First steps

Welcome to Rust computing. This chapter will be a bit longer than the next ones, because we need to introduce a number of basic concepts that you will likely all need to do subsequent exercises. Please read on carefully!

Hello world

Following an ancient tradition, let us start by displaying some text on stdout:

fn main() {
    println!("Hello world!");
}

Notice the following:

• In Rust, functions declarations start with the fn keyword.
• Like in C/++, the main function of a program is called main.
• It is legal to return nothing from main. Like return 0; in C/++, it indicates success.
• Sending a line of text to standard output can be done using the println!() macro. We’ll get back to why it is a macro later, but in a nutshell it allows controlling the formatting of values in a way that is similar to printf() in C and f-strings in Python.

Variables

Rust is in many ways a hybrid of programming languages from the C/++ family and the ML family (Ocaml, Haskell). Following the latter’s tradition, it uses a variable declaration syntax that is very reminiscent of mathematical notation:

#![allow(unused)]
fn main() {
let x = 4;
}

As in mathematics, variables are immutable by default, so the following code does not compile:

#![allow(unused)]
fn main() {
let x = 4;
x += 2;  // ERROR: Can't modify non-mut variable
}

If you want to modify a variable, you must make it mutable by adding a mut keyword after let:

#![allow(unused)]
fn main() {
let mut x = 4;
x += 2;  // Fine, variable is declared mutable
}

This design gently nudges you towards using immutable variables for most things, as in mathematics, which tends to make code easier to reason about.

Alternatively, Rust allows for variable shadowing, so you are allowed to define a new variable with the same name as a previously declared variable, even if it has a different type:

#![allow(unused)]
fn main() {
let foo = 123;  // Here "foo" is an integer
let foo = "456";  // Now "foo" is a string and old integer "foo" is not reachable
}

This pattern is commonly used in scenarios like parsing where the old value should not be needed after the new one has been declared. It is otherwise a bit controversial, and can make code harder to read, so please don’t abuse this possibility.

Type inference

What gets inferred

Rust is a strongly typed language like C++, yet you may have noticed that the variable declarations above contain no types. That’s because the language supports type inference as a core feature: the compiler can automatically determine the type of variables based on various information.

• First, the value that is affected to the variable may have an unambiguous type. For example, string literals in Rust are always of type &str (“reference-to-string”), so the compiler knows that the following variable s must be of type &str:

  #![allow(unused)]
  fn main() {
  let s = "a string literal of type &str";
  }

• Second, the way a variable is used after declaration may give its type away. If you use a variable in a context where a value of type T is expected, then that variable must be of type T.

  For example, Rust provides a heap-allocated variable-sized array type called Vec (similar to std::vector in C++), whose length is defined to be of type usize (similar to size_t in C/++). Therefore, if you use an integer as the length of a Vec, the compiler knows that this integer must be of type usize:

  #![allow(unused)]
  fn main() {
  let len = 7;  // This integer variable must be of type usize...
  let v = vec![4.2; len];  // ...because it's used as the length of a Vec here.
                           // (we'll introduce this vec![] macro later on)
  }

• Finally, numeric literals can be annotated to force them to be of a specific type. For example, the literal 42i8 is a 8-bit signed integer, the literal 666u64 is a 64-bit unsigned integer, and the 12.34f32 literal is a 32-bit (“single precision”) IEEE-754 floating point number. By this logic, the following variable x is a 16-bit unsigned integer:

  #![allow(unused)]
  fn main() {
  let x = 999u16;
  }

  If none of the above rules apply, then by default, integer literals will be inferred to be of type i32 (32-bit signed integer), and floating-point literals will be inferred to be of type f64 (double-precision floating-point number), as in C/++. This ensures that simple programs compile without requiring type annotations.

  Unfortunately, this fallback rule is not very reliable, as there are a number of common code patterns that will not trigger it, typically involving some form of genericity.

What does not get inferred

There are cases where these three rules will not be enough to determine a variable’s type. This happens in the presence of generic type and functions.

Getting back to the Vec type, for example, it is actually a generic type Vec<T> where T can be almost any Rust type¹. As with std::vector in C++, you can have a Vec<i32>, a Vec<f32>, or even a Vec<MyStruct> where MyStruct is a data structure that you have defined yourself.

This means that if you declare empty vectors like this…

#![allow(unused)]
fn main() {
// The following syntaxes are strictly equivalent. Neither compile. See below.
let empty_v1 = Vec::new();
let empty_v2 = vec![];
}

…the compiler has no way to know what kind of Vec you are dealing with. This cannot be allowed because the properties of a generic type like Vec<T> heavily depend on what concrete T parameter it’s instantiated with, therefore the above code does not compile.

