Edit Hir lowering

Author: pierwill

src/overview.md

@@ -169,6 +169,22 @@ The parser uses the standard `DiagnosticBuilder` API for error handling, but we
 try to recover, parsing a superset of Rust's grammar, while also emitting an error.
 `rustc_ast::ast::{Crate, Mod, Expr, Pat, ...}` AST nodes are returned from the parser.
 
+### HIR lowering
+
+We next take the AST and convert it to [High-Level Intermediate
+Representation (HIR)][hir], a more compiler-friendly representation of the
+AST. This process called "lowering". It involves a lot of desugaring of things
+like loops and `async fn`.
+
+We then use the HIR to do [*type inference*] (the process of automatic
+detection of the type of an expression), [*trait solving*] (the process
+of pairing up an impl with each reference to a trait), and [*type
+checking*]. Type checking is the process of converting the types found in the HIR
+([`hir::Ty`]), which represent what the user wrote,
+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.
+
 
 [String interning]: https://en.wikipedia.org/wiki/String_interning
 [`rustc_lexer`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lexer/index.html