Repository: rust
Issue: -Clinker-plugin-lto doesn't work without extra manual work
Labels: A-linkage,O-macos,T-compiler

```
$ cargo new testcase
$ cd testcase
$ RUSTFLAGS="-Clinker-plugin-lto" cargo run --release
```
Yields the following on mac:
```
 
   Compiling testcase v0.1.0 (/Users/glandium/testcase)
error: linking with `cc` failed: exit code: 1
  |
  = note: "cc" "-m64" "-Wl,-plugin-opt=O3" "-Wl,-plugin-opt=mcpu=core2" "-L" "/Users/glandium/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/x86_64-apple-darwin/lib" "/Users/glandium/testcase/target/release/deps/testcase-d97266bc083c0e9e.testcase.4ank1dht-cgu.0.rcgu.o" "/Users/glandium/testcase/target/release/deps/testcase-d97266bc083c0e9e.testcase.4ank1dht-cgu.1.rcgu.o" "-o" "/Users/glandium/testcase/target/release/deps/testcase-d97266bc083c0e9e" "/Users/glandium/testcase/target/release/deps/testcase-d97266bc083c0e9e.3pvtkuuqiy7p6wge.rcgu.o" "-Wl,-dead_strip" "-nodefaultlibs" "-L" "/Users/glandium/testcase/target/release/deps" "-L" "/Users/glandium/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/x86_64-apple-darwin/lib" "/Users/glandium/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/x86_64-apple-darwin/lib/libstd-64d1544b9dc8a8d7.rlib" "/Users/glandium/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/x86_64-apple-darwin/lib/libpanic_unwind-47702365139f147e.rlib" "/Users/glandium/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/x86_64-apple-darwin/lib/libbacktrace_sys-0aefa3a2bfa44649.rlib" "/Users/glandium/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/x86_64-apple-darwin/lib/libunwind-c2b22c88cacffeb6.rlib" "/Users/glandium/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/x86_64-apple-darwin/lib/librustc_demangle-fca4484aa9be2d09.rlib" "/Users/glandium/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/x86_64-apple-darwin/lib/liblibc-4728c64ee20d89f8.rlib" "/Users/glandium/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/x86_64-apple-darwin/lib/liballoc-1722fbf72ce989c9.rlib" "/Users/glandium/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/x86_64-apple-darwin/lib/librustc_std_workspace_core-0836ff3f3d6a6ee6.rlib" "/Users/glandium/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/x86_64-apple-darwin/lib/libcore-91c9fbc323ad09b7.rlib" "/Users/glandium/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/x86_64-apple-darwin/lib/libcompiler_builtins-43b96ba2cdcc7cb3.rlib" "-lSystem" "-lresolv" "-lc" "-lm"
  = note: ld: unknown option: -plugin-opt=O3
          clang: error: linker command failed with exit code 1 (use -v to see invocation)
          

error: aborting due to previous error

error: Could not compile `testcase`.

To learn more, run the command again with --verbose.
```

On mac, the default linker is ld64. rustc really invokes cc, which invokes ld64 with the flags passed with `-Wl` and some others depending on the other command line arguments. ld64 does support the LLVM plugin... but doesn't support the -plugin-opt option to pass arguments to it. I know some things can be passed with -Wl,-mllvm,... but I don't know if that includes things that rust is trying to pass here.

On Linux, it's funnier:
```
   Compiling testcase v0.1.0 (/tmp/testcase)
error: linking with `cc` failed: exit code: 1
  |
  = note: "cc" "-Wl,--as-needed" "-Wl,-z,noexecstack" "-m64" "-Wl,-plugin-opt=O3" "-Wl,-plugin-opt=mcpu=x86-64" "-L" "/home/glandium/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib" "/tmp/testcase/target/release/deps/testcase-576934ab8201a4ea.testcase.dl00diw9-cgu.0.rcgu.o" "/tmp/testcase/target/release/deps/testcase-576934ab8201a4ea.testcase.dl00diw9-cgu.1.rcgu.o" "-o" "/tmp/testcase/target/release/deps/testcase-576934ab8201a4ea" "/tmp/testcase/target/release/deps/testcase-576934ab8201a4ea.y29oqvxl224jt9k.rcgu.o" "-Wl,--gc-sections" "-pie" "-Wl,-zrelro" "-Wl,-znow" "-Wl,-O1" "-nodefaultlibs" "-L" "/tmp/testcase/target/release/deps" "-L" "/home/glandium/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib" "-Wl,--start-group" "-Wl,-Bstatic" "/home/glandium/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libstd-bbd8cb236ab3b537.rlib" "/home/glandium/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libpanic_unwind-334e405e4bdf1791.rlib" "/home/glandium/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libbacktrace_sys-1e14a089a9f63178.rlib" "/home/glandium/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libunwind-f1aae4818bd13556.rlib" "/home/glandium/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/librustc_demangle-a32c94e7da1105b4.rlib" "/home/glandium/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/liblibc-e214e2acd110aec9.rlib" "/home/glandium/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/liballoc-fbf429991e30afee.rlib" "/home/glandium/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/librustc_std_workspace_core-1734308ff05fb551.rlib" "/home/glandium/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libcore-b349c8b817f959a5.rlib" "-Wl,--end-group" "/home/glandium/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libcompiler_builtins-c4b4b16c70e666d9.rlib" "-Wl,-Bdynamic" "-ldl" "-lrt" "-lpthread" "-lgcc_s" "-lc" "-lm" "-lrt" "-lpthread" "-lutil" "-lutil"
  = note: /tmp/testcase/target/release/deps/testcase-576934ab8201a4ea.testcase.dl00diw9-cgu.0.rcgu.o: file not recognized: file format not recognized
          collect2: error: ld returned 1 exit status
          

error: aborting due to previous error

error: Could not compile `testcase`.

To learn more, run the command again with --verbose.
```

