rustc fails to link when the main function is inlined

[Eijebong]

This code fails to link:

#[inline(always)]
fn main() {}

with this nice linker error message

error: linking with `cc` failed: exit code: 1
  |
  = note: "cc" "-Wl,--as-needed" "-Wl,-z,noexecstack" "-m64" "-L" "/root/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib" "/playground/target/debug/deps/playground-b1780ebf501c82ef.playground0.rcgu.o" "-o" "/playground/target/debug/deps/playground-b1780ebf501c82ef" "/playground/target/debug/deps/playground-b1780ebf501c82ef.crate.allocator.rcgu.o" "-Wl,--gc-sections" "-pie" "-Wl,-z,relro,-z,now" "-nodefaultlibs" "-L" "/playground/target/debug/deps" "-L" "/playground/target/debug/build/backtrace-sys-65fded567eb95adc/out/.libs" "-L" "/playground/target/debug/build/miniz-sys-678f410ba3e047db/out" "-L" "/usr/lib/x86_64-linux-gnu" "-L" "/playground/target/debug/build/ring-c0358b5d62ddd508/out" "-L" "/root/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib" "-Wl,-Bstatic" "/root/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libstd-58a9e2944951d97f.rlib" "/root/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/liballoc_jemalloc-17deae1aa65c9467.rlib" "/root/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libpanic_unwind-167b5e977a2ab35c.rlib" "/root/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libunwind-b8892ba833aa6677.rlib" "/root/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/liballoc_system-17235785be0fea01.rlib" "/root/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/liblibc-2e9b8cf09e3563de.rlib" "/root/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/liballoc-d7195d5e94bc6586.rlib" "/root/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libstd_unicode-963f18f265d9b8d3.rlib" "/root/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libcore-48934815da5d2bfb.rlib" "/root/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libcompiler_builtins-a6b6fad6cc543169.rlib" "-Wl,-Bdynamic" "-l" "dl" "-l" "rt" "-l" "pthread" "-l" "pthread" "-l" "gcc_s" "-l" "c" "-l" "m" "-l" "rt" "-l" "pthread" "-l" "util" "-l" "util"
  = note: /usr/lib/gcc/x86_64-linux-gnu/5/../../../x86_64-linux-gnu/Scrt1.o: In function `_start':
          (.text+0x20): undefined reference to `main'
          collect2: error: ld returned 1 exit status
          

error: aborting due to previous error

error: Could not compile `playground`.

Here's a playground link: https://play.rust-lang.org/?gist=1b615b6d4160594fbb4e61b198de1dda&version=stable

[retep998]

Looks like a simple case of lazy inline function instantiation applying in a case where it shouldn't. The fix should be just marking the function as "used" for the sake of instantiating it.

cc @michaelwoerister