Linker error when main has a false trait bound

[matthewjasper]

This programme gives a linker error

fn main() where fn(&()): Eq {}

Compiler output:

   Compiling playground v0.0.1 (file:///playground)
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-0e82fec49f1d853f.playground0.rcgu.o" "-o" "/playground/target/debug/deps/playground-0e82fec49f1d853f" "/playground/target/debug/deps/playground-0e82fec49f1d853f.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-abbe17c29cebc5cc/out/.libs" "-L" "/playground/target/debug/build/miniz-sys-7a0db4796c5e7c10/out" "-L" "/usr/lib/x86_64-linux-gnu" "-L" "/playground/target/debug/build/ring-67ac7987243a7fd2/out" "-L" "/root/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib" "-Wl,--start-group" "-Wl,-Bstatic" "/root/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libstd-912509c5d8c60ccd.rlib" "/root/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libpanic_unwind-008c42aad548e7dd.rlib" "/root/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/liballoc_jemalloc-f68d8e8245e507c6.rlib" "/root/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libunwind-0fe53371ec419e32.rlib" "/root/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/liballoc_system-9dec1cbd51097ce1.rlib" "/root/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/liblibc-6a4fb915dd86d140.rlib" "/root/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/liballoc-7ebba6af2d3cc324.rlib" "/root/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libstd_unicode-7a26f8b3cf380464.rlib" "/root/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libcore-b0c2d164a9741309.rlib" "-Wl,--end-group" "/root/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libcompiler_builtins-66d072e25a9acee3.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`.

To learn more, run the command again with --verbose.

cc #48214 - which will make this issue easier to come across.

[nikomatsakis]

We... should not allow where clauses on main. That is definitely a bug. I think that a lot of type soundness stuff basically rests on the idea that the entry function has no where clauses in scope =)