Because `cc` is gcc, this just plain doesn't work, for no obvious reason. At the very least, it seems rust should try to use clang instead of cc in that case.

But that also fails:
```
RUSTFLAGS="-Clinker=clang-8 -Clinker-plugin-lto" cargo run --release
   Compiling testcase v0.1.0 (/tmp/testcase)
error: linking with `clang-8` failed: exit code: 1
  |
  = note: "clang-8" "-Wl,--as-needed" "-Wl,-z,noexecstack" "-m64" "-Wl,-plugin-opt=O3" "-Wl,-plugin-opt=mcpu=x86-64" "-L" "/home/glandium/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib" "/tmp/testcase/target/release/deps/testcase-e3dc53d119c25549.testcase.ci8upasg-cgu.0.rcgu.o" "/tmp/testcase/target/release/deps/testcase-e3dc53d119c25549.testcase.ci8upasg-cgu.1.rcgu.o" "-o" "/tmp/testcase/target/release/deps/testcase-e3dc53d119c25549" "/tmp/testcase/target/release/deps/testcase-e3dc53d119c25549.61wo3nlknhbro8d.rcgu.o" "-Wl,--gc-sections" "-pie" "-Wl,-zrelro" "-Wl,-znow" "-Wl,-O1" "-nodefaultlibs" "-L" "/tmp/testcase/target/release/deps" "-L" "/home/glandium/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib" "-Wl,--start-group" "-Wl,-Bstatic" "/home/glandium/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libstd-bbd8cb236ab3b537.rlib" "/home/glandium/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libpanic_unwind-334e405e4bdf1791.rlib" "/home/glandium/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libbacktrace_sys-1e14a089a9f63178.rlib" "/home/glandium/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libunwind-f1aae4818bd13556.rlib" "/home/glandium/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/librustc_demangle-a32c94e7da1105b4.rlib" "/home/glandium/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/liblibc-e214e2acd110aec9.rlib" "/home/glandium/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/liballoc-fbf429991e30afee.rlib" "/home/glandium/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/librustc_std_workspace_core-1734308ff05fb551.rlib" "/home/glandium/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libcore-b349c8b817f959a5.rlib" "-Wl,--end-group" "/home/glandium/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libcompiler_builtins-c4b4b16c70e666d9.rlib" "-Wl,-Bdynamic" "-ldl" "-lrt" "-lpthread" "-lgcc_s" "-lc" "-lm" "-lrt" "-lpthread" "-lutil" "-lutil"
  = note: /usr/bin/ld: bad -plugin-opt option
          clang: error: linker command failed with exit code 1 (use -v to see invocation)
          

error: aborting due to previous error

error: Could not compile `testcase`.

To learn more, run the command again with --verbose.
```

And here, the reason is essentially the same: the underlying linker doesn't support the `-plugin-opt` flag... except it does, but not when it's not passed `-plugin`, which happens when the compiler passes it, which happens when `-flto` was on its command line:

```
$ RUSTFLAGS="-Clinker=clang-8 -Clinker-plugin-lto -Clink-arg=-flto" cargo run --release   Compiling testcase v0.1.0 (/tmp/testcase)
    Finished release [optimized] target(s) in 0.28s
     Running `target/release/testcase`
Hello, world!
```

Using lld works too, because it doesn't need an explicit `-plugin`:
```
$ RUSTFLAGS="-Clinker=clang-8 -Clinker-plugin-lto -Clink-arg=-fuse-ld=lld" cargo run --release
   Compiling testcase v0.1.0 (/tmp/testcase)
    Finished release [optimized] target(s) in 0.36s
     Running `target/release/testcase`
Hello, world!
```