Auto merge of #108402 - clubby789:diag-bool-not-unit, r=davidtwco

Allow using `bool` instead of `Option<()>` in diagnostics

~~Disallow the unit type for `#[help]`, `#[note]` etc, instead using `bool` to express optional annotations without a span which I believe is more intuitive.~~

~~Test output ordering has changed in a few places, where a field was of type `()` and the annotation has been moved to the struct itself. If any of these changes are an issue, this can be restricted to allowing specifically `()`, and not `Option<()>`~~

~~Actual changes here: https://github.com/rust-lang/rust/pull/108402/files#diff-815b1d8debfc564112bd51093791d7c3f2ee288a37a8f5c0e89c11d1f609b4c0~~

Allows using `bool` in derive diagnostics to indicate an optional subdiagnostic without a span, where previously `Option<()>` had to be used

`@rustbot` label +A-diagnostics

compiler/rustc_macros/src/diagnostics/diagnostic_builder.rs

@@ -6,8 +6,8 @@ use crate::diagnostics::error::{
 };
 use crate::diagnostics::utils::{
     build_field_mapping, is_doc_comment, report_error_if_not_applied_to_span, report_type_error,
-    should_generate_set_arg, type_is_unit, type_matches_path, FieldInfo, FieldInnerTy, FieldMap,
-    HasFieldMap, SetOnce, SpannedOption, SubdiagnosticKind,
+    should_generate_set_arg, type_is_bool, type_is_unit, type_matches_path, FieldInfo,
+    FieldInnerTy, FieldMap, HasFieldMap, SetOnce, SpannedOption, SubdiagnosticKind,
 };
 use proc_macro2::{Ident, Span, TokenStream};
 use quote::{format_ident, quote};
@@ -414,12 +414,15 @@ impl<'a> DiagnosticDeriveVariantBuilder<'a> {
                 Ok(self.add_spanned_subdiagnostic(binding, &fn_ident, slug))
             }
             SubdiagnosticKind::Note | SubdiagnosticKind::Help | SubdiagnosticKind::Warn => {
-                if type_matches_path(info.ty.inner_type(), &["rustc_span", "Span"]) {
+                let inner = info.ty.inner_type();
+                if type_matches_path(inner, &["rustc_span", "Span"]) {
                     Ok(self.add_spanned_subdiagnostic(binding, &fn_ident, slug))
-                } else if type_is_unit(info.ty.inner_type()) {
+                } else if type_is_unit(inner)
+                    || (matches!(info.ty, FieldInnerTy::Plain(_)) && type_is_bool(inner))
+                {
                     Ok(self.add_subdiagnostic(&fn_ident, slug))
                 } else {
-                    report_type_error(attr, "`Span` or `()`")?
+                    report_type_error(attr, "`Span`, `bool` or `()`")?
                 }
             }
             SubdiagnosticKind::Suggestion {

compiler/rustc_macros/src/diagnostics/utils.rs

@@ -50,6 +50,11 @@ pub(crate) fn type_is_unit(ty: &Type) -> bool {
     if let Type::Tuple(TypeTuple { elems, .. }) = ty { elems.is_empty() } else { false }
 }
 
+/// Checks whether the type `ty` is `bool`.
+pub(crate) fn type_is_bool(ty: &Type) -> bool {
+    type_matches_path(ty, &["bool"])
+}
+
 /// Reports a type error for field with `attr`.
 pub(crate) fn report_type_error(
     attr: &Attribute,
@@ -192,6 +197,11 @@ impl<'ty> FieldInnerTy<'ty> {
                     #inner
                 }
             },
+            FieldInnerTy::Plain(t) if type_is_bool(t) => quote! {
+                if #binding {
+                    #inner
+                }
+            },
             FieldInnerTy::Plain(..) => quote! { #inner },
         }
     }

tests/ui-fulldeps/session-diagnostic/diagnostic-derive.rs

@@ -513,6 +513,19 @@ struct OptUnitField {
     bar: Option<()>,
 }
 
+#[derive(Diagnostic)]
+#[diag(no_crate_example)]
+struct BoolField {
+    #[primary_span]
+    spans: Span,
+    #[help]
+    foo: bool,
+    #[help(no_crate_help)]
+    //~^ ERROR the `#[help(...)]` attribute can only be applied to fields of type `Span`, `bool` or `()`
+    // only allow plain 'bool' fields
+    bar: Option<bool>,
+}
+
 #[derive(Diagnostic)]
 #[diag(no_crate_example, code = "E0123")]
 struct LabelWithTrailingPath {