#3: Should builder methods use `mut self` or `&mut self`?

bar.rs

#![crate_type = "lib"]
#![crate_name = "bar"]

extern crate foo;
use foo::Channel;

pub fn a<VALUE: Into<i32>>(mut builder: Channel, value: VALUE) -> Channel {
    builder.a(value.into()).clone()
}
pub fn b<VALUE: Into<i32>>(mut builder: Channel, value: VALUE) -> Channel {
    builder.b(value.into()).clone()
}
pub fn c<VALUE: Into<i32>>(mut builder: Channel, value: VALUE) -> Channel {
    builder.c(value.into()).clone()
}

/// consume variables with black_box to avoid optimization of trivial code.
#[inline(never)]
pub fn black_box(_x: Channel) {
}

baz.rs

#![crate_type = "bin"]
#![crate_name = "baz"]

extern crate foo;
use foo::Channel;

extern crate bar;

fn main() {
    let mut builder = Channel::default();

    builder = bar::a(builder, 1);
    builder = bar::b(builder, 2);
    builder = bar::c(builder, 3);

    let ch = builder.build();
    bar::black_box(ch);
}

foo.rs

#![crate_type = "lib"]
#![crate_name = "foo"]

#[derive(Default, Clone)]
pub struct Channel {
    a: i32,
    b: i32,
    c: i32,
}

impl Channel {
    pub fn a<VALUE: Into<i32>>(&mut self, value: VALUE) -> &mut Self {
        self.a = value.into();
        self
    }
    pub fn b<VALUE: Into<i32>>(&mut self, value: VALUE) -> &mut Self {
        self.b = value.into();
        self
    }
    pub fn c<VALUE: Into<i32>>(&mut self, value: VALUE) -> &mut Self {
        self.c = value.into();
        self
    }
}

impl Channel {
    pub fn build(&mut self) -> Self {
        Channel {
            a: self.a,
            b: self.b,
            c: self.c,
        }
    }
}

stable-release_ASM

	.text
	.file	"baz.cgu-0.rs"
	.section	.text._ZN3baz4main17h1acab71f10722165E,"ax",@progbits
	.p2align	4, 0x90
	.type	_ZN3baz4main17h1acab71f10722165E,@function
_ZN3baz4main17h1acab71f10722165E:
	.cfi_startproc
	pushq	%rbx
.Ltmp0:
	.cfi_def_cfa_offset 16
	subq	$32, %rsp
.Ltmp1:
	.cfi_def_cfa_offset 48
.Ltmp2:
	.cfi_offset %rbx, -16
	movabsq	$8589934593, %rax
	movq	%rax, (%rsp)
	movl	$3, 8(%rsp)
	leaq	16(%rsp), %rbx
	leaq	(%rsp), %rsi
	movq	%rbx, %rdi
	callq	_ZN3foo7Channel5build17hac6b33bde1470760E@PLT
	movq	%rbx, %rdi
	callq	_ZN3bar9black_box17hec8febb0a338c063E@PLT
	addq	$32, %rsp
	popq	%rbx
	retq
.Lfunc_end0:
	.size	_ZN3baz4main17h1acab71f10722165E, .Lfunc_end0-_ZN3baz4main17h1acab71f10722165E
	.cfi_endproc

	.section	.text.main,"ax",@progbits
	.globl	main
	.p2align	4, 0x90
	.type	main,@function
main:
	.cfi_startproc
	movq	%rsi, %rax
	movq	%rdi, %rcx
	leaq	_ZN3baz4main17h1acab71f10722165E(%rip), %rdi
	movq	%rcx, %rsi
	movq	%rax, %rdx
	jmp	_ZN3std2rt10lang_start17hfe9ab243c60ffb9bE@PLT
.Lfunc_end1:
	.size	main, .Lfunc_end1-main
	.cfi_endproc


	.section	".note.GNU-stack","",@progbits