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

main.rs

mod builder {
    #[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,
            }
        }
    }
}

mod foo {
    use super::builder::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)]
fn black_box(_x: builder::Channel) {
}

fn main() {
    let mut builder = builder::Channel::default();
    
    builder = foo::a(builder, 1);
    builder = foo::b(builder, 2);
    builder = foo::c(builder, 3);
    
    let ch = builder.build();
    black_box(ch);
}

stable-releae_LLVMIR-playpen

; ModuleID = 'rust_out.0.rs'
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

%"builder::Channel" = type { i32, i32, i32 }

; Function Attrs: noinline norecurse nounwind uwtable
define internal fastcc void @_ZN8rust_out9black_box17h4fbb43c364fffe15E(%"builder::Channel"* noalias nocapture dereferenceable(12) %_x) unnamed_addr #0 {
entry-block:
  %0 = bitcast %"builder::Channel"* %_x to i8*
  tail call void @llvm.lifetime.end(i64 12, i8* %0)
  ret void
}

; Function Attrs: argmemonly nounwind
declare void @llvm.lifetime.end(i64, i8* nocapture) #1

; Function Attrs: norecurse nounwind uwtable
define internal void @_ZN8rust_out4main17h4bd17b1d5c1469adE() unnamed_addr #2 {
entry-block:
  %arg3 = alloca %"builder::Channel", align 8
  %0 = bitcast %"builder::Channel"* %arg3 to i8*
  call void @llvm.lifetime.start(i64 12, i8* %0)
  %1 = getelementptr inbounds %"builder::Channel", %"builder::Channel"* %arg3, i64 0, i32 0
  store i32 1, i32* %1, align 8
  %2 = getelementptr inbounds %"builder::Channel", %"builder::Channel"* %arg3, i64 0, i32 1
  store i32 2, i32* %2, align 4
  %3 = getelementptr inbounds %"builder::Channel", %"builder::Channel"* %arg3, i64 0, i32 2
  store i32 3, i32* %3, align 8
  call fastcc void @_ZN8rust_out9black_box17h4fbb43c364fffe15E(%"builder::Channel"* noalias nocapture nonnull dereferenceable(12) %arg3)
  call void @llvm.lifetime.end(i64 12, i8* %0)
  ret void
}

; Function Attrs: argmemonly nounwind
declare void @llvm.lifetime.start(i64, i8* nocapture) #1

define i64 @main(i64, i8**) unnamed_addr {
top:
  %2 = tail call i64 @_ZN3std2rt10lang_start17hbcefdc316c2fbd45E(i8* bitcast (void ()* @_ZN8rust_out4main17h4bd17b1d5c1469adE to i8*), i64 %0, i8** %1)
  ret i64 %2
}

declare i64 @_ZN3std2rt10lang_start17hbcefdc316c2fbd45E(i8*, i64, i8**) unnamed_addr

attributes #0 = { noinline norecurse nounwind uwtable }
attributes #1 = { argmemonly nounwind }
attributes #2 = { norecurse nounwind uwtable }

stable-release_ASM-local

	.text
	.file	"aio.cgu-0.rs"
	.section	.text._ZN3aio4main17h1a1276b84a14ae38E,"ax",@progbits
	.p2align	4, 0x90
	.type	_ZN3aio4main17h1a1276b84a14ae38E,@function
_ZN3aio4main17h1a1276b84a14ae38E:
	.cfi_startproc
	retq
.Lfunc_end0:
	.size	_ZN3aio4main17h1a1276b84a14ae38E, .Lfunc_end0-_ZN3aio4main17h1a1276b84a14ae38E
	.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	_ZN3aio4main17h1a1276b84a14ae38E(%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

stable-release_ASM-playpen

	.text
	.file	"rust_out.0.rs"
	.section	.text._ZN8rust_out9black_box17h4fbb43c364fffe15E,"ax",@progbits
	.align	16, 0x90
	.type	_ZN8rust_out9black_box17h4fbb43c364fffe15E,@function
_ZN8rust_out9black_box17h4fbb43c364fffe15E:
	.cfi_startproc
	retq
.Lfunc_end0:
	.size	_ZN8rust_out9black_box17h4fbb43c364fffe15E, .Lfunc_end0-_ZN8rust_out9black_box17h4fbb43c364fffe15E
	.cfi_endproc

	.section	.text._ZN8rust_out4main17h4bd17b1d5c1469adE,"ax",@progbits
	.align	16, 0x90
	.type	_ZN8rust_out4main17h4bd17b1d5c1469adE,@function
_ZN8rust_out4main17h4bd17b1d5c1469adE:
	.cfi_startproc
	subq	$24, %rsp
.Ltmp0:
	.cfi_def_cfa_offset 32
	movabsq	$8589934593, %rax
	movq	%rax, 8(%rsp)
	movl	$3, 16(%rsp)
	leaq	8(%rsp), %rdi
	callq	_ZN8rust_out9black_box17h4fbb43c364fffe15E
	addq	$24, %rsp
	retq
.Lfunc_end1:
	.size	_ZN8rust_out4main17h4bd17b1d5c1469adE, .Lfunc_end1-_ZN8rust_out4main17h4bd17b1d5c1469adE
	.cfi_endproc

	.section	.text.main,"ax",@progbits
	.globl	main
	.align	16, 0x90
	.type	main,@function
main:
	.cfi_startproc
	movq	%rsi, %rax
	movq	%rdi, %rcx
	leaq	_ZN8rust_out4main17h4bd17b1d5c1469adE(%rip), %rdi
	movq	%rcx, %rsi
	movq	%rax, %rdx
	jmp	_ZN3std2rt10lang_start17hbcefdc316c2fbd45E@PLT
.Lfunc_end2:
	.size	main, .Lfunc_end2-main
	.cfi_endproc


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