Mention formatted strings

docs/versioned_docs/version-v0.19.4/language_concepts/data_types/03_strings.md

@@ -15,8 +15,7 @@ keywords:
 
 The string type is a fixed length value defined with `str<N>`.
 
-You can use strings in `assert()` functions or print them with
-`std::println()`. See more about [Logging](../../standard_library/logging).
+You can use strings in `assert()` functions or print them with `std::println()`. See more about [Logging](../../standard_library/logging).
 
 ```rust
 use dep::std;
@@ -42,7 +41,7 @@ fn main() {
 }
 ```
 
-## Escape characters
+## Escape Characters
 
 You can use escape characters for your strings:
 
@@ -58,6 +57,32 @@ You can use escape characters for your strings:
 Example:
 
 ```rust
-let s = "Hello \"world" // prints "Hello "world"
+let s = "Hello \"world"; // prints "Hello "world"
 let s = "hey \tyou"; // prints "hey   you"
 ```
+
+## Formatted Strings
+
+You can prepend a string with the singular `f` token to create a formatted string. This is useful when logging, as it allows injection of local variables:
+
+```rust
+let var = 15;
+std::println(f"var {var}") // prints "var 0x0F"
+
+let var = -1 as u8;
+std::println(f"var {var}") // prints "var 255"
+
+let var : i8 = -1;
+std::println(f"var {var}") // prints "var -1"
+
+// prints "Hello
+//world"
+std::println(f"Hello
+world");
+
+std::println(f"hey \tyou"); // prints "hey \tyou"
+```
+
+A type can be specified to print numbers either as hex via `Field`, unsigned via `u*` types and signed via `i*` types.
+
+Note that escaped characters in formatted strings `fmtstr` will be outputted as defined, i.e. "\n" will be printed `\n`, not as a new line. You can add a newline or other whitespace by creating a multiline string as in the example above.