Added example of impl Trait as an argument

src/trait/impl_trait.md

@@ -1,5 +1,56 @@
 # `impl Trait`
 
+`impl Trait` can be used in two locations:
+
+1. as an argument type
+2. as a return type
+
+## As an argument type
+
+If your function is generic over a trait but you don't mind the specific type, you can simplify the function declaration using `impl Trait` as the type of the argument.
+
+For example, consider the following code:
+
+```rust,editable
+fn parse_csv_document<R: std::io::BufRead>(src: R) -> std::io::Result<Vec<Vec<String>>> {
+    src.lines()
+        .map(|line| {
+            // For each line in the source
+            line.map(|line| {
+                // If the line was read successfully, process it, if not, return the error
+                line.split(',') // Split the line separated by commas
+                    .map(|entry| String::from(entry.trim())) // Remove leading and trailing whitespace
+                    .collect() // Collect all strings in a row into a Vec<String>
+            })
+        })
+        .collect() // Collect all lines into a Vec<Vec<String>>
+}
+```
+
+`parse_csv_document` is generic, allowing it to take any type which implements BufRead, such as `BufReader<File>` or `[u8]`,
+but it's not important what type `R` is, and `R` is only used to declare the type of `src`, so the function can also be written an
+
+```rust,editable
+fn parse_csv_document(src: impl std::io::BufRead) -> std::io::Result<Vec<Vec<String>>> {
+    src.lines()
+        .map(|line| {
+            // For each line in the source
+            line.map(|line| {
+                // If the line was read successfully, process it, if not, return the error
+                line.split(',') // Split the line separated by commas
+                    .map(|entry| String::from(entry.trim())) // Remove leading and trailing whitespace
+                    .collect() // Collect all strings in a row into a Vec<String>
+            })
+        })
+        .collect() // Collect all lines into a Vec<Vec<String>>
+}
+```
+
+Note that using `impl Trait` as an argument type means that you cannot explicitly state what form of the function you use, i.e. `parse_csv_document::<std::io::Empty>(std::io::empty())` will not work with the second example
+
+
+## As a return type
+
 If your function returns a type that implements `MyTrait`, you can write its
 return type as `-> impl MyTrait`. This can help simplify your type signatures quite a lot!