Inconsistent behaviour & peculiar error messages for `extern "Rust" fn main()`

[huonw]

extern "C" fn main() {}

gives

extern-rust.rs:1:0: 1:23 error: main function expects type: `fn()`: expected "Rust" fn but found "C" fn
extern-rust.rs:1 extern "C" fn main() {}
                 ^~~~~~~~~~~~~~~~~~~~~~~

but changing the "C" to a "Rust" as the error message implies might work gives

extern-rust.rs:1:0: 1:26 error: main function expects type: `fn()`: expected impure fn but found extern fn
extern-rust.rs:1 extern "Rust" fn main() {}
                 ^~~~~~~~~~~~~~~~~~~~~~~~~~

which is peculiar because it doesn't pass the entry-point-type-checker (although it'd hit #10763 even if it did), and it mentions impure fn even though this is now essentially a historical implementation detail of the AST (after the removal of pure fn).

[alexcrichton]

Note that this is not specific to the main function, error messages related to the Rust-abi are a little confusing in general

fn f(_: extern "Rust" fn()) {}
extern fn bar() {}            
fn main() { f(bar) }          

foo.rs:3:14: 3:17 error: mismatched types: expected `fn()` but found `extern "C" extern fn()` (expected "Rust" fn but found "C" fn)
foo.rs:3 fn main() { f(bar) }
                       ^~~
error: aborting due to previous error

[emberian]

Hah! I knew that that duplicate extern was going to show up somewhere.

[klutzy]

"impure fn": #9920