1 && node.fields.len > 1 {
 			// old style grouped consts, converted to the new style ungrouped const
diff --git a/vlib/v/parser/parser.v b/vlib/v/parser/parser.v
index 573024c7c8..9311f3595a 100644
--- a/vlib/v/parser/parser.v
+++ b/vlib/v/parser/parser.v
@@ -2368,10 +2368,20 @@ fn (mut p Parser) const_decl() ast.ConstDecl {
 			break
 		}
 		pos := p.tok.pos()
-		name := p.check_name()
+		mut name := p.check_name()
 		end_comments << p.eat_comments()
-		if !p.pref.translated && !p.is_translated && util.contains_capital(name) {
-			p.error_with_pos('const names cannot contain uppercase letters, use snake_case instead',
+		// Handle `const C.MY_CONST u16`
+		mut is_virtual_c_const := false
+		mut typ := ast.void_type
+		if name == 'C' && p.tok.kind == .dot {
+			p.next()
+			name += '.' + p.check_name()
+			typ = p.parse_type()
+			is_virtual_c_const = true
+		}
+		if !p.pref.translated && !p.is_translated && util.contains_capital(name)
+			&& !is_virtual_c_const {
+			p.error_with_pos('${name} const names cannot contain uppercase letters, use snake_case instead',
 				pos)
 		}
 		full_name := p.prepend_mod(name)
@@ -2379,14 +2389,17 @@ fn (mut p Parser) const_decl() ast.ConstDecl {
 			p.error_with_pos('const declaration do not support multiple assign yet', p.tok.pos())
 		}
 		// Allow for `const x := 123`, and for `const x = 123` too.
-		// Supporting `const x := 123` in addition to `const x = 123`, makes extracting local variables to constants much less annoying, while prototyping:
+		// Supporting `const x := 123` in addition to `const x = 123`, makes extracting local variables to constants
+		// much less annoying, while prototyping:
 		if p.tok.kind == .decl_assign {
 			p.check(.decl_assign)
 		} else {
-			p.check(.assign)
+			if !is_virtual_c_const {
+				p.check(.assign)
+			}
 		}
 		end_comments << p.eat_comments()
-		if p.tok.kind == .key_fn {
+		if p.tok.kind == .key_fn && !is_virtual_c_const {
 			p.error('const initializer fn literal is not a constant')
 			return ast.ConstDecl{}
 		}
@@ -2394,11 +2407,14 @@ fn (mut p Parser) const_decl() ast.ConstDecl {
 			p.unexpected(got: 'eof', expecting: 'an expression')
 			return ast.ConstDecl{}
 		}
-		expr := p.expr(0)
+		mut expr := ast.Expr{}
+		if !is_virtual_c_const {
+			expr = p.expr(0)
+		}
 		if is_block {
 			end_comments << p.eat_comments(same_line: true)
 		}
-		field := ast.ConstField{
+		mut field := ast.ConstField{
 			name:         full_name
 			mod:          p.mod
 			is_pub:       is_pub
@@ -2408,6 +2424,10 @@ fn (mut p Parser) const_decl() ast.ConstDecl {
 			comments:     comments
 			end_comments: end_comments
 			is_markused:  is_markused
+			is_virtual_c: is_virtual_c_const
+		}
+		if is_virtual_c_const {
+			field.typ = typ
 		}
 		fields << field
 		p.table.global_scope.register(field)
diff --git a/vlib/v/tests/c_const_u8_test.v b/vlib/v/tests/c_const_u8_test.v
new file mode 100644
index 0000000000..7e9f9ce4d1
--- /dev/null
+++ b/vlib/v/tests/c_const_u8_test.v
@@ -0,0 +1,9 @@
+const C.AF_INET u16
+
+fn x(n u16) bool {
+	return true
+}
+
+fn test_const() {
+	assert x(C.AF_INET) == true
+}