Sema: implement @export for arbitrary values

src/Sema.zig

@@ -5605,7 +5605,15 @@ fn zirExportValue(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError
     };
     const decl_index = switch (operand.val.tag()) {
         .function => operand.val.castTag(.function).?.data.owner_decl,
-        else => return sema.fail(block, operand_src, "TODO implement exporting arbitrary Value objects", .{}), // TODO put this Value into an anonymous Decl and then export it.
+        else => blk: {
+            var anon_decl = try block.startAnonDecl();
+            defer anon_decl.deinit();
+            break :blk try anon_decl.finish(
+                try operand.ty.copy(anon_decl.arena()),
+                try operand.val.copy(anon_decl.arena()),
+                0,
+            );
+        },
     };
     try sema.analyzeExport(block, src, options, decl_index);
 }
@@ -5641,6 +5649,14 @@ pub fn analyzeExport(
         return sema.failWithOwnedErrorMsg(msg);
     }
 
+    // TODO: some backends might support re-exporting extern decls
+    if (exported_decl.isExtern()) {
+        return sema.fail(block, src, "export target cannot be extern", .{});
+    }
+
+    // This decl is alive no matter what, since it's being exported
+    mod.markDeclAlive(exported_decl);
+
     const gpa = mod.gpa;
 
     try mod.decl_exports.ensureUnusedCapacity(gpa, 1);

test/behavior/export.zig

@@ -70,3 +70,22 @@ test "exporting using field access" {
 
     _ = S.Inner.x;
 }
+
+test "exporting comptime-known value" {
+    const x: u32 = 10;
+    @export(x, .{ .name = "exporting_comptime_known_value_foo" });
+    const S = struct {
+        extern const exporting_comptime_known_value_foo: u32;
+    };
+    try expect(S.exporting_comptime_known_value_foo == 10);
+}
+
+test "exporting comptime var" {
+    comptime var x: u32 = 5;
+    @export(x, .{ .name = "exporting_comptime_var_foo" });
+    x = 7; // modifying this now shouldn't change anything
+    const S = struct {
+        extern const exporting_comptime_var_foo: u32;
+    };
+    try expect(S.exporting_comptime_var_foo == 5);
+}