X-Git-Url: https://git.lizzy.rs/?a=blobdiff_plain;f=src%2Flibrustc%2Fty%2Fmaps.rs;h=da85d40b2c3e87cbec4bb6d3281741919acd9cd4;hb=6c4b961effdbec1bb2fc5aaed5aeb219eb7f3b19;hp=cfb9e648d3b7eb1dd4c0d18c649080decd35403d;hpb=4ed2edaafe82fb8d44e81e00ca3e4f7659855ba2;p=rust.git

diff --git a/src/librustc/ty/maps.rs b/src/librustc/ty/maps.rs
index cfb9e648d3b..da85d40b2c3 100644
--- a/src/librustc/ty/maps.rs
+++ b/src/librustc/ty/maps.rs
@@ -906,6 +906,12 @@ fn default() -> Self {
     [] specialization_graph_of: SpecializationGraph(DefId) -> Rc<specialization_graph::Graph>,
     [] is_object_safe: ObjectSafety(DefId) -> bool,
 
+    // Get the ParameterEnvironment for a given item; this environment
+    // will be in "user-facing" mode, meaning that it is suitabe for
+    // type-checking etc, and it does not normalize specializable
+    // associated types. This is almost always what you want,
+    // unless you are doing MIR optimizations, in which case you
+    // might want to use `reveal_all()` method to change modes.
     [] param_env: ParamEnv(DefId) -> ty::ParamEnv<'tcx>,
 
     // Trait selection queries. These are best used by invoking `ty.moves_by_default()`,