Add MAX_LEN_UTF8 and MAX_LEN_UTF16 Constants

[HTGAzureX1212]

This pull request adds the MAX_LEN_UTF8 and MAX_LEN_UTF16 constants as per #45795, gated behind the char_max_len feature.

The constants are currently applied in the alloc, core and std libraries.

[rustbot]

r? @m-ou-se

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[bors]

☔ The latest upstream changes (presumably #121295) made this pull request unmergeable. Please resolve the merge conflicts.

[joshtriplett]

In the tracking issue, please include a mention that when stabilizing this we should adjust the documentation of https://doc.rust-lang.org/std/primitive.char.html#method.encode_utf8 to use it.

[m-ou-se]

We briefly discussed this in the libs-api meeting and are happy to add these as unstable.
Please open a tracking issue and use that in the #[unstable] attributes. Thanks!

[HTGAzureX1212]

Tracking issue created. Will finish the remaining things later today.

[rust-log-analyzer]

The job x86_64-gnu-llvm-16 failed! Check out the build log: (web) (plain)

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---
#12 writing image sha256:b24da9fa4a2827aa3083adc10ce28cbc178780fc17f7cf9adb83836f5e2a7112 done
#12 naming to docker.io/library/rust-ci done
#12 DONE 10.0s
##[endgroup]
Setting extra environment values for docker:  --env ENABLE_GCC_CODEGEN=1 --env GCC_EXEC_PREFIX=/usr/lib/gcc/
[CI_JOB_NAME=x86_64-gnu-llvm-16]
##[group]Clock drift check
  local time: Thu Feb 29 13:16:18 UTC 2024
  network time: Thu, 29 Feb 2024 13:16:18 GMT
  network time: Thu, 29 Feb 2024 13:16:18 GMT
##[endgroup]
sccache: Starting the server...
##[group]Configure the build
configure: processing command line
configure: 
configure: build.configure-args := ['--build=x86_64-unknown-linux-gnu', '--llvm-root=/usr/lib/llvm-16', '--enable-llvm-link-shared', '--set', 'rust.thin-lto-import-instr-limit=10', '--set', 'change-id=99999999', '--enable-verbose-configure', '--enable-sccache', '--disable-manage-submodules', '--enable-locked-deps', '--enable-cargo-native-static', '--set', 'rust.codegen-units-std=1', '--set', 'dist.compression-profile=balanced', '--dist-compression-formats=xz', '--set', 'build.optimized-compiler-builtins', '--disable-dist-src', '--release-channel=nightly', '--enable-debug-assertions', '--enable-overflow-checks', '--enable-llvm-assertions', '--set', 'rust.verify-llvm-ir', '--set', 'rust.codegen-backends=llvm,cranelift,gcc', '--set', 'llvm.static-libstdcpp', '--enable-new-symbol-mangling']
configure: target.x86_64-unknown-linux-gnu.llvm-config := /usr/lib/llvm-16/bin/llvm-config
configure: llvm.link-shared     := True
configure: rust.thin-lto-import-instr-limit := 10
configure: change-id            := 99999999
---
##[endgroup]
Testing GCC stage1 (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
   Compiling y v0.1.0 (/checkout/compiler/rustc_codegen_gcc/build_system)
    Finished release [optimized] target(s) in 1.26s
     Running `/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-codegen/x86_64-unknown-linux-gnu/release/y test --use-system-gcc --use-backend gcc --out-dir /checkout/obj/build/x86_64-unknown-linux-gnu/stage1-tools/cg_gcc --release --no-default-features --mini-tests --std-tests`
Using system GCC
Using system GCC
[BUILD] example
[AOT] mini_core_hello_world
/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-tools/cg_gcc/mini_core_hello_world
abc
---
---- [run-make] tests/run-make/alloc-no-oom-handling stdout ----

error: make failed
status: exit status: 2
command: cd "/checkout/tests/run-make/alloc-no-oom-handling" && env -u CARGO_MAKEFLAGS -u MAKEFLAGS -u MFLAGS -u RUSTFLAGS AR="ar" CC="cc -ffunction-sections -fdata-sections -fPIC -m64" CXX="c++ -ffunction-sections -fdata-sections -fPIC -m64" HOST_RPATH_DIR="/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib" LD_LIB_PATH_ENVVAR="LD_LIBRARY_PATH" LLVM_BIN_DIR="/usr/lib/llvm-16/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 codegen core coroutines coverage debuginfocodeview debuginfodwarf debuginfogsym debuginfologicalview debuginfomsf debuginfopdb demangle dlltooldriver dwarflinker dwarflinkerparallel dwp engine executionengine extensions filecheck frontendhlsl frontendopenacc frontendopenmp fuzzercli fuzzmutate globalisel hexagon hexagonasmparser hexagoncodegen hexagondesc hexagondisassembler hexagoninfo instcombine instrumentation interfacestub interpreter ipo irprinter irreader jitlink lanai lanaiasmparser lanaicodegen lanaidesc lanaidisassembler lanaiinfo 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 orcjit orcshared orctargetprocess passes perfjitevents powerpc powerpcasmparser powerpccodegen powerpcdesc powerpcdisassembler powerpcinfo profiledata remarks riscv riscvasmparser riscvcodegen riscvdesc riscvdisassembler riscvinfo riscvtargetmca runtimedyld scalaropts selectiondag sparc sparcasmparser sparccodegen sparcdesc sparcdisassembler sparcinfo support symbolize systemz systemzasmparser systemzcodegen systemzdesc systemzdisassembler systemzinfo tablegen target targetparser textapi transformutils ve veasmparser vecodegen vectorize vedesc vedisassembler veinfo webassembly webassemblyasmparser webassemblycodegen webassemblydesc webassemblydisassembler webassemblyinfo webassemblyutils windowsdriver windowsmanifest x86 x86asmparser x86codegen x86desc x86disassembler x86info x86targetmca xcore xcorecodegen xcoredesc xcoredisassembler xcoreinfo xray" LLVM_FILECHECK="/usr/lib/llvm-16/bin/FileCheck" NODE="/usr/bin/node" 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" RUST_BUILD_STAGE="stage2-x86_64-unknown-linux-gnu" S="/checkout" TARGET="x86_64-unknown-linux-gnu" TARGET_RPATH_DIR="/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib/rustlib/x86_64-unknown-linux-gnu/lib" TMPDIR="/checkout/obj/build/x86_64-unknown-linux-gnu/test/run-make/alloc-no-oom-handling/alloc-no-oom-handling" "make"
--- stdout -------------------------------
--- stdout -------------------------------
LD_LIBRARY_PATH="/checkout/obj/build/x86_64-unknown-linux-gnu/test/run-make/alloc-no-oom-handling/alloc-no-oom-handling:/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/lib:/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-bootstrap-tools/x86_64-unknown-linux-gnu/release/deps:/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/lib" '/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc' --out-dir /checkout/obj/build/x86_64-unknown-linux-gnu/test/run-make/alloc-no-oom-handling/alloc-no-oom-handling -L /checkout/obj/build/x86_64-unknown-linux-gnu/test/run-make/alloc-no-oom-handling/alloc-no-oom-handling  -Ainternal_features --edition=2021 -Dwarnings --crate-type=rlib ../../../library/alloc/src/lib.rs --cfg no_global_oom_handling
--- stderr -------------------------------
error: unused import: `core::char::MAX_LEN_UTF8`
##[error]  --> ../../../library/alloc/src/string.rs:45:5
   |