Explicitly note any cast not defined in the documentation is illegal.

mcclure · mcclure · commit 4fd418652010 · 2021-03-16T18:04:45.000-04:00
diff --git a/src/expressions/operator-expr.md b/src/expressions/operator-expr.md
@@ -305,7 +305,7 @@ fn average(values: &[f64]) -> f64 {
 }
 ```
 
-`as` can be used to explicitly perform [coercions](../type-coercions.md), as well as the following additional casts.
+`as` can be used to explicitly perform [coercions](../type-coercions.md), as well as the following additional casts. Any cast that does not fit either a coercion rule or an entry in the table is a compiler error.
 Here `*T` means either `*const T` or `*mut T`.
 
 | Type of `e`           | `U`                   | Cast performed by `e as U`       |

Original file line number	Diff line number	Diff line change
`@@ -305,7 +305,7 @@ fn average(values: &[f64]) -> f64 {`
`305`	`305`	`}`
`306`	`306`	```
`307`	`307`
`308`		-`as` can be used to explicitly perform [coercions](../type-coercions.md), as well as the following additional casts.
	`308`	+`as` can be used to explicitly perform [coercions](../type-coercions.md), as well as the following additional casts. Any cast that does not fit either a coercion rule or an entry in the table is a compiler error.
`309`	`309`	Here `T` means either `const T` or `*mut T`.
`310`	`310`
`311`	`311`	\| Type of `e` \| `U` \| Cast performed by `e as U` \|