Improve error message

Author: Kobzol

compiler/rustc_builtin_macros/messages.ftl

@@ -222,7 +222,14 @@ builtin_macros_format_unused_args = multiple unused formatting arguments
 
 builtin_macros_format_use_positional = consider using a positional formatting argument instead
 
-builtin_macros_from_wrong_target = `#[derive(From)]` can only be used on structs with a single field
+builtin_macros_derive_from_wrong_target = `#[derive(From)]` used on {$kind}
+
+builtin_macros_derive_from_wrong_field_count = `#[derive(From)]` used on a struct with {$multiple_fields ->
+    [true] multiple fields
+    *[false] no fields
+}
+
+builtin_macros_derive_from_usage_note = `#[derive(From)]` can only be used on structs with exactly one field
 
 builtin_macros_multiple_default_attrs = multiple `#[default]` attributes
     .note = only one `#[default]` attribute is needed

compiler/rustc_builtin_macros/src/deriving/from.rs

@@ -1,5 +1,6 @@
 use rustc_ast as ast;
 use rustc_ast::{ItemKind, VariantData};
+use rustc_errors::MultiSpan;
 use rustc_expand::base::{Annotatable, DummyResult, ExtCtxt};
 use rustc_span::{Ident, Span, kw, sym};
 use thin_vec::thin_vec;
@@ -18,17 +19,18 @@ pub(crate) fn expand_deriving_from(
     cx: &ExtCtxt<'_>,
     span: Span,
     mitem: &ast::MetaItem,
-    item: &Annotatable,
+    annotatable: &Annotatable,
     push: &mut dyn FnMut(Annotatable),
     is_const: bool,
 ) {
+    let Annotatable::Item(item) = &annotatable else {
+        cx.dcx().bug("derive(From) used on something else than an item");
+    };
+
     // #[derive(From)] is currently usable only on structs with exactly one field.
-    let field = match &item {
-        Annotatable::Item(item) => match &item.kind {
-            ItemKind::Struct(_, _, data) => match data.fields() {
-                [field] => Some(field.clone()),
-                _ => None,
-            },
+    let field = match &item.kind {
+        ItemKind::Struct(_, _, data) => match data.fields() {
+            [field] => Some(field.clone()),
             _ => None,
         },
         _ => None,
@@ -70,7 +72,24 @@ pub(crate) fn expand_deriving_from(
             fieldless_variants_strategy: FieldlessVariantsStrategy::Default,
             combine_substructure: combine_substructure(Box::new(|cx, span, substructure| {
                 let Some(field) = &field else {
-                    let error = cx.dcx().emit_err(errors::DeriveFromWrongTarget { span });
+                    let item_span = item.kind.ident().map(|ident| ident.span).unwrap_or(item.span);
+                    let err_span = MultiSpan::from_spans(vec![span, item_span]);
+                    let error = match &item.kind {
+                        ItemKind::Struct(_, _, data) => {
+                            cx.dcx().emit_err(errors::DeriveFromWrongFieldCount {
+                                span: err_span,
+                                multiple_fields: data.fields().len() > 1,
+                            })
+                        }
+                        ItemKind::Enum(_, _, _) | ItemKind::Union(_, _, _) => {
+                            cx.dcx().emit_err(errors::DeriveFromWrongTarget {
+                                span: err_span,
+                                kind: &format!("{} {}", item.kind.article(), item.kind.descr()),
+                            })
+                        }
+                        _ => cx.dcx().bug("Invalid derive(From) ADT input"),
+                    };
+
                     return BlockOrExpr::new_expr(DummyResult::raw_expr(span, Some(error)));
                 };
 
@@ -109,5 +128,5 @@ pub(crate) fn expand_deriving_from(
         is_staged_api_crate: cx.ecfg.features.staged_api(),
     };
 
-    from_trait_def.expand(cx, mitem, item, push);
+    from_trait_def.expand(cx, mitem, annotatable, push);
 }

compiler/rustc_builtin_macros/src/errors.rs

@@ -447,10 +447,21 @@ pub(crate) struct DefaultHasArg {
 }
 
 #[derive(Diagnostic)]
-#[diag(builtin_macros_from_wrong_target)]
-pub(crate) struct DeriveFromWrongTarget {
+#[diag(builtin_macros_derive_from_wrong_target)]
+#[note(builtin_macros_derive_from_usage_note)]
+pub(crate) struct DeriveFromWrongTarget<'a> {
     #[primary_span]
-    pub(crate) span: Span,
+    pub(crate) span: MultiSpan,
+    pub(crate) kind: &'a str,
+}
+
+#[derive(Diagnostic)]
+#[diag(builtin_macros_derive_from_wrong_field_count)]
+#[note(builtin_macros_derive_from_usage_note)]
+pub(crate) struct DeriveFromWrongFieldCount {
+    #[primary_span]
+    pub(crate) span: MultiSpan,
+    pub(crate) multiple_fields: bool,
 }
 
 #[derive(Diagnostic)]