Ensure code= in #[suggestion(...)] is only set once

Author: Xiretza

compiler/rustc_macros/src/diagnostics/diagnostic_builder.rs

@@ -476,7 +476,7 @@ impl DiagnosticDeriveBuilder {
                     match nested_name {
                         "code" => {
                             let formatted_str = self.build_format(&s.value(), s.span());
-                            code = Some(formatted_str);
+                            code.set_once((formatted_str, span));
                         }
                         "applicability" => {
                             applicability = match applicability {
@@ -524,7 +524,7 @@ impl DiagnosticDeriveBuilder {
 
         let msg = msg.unwrap_or_else(|| parse_quote! { _subdiag::suggestion });
         let msg = quote! { rustc_errors::fluent::#msg };
-        let code = code.unwrap_or_else(|| quote! { String::new() });
+        let code = code.value().unwrap_or_else(|| quote! { String::new() });
 
         Ok(quote! { #diag.#method(#span_field, #msg, #code, #applicability); })
     }

src/test/ui-fulldeps/session-diagnostic/diagnostic-derive.rs

@@ -581,3 +581,11 @@ struct LintAttributeOnSessionDiag {}
 //~| ERROR diagnostic slug not specified
 //~| ERROR cannot find attribute `lint` in this scope
 struct LintAttributeOnLintDiag {}
+
+#[derive(Diagnostic)]
+#[diag(typeck::ambiguous_lifetime_bound, code = "E0123")]
+struct DuplicatedSuggestionCode {
+    #[suggestion(typeck::suggestion, code = "...", code = ",,,")]
+    //~^ ERROR specified multiple times
+    suggestion: Span,
+}

src/test/ui-fulldeps/session-diagnostic/diagnostic-derive.stderr

@@ -403,6 +403,18 @@ LL | | struct LintAttributeOnLintDiag {}
    |
    = help: specify the slug as the first argument to the attribute, such as `#[diag(typeck::example_error)]`
 
+error: specified multiple times
+  --> $DIR/diagnostic-derive.rs:588:52
+   |
+LL |     #[suggestion(typeck::suggestion, code = "...", code = ",,,")]
+   |                                                    ^^^^^^^^^^^^
+   |
+note: previously specified here
+  --> $DIR/diagnostic-derive.rs:588:38
+   |
+LL |     #[suggestion(typeck::suggestion, code = "...", code = ",,,")]
+   |                                      ^^^^^^^^^^^^
+
 error: cannot find attribute `nonsense` in this scope
   --> $DIR/diagnostic-derive.rs:53:3
    |
@@ -459,7 +471,7 @@ LL |         arg: impl IntoDiagnosticArg,
    |                   ^^^^^^^^^^^^^^^^^ required by this bound in `DiagnosticBuilder::<'a, G>::set_arg`
    = note: this error originates in the derive macro `Diagnostic` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: aborting due to 55 previous errors
+error: aborting due to 56 previous errors
 
 Some errors have detailed explanations: E0277, E0425.
 For more information about an error, try `rustc --explain E0277`.