Experiment: New fmt::Arguments implementation (another one)

[m-ou-se]

Part of #99012

Alternative to #148529

This is yet another an experimental new implementation of fmt::Arguments. In this implementation, fmt::Arguments is only two pointers in size. (Instead of six, currently.) This makes it the same size as a &str and makes it fit in a register pair.

---

Unlike #148529, this implementation stores all static information as just a single (byte) string, without any indirection:

code:

format_args!("Hello, {name:-^20}!")

lowering before:

fmt::Arguments::new_v1_formatted(
    &["Hello, ", "!\n"],
    &args,
    &[
        Placeholder {
            position: 0usize,
            flags: 3355443245u32,
            precision: format_count::Implied,
            width: format_count::Is(20u16),
        },
    ],
)

lowering in #148529:

format_arguments::new(
    &[
        Piece::num(7usize),
        Piece::str("Hello, "),
        Piece::num(14411519000859136000usize),
        Piece::num(2usize),
        Piece::str("!\n"),
        Piece::num(0usize),
    ],
    &args,
)

lowering in this PR:

fmt::Arguments::new(
    b"\x07Hello, \x83-\x00\x00\xc8\x14\x00\x02!\n\x00",
    &args,
)

This saves a ton of pointers and simplifies the expansion significantly, but does mean that individual pieces (e.g. "Hello, " and "!\n") cannot be reused.

---

Like #148529, this fmt::Arguments can store a &'static str without any indirection or additional storage. This means that simple cases like print_fmt(format_args!("hello")) are now just as efficient for the caller as print_str("hello"), as shown by this example:

code:

fn main() {
    println!("Hello, world!");
}

before:

main:
 sub     rsp, 56
 lea     rax, [rip + .Lanon_hello_world]
 mov     qword ptr [rsp + 8], rax
 mov     qword ptr [rsp + 16], 1
 mov     qword ptr [rsp + 24], 8
 xorps   xmm0, xmm0
 movups  xmmword ptr [rsp + 32], xmm0
 lea     rdi, [rsp + 8]
 call    qword ptr [rip + std::io::stdio::_print]
 add     rsp, 56
 ret

after:

main:
 lea     rsi, [rip + .Lanon_hello_world]
 mov     edi, 29
 jmp     qword ptr [rip + std::io::stdio::_print]

Similarly, panic!("Hello, world!"); shows the same change.

---

To do:

• Performance testing
• Fix compilation performance?
• Documentation / comments

[m-ou-se]

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@rustbot label: +S-waiting-on-perf

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The job x86_64-gnu-gcc failed! Check out the build log: (web) (plain enhanced) (plain)

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---- [run-make] tests/run-make/symbol-mangling-hashed stdout ----

