Tested with the Gaisler bcc2 toolchain (both gcc and clang) and the Leon3 simulator.

Generate `match *self {}` instead of `unsafe { core::intrinsics::unreachable() }`.

This is:

    1. safe
    2. stable

for the benefit of everyone looking at these derived impls through `cargo expand`.

Both expansions compile to the same code at all optimization levels (including `0`).

fix typo in `rustdoc/src/what-is-rustdoc.md`

…r=jackh726

Add a sparc-unknown-none-elf target.

# `sparc-unknown-none-elf`

**Tier: 3**

Rust for bare-metal 32-bit SPARC V7 and V8 systems, e.g. the Gaisler LEON3.

## Target maintainers

- Jonathan Pallant, `[email protected]`, https://ferrous-systems.com

## Requirements

> Does the target support host tools, or only cross-compilation?

Only cross-compilation.

> Does the target support std, or alloc (either with a default allocator, or if the user supplies an allocator)?

Only tested with `libcore` but I see no reason why you couldn't also support `liballoc`.

> Document the expectations of binaries built for the target. Do they assume
specific minimum features beyond the baseline of the CPU/environment/etc? What
version of the OS or environment do they expect?

Tested by linking with a standard SPARC bare-metal toolchain - specifically I used the [BCC2] toolchain from Gaisler (both GCC and clang variants, both pre-compiled for x64 Linux and compiling my own SPARC GCC from source to run on `aarch64-apple-darwin`).

The target is set to use the lowest-common-denominator `SPARC V7` architecture (yes, they started at V7 - see [Wikipedia](https://en.wikipedia.org/wiki/SPARC#History)).

[BCC2]: https://www.gaisler.com/index.php/downloads/compilers

> Are there notable `#[target_feature(...)]` or `-C target-feature=` values that
programs may wish to use?

`-Ctarget-cpu=v8` adds the instructions added in V8.

`-Ctarget-cpu=leon3` adds the V8 instructions and sets up scheduling to suit the Gaisler LEON3.

> What calling convention does `extern "C"` use on the target?

I believe this is defined by the SPARC architecture reference manuals and V7, V8 and V9 are all compatible.

> What format do binaries use by default? ELF, PE, something else?

ELF

## Building the target

> If Rust doesn't build the target by default, how can users build it? Can users
just add it to the `target` list in `config.toml`?

Yes. I did:

```toml
target = ["aarch64-apple-darwin", "sparc-unknown-none-elf"]
```

## Building Rust programs

> Rust does not yet ship pre-compiled artifacts for this target. To compile for
this target, you will either need to build Rust with the target enabled (see
"Building the target" above), or build your own copy of `core` by using
`build-std` or similar.

Correct.

## Testing

> Does the target support running binaries, or do binaries have varying
expectations that prevent having a standard way to run them?

No - it's a bare metal platform.

> If users can run binaries, can they do so in some common emulator, or do they need native
hardware?

But if you use [BCC2] as the linker, you get default memory map suitable for the LEON3, and a default BSP for the LEON3, and so you can run the binaries in the `tsim-leon3` simulator from Gaisler.

```console
$ cat .cargo/config.toml | grep runner
runner = "tsim-leon3 -c sim-commands.txt"
$ cat sim-commands.txt
run
quit
$ cargo +sparcrust run --targe=sparc-unknown-none-elf
   Compiling sparc-demo-rust v0.1.0 (/work/sparc-demo-rust)
    Finished dev [unoptimized + debuginfo] target(s) in 3.44s
     Running `tsim-leon3 -c sim-commands.txt target/sparc-unknown-none-elf/debug/sparc-demo-rust`

 TSIM3 LEON3 SPARC simulator, version 3.1.9 (evaluation version)

 Copyright (C) 2023, Frontgrade Gaisler - all rights reserved.
 This software may only be used with a valid license.
 For latest updates, go to https://www.gaisler.com/
 Comments or bug-reports to [email protected]

 This TSIM evaluation version will expire 2023-11-28

Number of CPUs: 2
system frequency: 50.000 MHz
icache: 1 * 4 KiB, 16 bytes/line (4 KiB total)
dcache: 1 * 4 KiB, 16 bytes/line (4 KiB total)
Allocated 8192 KiB SRAM memory, in 1 bank at 0x40000000
Allocated 32 MiB SDRAM memory, in 1 bank at 0x60000000
Allocated 8192 KiB ROM memory at 0x00000000
section: .text, addr: 0x40000000, size: 104400 bytes
section: .rodata, addr: 0x400197d0, size: 15616 bytes
section: .data, addr: 0x4001d4d0, size: 1176 bytes
read 1006 symbols

  Initializing and starting from 0x40000000
Hello, this is Rust!
PANIC: PanicInfo { payload: Any { .. }, message: Some(I am a panic), location: Location { file: "src/main.rs", line: 33, col: 5 }, can_unwind: true }

  Program exited normally on CPU 0.
```

> Does the target support running the Rust testsuite?

I don't think so, the testsuite requires `libstd` IIRC.

## Cross-compilation toolchains and C code

> Does the target support C code?

Yes.

> If so, what toolchain target should users use to build compatible C code? (This may match the target triple, or it may be a toolchain for a different target triple, potentially with specific options or caveats.)

I suggest [BCC2] from Gaisler. It comes in both GCC and Clang variants.

self type param infer, avoid ICE

fixes rust-lang#113610, which is caused by https://github.com/rust-lang/rust/blob/33a2c2487ac5d9927830ea4c1844335c6b9f77db/compiler/rustc_hir_analysis/src/collect/generics_of.rs#L190-L205

…ors,nnethercote

Generate safe stable code for derives on empty enums

Generate `match *self {}` instead of `unsafe { core::intrinsics::unreachable() }`.

This is:

1. safe
2. stable

for the benefit of everyone looking at these derived impls through `cargo expand`.

[Both expansions compile to the same code at all optimization levels (including `0`).](https://rust.godbolt.org/z/P79joGMh3)