In that case, you can enforce a specific variable type using type ascription:

#![allow(unused)]
fn main() {
// The compiler knows this is a Vec<bool> because we said so
let empty_vb: Vec<bool> = Vec::new();
}

Inferring most things is the idiom

If you are coming from another programming language where type inference is either not provided, or very hard to reason about as in C++, you may be tempted to use type ascription to give an explicit type to every single variable. But I would advise resisting this temptation for a few reasons:

• Rust type inference rules are much simpler than those of C++. It only takes a small amount of time to “get them in your head”, and once you do, you will get more concise code that is less focused on pleasing the type system and more on performing the task at hand.
• Doing so is the idiomatic style in the Rust ecosystem. If you don’t follow it, your code will look odd to other Rust developers, and you will have a harder time reading code written by other Rust developers.
• If you have any question about inferred types in your program, Rust comes with excellent IDE support, so it is very easy to configure your code editor so that it displays inferred types, either all the time or on mouse hover.

But of course, there are limits to this approach. If every single type in the program was inferred, then a small change somewhere in the implementation your program could non-locally change the type of many other variables in the program, or even in client code, resulting in accidental API breakages, as commonly seen in dynamically typed programming language.

For this reason, Rust will not let you use type inference in entities that may appear in public APIs, like function signatures or struct declarations. This means that in Rust code, type inference will be restricted to the boundaries of a single function’s implementation. Which makes it more reliable and easier to reason about, as long as you do not write huge functions.

Back to println!()

With variable declarations out of the way, let’s go back to our hello world example and investigate the Rust text formatting macro in more details.

Remember that at the beginning of this chapter, we wrote this hello world statement:

#![allow(unused)]
fn main() {
println!("Hello world!");
}

This program called the println macro with a string literal as an argument. Which resulted in that string being written to the program’s standard output, followed by a line feed.

If all we could pass to println was a string literal, however, it wouldn’t need to be a macro. It would just be a regular function.

But like f-strings in Python, the println provides a variety of text formatting operations, accessible via curly braces. For example, you can interpolate variables from the outer scope…

#![allow(unused)]
fn main() {
let x = 4;
// Will print "x is 4"
println!("x is {x}");
}

…pass extra arguments in a style similar to printf in C…

#![allow(unused)]
fn main() {
let s = "string";
println!("s is {}", s);
}

…and name arguments so that they are easier to identify in complex usage.

#![allow(unused)]
fn main() {
println!("x is {x} and y is {y}. Their sum is {x} + {y} = {sum}",
         x = 4,
         y = 5,
         sum = 4 + 5);
}

You can also control how these arguments are converted to strings, using a mini-language that is described in the documentation of the std::fmt module from the standard library.

For example, you can enforce that floating-point numbers are displayed with a certain number of decimal digits:

#![allow(unused)]
fn main() {
let x = 4.2;
// Will print "x is 4.200000"
println!("x is {x:.6}");
}

println!() is part of a larger family of text formatting and text I/O macros that includes…

• print!(), which differs from println!() by not adding a trailing newline at the end. Beware that since stdout is line buffered, this will result in no visible output until the next println!(), unless the text that you are printing contains the \n line feed character.
• eprint!() and eprintln!(), which work like print!() and println!() but write their output to stderr instead of stdout.
• write!() and writeln!(), which take a byte or text output stream² as an extra argument and write down the specified text there. This is the same idea as fprintf() in C.
• format!(), which does not write the output string to any I/O stream, but instead builds a heap-allocated String containing it for later use.

All of these macros use the same format string mini-language as println!(), although their semantics differ. For example, write!() takes an extra output stream arguments, and returns a Result to account for the possibility of I/O errors. Since these errors are rare on stdout and stderr, they are just treated as fatal by the print!() family, keeping the code that uses them simpler.

From Display to Debug

So far, we have been printing simple numerical types. What they have in common is that there is a single, universally agreed upon way to display them, modulo formatting options. So the Rust standard library can afford to incorporate this display logic into its stability guarantees.

But some other types are in a muddier situation. For example, take the Vec dynamically-sized array type. You may think that something like “[1, 2, 3, 4, 5]” would be a valid way to display an array containing the numbers containing numbers from 1 to 5. But what happens when the array contains billions of numbers? Should we attempt to display all of them, drowning the user’s terminal in endless noise and slowing down the application to a crawl? Or should we summarize the display in some way like numpy does in Python?

