adjust overview slightly

Author: nikomatsakis

src/borrow_check/region_inference.md

@@ -24,20 +24,19 @@ The MIR-based region analysis consists of two major functions:
 - [`compute_regions`], invoked second: this is given as argument the
   results of move analysis. It has the job of computing values for all
   the inference variables that `replace_regions_in_mir` introduced.
-  - To do that, it first runs the [MIR type checker]. This
-    is basically a normal type-checker but specialized to MIR, which
-    is much simpler than full Rust, of course. Running the MIR type
-    checker will however create **outlives constraints** between
-    region variables (e.g., that one variable must outlive another
-    one) to reflect the subtyping relationships that arise.
-  - It also adds **liveness constraints** that arise from where variables
-    are used.
-  - After this, we create a [`RegionInferenceContext`] with the constraints we
-    have computed and the inference variables we introduced and use the
-    [`solve`] method to infer values for all region inference varaibles.
+  - To do that, it first runs the [MIR type checker]. This is
+    basically a normal type-checker but specialized to MIR, which is
+    much simpler than full Rust, of course. Running the MIR type
+    checker will however create various [constraints][cp] between region
+    variables, indicating their potential values and relationships to
+    one another.
+  - After this, we perform [constraint propagation][cp] by creating a
+    [`RegionInferenceContext`] and invoking its [`solve`]
+    method.
   - The [NLL RFC] also includes fairly thorough (and hopefully readable)
     coverage.
 
+[cp]: ./region_inference/constraint_propagation.html
 [fvb]: ../appendix/background.html#free-vs-bound
 [`replace_regions_in_mir`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/borrow_check/nll/fn.replace_regions_in_mir.html
 [`compute_regions`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir/borrow_check/nll/fn.compute_regions.html
@@ -105,8 +104,10 @@ The kinds of region elements are as follows:
 
 ## Constraints
 
-Before we can infer the value of regions, we need to collect constraints on the
-regions. There are two primary types of constraints.
+Before we can infer the value of regions, we need to collect
+constraints on the regions. The full set of constraints is described
+in [the section on constraint propagation][cp], but the two most
+common sorts of constraints are:
 
 1. Outlives constraints. These are constraints that one region outlives another
    (e.g. `'a: 'b`). Outlives constraints are generated by the [MIR type