avoid `&format` in error message code

compiler/rustc_borrowck/src/borrowck_errors.rs

@@ -278,7 +278,7 @@ impl<'cx, 'tcx> crate::MirBorrowckCtxt<'cx, 'tcx> {
         move_from_span: Span,
         move_from_desc: &str,
     ) -> DiagnosticBuilder<'cx, ErrorGuaranteed> {
-        struct_span_err!(self, move_from_span, E0507, "cannot move out of {}", move_from_desc,)
+        struct_span_err!(self, move_from_span, E0507, "cannot move out of {}", move_from_desc)
     }
 
     /// Signal an error due to an attempt to move out of the interior

compiler/rustc_macros/src/diagnostics/error.rs

@@ -54,7 +54,7 @@ fn path_to_string(path: &syn::Path) -> String {
 
 /// Returns an error diagnostic on span `span` with msg `msg`.
 #[must_use]
-pub(crate) fn span_err(span: impl MultiSpan, msg: &str) -> Diagnostic {
+pub(crate) fn span_err<T: Into<String>>(span: impl MultiSpan, msg: T) -> Diagnostic {
     Diagnostic::spanned(span, Level::Error, msg)
 }
 
@@ -77,11 +77,9 @@ pub(crate) fn invalid_attr(attr: &Attribute) -> Diagnostic {
     let span = attr.span().unwrap();
     let path = path_to_string(attr.path());
     match attr.meta {
-        Meta::Path(_) => span_err(span, &format!("`#[{path}]` is not a valid attribute")),
-        Meta::NameValue(_) => {
-            span_err(span, &format!("`#[{path} = ...]` is not a valid attribute"))
-        }
-        Meta::List(_) => span_err(span, &format!("`#[{path}(...)]` is not a valid attribute")),
+        Meta::Path(_) => span_err(span, format!("`#[{path}]` is not a valid attribute")),
+        Meta::NameValue(_) => span_err(span, format!("`#[{path} = ...]` is not a valid attribute")),
+        Meta::List(_) => span_err(span, format!("`#[{path}(...)]` is not a valid attribute")),
     }
 }
 

compiler/rustc_macros/src/diagnostics/subdiagnostic.rs

@@ -200,7 +200,7 @@ impl<'parent, 'a> SubdiagnosticDeriveVariantBuilder<'parent, 'a> {
 
                 throw_span_err!(
                     attr.span().unwrap(),
-                    &format!(
+                    format!(
                         "diagnostic slug must be first argument of a `#[{name}(...)]` attribute"
                     )
                 );

compiler/rustc_macros/src/diagnostics/utils.rs

@@ -347,7 +347,7 @@ pub(crate) trait HasFieldMap {
                 None => {
                     span_err(
                         span.unwrap(),
-                        &format!("`{field}` doesn't refer to a field on this type"),
+                        format!("`{field}` doesn't refer to a field on this type"),
                     )
                     .emit();
                     quote! {