Mozilla's systems programming language Rust

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© Lead Image © Paul Maydikov, 123RF.com

© Lead Image © Paul Maydikov, 123RF.com

Article from Issue 179/2015
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We look at a few features of Rust, Mozilla's systems programming language, and its similarity to other languages.

The statically typed programming language Rust [1] places much emphasis on speed and security. Rust, like its inspiration C, stores fixed-sized variables in a stack. However, it stores pointer values in the main memory heap. The code in Listing 1 demonstrates controlled handling of allocated memory in Rust. If you compile Listing 1 in the shell via

rustc moved.rs

the compiler will abort the process with the error message moved_type.rs:8:3: 8:6 error: use of moved value: `vec` for line 8.

In Listing 1, the pointer vec (line 6) generated in the main part of the program refers to the memory area in the heap that saves the representation of the vector with the components 1, 2, and 3. Line 7 passes the pointer to print_length() and makes the function the exclusive owner. As a consequence, the program can no longer use the pointer from line 7 onward, resulting in the error message.

Listing 1

Moved Value (moved_type.rs)

01 fn print_length(vec:Vec<i32>) {
02   println!("The length of the vector is {}", vec.len());
03 }
04
05 fn main() {
06   let mut vec:Vec<i32> = vec![1,2,3];
07   print_length(vec);
08   vec.push(4);
09 }

However, if you want to carry on using the pointer, you can temporarily lend it to print_length() by passing in a "borrowed" pointer, &vec, on line 7; also, the print_length() function declaration in line 1 should be changed to vec: &Vec<i32>. If print_length() also needs to be able to change the pointer value, you can alternatively use the fragment vec: &mut Vec<i32>.

However, Rust only allows developers to pass in a variable reference. If you want to handle resource borrowing correctly in all aspects of programming, Rust helps you do so with lifetimes [2]. Rust follows its own approach when releasing allocated memory: The Rust compiler releases memory automatically and does without a garbage collector as used in Java. The language also does not need to use the free() function as in C and C++ to free memory programmatically.

Expressive

If Rust evaluates an expression, the return value is the result. However, all assignments have a return value: an empty tuple. As in many other languages, the semicolon acts as a grouping device to separate expressions and functions.

The following function determines the maximum of two integers, which it adds to the parameter list of the function named in the first line:

fn max(a:i32, b:i32) -> i32 {
  if a > b { return a };
  b
}

The max function requires a 32-bit number as a data type for each parameter a and b. The return value to the right of the -> operator is of the same type.

In the second line, the keyword return (ret in version 0.3) returns a as the result. Otherwise, the next line returns the value b. The last line of the function body does without a return.

If the programmer were to mistakenly finish line 3 with a semicolon, the function would be invalid because that would convert the expression b into an assignment. The return value type would thus be a tuple and not, as required, an integer.

Exemplary

Rust draws heavily on the repertoires of other languages. Like the function programming languages Haskell [3] and Standard ML [4], it uses patterns in various contexts. The assignment using the pattern (x,y,z) on the left-hand side of the allocation

let (x,y,z) = (1,2,3);

declares the three variables x, y, and z in one fell swoop. This assignment, also known as a destructuring assignment, assigns the value of 1 to x, 2 to y, and 3 to z. The match expression in

let w = match x {
    0 => y+z,
    _ => y*z
}

uses a pattern for case distinction to the left of the => operators. If the expression x in the first case yields the value  , the variable w stores the sum of y and z. In any other case, w stores the product from the values of both variables.

Classless

Rust 1.0 scraps the concept of classes. Instead of class definitions, version 1.0 works with user-defined compound data types supplemented by methods, as required.

The keyword struct introduces the definition of the user-defined data type container in Listing 2. In line 2, an instance of container stores a vector of 64-bit-wide floating-point numbers internally in content.

Listing 2

Data Without Classes (impl.rs)

01 struct Container {
02   content: Vec<f64>
03 }
04
05 impl Container {
06   fn print_content(&self) {
07     println!("The container stores {} floats", \
                self.content.len());
08   }
09 }
10
11 fn main() {
12   let c = Container {
13      content: vec!(1.0, 2.0, 3.0)
14   };
15
16   c.print_content();
17 }

The block after the keyword impl in line 5 defines the method that matches the Container data type, such as print_content() (lines 6-8). Thanks to the self variable, it adopts a pointer to itself.

The next line indicates the number of floating-point numbers saved in content using the macro println to output via standard output in the shell. It also formats the results straightaway. However, before the macro does this, Rust replaces the placeholder {} in the character string with the value of the len method call for the vector from the content component.

In the main part of the program, line 12 generates variable c of type Container. It then gets the vector – stored in the component content – in line 13 after the colon. Line 16 also makes use of the print_content() method with familiar dot notation. If a developer now compiles the file impl.rs with

rustc impl.rs

and then executes it, the message The container stores 3 floats appears in the shell.

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