[rust-dev] Why doesn't rust require "mut" param prefix at call site?

Martin Olsson martin at minimum.se
Wed Jan 1 15:49:29 PST 2014

Short version of my question:

Why doesn't rust require "mut" param prefix at call sites? i.e. to 
avoid "non-const ref badness" that C++ has?

Longer version of my question:

Since this question was asked recently by vadim and not really 
answered clearly (imo), I'm also including this longer verbose version 
of my question.

For example in C the call "f(a,&b);" might modify "b" but not "a" so 
the "&" token acts as a "call site heads-up flag" when reading the 
code. In C# the "out/ref" keywords are mandatory at the call site if 
the callee uses them in its param declaration so there you also get a 
little in hint when reading the code near the call site. C++ of course 
has non-const references so "f(a,&b);" might modify both "a" and "b" 
so the hint is missing and I really have to look up the code for "f()" 
to be sure. If some function foo() passes "a" to a bunch of functions 
then I have to find each such function and check if "a" can be 
modified or not, so potentially I have to open a bunch of files and 
read code there before I can fully understand the code near the call 

Because of this many large C++ projects have coding styles that 
disallow non-const refs. See for example the google C++ coding style 

Right now, it seems that rust works similar to C++ in this regard, 
meaning that there is no hint at the call site that a parameter may or 
may not be modified by the function.

In the snippet below, if I'm reading foo() in main.rs and I wonder 
which lines in foo() could possibly change the value of "i", then I 
have to open up 4 additional files and find the relevant source 
locations to double check which functions might mutate their arguments.

Why isn't it a good idea to require some parameter prefix like "mut" 
at the call site so that when I read main.rs I immediately will know 
which lines among the calls to funcA()..funcD() that might change the 
value of "i" ?

// ---[ funcA.rs ]-----------------------
fn funcA(i: &int) -> int{
     return 2**i;
// ---[ funcB.rs ]-----------------------
fn funcB(i: &mut int) -> int {
     *i += 1;
     return 0;
// ---[ funcC.rs ]-----------------------
fn funcC(i: &int) -> int {
     return 3**i;
// ---[ funcD.rs ]-----------------------
fn funcD(i: &int) -> int{
     return 2**i;
// ---[ main.rs ]-----------------------
fn foo(i: &mut int) {
     *i += 1;
     funcB(i); // no mut!
fn main() {
     let mut i: int = 0;
     foo(&mut i);
     println!("{}", i);


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