diff --git a/compiler/rustc_middle/src/query/mod.rs b/compiler/rustc_middle/src/query/mod.rs
index 97e21c152ca2c..e94e0332aa464 100644
--- a/compiler/rustc_middle/src/query/mod.rs
+++ b/compiler/rustc_middle/src/query/mod.rs
@@ -1375,8 +1375,9 @@ rustc_queries! {
     /// type-checking etc, and it does not normalize specializable
     /// associated types.
     ///
-    /// You should pretty much only use this if an `infcx` is available,
-    /// otherwise use a `TypingEnv`.
+    /// You should almost certainly not use this. If you already have an InferCtxt, then
+    /// you should also probably have a `ParamEnv` from when it was built. If you don't,
+    /// then you should take a `TypingEnv` to ensure that you handle opaque types correctly.
     query param_env(def_id: DefId) -> ty::ParamEnv<'tcx> {
         desc { |tcx| "computing normalized predicates of `{}`", tcx.def_path_str(def_id) }
         feedable
diff --git a/compiler/rustc_mir_transform/src/post_analysis_normalize.rs b/compiler/rustc_mir_transform/src/post_analysis_normalize.rs
index 4469050069096..c842e92b5270d 100644
--- a/compiler/rustc_mir_transform/src/post_analysis_normalize.rs
+++ b/compiler/rustc_mir_transform/src/post_analysis_normalize.rs
@@ -1,5 +1,6 @@
-//! Normalizes MIR in TypingMode::PostAnalysis mode, most notably revealing
-//! its opaques.
+//! Normalizes MIR in `TypingMode::PostAnalysis`` mode, most notably revealing
+//! its opaques. We also only normalize specializable associated items once in
+//! `PostAnalysis` mode.
 
 use rustc_middle::mir::visit::*;
 use rustc_middle::mir::*;
diff --git a/compiler/rustc_type_ir/src/infer_ctxt.rs b/compiler/rustc_type_ir/src/infer_ctxt.rs
index c2b43416623ed..13ad505bc0483 100644
--- a/compiler/rustc_type_ir/src/infer_ctxt.rs
+++ b/compiler/rustc_type_ir/src/infer_ctxt.rs
@@ -39,9 +39,38 @@ pub enum TypingMode<I: Interner> {
     ///
     /// We only normalize opaque types which may get defined by the current body,
     /// which are stored in `defining_opaque_types`.
+    ///
+    /// We also refuse to project any associated type that is marked `default`.
+    /// Non-`default` ("final") types are always projected. This is necessary in
+    /// general for soundness of specialization. However, we *could* allow projections
+    /// in fully-monomorphic cases. We choose not to, because we prefer for `default type`
+    /// to force the type definition to be treated abstractly by any consumers of the
+    /// impl. Concretely, that means that the following example will
+    /// fail to compile:
+    ///
+    /// ```compile_fail,E0308
+    /// #![feature(specialization)]
+    /// trait Assoc {
+    ///     type Output;
+    /// }
+    ///
+    /// impl<T> Assoc for T {
+    ///     default type Output = bool;
+    /// }
+    ///
+    /// fn main() {
+    ///     let x: <() as Assoc>::Output = true;
+    /// }
+    /// ```
     Analysis { defining_opaque_types: I::DefiningOpaqueTypes },
     /// After analysis, mostly during codegen and MIR optimizations, we're able to
-    /// reveal all opaque types.
+    /// reveal all opaque types. As the concrete type should *never* be observable
+    /// directly by the user, this should not be used by checks which may expose
+    /// such details to the user.
+    ///
+    /// There are some exceptions to this as for example `layout_of` and const-evaluation
+    /// always run in `PostAnalysis` mode, even when used during analysis. This exposes
+    /// some information about the underlying type to users, but not the type itself.
     PostAnalysis,
 }