There is no single right answer to this kind of question, and attempting to account for all use cases would bloat up Rust’s text formatting mini-language very quickly. So instead, Rust does not provide a standard text display for these types, and therefore the following code does not compile:

#![allow(unused)]
fn main() {
// ERROR: Type Vec does not implement Display
println!("{}", vec![1, 2, 3, 4, 5]);
}

All this is fine and good, but we all know that in real-world programming, it is very convenient during program debugging to have a way to exhaustively display the contents of a variable. Unlike C++, Rust acknowledges this need by distinguishing two different ways to translate a typed value to text:

• The Display trait provides, for a limited set of types, an “official” value-to-text translation logic that should be fairly consensual, general-purpose, suitable for end-user consumption, and can be covered by library stability guarantees.
• The Debug trait provides, for almost every type, a dumber value-to-text translation logic which prints out the entire contents of the variable, including things which are considered implementation details and subjected to change. It is purely meant for developer use, and showing debug strings to end users is somewhat frowned upon, although they are tolerated in developer-targeted output like logs or error messages.

As you may guess, although Vec does not implement the Display operation, it does implement Debug, and in the mini-language of println!() et al, you can access this alternate Debug logic using the ? formatting option:

#![allow(unused)]
fn main() {
println!("{:?}", vec![1, 2, 3, 4, 5]);
}

As a more interesting example, strings implement both Display and Debug. The Display variant displays the text as-is, while the Debug logic displays it as you would type it in a program, with quotes around it and escape sequences for things like line feeds and tab stops:

#![allow(unused)]
fn main() {
let s = "This is a \"string\".\nWell, actually more of an str.";
println!("String display: {s}");
println!("String debug: {s:?}");
}

Both Display and Debug additionally support an alternate display mode, accessible via the # formatting option. For composite types like Vec, this has the effect of switching to a multi-line display (one line feed after each inner value), which can be more readable for complex values:

#![allow(unused)]
fn main() {
let v = vec![1, 2, 3, 4, 5];
println!("Normal debug: {v:?}");
println!("Alternate debug: {v:#?}");
}

For simpler types like integers, this may have no effect. It’s ultimately up to implementations of Display and Debug to decide what this formatting option does, although staying close to the standard library’s convention is obviously strongly advised.

Finally, for an even smoother printout-based debugging experience, you can use the dbg!() macro. It takes an expression as input and prints out…

• Where you are in the program (source file, line, column).
• What is the expression that was passed to dbg, in un-evaluated form (e.g. dbg!(2 * x) would literally print “2 * x”).
• What is the result of evaluating this expression, with alternate debug formatting.

…then the result is re-emitted as an output, so that the program can keep using it. This makes it easy to annotate existing code with dbg!() macros, with minimal disruption:

#![allow(unused)]
fn main() {
let y = 3 * dbg!(4 + 2);
println!("{y}");
}

Assertions

With debug formatting covered, we are now ready to cover the last major component of the exercises, namely assertions.

Rust has an assert!() macro which can be used similarly to the C/++ macro of the same name: make sure that a condition that should always be true if the code is correct, is indeed true. If the condition is not true, the thread will panic. This is a process analogous to throwing a C++ exception, which in simple cases will just kill the program.

#![allow(unused)]
fn main() {
assert!(1 == 1);  // This will have no user-visible effect
assert!(2 == 3);  // This will trigger a panic, crashing the program
}

There are, however, a fair number of differences between C/++ and Rust assertions:

• Although well-intentioned, the C/++ practice of only checking assertions in debug builds has proven to be tracherous in practice. Therefore, most Rust assertions are checked in all builds. When the runtime cost of checking an assertion in release builds proves unacceptable, you can use debug_assert!() instead, for assertions which are only checked in debug builds.

• Rust assertions do not abort the process in the default compiler configuration. Cleanup code will still run, so e.g. files and network conncections will be closed properly, reducing system state corruption in the event of a crash. Also, unlike unhandled C++ exceptions, Rust panics make it trivial to get a stack trace at the point of assertion failure by setting the RUST_BACKTRACE environment variable to 1.

• Rust assertions allow you to customize the error message that is displayed in the event of a failure, using the same formatting mini-language as println!():

  #![allow(unused)]
  fn main() {
  let sonny = "dead";
  assert!(sonny == "alive",
          "Look how they massacred my boy :( Why is Sonny {}?",
          sonny);
  }

Finally, one common case for wanting custom error messages in C++ is when checking two variables for equality. If they are not equal, you will usually want to know what are their actual values. In Rust, this is natively supported by the assert_eq!() and assert_ne!(), which respectively check that two values are equal or not equal.

If the comparison fails, the two values being compared will be printed out with Debug formatting.

#![allow(unused)]
fn main() {
assert_eq!(2 + 2,
           5,
           "Don't you dare make Big Brother angry! >:(");
}

Exercise

Now, go to your code editor, open the examples/01-let-it-be.rs source file, and address the TODOs in it. The code should compile and runs successfully at the end.

To attempt to compile and run the file after making corrections, you may use the following command in the VSCode terminal:

cargo run --example 01-let-it-be