improve help for multiple #[default] variants

compiler/rustc_builtin_macros/src/deriving/default.rs

@@ -127,18 +127,17 @@ fn extract_default_variant<'a>(
         [first, rest @ ..] => {
             let suggs = default_variants
                 .iter()
-                .map(|variant| {
-                    let spans = default_variants
+                .filter_map(|variant| {
+                    let keep = attr::find_by_name(&variant.attrs, kw::Default)?.span;
+                    let spans: Vec<Span> = default_variants
                         .iter()
-                        .filter_map(|v| {
-                            if v.span == variant.span {
-                                None
-                            } else {
-                                Some(attr::find_by_name(&v.attrs, kw::Default)?.span)
-                            }
+                        .flat_map(|v| {
+                            attr::filter_by_name(&v.attrs, kw::Default)
+                                .filter_map(|attr| (attr.span != keep).then_some(attr.span))
                         })
                         .collect();
-                    errors::MultipleDefaultsSugg { spans, ident: variant.ident }
+                    (!spans.is_empty())
+                        .then_some(errors::MultipleDefaultsSugg { spans, ident: variant.ident })
                 })
                 .collect();
             cx.emit_err(errors::MultipleDefaults {

tests/ui/deriving/issue-105101.rs

@@ -1,9 +0,0 @@
-// compile-flags: --crate-type=lib
-
-#[derive(Default)] //~ ERROR multiple declared defaults
-enum E {
-    #[default]
-    A,
-    #[default]
-    A, //~ ERROR defined multiple times
-}

tests/ui/deriving/issue-105101.stderr

@@ -1,29 +0,0 @@
-error: multiple declared defaults
-  --> $DIR/issue-105101.rs:3:10
-   |
-LL | #[derive(Default)]
-   |          ^^^^^^^
-...
-LL |     A,
-   |     - first default
-LL |     #[default]
-LL |     A,
-   |     - additional default
-   |
-   = note: only one variant can be default
-   = note: this error originates in the derive macro `Default` (in Nightly builds, run with -Z macro-backtrace for more info)
-
-error[E0428]: the name `A` is defined multiple times
-  --> $DIR/issue-105101.rs:8:5
-   |
-LL |     A,
-   |     - previous definition of the type `A` here
-LL |     #[default]
-LL |     A,
-   |     ^ `A` redefined here
-   |
-   = note: `A` must be defined only once in the type namespace of this enum
-
-error: aborting due to 2 previous errors
-
-For more information about this error, try `rustc --explain E0428`.

tests/ui/deriving/multiple-defaults.rs

@@ -0,0 +1,41 @@
+// compile-flags: --crate-type=lib
+
+// When we get multiple `#[default]` variants, we emit several tool-only suggestions
+// to remove all except one of the `#[default]`s.
+
+#[derive(Default)] //~ ERROR multiple declared defaults
+enum A {
+    #[default] //~ HELP make `B` default
+    #[default] //~ HELP make `A` default
+    A,
+    #[default] // also "HELP make `A` default", but compiletest can't handle multispans
+    B,
+}
+
+// Originally, we took each defaulted variant and emitted the suggestion for every variant
+// with a different identifier, causing an ICE when multiple variants have the same identifier:
+// https://github.com/rust-lang/rust/pull/105106
+#[derive(Default)] //~ ERROR multiple declared defaults
+enum E {
+    #[default] //~ HELP make `A` default
+    A,
+    #[default] //~ HELP make `A` default
+    A, //~ ERROR defined multiple times
+}
+
+// Then, we took each defaulted variant and emitted the suggestion for every variant
+// with a different span, causing an ICE when multiple variants have the same span:
+// https://github.com/rust-lang/rust/issues/118119
+macro_rules! m {
+    { $($id:ident)* } => {
+        #[derive(Default)] //~ ERROR multiple declared defaults
+        enum F {
+            $(
+                #[default]
+                $id,
+            )*
+        }
+    }
+}
+
+m! { A B }

tests/ui/deriving/multiple-defaults.stderr

@@ -0,0 +1,62 @@
+error: multiple declared defaults
+  --> $DIR/multiple-defaults.rs:6:10
+   |
+LL | #[derive(Default)]
+   |          ^^^^^^^
+...
+LL |     A,
+   |     - first default
+LL |     #[default] // also "HELP make `A` default", but compiletest can't handle multispans
+LL |     B,
+   |     - additional default
+   |
+   = note: only one variant can be default
+   = note: this error originates in the derive macro `Default` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: multiple declared defaults
+  --> $DIR/multiple-defaults.rs:18:10
+   |
+LL | #[derive(Default)]
+   |          ^^^^^^^
+...
+LL |     A,
+   |     - first default
+LL |     #[default]
+LL |     A,
+   |     - additional default
+   |
+   = note: only one variant can be default
+   = note: this error originates in the derive macro `Default` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0428]: the name `A` is defined multiple times
+  --> $DIR/multiple-defaults.rs:23:5
+   |
+LL |     A,
+   |     - previous definition of the type `A` here
+LL |     #[default]
+LL |     A,
+   |     ^ `A` redefined here
+   |
+   = note: `A` must be defined only once in the type namespace of this enum
+
+error: multiple declared defaults
+  --> $DIR/multiple-defaults.rs:31:18
+   |
+LL |         #[derive(Default)]
+   |                  ^^^^^^^
+...
+LL |                 $id,
+   |                 ---
+   |                 |
+   |                 first default
+   |                 additional default
+...
+LL | m! { A B }
+   | ---------- in this macro invocation
+   |
+   = note: only one variant can be default
+   = note: this error originates in the derive macro `Default` which comes from the expansion of the macro `m` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0428`.