`T: VaArgSafe` is relied on for soundness. Safe impls promise nothing.
Therefore this must be an unsafe trait. Slightly pedantic, as
only core can impl this, but we could choose to unseal the trait.
That would allow soundly (but unsafely) implementing this for e.g.
a `#[repr(C)] struct` that should be passable by varargs.

It was somewhat confusing that the old constructor would create a `FlatPat` in
a (possibly) non-simplified state, and then simplify its contents in-place.

So instead we now create its fields as local variables, perform simplification,
and then create the struct afterwards.

This doesn't affect correctness, but is less confusing.

…thBuf::_push`

…orms rust-lang#126333

…=michaelwoerister

Detect unused structs which derived Default

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Fixes rust-lang#98871

…ec, r=workingjubilee

Remove internal `PathBuf::as_mut_vec`

closes rust-lang#126333

…specify-linker, r=Kobzol

Specify target specific linker for `riscv64gc-gnu` job

In rust-lang#125669 I updated the `riscv64gc-gnu` job to be more in line with other jobs, like the `armhf-gnu` one.

I missed something important, though! Since the `riscv64gc-unknown-linux-gnu` builds go through a cross compiler, invoking `cc` is not correct! That would invoke the `x86_64-unknown-linux-gnu` linker, instead of the `riscv64gc-unknown-linux-gnu` linker.

While developing rust-lang#125669 I assumed this line would have been sufficient, however it was not:

https://github.com/rust-lang/rust/blob/553a69030e5a086eb3841d020db8c9c463948c72/src/ci/docker/host-x86_64/disabled/riscv64gc-gnu/Dockerfile#L27-L28

Unfortunately, without the linker set, `cc` is invoked, and the following linker error occurs:

<details>

<summary>Long output</summary>

```
2024-06-21T14:37:54.4786899Z === HAYSTACK ===
2024-06-21T14:37:54.4787240Z error: linking with `cc` failed: exit status: 1
2024-06-21T14:37:54.4787589Z   |
2024-06-21T14:37:54.4806781Z   = note: LC_ALL="C" PATH="/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/x86_64-unknown-linux-gnu/bin:/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/x86_64-unknown-linux-gnu/bin:/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/x86_64-unknown-linux-gnu/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin" VSLANG="1033" "cc" "/tmp/rustc7Ju6va/symbols.o" "inaccessible/tmp/program.program.45572bc5f2b14090-cgu.0.rcgu.o" "inaccessible/tmp/program.dv9uftjrq86w5xa7l2eo7g9l7.rcgu.o" "-Wl,--as-needed" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/run-make/inaccessible-temp-dir/inaccessible-temp-dir/rmake_out" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/riscv64gc-unknown-linux-gnu/lib" "-Wl,-Bstatic" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/riscv64gc-unknown-linux-gnu/lib/libstd-bb9ed017b7d70437.rlib" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/riscv64gc-unknown-linux-gnu/lib/libpanic_unwind-5b34ef68c22a7b9b.rlib" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/riscv64gc-unknown-linux-gnu/lib/libobject-4b9224407e296f51.rlib" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/riscv64gc-unknown-linux-gnu/lib/libmemchr-1eaaa8d74bbe0b36.rlib" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/riscv64gc-unknown-linux-gnu/lib/libaddr2line-b34292f7086867e9.rlib" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/riscv64gc-unknown-linux-gnu/lib/libgimli-1927540c5dff6690.rlib" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/riscv64gc-unknown-linux-gnu/lib/librustc_demangle-ab493befe3742702.rlib" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/riscv64gc-unknown-linux-gnu/lib/libstd_detect-41195694489636c4.rlib" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/riscv64gc-unknown-linux-gnu/lib/libhashbrown-6905da49c8a1542a.rlib" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/riscv64gc-unknown-linux-gnu/lib/librustc_std_workspace_alloc-bbc6290f03f92349.rlib" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/riscv64gc-unknown-linux-gnu/lib/libminiz_oxide-7a4683d4a82cf47c.rlib" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/riscv64gc-unknown-linux-gnu/lib/libadler-071583516cdbce90.rlib" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/riscv64gc-unknown-linux-gnu/lib/libunwind-d84b176d2f4c1334.rlib" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/riscv64gc-unknown-linux-gnu/lib/libcfg_if-92b7de0160f87442.rlib" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/riscv64gc-unknown-linux-gnu/lib/liblibc-cff1533ea7c26263.rlib" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/riscv64gc-unknown-linux-gnu/lib/liballoc-49e374c13f9e6132.rlib" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/riscv64gc-unknown-linux-gnu/lib/librustc_std_workspace_core-9389dcc15b2f7f7b.rlib" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/riscv64gc-unknown-linux-gnu/lib/libcore-46f0fa14905fdc5c.rlib" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/riscv64gc-unknown-linux-gnu/lib/libcompiler_builtins-20e1af401f90cd1c.rlib" "-Wl,-Bdynamic" "-lgcc_s" "-lutil" "-lrt" "-lpthread" "-lm" "-ldl" "-lc" "-Wl,--eh-frame-hdr" "-Wl,-z,noexecstack" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/riscv64gc-unknown-linux-gnu/lib" "-o" "program" "-Wl,--gc-sections" "-pie" "-Wl,-z,relro,-z,now" "-nodefaultlibs"
2024-06-21T14:37:54.4823900Z   = note: /usr/bin/ld: inaccessible/tmp/program.program.45572bc5f2b14090-cgu.0.rcgu.o: Relocations in generic ELF (EM: 243)
2024-06-21T14:37:54.4825032Z           /usr/bin/ld: inaccessible/tmp/program.program.45572bc5f2b14090-cgu.0.rcgu.o: Relocations in generic ELF (EM: 243)
2024-06-21T14:37:54.4826130Z           /usr/bin/ld: inaccessible/tmp/program.program.45572bc5f2b14090-cgu.0.rcgu.o: Relocations in generic ELF (EM: 243)
2024-06-21T14:37:54.4827200Z           /usr/bin/ld: inaccessible/tmp/program.program.45572bc5f2b14090-cgu.0.rcgu.o: Relocations in generic ELF (EM: 243)
2024-06-21T14:37:54.4828279Z           /usr/bin/ld: inaccessible/tmp/program.program.45572bc5f2b14090-cgu.0.rcgu.o: Relocations in generic ELF (EM: 243)
2024-06-21T14:37:54.4829365Z           /usr/bin/ld: inaccessible/tmp/program.program.45572bc5f2b14090-cgu.0.rcgu.o: Relocations in generic ELF (EM: 243)
2024-06-21T14:37:54.4830474Z           /usr/bin/ld: inaccessible/tmp/program.program.45572bc5f2b14090-cgu.0.rcgu.o: Relocations in generic ELF (EM: 243)
2024-06-21T14:37:54.4831554Z           /usr/bin/ld: inaccessible/tmp/program.program.45572bc5f2b14090-cgu.0.rcgu.o: Relocations in generic ELF (EM: 243)
2024-06-21T14:37:54.4833002Z           /usr/bin/ld: inaccessible/tmp/program.program.45572bc5f2b14090-cgu.0.rcgu.o: Relocations in generic ELF (EM: 243)
2024-06-21T14:37:54.4834116Z           /usr/bin/ld: inaccessible/tmp/program.program.45572bc5f2b14090-cgu.0.rcgu.o: Relocations in generic ELF (EM: 243)
2024-06-21T14:37:54.4835237Z           /usr/bin/ld: inaccessible/tmp/program.program.45572bc5f2b14090-cgu.0.rcgu.o: error adding symbols: file in wrong format
2024-06-21T14:37:54.4835971Z           collect2: error: ld returned 1 exit status
2024-06-21T14:37:54.4836449Z
2024-06-21T14:37:54.4836584Z
2024-06-21T14:37:54.4836744Z error: aborting due to 1 previous error
2024-06-21T14:37:54.4836983Z
2024-06-21T14:37:54.4836990Z
2024-06-21T14:37:54.4837083Z === NEEDLE ===
2024-06-21T14:37:54.4837518Z failed to find or create the directory specified by `--temps-dir`
2024-06-21T14:37:54.4838250Z thread 'main' panicked at /checkout/tests/run-make/inaccessible-temp-dir/rmake.rs:35:14:
2024-06-21T14:37:54.4838801Z needle was not found in haystack
2024-06-21T14:37:54.4839270Z note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
2024-06-21T14:37:54.4840125Z thread 'main' panicked at /checkout/tests/run-make/inaccessible-temp-dir/rmake.rs:27:5:
2024-06-21T14:37:54.4840759Z called `Result::unwrap()` on an `Err` value: Any { .. }
2024-06-21T14:37:54.4841194Z ------------------------------------------
```

