More doctest fixes.

Author: calder

library/core/src/primitive_docs.rs

@@ -1335,16 +1335,20 @@ mod prim_f16 {}
 /// For example:
 ///
 /// ```
-/// let x = 12.0;
-/// let a = 34.0;
-/// let b = 56.0;
+/// # let mut x: f32 = 12.0;
+/// # let a: f32 = 34.0;
+/// # let b: f32 = 56.0;
+/// x = x.algebraic_add(x, a.algebraic_mul(b));
+/// ```
 ///
-/// // This:
-/// let x = x.algebraic_add(x, a.algebraic_mul(b));
+/// May be rewritten as either:
 ///
-/// // May be rewritten as either:
-/// let x = x + (a * b); // As written
-/// let x = (a * b) + x; // Reordered to allow using a single `fma`
+/// ```
+/// # let mut x: f32 = 12.0;
+/// # let a: f32 = 34.0;
+/// # let b: f32 = 56.0;
+/// x = x + (a * b); // As written
+/// x = (a * b) + x; // Reordered to allow using a single `fma`
 /// ```
 
 #[stable(feature = "rust1", since = "1.0.0")]