translate-c: Struct fields default to zero value

C99 introduced designated initializers for structs. Omitted fields are
implicitly initialized to zero. Some C APIs are designed with this in
mind. Defaulting to zero values for translated struct fields permits Zig
code to comfortably use such an API.

Closes #8165

Author: jayschwa

src/translate_c.zig

@@ -1229,6 +1229,14 @@ fn transRecordDecl(c: *Context, scope: *Scope, record_decl: *const clang.RecordD
             else
                 ClangAlignment.forField(c, field_decl, record_def).zigAlignment();
 
+            // C99 introduced designated initializers for structs. Omitted fields are implicitly
+            // initialized to zero. Some C APIs are designed with this in mind. Defaulting to zero
+            // values for translated struct fields permits Zig code to comfortably use such an API.
+            const default_value = if (record_decl.isStruct())
+                try Tag.std_mem_zeroes.create(c.arena, field_type)
+            else
+                null;
+
             if (is_anon) {
                 try c.decl_table.putNoClobber(c.gpa, @intFromPtr(field_decl.getCanonicalDecl()), field_name);
             }
@@ -1237,6 +1245,7 @@ fn transRecordDecl(c: *Context, scope: *Scope, record_decl: *const clang.RecordD
                 .name = field_name,
                 .type = field_type,
                 .alignment = alignment,
+                .default_value = default_value,
             });
         }
 

src/translate_c/ast.zig

@@ -576,6 +576,7 @@ pub const Payload = struct {
             name: []const u8,
             type: Node,
             alignment: ?c_uint,
+            default_value: ?Node,
         };
     };
 
@@ -2095,34 +2096,47 @@ fn renderRecord(c: *Context, node: Node) !NodeIndex {
         _ = try c.addToken(.colon, ":");
         const type_expr = try renderNode(c, field.type);
 
-        const alignment = field.alignment orelse {
-            members[i] = try c.addNode(.{
-                .tag = .container_field_init,
-                .main_token = name_tok,
-                .data = .{
-                    .lhs = type_expr,
-                    .rhs = 0,
-                },
+        const align_expr = if (field.alignment) |alignment| blk: {
+            _ = try c.addToken(.keyword_align, "align");
+            _ = try c.addToken(.l_paren, "(");
+            const align_expr = try c.addNode(.{
+                .tag = .number_literal,
+                .main_token = try c.addTokenFmt(.number_literal, "{d}", .{alignment}),
+                .data = undefined,
             });
-            _ = try c.addToken(.comma, ",");
-            continue;
-        };
-        _ = try c.addToken(.keyword_align, "align");
-        _ = try c.addToken(.l_paren, "(");
-        const align_expr = try c.addNode(.{
-            .tag = .number_literal,
-            .main_token = try c.addTokenFmt(.number_literal, "{d}", .{alignment}),
-            .data = undefined,
-        });
-        _ = try c.addToken(.r_paren, ")");
+            _ = try c.addToken(.r_paren, ")");
+            break :blk align_expr;
+        } else 0;
 
-        members[i] = try c.addNode(.{
+        const value_expr = if (field.default_value) |value| blk: {
+            _ = try c.addToken(.equal, "=");
+            break :blk try renderNode(c, value);
+        } else 0;
+
+        members[i] = try c.addNode(if (align_expr == 0) .{
+            .tag = .container_field_init,
+            .main_token = name_tok,
+            .data = .{
+                .lhs = type_expr,
+                .rhs = value_expr,
+            },
+        } else if (value_expr == 0) .{
             .tag = .container_field_align,
             .main_token = name_tok,
             .data = .{
                 .lhs = type_expr,
                 .rhs = align_expr,
             },
+        } else .{
+            .tag = .container_field,
+            .main_token = name_tok,
+            .data = .{
+                .lhs = type_expr,
+                .rhs = try c.addExtra(std.zig.Ast.Node.ContainerField{
+                    .align_expr = align_expr,
+                    .value_expr = value_expr,
+                }),
+            },
         });
         _ = try c.addToken(.comma, ",");
     }