better error for rustc_strict_coherence misuse

compiler/rustc_error_messages/locales/en-US/middle.ftl

@@ -27,3 +27,7 @@ middle_values_too_big =
 
 middle_cannot_be_normalized =
     unable to determine layout for `{$ty}` because `{$failure_ty}` cannot be normalized
+
+middle_strict_coherence_needs_negative_coherence =
+    to use `strict_coherence` on this trait, the `with_negative_coherence` feature must be enabled
+    .label = due to this attribute

compiler/rustc_middle/src/error.rs

@@ -55,3 +55,12 @@ pub struct ConstEvalNonIntError {
     #[primary_span]
     pub span: Span,
 }
+
+#[derive(Diagnostic)]
+#[diag(middle_strict_coherence_needs_negative_coherence)]
+pub(crate) struct StrictCoherenceNeedsNegativeCoherence {
+    #[primary_span]
+    pub span: Span,
+    #[label]
+    pub attr_span: Option<Span>,
+}

compiler/rustc_middle/src/traits/specialization_graph.rs

@@ -1,3 +1,4 @@
+use crate::error::StrictCoherenceNeedsNegativeCoherence;
 use crate::ty::fast_reject::SimplifiedType;
 use crate::ty::visit::TypeVisitable;
 use crate::ty::{self, TyCtxt};
@@ -65,9 +66,21 @@ impl OverlapMode {
 
         if with_negative_coherence {
             if strict_coherence { OverlapMode::Strict } else { OverlapMode::WithNegative }
-        } else if strict_coherence {
-            bug!("To use strict_coherence you need to set with_negative_coherence feature flag");
         } else {
+            if strict_coherence {
+                let attr_span = trait_id
+                    .as_local()
+                    .into_iter()
+                    .flat_map(|local_def_id| {
+                        tcx.hir().attrs(tcx.hir().local_def_id_to_hir_id(local_def_id))
+                    })
+                    .find(|attr| attr.has_name(sym::rustc_strict_coherence))
+                    .map(|attr| attr.span);
+                tcx.sess.emit_err(StrictCoherenceNeedsNegativeCoherence {
+                    span: tcx.def_span(trait_id),
+                    attr_span,
+                });
+            }
             OverlapMode::Stable
         }
     }

src/test/ui/coherence/strict-coherence-needs-negative-coherence.rs

@@ -0,0 +1,7 @@
+#![feature(rustc_attrs)]
+
+#[rustc_strict_coherence]
+trait Foo {}
+//~^ ERROR to use `strict_coherence` on this trait, the `with_negative_coherence` feature must be enabled
+
+fn main() {}

src/test/ui/coherence/strict-coherence-needs-negative-coherence.stderr

@@ -0,0 +1,10 @@
+error: to use `strict_coherence` on this trait, the `with_negative_coherence` feature must be enabled
+  --> $DIR/strict-coherence-needs-negative-coherence.rs:4:1
+   |
+LL | #[rustc_strict_coherence]
+   | ------------------------- due to this attribute
+LL | trait Foo {}
+   | ^^^^^^^^^
+
+error: aborting due to previous error
+