error: rmake recipe failed to complete
status: exit status: 101
command: cd "/checkout/obj/build/x86_64-unknown-linux-gnu/test/run-make/symbol-mangling-hashed/rmake_out" && env -u RUSTFLAGS -u __RUSTC_DEBUG_ASSERTIONS_ENABLED -u __STD_DEBUG_ASSERTIONS_ENABLED AR="ar" BUILD_ROOT="/checkout/obj/build/x86_64-unknown-linux-gnu" CC="cc" CC_DEFAULT_FLAGS="-ffunction-sections -fdata-sections -fPIC -m64" CXX="c++" CXX_DEFAULT_FLAGS="-ffunction-sections -fdata-sections -fPIC -m64" HOST_RUSTC_DYLIB_PATH="/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib" LD_LIBRARY_PATH="/checkout/obj/build/x86_64-unknown-linux-gnu/bootstrap-tools/x86_64-unknown-linux-gnu/release/deps:/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/lib/rustlib/x86_64-unknown-linux-gnu/lib" LD_LIB_PATH_ENVVAR="LD_LIBRARY_PATH" LLVM_BIN_DIR="/checkout/obj/build/x86_64-unknown-linux-gnu/ci-llvm/bin" LLVM_COMPONENTS="aarch64 aarch64asmparser aarch64codegen aarch64desc aarch64disassembler aarch64info aarch64utils aggressiveinstcombine all all-targets amdgpu amdgpuasmparser amdgpucodegen amdgpudesc amdgpudisassembler amdgpuinfo amdgputargetmca amdgpuutils analysis arm armasmparser armcodegen armdesc armdisassembler arminfo armutils asmparser asmprinter avr avrasmparser avrcodegen avrdesc avrdisassembler avrinfo binaryformat bitreader bitstreamreader bitwriter bpf bpfasmparser bpfcodegen bpfdesc bpfdisassembler bpfinfo cfguard cgdata codegen codegentypes core coroutines coverage csky cskyasmparser cskycodegen cskydesc cskydisassembler cskyinfo debuginfobtf debuginfocodeview debuginfodwarf debuginfodwarflowlevel debuginfogsym debuginfologicalview debuginfomsf debuginfopdb demangle dlltooldriver dwarfcfichecker dwarflinker dwarflinkerclassic dwarflinkerparallel dwp engine executionengine extensions filecheck frontendatomic frontenddirective frontenddriver frontendhlsl frontendoffloading frontendopenacc frontendopenmp fuzzercli fuzzmutate globalisel hexagon hexagonasmparser hexagoncodegen hexagondesc hexagondisassembler hexagoninfo hipstdpar instcombine instrumentation interfacestub interpreter ipo irprinter irreader jitlink libdriver lineeditor linker loongarch loongarchasmparser loongarchcodegen loongarchdesc loongarchdisassembler loongarchinfo lto m68k m68kasmparser m68kcodegen m68kdesc m68kdisassembler m68kinfo mc mca mcdisassembler mcjit mcparser mips mipsasmparser mipscodegen mipsdesc mipsdisassembler mipsinfo mirparser msp430 msp430asmparser msp430codegen msp430desc msp430disassembler msp430info native nativecodegen nvptx nvptxcodegen nvptxdesc nvptxinfo objcarcopts objcopy object objectyaml option orcdebugging orcjit orcshared orctargetprocess passes powerpc powerpcasmparser powerpccodegen powerpcdesc powerpcdisassembler powerpcinfo profiledata remarks riscv riscvasmparser riscvcodegen riscvdesc riscvdisassembler riscvinfo riscvtargetmca runtimedyld sandboxir scalaropts selectiondag sparc sparcasmparser sparccodegen sparcdesc sparcdisassembler sparcinfo support symbolize systemz systemzasmparser systemzcodegen systemzdesc systemzdisassembler systemzinfo tablegen target targetparser telemetry textapi textapibinaryreader transformutils vectorize webassembly webassemblyasmparser webassemblycodegen webassemblydesc webassemblydisassembler webassemblyinfo webassemblyutils windowsdriver windowsmanifest x86 x86asmparser x86codegen x86desc x86disassembler x86info x86targetmca xray xtensa xtensaasmparser xtensacodegen xtensadesc xtensadisassembler xtensainfo" LLVM_FILECHECK="/checkout/obj/build/x86_64-unknown-linux-gnu/ci-llvm/bin/FileCheck" PYTHON="/usr/bin/python3" RUSTC="/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" RUSTDOC="/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustdoc" SOURCE_ROOT="/checkout" TARGET="x86_64-unknown-linux-gnu" TARGET_EXE_DYLIB_PATH="/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/x86_64-unknown-linux-gnu/lib" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/run-make/symbol-mangling-hashed/rmake"
--- stdout -------------------------------
checking dylib `libhashed_dylib.so`
------------------------------------------
--- stderr -------------------------------
exported dynamic symbols: [
    "_RNxC12hashed_dylib12HcerJokNWwiK",
    "rust_metadata_hashed_dylib_a4dc49c23f628cc7",
]

thread 'main' (57383) panicked at /checkout/tests/run-make/symbol-mangling-hashed/rmake.rs:66:13:
expected two dynamic symbols starting with `_RNxC12hashed_dylib`
stack backtrace:
   0: __rustc::rust_begin_unwind
             at /rustc/3b4dd9bf1410f8da6329baa36ce5e37673cbbd1f/library/std/src/panicking.rs:698:5
   1: core::panicking::panic_fmt
             at /rustc/3b4dd9bf1410f8da6329baa36ce5e37673cbbd1f/library/core/src/panicking.rs:80:14

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[rust-bors]

☀️ Try build successful (CI)
Build commit: 6e6ba94 (6e6ba949d24fbfbd9cd48ca4c98adf59fbd04482, parent: a7b3715826827677ca8769eb88dc8052f43e734b)

[rust-timer]

Queued 6e6ba94 with parent a7b3715, future comparison URL.
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