</details>

By setting `--set target.riscv64gc-unknown-linux-gnu.linker=riscv64-linux-gnu-gcc` the correct linker is invoked.

## Testing

> [!NOTE]
> `riscv64gc-unknown-linux-gnu` is a [**Tier 2 with Host Tools** platform](https://doc.rust-lang.org/beta/rustc/platform-support.html), all tests may not necessarily pass! This change should only make fix existing broken tests (like `inaccessible-temp-dir`) while introducing no new breakage.

You can test out the job locally:

```sh
mv src/ci/docker/host-x86_64/disabled/riscv64gc-gnu src/ci/docker/host-x86_64/riscv64gc-gnu
DEPLOY=1 ./src/ci/docker/run.sh riscv64gc-gnu
```

…ieril

Tweak a confusing comment in `create_match_candidates`

This comment was accurate at the time it was written, but various later changes reshuffled things in ways that caused the existing comment to become confusing.

I've therefore tried to clarify that *these* candidates are 1:1 with match arms, while also warning that that isn't the case in general.

…, r=joboet

core: VaArgSafe is an unsafe trait

`T: VaArgSafe` is relied on for soundness. Safe impls promise nothing. Therefore this must be an unsafe trait. Slightly pedantic, as only core can impl this, but we *could* choose to unseal the trait. That would allow soundly (but unsafely) implementing this for e.g. a `#[repr(C)] struct` that should be passable by varargs.

Relates to rust-lang#44930

Tweak `FlatPat::new` to avoid a temporarily-invalid state

It was somewhat confusing that the old constructor would create a `FlatPat` in a (possibly) non-simplified state, and then simplify its contents in-place.

So instead we now create its fields as local variables, perform simplification, and then create the struct afterwards.

This doesn't affect correctness, but is less confusing.

---

I've also included some semi-related comments that I made while trying to navigate this code.

Add missing slash in `const_eval_select` doc comment

In the middle of the doc comment, one line has only two slashes instead of three and isn't included in the [rendered documentation](https://doc.rust-lang.org/std/intrinsics/fn.const_eval_select.html#stability-concerns). This PR adds the missing slash.

…ctoring, r=petrochenkov

Delegation: ast lowering refactor

refactoring changes for rust-lang#126699

r? ```@petrochenkov```