Add links to the rustc docs (#578)

Author: loris-intergalactique

src/query.md

@@ -76,8 +76,8 @@ dependencies of the local crate). Note that what determines the crate
 that a query is targeting is not the *kind* of query, but the *key*.
 For example, when you invoke `tcx.type_of(def_id)`, that could be a
 local query or an external query, depending on what crate the `def_id`
-is referring to (see the `self::keys::Key` trait for more information
-on how that works).
+is referring to (see the [`self::keys::Key`][Key] trait for more
+information on how that works).
 
 Providers always have the same signature:
 
@@ -96,8 +96,10 @@ They return the result of the query.
 ####  How providers are setup
 
 When the tcx is created, it is given the providers by its creator using
-the `Providers` struct. This struct is generated by the macros here, but it
-is basically a big list of function pointers:
+the [`Providers`][providers_struct] struct. This struct is generated by
+the macros here, but it is basically a big list of function pointers:
+
+[providers_struct]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/ty/query/struct.Providers.html
 
 ```rust,ignore
 struct Providers {
@@ -110,11 +112,13 @@ At present, we have one copy of the struct for local crates, and one
 for external crates, though the plan is that we may eventually have
 one per crate.
 
-These `Provider` structs are ultimately created and populated by
+These `Providers` structs are ultimately created and populated by
 `librustc_driver`, but it does this by distributing the work
 throughout the other `rustc_*` crates. This is done by invoking
-various `provide` functions. These functions tend to look something
-like this:
+various [`provide`][provide_fn] functions. These functions tend to look
+something like this:
+
+[provide_fn]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/hir/fn.provide.html
 
 ```rust,ignore
 pub fn provide(providers: &mut Providers) {
@@ -147,13 +151,16 @@ fn fubar<'tcx>(tcx: TyCtxt<'tcx>, key: DefId) -> Fubar<'tcx> { ... }
 
 N.B. Most of the `rustc_*` crates only provide **local
 providers**. Almost all **extern providers** wind up going through the
-[`rustc_metadata` crate][rustc_metadata], which loads the information from the
-crate metadata. But in some cases there are crates that provide queries for
-*both* local and external crates, in which case they define both a
-`provide` and a `provide_extern` function that `rustc_driver` can
-invoke.
+[`rustc_metadata` crate][rustc_metadata], which loads the information
+from the crate metadata. But in some cases there are crates that
+provide queries for *both* local and external crates, in which case
+they define both a [`provide`][ext_provide] and a
+[`provide_extern`][ext_provide_extern] function that `rustc_driver`
+can invoke.
 
 [rustc_metadata]: https://github.com/rust-lang/rust/tree/master/src/librustc_metadata
+[ext_provide]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_llvm/attributes/fn.provide.html
+[ext_provide_extern]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_llvm/attributes/fn.provide_extern.html
 
 ### Adding a new kind of query
 
@@ -205,7 +212,7 @@ Let's go over them one by one:
   (`ty::queries::type_of`) that will be generated to represent
   this query.
 - **Query key type:** the type of the argument to this query.
-  This type must implement the `ty::query::keys::Key` trait, which
+  This type must implement the [`ty::query::keys::Key`][Key] trait, which
   defines (for example) how to map it to a crate, and so forth.
 - **Result type of query:** the type produced by this query. This type
   should (a) not use `RefCell` or other interior mutability and (b) be
@@ -218,6 +225,8 @@ Let's go over them one by one:
 - **Query modifiers:** various flags and options that customize how the
   query is processed.
 
+[Key]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/ty/query/keys/trait.Key.html
+
 So, to add a query:
 
 - Add an entry to `rustc_queries!` using the format above.
@@ -229,7 +238,7 @@ So, to add a query:
 For each kind, the `rustc_queries` macro will generate a "query struct"
 named after the query. This struct is a kind of a place-holder
 describing the query. Each such struct implements the
-`self::config::QueryConfig` trait, which has associated types for the
+[`self::config::QueryConfig`][QueryConfig] trait, which has associated types for the
 key/value of that particular query. Basically the code generated looks something
 like this:
 
@@ -247,13 +256,16 @@ impl<'tcx> QueryConfig for type_of<'tcx> {
 ```
 
 There is an additional trait that you may wish to implement called
-`self::config::QueryDescription`. This trait is used during cycle
-errors to give a "human readable" name for the query, so that we can
-summarize what was happening when the cycle occurred. Implementing
-this trait is optional if the query key is `DefId`, but if you *don't*
-implement it, you get a pretty generic error ("processing `foo`...").
+[`self::config::QueryDescription`][QueryDescription]. This trait is
+used during cycle errors to give a "human readable" name for the query,
+so that we can summarize what was happening when the cycle occurred.
+Implementing this trait is optional if the query key is `DefId`, but
+if you *don't* implement it, you get a pretty generic error ("processing `foo`...").
 You can put new impls into the `config` module. They look something like this:
 
+[QueryConfig]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/ty/query/trait.QueryConfig.html
+[QueryDescription]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/ty/query/config/trait.QueryDescription.html
+
 ```rust,ignore
 impl<'tcx> QueryDescription for queries::type_of<'tcx> {
     fn describe(tcx: TyCtxt, key: DefId) -> String {