{"slug":"carbon-lang","title":"carbon-lang","summary":"Use when evaluating Carbon for a C++ code base, running the carbon toolchain from a nightly or Bazel build, or comparing Carbon with staying on C++. Not for C++ modules: use cpp-modules.","platform":"Claude","tags":[],"authorName":"LLM Mart","authorSlug":"llm-mart","score":0,"source":"github","price":null,"verified":false,"createdAt":"2026-09-30T19:49:59.215436Z","repo":{"url":"https://github.com/OutlineDriven/outline-driven-development","stars":54,"forks":10,"license":"Apache-2.0","updatedAt":"2026-09-28T03:16:21Z"},"bodyHtml":"
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name: carbon-lang\ndescription: 'Use when evaluating Carbon for a C++ code base, running the carbon toolchain from a nightly or Bazel build, or comparing Carbon with staying on C++. Not for C++ modules: use cpp-modules.'\ndisable-model-invocation: true

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Carbon

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Carbon is an experimental successor language to C++ from the carbon-language project. As of 2026-09-05 there is no 1.0 and no 0.1: the 0.1 milestone is the minimum language for evaluation, and the 2025 roadmap revision moved its earliest realistic date to the end of 2026 after memory-safety design was added to its scope. The only downloads are nightly toolchain tarballs tagged v0.0.0-0.nightly.YYYY.MM.DD, on limited platforms. Treat every finding below as dated to the nightly you ran.

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Contract

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FieldBound contract
TriggerA team with a large C++ code base asks whether to evaluate Carbon, wants to run the toolchain, wants to call C++ from Carbon, or asks how Carbon's safety model and readiness compare with C++.
AuthorityReversible local: writes only the toolchain checkout or extracted nightly tarball, Carbon source files, and --output artifacts in the working directory; rollback is deleting them. No remote mutation.
Side effectA Bazel build of the toolchain compiles a pinned LLVM commit and takes substantial disk and time.
DoneThe evaluation report states the nightly date or commit used, what compiled and what did not, and a decision (evaluate further or stay on C++) tied to the constraints in step 7.
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Inputs

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Procedure

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  1. Record the toolchain identity. From a nightly: extract the tarball, take the version from the carbon_toolchain-${VERSION} directory name, and run ./carbon_toolchain-${VERSION}/bin/carbon help. From source: ./scripts/run_bazelisk.py run //toolchain -- help builds and runs the driver (target //toolchain:carbon, alias //toolchain:toolchain); //toolchain/install:carbon_toolchain packages the tarball. Done when: the nightly tag or commit hash is written into the report.

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    2. \n

  2. Compile the README example to prove the toolchain works:

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    import Core library \"io\";\n\nfn Run() {\n  Core.Print(42);\n}\n
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    carbon compile --output=forty_two.o forty_two.carbon then carbon link --output=forty_two forty_two.o. Type-check only with carbon compile --phase=check file.carbon (phases: lex, parse, check, lower, optimize, codegen). Done when: the binary prints 42.

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    3. \n

  3. Read the syntax against the design docs, not against tutorials. A package declaration is [impl] [package Name] [library \"Name\"];; there is no api keyword, and an API file is one that omits impl. The entry point is a function named Run in the default library of the Main package; its valid signatures are undecided. Variables are var x: i32 = 3;. Printing is Core.Print(...) after import Core library \"io\";. Done when: each construct in the evaluation has a line in docs/design/README.md behind it.

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  4. Test C++ interop with the documented form. Import a header into the Cpp namespace and call through it:

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    import Cpp library \"circle.h\";\n\nfn Area(r: f64) -> f64 {\n  return Cpp.circle_area(r);\n}\n
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    import Cpp library \"<cstdio>\"; works for standard headers. Interop is semantic: C++ functions, classes, and macros map into Cpp. with explicit-cast rules for types such as Cpp.int. The design says Carbon functions and types may be marked as exported to C, comparable to extern \"C\", but no exported-to-C++ header generator exists in the docs; Carbon.h is not a real artifact. Done when: at least one C++ function is called from Carbon on the recorded nightly, or the failure is quoted.

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    5. \n

  5. State the safety model as designed, not as marketed. Safe and unsafe code are split by a narrow unsafe keyword, with two modes: Strict Carbon marks all unsafe code, Permissive Carbon relaxes that for C++ interop and migration. Type safety and temporal and data-race safety are meant to come from the type system at compile time, spatial safety from run-time checks. The docs state the temporal model is expected to follow Rust's direction at the highest level. None of this is implemented enough to evaluate; say so. Done when: the report cites docs/design/safety/README.md and marks the model as design-stage.

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  6. List the limitations that apply today: no standard library comparable to C++'s, toolchain and syntax change nightly, editor support is thin, and the platform list is short. Done when: each limitation is tied to something observed on the recorded nightly.

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    7. \n

  7. Decide. Evaluate further when the team owns a large C++ code base it intends to migrate gradually, wants C++ interop without an FFI layer, and can absorb nightly churn on an evaluation branch. Stay on C++ when production stability is needed now, when the code leans on the standard library, Boost, or deep template metaprogramming, or when the target platforms exceed the nightly's list. cpp-modules and cpp-templates cover the C++ side of that comparison. Done when: the decision names the constraint that settled it.

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Failure and recovery

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FailureCauseFix
Bazel build fails on LLVMHost Clang older than 19, or libc++, libc++abi, lld missingInstall the listed prerequisites; let Bazel fetch the pinned LLVM commit rather than pointing at a system LLVM
C++ import rejects a headerUnsupported C++ feature in the headerWrap the needed calls in a plain C or simpler C++ shim header and import that
Tutorial syntax fails to parsePre-0.1 churnRewrite against docs/design/README.md at the same commit as the toolchain
Compiler Explorer and local results differDifferent nightliesPin both to the same date; carbon.compiler-explorer.com is the project's own instance
Needed library function does not existNo standard library yetCall the C++ library through import Cpp library
Link error mixing Carbon and C++ objectsToolchain mismatchLink with carbon link from the same nightly that compiled the objects
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Output

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An evaluation report naming the nightly tag or commit, the examples that compiled and ran, the C++ interop result, the safety model status as design-stage, the observed limitations, and the decision from step 7 with its deciding constraint.

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