This commit improves the diagnostic when a type parameter is expected
and a closure is found, noting that each closure has a distinct type and
therefore could not always match the caller-chosen type of the
parameter.

Signed-off-by: David Wood <david@davidtw.co>

fold is known in python and javascript as reduce,
not sure about inject but it was written in doc there.

…-in macros

This was a big hack, and built-in macros should be able to deal with `NtIdents` in the input by themselves like any other parser code.

This was a hack to work around the lack of an escape hatch for the "min
`const fn`" checks in const-stable functions. Now that we have co-opted
`allow_internal_unstable` for this purpose, we no longer need the
bespoke attribute.

…atklad

UI to unit test for those using Cell/RefCell/UnsafeCell

Helps with rust-lang#76268.

I'm working on all files using `Cell` and moving them to unit tests when possible.

r? @matklad

…li-obk

Add option to pass a custom codegen backend from a driver

This allows the driver to pass information to the codegen backend. For example the headcrab debugger may in the future want to use cg_clif to JIT code to be injected in the debuggee. This would PR make it possible to tell cg_clif which symbol can be found at which address and to tell it to inject the JITed code into the right process.

This PR may also help with rust-lang/miri#1540 by allowing miri to provide a codegen backend that only emits metadata and doesn't perform any codegen.

cc @nbaksalyar (headcrab)
cc @RalfJung (miri)

…, r=estebank

diag: improve closure/generic parameter mismatch

Fixes rust-lang#51154.

This PR improves the diagnostic when a type parameter is expected and a closure is found, noting that each closure has a distinct type and therefore could not always match the caller-chosen type of the parameter.

r? @estebank

Remove `#[rustc_allow_const_fn_ptr]` and add `#![feature(const_fn_fn_ptr_basics)]`

`rustc_allow_const_fn_ptr` was a hack to work around the lack of an escape hatch for the "min `const fn`" checks in const-stable functions. Now that we have co-opted `allow_internal_unstable` for this purpose, we no longer need a bespoke attribute.

Now this functionality is gated under `const_fn_fn_ptr_basics` (how concise!), and `#[allow_internal_unstable(const_fn_fn_ptr_basics)]` replaces `#[rustc_allow_const_fn_ptr]`. `const_fn_fn_ptr_basics` allows function pointer types to appear in the arguments and locals of a `const fn` as well as function pointer casts to be performed inside a `const fn`. Both of these were allowed in constants and statics already. Notably, this does **not** allow users to invoke function pointers in a const context. Presumably, we will use a nicer name for that (`const_fn_ptr`?).

r? @oli-obk

Add doc alias for iterator fold

fold is known in python and javascript as reduce,
not sure about inject but it was written in doc there.

This was my first confusion when coming into rust, I somehow cannot find where is reduce, sometimes I still forget that it is known as `fold`.

expand: Stop normalizing `NtIdent`s before passing them to built-in macros

Built-in macros should be able to deal with `NtIdents` in the input by themselves like any other parser code.

You can't imagine how bad mutable AST visitors are, *especially* if they are modifying tokens.
This is one step towards removing token visiting from the visitor infrastructure (rust-lang#77271 also works in this direction.)

fix building libstd for Miri on macOS

Fixes a Miri regression introduced by rust-lang#75295
Cc @tmiasko @Amanieu