Represent diagnostic side effects as dep nodes

compiler/rustc_middle/src/dep_graph/dep_node.rs

@@ -79,6 +79,7 @@ rustc_query_append!(define_dep_nodes![
     [] fn Null() -> (),
     /// We use this to create a forever-red node.
     [] fn Red() -> (),
+    [] fn SideEffect() -> (),
     [] fn TraitSelect() -> (),
     [] fn CompileCodegenUnit() -> (),
     [] fn CompileMonoItem() -> (),

compiler/rustc_middle/src/dep_graph/mod.rs

@@ -46,6 +46,7 @@ impl Deps for DepsType {
 
     const DEP_KIND_NULL: DepKind = dep_kinds::Null;
     const DEP_KIND_RED: DepKind = dep_kinds::Red;
+    const DEP_KIND_SIDE_EFFECT: DepKind = dep_kinds::SideEffect;
     const DEP_KIND_MAX: u16 = dep_node::DEP_KIND_VARIANTS - 1;
 }