Building object file with `swiftc -emit-object` also emitting `_main` implementation - swift

Question

I'm currently building object files using swiftc, with:
swiftc -emit-object bar.swift
where bar.swift is something simple like:
class Bar {
var value: Int
init(value: Int) {
self.value = value
}
func plusValue(_ value: Int) -> Int {
return self.value + value
}
}
When I then move on to linking this against my main object to create an executable, I get the following error:
$ cc -o foobar foo.o bar.o
duplicate symbol '_main' in:
foo.o
bar.o
ld: 1 duplicate symbol for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
This indicates that swiftc is adding a main implementation to the object file, which can be confirmed with:
$ nm bar.o | grep _main
0000000000000000 T _main
As far as I can tell, this added function does very little:
$ otool -tV bar.o
bar.o:
(__TEXT,__text) section
_main:
0000000000000000 pushq %rbp
0000000000000001 movq %rsp, %rbp
0000000000000004 xorl %eax, %eax
0000000000000006 movl %edi, -0x4(%rbp)
0000000000000009 movq %rsi, -0x10(%rbp)
000000000000000d popq %rbp
000000000000000e retq
000000000000000f nop
....snip...
Is there a way to tell swiftc -emit-object to not add this vestigial implementation of main?

Answer 1

Short answer
The command line argument that I was missing was:
-parse-as-library
Long answer
In my quest to find an answer, I resorted to looking at the Swift source code on Github, and with a bit of luck found the following compiler invocation in the test suite:
// RUN: %target-build-swift %S/Inputs/CommandLineStressTest/CommandLineStressTest.swift -parse-as-library -force-single-frontend-invocation -module-name CommandLineStressTestSwift -emit-object -o %t/CommandLineStressTestSwift.o
According to swiftc --help, -parse-as-library causes the compiler to:
Parse the input file(s) as libraries, not scripts
This has proven to work for me, and the only difference in exported symbols is the removal of _main:
$ diff -U1 <(swiftc -emit-object -module-name bar -o a.o bar.swift && nm a.o | cut -c18-) <(swiftc -emit-object -parse-as-library -module-name bar -o b.o bar.swift && nm b.o | cut -c18-)
--- /dev/fd/63 2020-03-28 17:13:08.000000000 +1100
+++ /dev/fd/62 2020-03-28 17:13:08.000000000 +1100
## -23,3 +23,2 ##
U __objc_empty_cache
-T _main
s _objc_classes
and in the generated assembly the only change is the removal of the _main code:
$ diff -U1 <(swiftc -emit-object -module-name bar -o a.o bar.swift && objdump -d -no-leading-addr -no-show-raw-insn a.o) <(swiftc -emit-object -parse-as-library -module-name bar -o b.o bar.swift && objdump -d -no-leading-addr -no-show-raw-insn b.o)
--- /dev/fd/63 2020-03-28 17:19:03.000000000 +1100
+++ /dev/fd/62 2020-03-28 17:19:03.000000000 +1100
## -1,15 +1,5 ##
-a.o: file format Mach-O 64-bit x86-64
+b.o: file format Mach-O 64-bit x86-64
Disassembly of section __TEXT,__text:
-_main:
- pushq %rbp
- movq %rsp, %rbp
- xorl %eax, %eax
- movl %edi, -4(%rbp)
- movq %rsi, -16(%rbp)
- popq %rbp
- retq
- nop
-
_$S3bar3BarC5valueSivg: