fix mdbook test

Author: mark-i-m

src/borrow_check/region_inference/constraint_propagation.md

@@ -88,7 +88,7 @@ have to compute `Values(R1) = Values(R1) union Values(R2)`.
 One observation that follows from this is that if you have `R1: R2`
 and `R2: R1`, then `R1 = R2` must be true. Similarly, if you have:
 
-```
+```txt
 R1: R2
 R2: R3
 R3: R4
@@ -119,7 +119,7 @@ context.
 When using a graph representation, we can detect regions that must be equal
 by looking for cycles. That is, if you have a constraint like
 
-```
+```txt
 'a: 'b
 'b: 'c
 'c: 'd
@@ -153,7 +153,7 @@ When we compute SCCs, we not only figure out which regions are a
 member of each SCC, we also figure out the edges between them. So for example
 consider this set of outlives constraints:
 
-```
+```txt
 'a: 'b
 'b: 'a
 
@@ -178,7 +178,7 @@ expressed in terms of regions -- that is, we have a map like
 in terms of SCCs -- we can integrate these liveness constraints very
 easily just by taking the union:
 
-```
+```txt
 for each region R:
   let S be the SCC that contains R
   Values(S) = Values(S) union Liveness(R)
@@ -195,7 +195,7 @@ the value of `S1`, we first compute the values of each successor `S2`.
 Then we simply union all of those values together. To use a
 quasi-iterator-like notation:
 
-```
+```txt
 Values(S1) =
   s1.successors()
     .map(|s2| Values(s2))

src/borrow_check/region_inference/lifetime_parameters.md

@@ -7,7 +7,7 @@ derives from the fact that such lifetimes are "universally quantified"
 lifetimes). It is worth spending a bit of discussing how lifetime
 parameters are handled during region inference. Consider this example:
 
-```rust
+```rust,ignore
 fn foo<'a, 'b>(x: &'a u32, y: &'b u32) -> &'b u32 {
   x
 }
@@ -92,7 +92,7 @@ itself). In the code, these liveness constraints are setup in
 
 So, consider the first example of this section:
 
-```rust
+```rust,ignore
 fn foo<'a, 'b>(x: &'a u32, y: &'b u32) -> &'b u32 {
   x
 }
@@ -102,7 +102,7 @@ Here, returning `x` requires that `&'a u32 <: &'b u32`, which gives
 rise to an outlives constraint `'a: 'b`. Combined with our default liveness
 constraints we get:
 
-```
+```txt
 'a live at {B, end('a)} // B represents the "function body"
 'b live at {B, end('b)}
 'a: 'b

src/borrow_check/region_inference/member_constraints.md

@@ -6,7 +6,7 @@ some `i`). These constraints cannot be expressed by users, but they
 arise from `impl Trait` due to its lifetime capture rules. Consider a
 function such as the following:
 
-```rust
+```rust,ignore
 fn make(a: &'a u32, b: &'b u32) -> impl Trait<'a, 'b> { .. }
 ```
 
@@ -15,7 +15,7 @@ permitted to capture the lifetimes `'a` or `'b`. You can kind of see
 this more clearly by desugaring that `impl Trait` return type into its
 more explicit form:
 
-```rust
+```rust,ignore
 type MakeReturn<'x, 'y> = impl Trait<'x, 'y>;
 fn make(a: &'a u32, b: &'b u32) -> MakeReturn<'a, 'b> { .. }
 ```
@@ -34,14 +34,14 @@ To help us explain member constraints in more detail, let's spell out
 the `make` example in a bit more detail. First off, let's assume that
 you have some dummy trait:
 
-```rust
+```rust,ignore
 trait Trait<'a, 'b> { }
 impl<T> Trait<'_, '_> for T { }
 ```
 
 and this is the `make` function (in desugared form):
 
-```rust
+```rust,ignore
 type MakeReturn<'x, 'y> = impl Trait<'x, 'y>;
 fn make(a: &'a u32, b: &'b u32) -> MakeReturn<'a, 'b> {
   (a, b)
@@ -52,7 +52,7 @@ What happens in this case is that the return type will be `(&'0 u32, &'1 u32)`,
 where `'0` and `'1` are fresh region variables. We will have the following
 region constraints:
 
-```
+```txt
 '0 live at {L}
 '1 live at {L}
 'a: '0
@@ -73,7 +73,7 @@ u32)` -- the region variables reflect that the lifetimes of these
 references could be made smaller. For this value to be created from
 `a` and `b`, however, we do require that:
 
-```
+```txt
 (&'a u32, &'b u32) <: (&'0 u32, &'1 u32)
 ```