Improve error for missing expression in struct update syntax

Author: clubby789

compiler/rustc_ast_lowering/messages.ftl

@@ -40,6 +40,7 @@ ast_lowering_bad_return_type_notation_output =
 ast_lowering_base_expression_double_dot =
     base expression required after `..`
     .label = add a base expression here
+    .suggestion = using struct update syntax requires a base expression of the same type
 
 ast_lowering_clobber_abi_not_supported =
     `clobber_abi` is not supported on this target

compiler/rustc_ast_lowering/src/errors.rs

@@ -100,6 +100,7 @@ pub struct UnderscoreExprLhsAssign {
 pub struct BaseExpressionDoubleDot {
     #[primary_span]
     #[label]
+    #[suggestion(code = "/* expr */", applicability = "has-placeholders", style = "verbose")]
     pub span: Span,
 }
 

tests/ui/destructuring-assignment/struct_destructure_fail.stderr

@@ -17,6 +17,11 @@ error: base expression required after `..`
    |
 LL |     Struct { a, .. };
    |                   ^ add a base expression here
+   |
+help: using struct update syntax requires a base expression of the same type
+   |
+LL |     Struct { a, ../* expr */ };
+   |                   ++++++++++
 
 error[E0026]: struct `Struct` does not have a field named `c`
   --> $DIR/struct_destructure_fail.rs:10:20