From 6e670ec90818c1aea158b9d73f74cbc58f97da93 Mon Sep 17 00:00:00 2001
From: ChAoS_UnItY <minecraft.kyle.train@gmail.com>
Date: Tue, 7 Mar 2023 14:52:40 +0800
Subject: [PATCH] v: support interface field compile time reflection (#17503)

---
 vlib/v/checker/comptime.v                     | 19 +++++++++++++++---
 vlib/v/gen/c/comptime.v                       | 20 +++++++++++++++----
 .../c/testdata/comptime_interface_field.out   |  2 ++
 .../c/testdata/comptime_interface_field.vv    | 10 ++++++++++
 4 files changed, 44 insertions(+), 7 deletions(-)
 create mode 100644 vlib/v/gen/c/testdata/comptime_interface_field.out
 create mode 100644 vlib/v/gen/c/testdata/comptime_interface_field.vv

diff --git a/vlib/v/checker/comptime.v b/vlib/v/checker/comptime.v
index f22902b17..f8709f1f1 100644
--- a/vlib/v/checker/comptime.v
+++ b/vlib/v/checker/comptime.v
@@ -180,10 +180,23 @@ fn (mut c Checker) comptime_for(node ast.ComptimeFor) {
 		c.error('unknown type `${sym.name}`', node.typ_pos)
 	}
 	if node.kind == .fields {
-		if sym.kind == .struct_ {
-			sym_info := sym.info as ast.Struct
+		if sym.kind in [.struct_, .interface_] {
+			mut fields := []ast.StructField{}
+			match sym.info {
+				ast.Struct {
+					fields = sym.info.fields.clone()
+				}
+				ast.Interface {
+					fields = sym.info.fields.clone()
+				}
+				else {
+					c.error('comptime field lookup supports only structs and interfaces currently, and ${sym.name} is neither',
+						node.typ_pos)
+					return
+				}
+			}
 			c.inside_comptime_for_field = true
-			for field in sym_info.fields {
+			for field in fields {
 				c.comptime_for_field_value = field
 				c.comptime_for_field_var = node.val_var
 				c.comptime_fields_type[node.val_var] = node.typ
diff --git a/vlib/v/gen/c/comptime.v b/vlib/v/gen/c/comptime.v
index 8f462df1b..bb5b82710 100644
--- a/vlib/v/gen/c/comptime.v
+++ b/vlib/v/gen/c/comptime.v
@@ -763,12 +763,24 @@ fn (mut g Gen) comptime_for(node ast.ComptimeFor) {
 		}
 	} else if node.kind == .fields {
 		// TODO add fields
-		if sym.kind == .struct_ {
-			sym_info := sym.info as ast.Struct
-			if sym_info.fields.len > 0 {
+		if sym.kind in [.struct_, .interface_] {
+			fields := match sym.info {
+				ast.Struct {
+					sym.info.fields
+				}
+				ast.Interface {
+					sym.info.fields
+				}
+				else {
+					g.error('comptime field lookup is supported only for structs and interfaces, and ${sym.name} is neither',
+						node.pos)
+					[]ast.StructField{len: 0}
+				}
+			}
+			if fields.len > 0 {
 				g.writeln('\tFieldData ${node.val_var} = {0};')
 			}
-			for field in sym_info.fields {
+			for field in fields {
 				g.push_existing_comptime_values()
 				g.inside_comptime_for_field = true
 				g.comptime_for_field_var = node.val_var
diff --git a/vlib/v/gen/c/testdata/comptime_interface_field.out b/vlib/v/gen/c/testdata/comptime_interface_field.out
new file mode 100644
index 000000000..759a31c98
--- /dev/null
+++ b/vlib/v/gen/c/testdata/comptime_interface_field.out
@@ -0,0 +1,2 @@
+name
+age
diff --git a/vlib/v/gen/c/testdata/comptime_interface_field.vv b/vlib/v/gen/c/testdata/comptime_interface_field.vv
new file mode 100644
index 000000000..dfe85ab98
--- /dev/null
+++ b/vlib/v/gen/c/testdata/comptime_interface_field.vv
@@ -0,0 +1,10 @@
+interface User {
+	name string
+	age int
+}
+
+fn main() {
+	$for field in User.fields {
+		println(field.name)
+	}
+}
-- 
2.39.5