fixes rust-lang#124669

People keep making fun of this signature for being so gnarly.
Associated type bounds lend it a much simpler scribbling.
ChangeOutputType can also come along for the ride.

Instead of not generating the function at all on big endian (which
makes the CHECK lines fail), instead use to_le() on big endian,
so that we essentially perform a bswap for both endiannesses.

Currently it can be made to fail by rearranging code within
`compiler/rustc_mir_transform/src/lint.rs`.

…=scottmcm

DOC: Add FFI example for slice::from_raw_parts()

For some discussion, see https://users.rust-lang.org/t/missing-guidance-on-converting-ffi-ptr-length-to-slice/106048

See also rust-lang#120608.

…=workingjubilee

docs(core): make more const_ptr doctests assert instead of printing

improves on rust-lang#124669

…s, r=compiler-errors

Provide correct parent for nested anon const

Fixes rust-lang#126147

99% of this PR is just comments explaining what the issue is.

`tcx.parent(` and `hir().get_parent_item(` give different results as the hir owner for all the hir of anon consts is the enclosing function. I didn't attempt to change that as being a hir owner requires a `DefId` and long term we want to stop creating anon consts' `DefId`s before hir ty lowering.

So i just opted to change `generics_of` to use `tcx.parent` to get the parent for `AnonConst`'s. I'm not entirely sure about this being what we want, it does seem weird that we have two ways of getting the parent of an `AnonConst` and they both give different results.

Alternatively we could just go ahead and make `const_evaluatable_unchecked` a hard error and stop providing generics to repeat exprs. Then this isn't an issue. (The FCW has been around for almost 4 years now)

r? ``@compiler-errors``

Simplify provider api to improve llvm ir

This PR seeks to resolve the last concern in rust-lang#99301 (comment)

We resolve the issue by moving the type_id to be stored in the `Request` itself rather than being accessed through the `Erased` trait, letting the compiler infer that the value of the type id will not change between lookups.

### LLVM Codegen

**Before**

```
; <provider_test::MyError as core::error::Error>::provide
; Function Attrs: nonlazybind uwtable
define void `@"_ZN61_$LT$provider_test..MyError$u20$as$u20$core..error..Error$GT$7provide17hd9c9de412063aa73E"(ptr` noalias nocapture noundef nonnull readonly align 1 %self, ptr noundef nonnull align 1 %request.0, ptr noalias nocapture noundef readonly align 8 dereferenceable(32) %request.1) unnamed_addr #0 personality ptr `@rust_eh_personality` {
start:
  %0 = getelementptr inbounds i8, ptr %request.1, i64 24
  %self.1.val.i = load ptr, ptr %0, align 8
  %1 = tail call { i64, i64 } %self.1.val.i(ptr noundef nonnull align 1 %request.0), !noalias !15
  %2 = extractvalue { i64, i64 } %1, 0
  %3 = extractvalue { i64, i64 } %1, 1
  %_18.i.i = icmp ne i64 %2, 1101338453689927725
  %_2.i.i = icmp ne i64 %3, 472224167662714873
  %or.cond.i.not.i = select i1 %_18.i.i, i1 true, i1 %_2.i.i
  br i1 %or.cond.i.not.i, label %_ZN4core5error7Request7provide17h8f8125d2543333e0E.exit, label %bb2.i
```

**After**

```
; <provider_test::MyError as provider_test::Error>::provide
; Function Attrs: mustprogress nofree norecurse nosync nounwind nonlazybind willreturn memory(argmem: readwrite) uwtable
define void `@"_ZN63_$LT$provider_test..MyError$u20$as$u20$provider_test..Error$GT$7provide17h5bbf091795a6d359E"(ptr` noalias nocapture noundef nonnull readonly align 1 %self, ptr nocapture noundef nonnull align 8 %request.0, ptr noalias nocapture noundef readonly align 8 dereferenceable(24) %request.1) unnamed_addr rust-lang#2 personality ptr `@rust_eh_personality` {
start:
  %_19.i = load i64, ptr %request.0, align 8, !noalias !3, !noundef !3
  switch i64 %_19.i, label %_ZN13provider_test7Request7provide17heb3ee140962e3b2fE.exit [
    i64 7665305208997882008, label %bb12.i
    i64 7050211241160863540, label %bb12.i3
    i64 9112786072622981063, label %bb12.i11
  ]
```

…gnature, r=scottmcm

Simplify `[T; N]::try_map` signature

People keep making fun of this signature for being so gnarly.
Associated type bounds admit a much simpler scribbling.

r? ``@scottmcm``

…tlarsan68

Add {{target}} substitution to compiletest

In ferrocene we have ui tests testing the cli interface of the compiler, one of which tests the `--target` flag. To be able to run this on all targets we require a way to specify a valid target in the `compile-flags` directive that is target independent, as otherwise we can only run the test against the one target we choose to supply in the flags. See https://github.com/ferrocene/ferrocene/blob/383cbc80f4e85859a4055f121f15dac329908346/tests/ui/ferrocene/compiler-arguments/target/target.rs

We figured the project might be able to make use of this substitution as well in the future.

try-job: dist-x86_64-msvc

…eyouxu

Make issue-122805.rs big endian compatible

Instead of not generating the function at all on big endian (which makes the CHECK lines fail), instead use to_le() on big endian, so that we essentially perform a bswap for both endiannesses.

Make `storage-live.rs` robust against rustc internal changes.

Currently it can be made to fail by rearranging code within `compiler/rustc_mir_transform/src/lint.rs`.

This is a precursor to rust-lang#125443.

r? `@lqd`