…henkov

Complete removal of #[main] attribute from compiler

resolves rust-lang#93786

---

The `#[main]` attribute was mostly removed from the language in rust-lang#84217, but not completely. It is still recognized as a builtin attribute by the compiler, but it has no effect. However, this no-op attribute is no longer gated by `#[feature(main)]` (which no longer exists), so it's possible to include it in code *on stable* without any errors, which seems unintentional. For example, the following code is accepted ([playground link](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&code=%23%5Bmain%5D%0Afn%20main()%20%7B%0A%20%20%20%20println!(%22hello%20world%22)%3B%0A%7D%0A)).

```rust
#[main]
fn main() {
    println!("hello world");
}
```

Aside from that oddity, the existence of this attribute causes code like the following to fail ([playground link](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&code=use%20tokio%3A%3Amain%3B%0A%0A%23%5Bmain%5D%0Afn%20main()%20%7B%0A%20%20%20%20println!(%22hello%20world%22)%3B%0A%7D%0A)). According rust-lang#84062 (comment), the removal of `#[main]` was expected to eliminate this conflict (previously reported as rust-lang#62127).

```rust
use tokio::main;

#[main]
fn main() {
    println!("hello world");
}
```

```
error[E0659]: `main` is ambiguous
 --> src/main.rs:3:3
  |
3 | #[main]
  |   ^^^^ ambiguous name
  |
  = note: ambiguous because of a name conflict with a builtin attribute
  = note: `main` could refer to a built-in attribute
```

[This error message can be confusing](https://stackoverflow.com/q/71024443/1114), as the mostly-removed `#[main]` attribute is not mentioned in any documentation.

Since the current availability of `#[main]` on stable seems unintentional, and to needlessly block use of the `main` identifier in the attribute namespace, this PR finishes removing the `#[main]` attribute as described in rust-lang#29634 (comment) by deleting it from `builtin_attrs.rs`, and adds two test cases to ensure that the attribute is no longer accepted and no longer conflicts with other attributes imported as `main`.

…askrgr

Rollup of 6 pull requests

Successful merges:

 - rust-lang#91443 (Better suggestions when user tries to collect into an unsized `[_]`)
 - rust-lang#91504 (`#[used(linker)]` attribute)
 - rust-lang#93503 (debuginfo: Fix DW_AT_containing_type vtable debuginfo regression)
 - rust-lang#93753 (Complete removal of #[main] attribute from compiler)
 - rust-lang#93799 (Fix typo in `std::fmt` docs)
 - rust-lang#93813 (Make a few cleanup MIR passes public)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup

…ulacrum

[beta] backports

This backports:

*  Complete removal of #[main] attribute from compiler rust-lang#93753
*  Resolve lifetimes for const generic defaults rust-lang#93669
*  backport llvm fix for issue 91671. rust-lang#93426
*  Fix invalid special casing of the unreachable! macro rust-lang#93179
*  Fix hashing for windows paths containing a CurDir component rust-lang#93697

r? `@Mark-Simulacrum`