Add error code for missing base expression in struct update syntax

compiler/rustc_error_codes/src/error_codes/E0797.md

@@ -0,0 +1,26 @@
+Struct update syntax was used without a base expression.
+
+Erroneous code example:
+
+```compile_fail,E0797
+struct Foo {
+    fizz: u8,
+    buzz: u8
+}
+
+let f1 = Foo { fizz: 10, buzz: 1};
+let f2 = Foo { fizz: 10, .. }; // error
+```
+
+Using struct update syntax requires a 'base expression'.
+This will be used to fill remaining fields.
+
+```
+struct Foo {
+    fizz: u8,
+    buzz: u8
+}
+
+let f1 = Foo { fizz: 10, buzz: 1};
+let f2 = Foo { fizz: 10, ..f1 };
+```