Support `#[start]`

The start feature allows to mark a function as the entry point and override the start lang item. It's currently not supported in Miri:

error: internal compiler error: src/tools/miri/src/eval.rs:110:9: main function must not take any arguments

thread 'rustc' panicked at 'Box<Any>', compiler/rustc_errors/src/lib.rs:1007:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: rustc 1.54.0-nightly (c79419af0 2021-06-04) running on x86_64-unknown-linux-gnu

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=b867b78dd787c573bdfb44c88fa856dc

miri/src/eval.rs

Line 110 in ef99830

bug!("main function must not take any arguments");

[RalfJung]

Interesting, so #[start] must have 2 arguments even though the start lang-item defined inside std has 0 arguments.

error[E0308]: `#[start]` function has wrong type
 --> src/main.rs:4:1
  |
4 | fn start() -> isize {
  | ^^^^^^^^^^^^^^^^^^^ incorrect number of function parameters
  |
  = note: expected fn pointer `fn(isize, *const *const u8) -> _`
             found fn pointer `fn() -> _`

the start lang-item defined inside std has 0 arguments

It's not the start lang item, but the main function, I think. The start lang item takes 3 arguments:

miri/src/eval.rs

Lines 195 to 199 in ef99830

	// Call start function.
	ecx.call_function(
	start_instance,
	Abi::Rust,
	&[main_ptr.into(), argc.into(), argv.into()],

[RalfJung]

Ah, right. And then we pass a pointer to the main function. That one must indeed have no arguments or else the call you are citing above will not even be well-typed.