Add str::strip_prefix and str::strip_suffix

Introduces a counterpart for `Path::strip_prefix` on `str`.

This was also discussed in https://internals.rust-lang.org/t/pre-pr-path-strip-prefix-counterpart-in-str/11364/.

stabilize Result::map_or

r? @SimonSapin
Closes #66293

Stabilize `std::{rc,sync}::Weak::{weak_count, strong_count}`

* Original PR: #56696
* Tracking issue: #57977

Closes: #57977

Supporting comments:

> Although these were added for testing, it is occasionally useful to have a way to probe optimistically for whether a weak pointer has become dangling, without actually taking the overhead of manipulating atomics. Are there any plans to stabilize this?

_Originally posted by @bdonlan in https://github.com/rust-lang/rust/issues/57977#issuecomment-516970921_

> Having this stabilized would help. Currently, the only way to check if a weak pointer has become dangling is to call `upgrade`, which is by far expensive.

_Originally posted by @glebpom in https://github.com/rust-lang/rust/issues/57977#issuecomment-526934709_

Not sure if stabilizing these warrants a full RFC, so throwing this out here as a start for now.

Note: per CONTRIBUTING.md, I ran the tidy checks, but they seem to be failing on unchanged files (primarily in `src/stdsimd`).

Rollup of 6 pull requests

Successful merges:

 - #67255 (Remove i686-unknown-dragonfly target)
 - #67267 (Fix signature of `__wasilibc_find_relpath`)
 - #67282 (Fix example code of OpenOptions::open)
 - #67289 (Do not ICE on unnamed future)
 - #67300 (Restore original implementation of Vec::retain)
 - #67305 (Doc typo)

Failed merges:

r? @ghost

Doc typo

Restore original implementation of Vec::retain

This PR reverts #48065, which aimed to optimize `Vec::retain` by making use of `Vec::drain_filter`. Unfortunately at that time, `drain_filter` was unsound.

The soundness hole in `Vec::drain_filter` was fixed in #61224 by guaranteeing that cleanup logic runs via a nested `Drop`, even in the event of a panic. Implementing this nested drop affects codegen (apparently?) and results in slower code.

Fixes #65970

Do not ICE on unnamed future

Fix #67252.

Fix example code of OpenOptions::open

The example didn't set the access mode flag, which resulted in an `Err(InvalidInput)`.

r? @steveklabnik

Fix signature of `__wasilibc_find_relpath`

Looks like this function changed upstream, so it needs to be adjusted
for when used by libstd.

Remove i686-unknown-dragonfly target

DragonFly BSD removed support for i386 a while ago. It only supports x86_64 right now.

Enable `loop` and `while` in constants behind a feature flag

This PR is an initial implementation of #52000. It adds a `const_loop` feature gate, which allows `while` and `loop` expressions through both HIR and MIR const-checkers if enabled. `for` expressions remain forbidden by the HIR const-checker, since they desugar to a call to `IntoIterator::into_iter`, which will be rejected anyways.

`while` loops also require [`#![feature(const_if_match)]`](https://github.com/rust-lang/rust/pull/66507), since they have a conditional built into them. The diagnostics from the HIR const checker will suggest this to the user.

r? @oli-obk
cc @rust-lang/wg-const-eval

Revert stabilization of never type

Fixes https://github.com/rust-lang/rust/issues/66757

I decided to keep the separate `never-type-fallback` feature gate, but tried to otherwise revert https://github.com/rust-lang/rust/pull/65355. Seemed pretty clean.

( cc @Centril, author of #65355, you may want to check this over briefly )

Actually to commit <69f99e74>.

This PR reverts #48065, which aimed to optimize `Vec::retain` by
making use of `Vec::drain_filter`. Unfortunately at that time,
`drain_filter` was unsound.

The soundness hole in `Vec::drain_filter` was fixed in #61224 by
guaranteeing that cleanup logic runs via a nested `Drop`, even in
the event of a panic. Implementing this nested drop affects codegen
(apparently?) and results in slower code.

Fixes #65970

This reverts commit 15c30ddd.

This reverts commit 8f6197f3.

This reverts commit 089229a1.

Require stable/unstable annotations for the constness of all stable fns with a const modifier

r? @RalfJung @Centril

Every `#[stable]` const fn now needs either a `#[rustc_const_unstable]` attribute or a `#[rustc_const_stable]` attribute. You can't silently stabilize the constness of a function anymore.

SGX: Change ELF entrypoint

This fixes [rust-sgx issue #148](https://github.com/fortanix/rust-sgx/issues/148).

A new entry point is created for the ELF file generated by `rustc`, separate from the enclave entry point. When the ELF file is executed as a Linux binary, the error message below is written to stderr.

> Error: This file is an SGX enclave which cannot be executed as a standard Linux binary.
> See the installation guide at https://edp.fortanix.com/docs/installation/guide/ on how to use 'cargo run' or follow the steps at https://edp.fortanix.com/docs/tasks/deployment/ for manual deployment.

When the ELF file is converted to an SGXS using `elf2sgxs`, the old entry point is still set as the enclave entry point. In a future pull request in the rust-sgx repository, `elf2sgxs` will be modified to remove the code in the ELF entry point, since this code is not needed in the enclave.

Point at method call when type annotations are needed

- Point at method call instead of whole expression when type annotations are needed.
- Suggest use of turbofish on function and methods.

Fix #49391, fix #46333, fix #48089. CC #58517, #63502, #63082.

Fixes https://github.com/rust-lang/rust/issues/40015

r? @nikomatsakis

Rollup of 7 pull requests

Successful merges:

 - #67026 (Improve diagnostics and code for exhaustiveness of empty matches)
 - #67235 (VecDeque: drop remaining items on destructor panic)
 - #67254 (dont ICE in case of invalid drop fn)
 - #67256 (Reduce allocs for validation errors)
 - #67274 (be explicit that mem::uninitialized is the same as MaybeUninit::uninit().assume_init())
 - #67278 (`coerce_inner`: use initial `expected_ty`)
 - #67280 (docs: std::convert::From: Fix typo)

Failed merges:

r? @ghost

docs: std::convert::From: Fix typo

Fix a minor typo

`coerce_inner`: use initial `expected_ty`

Fixes #67273.
Follow-up to #59439.

r? @oli-obk

be explicit that mem::uninitialized is the same as MaybeUninit::uninit().assume_init()

Cc @Centril @nikomatsakis

Reduce allocs for validation errors

This probably doesn't really matter, but I just felt like I had to do this...

r? @oli-obk

dont ICE in case of invalid drop fn

Fixes https://github.com/rust-lang/miri/issues/1112

r? @oli-obk

VecDeque: drop remaining items on destructor panic

Closes https://github.com/rust-lang/rust/issues/67232

Improve diagnostics and code for exhaustiveness of empty matches

There was a completely separate check and diagnostics for the case of an empty match. This led to slightly different error messages and duplicated code.
This improves code reuse and generally clarifies what happens for empty matches. This also clarifies the action of the `exhaustive_patterns` feature, and ensures that this feature doesn't change diagnostics in places it doesn't need to.

Co-Authored-By: NMazdak Farrokhzad <twingoow@gmail.com>