auto merge of #18339 : chastell/rust/guide_pattern_fixes, r=nikomatsakis

I think it helps to show that the variables introduced in match blocks are indeed independent from the matched variable `x` (especially when `x` is still reachable inside those blocks and might be useful), so this renames them accordingly. Maybe some linter (or language-level warning?) will eventually warn about shadowing `x` in such cases. ;)

I’m not super happy about the matching-on-range example, as it’s too contrived (`e` and `x` are exactly the same here), but I couldn’t come up with something both simple and non-redundant.

src/doc/guide.md

@@ -3852,7 +3852,7 @@ the value to a name with `@`:
 let x = 1i;
 
 match x {
-    x @ 1 ... 5 => println!("got {}", x),
+    e @ 1 ... 5 => println!("got a range element {}", e),
     _ => println!("anything"),
 }
 ```
@@ -3885,7 +3885,7 @@ enum OptionalInt {
 let x = Value(5i);
 
 match x {
-    Value(x) if x > 5 => println!("Got an int bigger than five!"),
+    Value(i) if i > 5 => println!("Got an int bigger than five!"),
     Value(..) => println!("Got an int!"),
     Missing   => println!("No such luck."),
 }
@@ -3898,12 +3898,12 @@ with. First, `&`:
 let x = &5i;
 
 match x {
-    &x => println!("Got a value: {}", x),
+    &val => println!("Got a value: {}", val),
 }
 ```
 
-Here, the `x` inside the `match` has type `int`. In other words, the left hand
-side of the pattern destructures the value. If we have `&5i`, then in `&x`, `x`
+Here, the `val` inside the `match` has type `int`. In other words, the left hand
+side of the pattern destructures the value. If we have `&5i`, then in `&val`, `val`
 would be `5i`.
 
 If you want to get a reference, use the `ref` keyword:
@@ -3912,11 +3912,11 @@ If you want to get a reference, use the `ref` keyword:
 let x = 5i;
 
 match x {
-    ref x => println!("Got a reference to {}", x),
+    ref r => println!("Got a reference to {}", r),
 }
 ```
 
-Here, the `x` inside the `match` has the type `&int`. In other words, the `ref`
+Here, the `r` inside the `match` has the type `&int`. In other words, the `ref`
 keyword _creates_ a reference, for use in the pattern. If you need a mutable
 reference, `ref mut` will work in the same way:
 
@@ -3924,7 +3924,7 @@ reference, `ref mut` will work in the same way:
 let mut x = 5i;
 
 match x {
-    ref mut x => println!("Got a mutable reference to {}", x),
+    ref mut mr => println!("Got a mutable reference to {}", mr),
 }
 ```