this post was submitted on 07 Feb 2024
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Learning Rust and Lemmy
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Thanks!
Seems I gotta dig into the borrow checker before thinking too much about this!
Otherwise, compiling your code snippet is a nice illustration of how helpful the compiler tries to be ... lots of tips in the output there! To anyone else, just try running
rustc
on this, with the secondprintln!
uncommented and see the output, which is half error half linting.Yea. So if the second
println!
were uncommented, how could we compile this? From what you've said, I'd guess that&
means "borrow" (ie, not "move" ownership).So if we alter
abc
to take a&String
type and notString
, and therefore only "borrow" the variable, and then pass in&v
and notv
to pass in a "borrowed" variable, it should compile.It seems to!
Of course, as the compiler suggests, we could instead just pass in
v.clone()
which presumably creates a new variable and effectively "passes by value".Digging in a bit more, what happens if
abc
(tries to) mutate the variable?We can add
v.push('X')
toabc
and see if we get different printouts. As the compiler would tell us, we would need to make the argumentv
mutable for this to work.I'm not clear on why I don't have to declare that the
v.clone()
is mutable in anyway though.What about trying the same with a "borrowed' variable?
Well we need mutable borrowed variables, so necessary adjustments to the types of
abc
and its call inmain
. And adding an additional mutation ofv
inmain
afterabc
is called, and we get two different println outputs, with each mutation applying to the same variable.It's covered in detail in chapter 4.