Add a useful comment about InferOk.

Prompted by rust-lang#131134, which tried to remove `InferOk<'tcx, ()>`
occurrences.

Author: nnethercote

compiler/rustc_infer/src/infer/mod.rs

@@ -68,6 +68,12 @@ pub mod resolve;
 pub(crate) mod snapshot;
 mod type_variable;
 
+// `InferOk<'tcx, ()>` is used a lot. It may seem like a useless wrapper around
+// `Vec<PredicateObligation<'tcx>>`, but it has one important property: because
+// `InferOk` is marked with `#[must_use]`, if you have a method `InferCtxt::f`
+// that returns `InferResult<'tcx, ()>` and you call it with `infcx.f()?;`
+// you'll get a warning about the obligations being discarded without use,
+// which is probably unintentional and has been a source of bugs in the past.
 #[must_use]
 #[derive(Debug)]
 pub struct InferOk<'tcx, T> {