Handle several #[diagnostic::on_unimplemented] attributes correctly

This PR fixes an issues where rustc would ignore subsequent
`#[diagnostic::on_unimplemented]` attributes. The [corresponding
RFC](https://rust-lang.github.io/rfcs/3368-diagnostic-attribute-namespace.html)
specifies that the first matching instance of each option is used.
Invalid attributes are linted and otherwise ignored.

compiler/rustc_trait_selection/src/traits/error_reporting/on_unimplemented.rs

@@ -1,6 +1,6 @@
 use super::{ObligationCauseCode, PredicateObligation};
 use crate::infer::error_reporting::TypeErrCtxt;
-use rustc_ast::{MetaItem, NestedMetaItem};
+use rustc_ast::{Attribute, MetaItem, NestedMetaItem};
 use rustc_attr as attr;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_errors::{struct_span_err, ErrorGuaranteed};
@@ -474,18 +474,40 @@ fn parse(
     }
 
     pub fn of_item(tcx: TyCtxt<'tcx>, item_def_id: DefId) -> Result<Option<Self>, ErrorGuaranteed> {
-        let mut is_diagnostic_namespace_variant = false;
-        let Some(attr) = tcx.get_attr(item_def_id, sym::rustc_on_unimplemented).or_else(|| {
-            if tcx.features().diagnostic_namespace {
-                is_diagnostic_namespace_variant = true;
-                tcx.get_attrs_by_path(item_def_id, &[sym::diagnostic, sym::on_unimplemented]).next()
-            } else {
-                None
-            }
-        }) else {
-            return Ok(None);
-        };
+        if let Some(attr) = tcx.get_attr(item_def_id, sym::rustc_on_unimplemented) {
+            return Self::parse_attribute(attr, false, tcx, item_def_id);
+        } else if tcx.features().diagnostic_namespace {
+            tcx.get_attrs_by_path(item_def_id, &[sym::diagnostic, sym::on_unimplemented])
+                .filter_map(|attr| Self::parse_attribute(attr, true, tcx, item_def_id).transpose())
+                .try_fold(None, |aggr: Option<Self>, directive| {
+                    let directive = directive?;
+                    if let Some(aggr) = aggr {
+                        let mut subcommands = aggr.subcommands;
+                        subcommands.extend(directive.subcommands);
+                        Ok(Some(Self {
+                            condition: aggr.condition.or(directive.condition),
+                            subcommands,
+                            message: aggr.message.or(directive.message),
+                            label: aggr.label.or(directive.label),
+                            note: aggr.note.or(directive.note),
+                            parent_label: aggr.parent_label.or(directive.parent_label),
+                            append_const_msg: aggr.append_const_msg.or(directive.append_const_msg),
+                        }))
+                    } else {
+                        Ok(Some(directive))
+                    }
+                })
+        } else {
+            Ok(None)
+        }
+    }
 
+    fn parse_attribute(
+        attr: &Attribute,
+        is_diagnostic_namespace_variant: bool,
+        tcx: TyCtxt<'tcx>,
+        item_def_id: DefId,
+    ) -> Result<Option<Self>, ErrorGuaranteed> {
         let result = if let Some(items) = attr.meta_item_list() {
             Self::parse(tcx, item_def_id, &items, attr.span, true, is_diagnostic_namespace_variant)
         } else if let Some(value) = attr.value_str() {

tests/ui/diagnostic_namespace/on_unimplemented/ignore_unsupported_options_and_continue_to_use_fallback.rs

@@ -0,0 +1,22 @@
+#![feature(diagnostic_namespace)]
+
+#[diagnostic::on_unimplemented(
+    //~^WARN malformed `on_unimplemented` attribute
+    //~|WARN malformed `on_unimplemented` attribute
+    if(Self = ()),
+    message = "not used yet",
+    label = "not used yet",
+    note = "not used yet"
+)]
+#[diagnostic::on_unimplemented(message = "fallback!!")]
+#[diagnostic::on_unimplemented(label = "fallback label")]
+#[diagnostic::on_unimplemented(note = "fallback note")]
+#[diagnostic::on_unimplemented(message = "fallback2!!")]
+trait Foo {}
+
+fn takes_foo(_: impl Foo) {}
+
+fn main() {
+    takes_foo(());
+    //~^ERROR fallback!!
+}

tests/ui/diagnostic_namespace/on_unimplemented/ignore_unsupported_options_and_continue_to_use_fallback.stderr

@@ -0,0 +1,52 @@
+warning: malformed `on_unimplemented` attribute
+  --> $DIR/ignore_unsupported_options_and_continue_to_use_fallback.rs:3:1
+   |
+LL | / #[diagnostic::on_unimplemented(
+LL | |
+LL | |
+LL | |     if(Self = ()),
+...  |
+LL | |     note = "not used yet"
+LL | | )]
+   | |__^
+   |
+   = note: `#[warn(unknown_or_malformed_diagnostic_attributes)]` on by default
+
+warning: malformed `on_unimplemented` attribute
+  --> $DIR/ignore_unsupported_options_and_continue_to_use_fallback.rs:3:1
+   |
+LL | / #[diagnostic::on_unimplemented(
+LL | |
+LL | |
+LL | |     if(Self = ()),
+...  |
+LL | |     note = "not used yet"
+LL | | )]
+   | |__^
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+error[E0277]: fallback!!
+  --> $DIR/ignore_unsupported_options_and_continue_to_use_fallback.rs:20:15
+   |
+LL |     takes_foo(());
+   |     --------- ^^ fallback label
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = help: the trait `Foo` is not implemented for `()`
+   = note: fallback note
+help: this trait has no implementations, consider adding one
+  --> $DIR/ignore_unsupported_options_and_continue_to_use_fallback.rs:15:1
+   |
+LL | trait Foo {}
+   | ^^^^^^^^^
+note: required by a bound in `takes_foo`
+  --> $DIR/ignore_unsupported_options_and_continue_to_use_fallback.rs:17:22
+   |
+LL | fn takes_foo(_: impl Foo) {}
+   |                      ^^^ required by this bound in `takes_foo`
+
+error: aborting due to previous error; 2 warnings emitted
+
+For more information about this error, try `rustc --explain E0277`.