Const generics with function pointers produces linker error.

[Mr-Byte]

When attempting to compile a program using const generic function pointers, I get the following linker error:

error: linking with `cc` failed: exit code: 1
  |
  = note: "cc" "-Wl,--as-needed" "-Wl,-z,noexecstack" "-m64" "-L" "/root/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib" "/playground/target/debug/deps/playground-343407c725b154da.playground.9gij2ntw-cgu.0.rcgu.o" "-o" "/playground/target/debug/deps/playground-343407c725b154da" "/playground/target/debug/deps/playground-343407c725b154da.3lfkt68d9bwnd0w.rcgu.o" "-Wl,--gc-sections" "-pie" "-Wl,-zrelro" "-Wl,-znow" "-nodefaultlibs" "-L" "/playground/target/debug/deps" "-L" "/playground/target/debug/build/backtrace-sys-0bad857f246b8d46/out" "-L" "/playground/target/debug/build/miniz-sys-cc52677d966ce23b/out" "-L" "/playground/target/debug/build/ring-3ec1d93b15dca95a/out" "-L" "/root/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib" "-Wl,--start-group" "-Wl,-Bstatic" "/root/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libstd-3155131507cc46c3.rlib" "/root/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libpanic_unwind-d295083df0b8f9bc.rlib" "/root/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libbacktrace-ee721caa5e399a5b.rlib" "/root/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libbacktrace_sys-f522ac12e8b57785.rlib" "/root/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/librustc_demangle-fde635d685e9d5b4.rlib" "/root/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libhashbrown-400b98aa42a3327b.rlib" "/root/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/librustc_std_workspace_alloc-8d4d01f94c0ba00e.rlib" "/root/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libunwind-bbe52d74e07e8ddb.rlib" "/root/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libcfg_if-7809a3c56dc793e3.rlib" "/root/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/liblibc-eb4f3199c7861934.rlib" "/root/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/liballoc-33ebcd88f3886c16.rlib" "/root/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/librustc_std_workspace_core-a2ceb2a4c1b13237.rlib" "/root/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libcore-c19151a56f15ba15.rlib" "-Wl,--end-group" "/root/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libcompiler_builtins-0ce4526441314674.rlib" "-Wl,-Bdynamic" "-ldl" "-lrt" "-lpthread" "-lgcc_s" "-lc" "-lm" "-lrt" "-lpthread" "-lutil" "-lutil"
  = note: /playground/target/debug/deps/playground-343407c725b154da.playground.9gij2ntw-cgu.0.rcgu.o: In function `playground::Wrapper<_>::call':
          /playground/src/main.rs:11: undefined reference to `playground::hello_world'
          collect2: error: ld returned 1 exit status

Source:

#![feature(const_generics)]

fn hello_world() {
    println!("Hello, world!");
}

struct Wrapper<const F: fn() -> ()>;

impl<const F: fn() -> ()> Wrapper<{F}> {
    fn call() {
        F();
    }
}

fn main() {
    Wrapper::<{hello_world}>::call();
}

I'm not sure what the intended behavior should be here, but I was expecting to be able to call the function that I passed as a const generic parameter, which the compiler seems to accept as valid, but is failing during the linking step.

[Aaron1011]

Possibly related: The following code:

#![feature(const_generics)]
struct Checked<const F: fn(usize) -> bool>;

fn not_one(val: usize) -> bool { val != 1 }

fn new() {
    let a: Option<Checked<{not_one}>> = None;
}

produces some strange errors:

error[E0308]: mismatched types
  --> src/lib.rs:36:41
   |
36 |     let a: Option<Checked<{not_one}>> = None;
   |                                         ^^^^ expected `Scalar(AllocId(1).0x0) : fn(usize) -> bool`, found `Scalar(AllocId(1).0x0) : fn(usize) -> bool`
   |
   = note: expected type `std::option::Option<Checked<Scalar(AllocId(1).0x0) : fn(usize) -> bool>>`
              found type `std::option::Option<_>`

error[E0308]: mismatched types
  --> src/lib.rs:36:9
   |
36 |     let a: Option<Checked<{not_one}>> = None;
   |         ^ expected `Scalar(AllocId(1).0x0) : fn(usize) -> bool`, found `Scalar(AllocId(1).0x0) : fn(usize) -> bool`
   |
   = note: expected type `std::option::Option<Checked<Scalar(AllocId(1).0x0) : fn(usize) -> bool>>`
              found type `_`