Minor cleanups.

- Reduce some function exposure.
- Fix some comment formatting.

compiler/rustc_macros/src/diagnostics/diagnostic_builder.rs

@@ -53,6 +53,7 @@ pub(crate) struct DiagnosticDeriveVariantBuilder<'parent> {
     /// Slug is a mandatory part of the struct attribute as corresponds to the Fluent message that
     /// has the actual diagnostic message.
     pub slug: SpannedOption<Path>,
+
     /// Error codes are a optional part of the struct attribute - this is only set to detect
     /// multiple specifications.
     pub code: SpannedOption<()>,
@@ -68,7 +69,7 @@ impl DiagnosticDeriveBuilder {
     /// Call `f` for the struct or for each variant of the enum, returning a `TokenStream` with the
     /// tokens from `f` wrapped in an `match` expression. Emits errors for use of derive on unions
     /// or attributes on the type itself when input is an enum.
-    pub fn each_variant<'s, F>(&mut self, structure: &mut Structure<'s>, f: F) -> TokenStream
+    pub(crate) fn each_variant<'s, F>(&mut self, structure: &mut Structure<'s>, f: F) -> TokenStream
     where
         F: for<'a, 'v> Fn(DiagnosticDeriveVariantBuilder<'a>, &VariantInfo<'v>) -> TokenStream,
     {
@@ -121,7 +122,7 @@ impl DiagnosticDeriveBuilder {
 impl<'a> DiagnosticDeriveVariantBuilder<'a> {
     /// Generates calls to `code` and similar functions based on the attributes on the type or
     /// variant.
-    pub fn preamble(&mut self, variant: &VariantInfo<'_>) -> TokenStream {
+    pub(crate) fn preamble(&mut self, variant: &VariantInfo<'_>) -> TokenStream {
         let ast = variant.ast();
         let attrs = &ast.attrs;
         let preamble = attrs.iter().map(|attr| {
@@ -135,7 +136,7 @@ impl<'a> DiagnosticDeriveVariantBuilder<'a> {
 
     /// Generates calls to `span_label` and similar functions based on the attributes on fields or
     /// calls to `set_arg` when no attributes are present.
-    pub fn body(&mut self, variant: &VariantInfo<'_>) -> TokenStream {
+    pub(crate) fn body(&mut self, variant: &VariantInfo<'_>) -> TokenStream {
         let mut body = quote! {};
         // Generate `set_arg` calls first..
         for binding in variant.bindings().iter().filter(|bi| should_generate_set_arg(bi.ast())) {