Added the mdbook-linkcheck backend

Author: Michael-F-Bryan

book.toml

@@ -3,3 +3,6 @@ title = "Guide to Rustc Development"
 author = "Rustc developers"
 description = "A guide to developing rustc "
 
+[output.html]
+
+[output.linkcheck]

src/high-level-overview.md

@@ -43,7 +43,7 @@ The `rustc_driver` crate, at the top of this lattice, is effectively
 the "main" function for the rust compiler. It doesn't have much "real
 code", but instead ties together all of the code defined in the other
 crates and defines the overall flow of execution. (As we transition
-more and more to the [query model](ty/maps/README.md), however, the
+more and more to the [query model], however, the
 "flow" of compilation is becoming less centrally defined.)
 
 At the other extreme, the `rustc` crate defines the common and
@@ -134,3 +134,5 @@ take:
       (one for each "codegen unit").
 6. **Linking**
     - Finally, those `.o` files are linked together.
+
+[query model]: https://github.com/rust-lang/rust/blob/master/src/librustc/ty/maps/README.md

src/ty.md

@@ -78,7 +78,7 @@ is in fact a simple type alias for a reference with `'tcx` lifetime:
 pub type Ty<'tcx> = &'tcx TyS<'tcx>;
 ```
 
-[the HIR]: ../hir/README.md
+[the HIR]: https://github.com/rust-lang/rust/blob/master/src/librustc/hir/README.md
 
 You can basically ignore the `TyS` struct -- you will basically never
 access it explicitly. We always pass it by reference using the