edit MIR lowering

Author: pierwill

src/overview.md

@@ -185,6 +185,15 @@ into the internal representation used by the compiler ([`Ty<'tcx>`]).
 That information is usedto verify the type safety, correctness and
 coherence of the types used in the program.
 
+### MIR lowering
+
+The HIR is then [lowered to Mid-level Intermediate Representation (MIR)][mir],
+which is used for [borrow checking].
+
+Along the way, we also construct the THIR, which is an even more desugared HIR.
+THIR is used for pattern and exhaustiveness checking. It is also more
+convenient to convert into MIR than HIR is.
+
 
 [String interning]: https://en.wikipedia.org/wiki/String_interning
 [`rustc_lexer`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lexer/index.html