mut and & confusion

[pd-giz-dave]

Describe the bug

Attempt to compile this:

module main
@[heap]
struct Logger {}
struct Thing {
mut:
	logger &Logger
}
fn new_logger() &Logger {
	return &Logger{}
}
fn step(mut logr &Logger) Thing {
	return Thing{logger: logr}
}
fn main()  {
	mut logr := new_logger()
	thing := step(mut logr)
}

It generates a compiler error of:

compiler_bugs/main.v:12:15: error: cannot assign to field `logger`: expected `&Logger`, not `&&Logger`
   10 | }
   11 | fn step(mut logr &Logger) Thing {
   12 |     return Thing{logger: logr}
      |                  ~~~~~~~~~~~~
   13 | }
   14 | fn main()  {

Reproduction Steps

Attempt to compile the source of above

Expected Behavior

I expect it to compile

Current Behavior

Compiler error of: error: cannot assign to field logger: expected &Logger, not `&&Logger'

Possible Solution

No response

Additional Information/Context

No response

V version

V 0.4.6 66ea826

Environment details (OS name and version, etc.)

dave@dave-mini-pc~/.../fellsafe/v >>> v doctor
V full version: V 0.4.6 294f7e4.66ea826
OS: linux, "EndeavourOS Linux"
Processor: 16 cpus, 64bit, little endian, AMD Ryzen 7 5800H with Radeon Graphics

getwd: /home/dave/precious/fellsafe/v
vexe: /home/dave/v/v
vexe mtime: 2024-07-08 15:23:28

vroot: OK, value: /home/dave/v
VMODULES: OK, value: /home/dave/.vmodules
VTMP: OK, value: /tmp/v_1000

Git version: git version 2.45.2
Git vroot status: weekly.2024.27-32-g66ea8260
.git/config present: true

CC version: cc (GCC) 14.1.1 20240522
thirdparty/tcc status: thirdparty-linux-amd64 40e5cbb5

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[JalonSolov]

mut adds an implicit &, since you have to pass by reference to access the original in order to modify it. So mut log &Logger == log &&Logger... in other words, a pointer to a pointer to a Logger instance,

[pd-giz-dave]

Yes, but the compiler insists on the 'mut' in the step call.
Coming from a 'C' like environment I get very confused in V about the use of mut and &
So is the 'fix' to remove the '&' everywhere and just rely on 'mut' to ensure aobjects are passed by reference and not copied?
In my struct I want a reference to an object, not a copy.

[JalonSolov]

Yes, it is confusing... until you realize a mut parameter has to be a reference.

mut implies a reference, since you can't modify the original without a reference. Without a reference, you get a copy on the stack.

So yes, using mut logr Logger is correct to get a reference to a Logger instance named logr.

[pd-giz-dave]

Thanks. I've unconfused myself.
In my mind a 'mut' was merely allowing the compiler to tolerate an assign; reference, or not, was up to me.
That is clearly incorrect.
It may be helpful if the documentation explicitly stated that mut implies a reference.