The size assertion in the comment was inverted compared to the code. After fixing that the implication that `(new_size >= old_size) => new_size != 0` still doesn't hold so explain why `old_size != 0` at this point.

On structured suggestion for `let` -> `const`  and `const` -> `let`, use
a proper `Span` and update tests to check the correct application.

Follow up to rust-lang#80012.

It was only used once, in a function that was otherwise trivial.

In particular, the CI built `libLLVM-*.so` needs to have `libz.so`
RPATHed so that binaries like `llvm-config` work at all.

This makes it possible to have both std::panic and core::panic as a
builtin macro, by using different builtin macro names for each.

Also removes SyntaxExtension::is_derive_copy, as the macro name (e.g.
sym::Copy) is now tracked and provides that information directly.

This used to be necessary for `is_builtin` in the past, but is no longer required.

Co-authored-by: Vadim Petrochenkov <[email protected]>

Hopefully this will prevent people from continuing to ask about this
over and over again :)

See [this Zulip discussion][1] for more.

[1]: https://rust-lang.zulipchat.com/#narrow/stream/219381-t-libs/topic/Vec.3A.3Atruncate.20implementation

…u64, r=withoutboats

Implement From<char> for u64 and u128.

With this PR you can write
```
let u = u64::from('👤');
let u = u128::from('👤');
```

Previously, you could already write `as` conversions ([Playground link](https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=cee18febe28e69024357d099f07ca081)):
```
// Lossless conversions
dbg!('👤' as u32);    // Prints 128100
dbg!('👤' as u64);    // Prints 128100
dbg!('👤' as u128);   // Prints 128100

// truncates, thus no `From` impls.
dbg!('👤' as u8);     // Prints 100
dbg!('👤' as u16);    // Prints 62564

// These `From` impls already exist.
dbg!(u32::from('👤'));               // Prints 128100
dbg!(u64::from(u32::from('👤')));    // Prints 128100
```

The idea is from ``@gendx`` who opened [this Internals thread](https://internals.rust-lang.org/t/implement-from-char-for-u64/13454), and ``@withoutboats`` responded that someone should open a PR for it.
Some people mentioned `From<char>` impls for `f32` and `f64`, but that doesn't seem correct to me, so I didn't include them here.

I don't know what the feature should be named. Must it be registered somewhere, like unstable features?

r? ``@withoutboats``

…r=matthewjasper

Improve core::ptr::drop_in_place debuginfo

* Use span of the dropped type as function span when possible.
* Rename symbol from `core::ptr::drop_in_place::$hash` to `{{drop}}::<$TYPE>::$hash`.

Fixes rust-lang#77465

(I haven't yet updated the tests)

Fix safety comment

The size assertion in the comment was inverted compared to the code. After fixing that the implication that `(new_size >= old_size) => new_size != 0` still doesn't hold so explain why `old_size != 0` at this point.

… r=petrochenkov

Use correct span for structured suggestion

On structured suggestion for `let` -> `const`  and `const` -> `let`, use
a proper `Span` and update tests to check the correct application.

Follow up to rust-lang#80012.

…Gomez

Remove useless `fill_in` function

It was only used once, in a function that was otherwise trivial.

…ulacrum

Support `download-ci-llvm` on NixOS

In particular, the CI built `libLLVM-*.so` needs to have `libz.so`
RPATHed so that binaries like `llvm-config` work at all.

…repetita, r=jyn514

Remove under-used ImplPolarity enum

It doesn't make much sense to have an enum with only two possible values and to store it inside an `Option` in my opinion when you can do all the same with a simple boolean. I don't expect any chances, performance or RSS usage wise.

r? ``@jyn514``

…=petrochenkov

Allow #[rustc_builtin_macro = "name"]

This adds the option of specifying the name of a builtin macro in the `#[rustc_builtin_macro]` attribute: `#[rustc_builtin_macro = "name"]`.

This makes it possible to have both `std::panic!` and `core::panic!` as a builtin macro, by using different builtin macro names for each. This is needed to implement the edition-specific behaviour of the panic macros of RFC 3007.

Also removes `SyntaxExtension::is_derive_copy`, as the macro name (e.g. `sym::Copy`) is now tracked and provides that information directly.

r? ``@petrochenkov``

…ttmcm

Add comment to `Vec::truncate` explaining `>` vs `>=`

Hopefully this will prevent people from continuing to ask about this
over and over again :)

See [this Zulip discussion][1] for more.

[1]: https://rust-lang.zulipchat.com/#narrow/stream/219381-t-libs/topic/Vec.3A.3Atruncate.20implementation

r? ``@scottmcm``