Move explorer out of toolchain git repo (#5270)

The explorer is an archived codebase, without a plan to restart
development on it. The costs incurred by keeping it in the main git repo
can be alleviated by moving it to a new sibling repo, without
diminishing the usefulness of the explorer codebase for demonstrating
implementation of the carbon language design.

---------

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>

proposals/p5270.md

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+# Move explorer out of toolchain git repository
+
+<!--
+Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+Exceptions. See /LICENSE for license information.
+SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+-->
+
+[Pull request](https://github.com/carbon-language/carbon-lang/pull/5270)
+
+<!-- toc -->
+
+## Table of contents
+
+-   [Abstract](#abstract)
+-   [Problem](#problem)
+-   [Background](#background)
+-   [Proposal](#proposal)
+-   [Rationale](#rationale)
+-   [Alternatives considered](#alternatives-considered)
+    -   [Use a branch to refer to the explorer codebase](#use-a-branch-to-refer-to-the-explorer-codebase)
+    -   [Do nothing](#do-nothing)
+
+<!-- tocstop -->
+
+## Abstract
+
+The explorer codebase is no longer actively developed or maintained, but retains
+value as a place to look for implementations of various parts of the design.
+Having both the explorer and toolchain in the working git repository causes
+confusion for developers and users, and increases our incremental maintenance
+burden. We will move explorer out of the working git repository so that it
+remains accessible but is clearly deliniated from the toolchain and is more
+clearly frozen or archived.
+
+## Problem
+
+Users have expressed confusion when they come across examples or tests for the
+explorer that involve `Main()`, whereas the design and implementation of the
+toolchain have moved on to `Run()` as the entry point.
+
+Developers have attempted to update the explorer codebase to fix style guide
+issues[^1], as tools like grep or clang-tidy do not differentiate our active
+toolchain codebase from the frozen explorer one.
+
+As we update Bazel, Clang, Clang-tidy, etc, we also have to update the explorer
+codebase to keep it building cleanly.
+
+As we modify the `//testing` harness, we have to accommodate and work around the
+explorer tests[^2][^3][^4].
+
+[^1]: https://github.com/carbon-language/carbon-lang/pull/5224
+
+[^2]: https://github.com/carbon-language/carbon-lang/pull/4979
+
+[^3]: https://github.com/carbon-language/carbon-lang/pull/5025
+
+[^4]: https://github.com/carbon-language/carbon-lang/pull/5036
+
+## Background
+
+-   Previous proposal:
+    [#3532: Focus implementation effort on the toolchain](https://github.com/carbon-language/carbon-lang/blob/trunk/proposals/p3532.md)
+-   Kick-off of this discussion
+    [#5224](https://github.com/carbon-language/carbon-lang/pull/5224#pullrequestreview-2730512195)
+-   Discord discussion:
+    [2025-04-02 #explorer](https://discord.com/channels/655572317891461132/763516049710120960/1356979197070803095)
+
+## Proposal
+
+1. Add a tag `explorer-archived` in the main `carbon-lang` git repository.
+2. Create a new `explorer` repository under the `carbon-language` organization
+   that only contains the `//explorer` and `//installers` directories and their
+   dependencies at head.
+3. Locally ensure the explorer tests build and pass under the `explorer`
+   repository.
+4. Add a `README.md` to the `explorer` repository that explains explorer is
+   archived and not under active development.
+5. Stop building, or remove the "Explorer (trunk)" compiler option from
+   [carbon.compiler-explorer.com](https://carbon.compiler-explorer.com).
+6. Delete `//explorer` and `//installers` in the main `carbon-lang` repository.
+7. Archive the `explorer` repository in GitHub, making it read-only.
+
+Note that fuzzer test cases from the explorer are already relocated under
+`//toolchain/*/fuzzer_corpus/`.
+
+## Rationale
+
+The primary purpose of the explorer codebase at this time is as a demonstration
+of past work implementing the carbon language design. This can help to inform
+the implementation of the toolchain as it catches up in various areas, as long
+as the design has not deviated from the explorer implementation.
+
+Searching in, and providing links to the explorer codebase comes up in design
+discussions occasionally [^5][^6][^7][^8][^9][^10], and we should maintain the
+ability of toolchain developers to look through the explorer easily. The primary
+place they do so is on GitHub, where the code can be linked to. GitHub search
+only works
+[on the main branch](https://docs.github.com/en/search-github/github-code-search/understanding-github-code-search-syntax#using-qualifiers)
+of a repository, so the `trunk` branch for the `carbon-lang` repository. To
+maintain searchability, the explorer codebase must either remain on `trunk` in
+`carbon-lang` or in a sibling repository. GitHub search does continue to work in
+archived repositories.
+
+Proposal
+[p3532](https://github.com/carbon-language/carbon-lang/blob/trunk/proposals/p3532.md)
+directed to keep the explorer codebase active in the main repository, with its
+tests building and running:
+
+> We should keep the Explorer's code in place, building, and passing its basic
+> regression tests because the built artifacts of the Explorer remain really
+> valuable given its coverage of our design's feature sets.
+
+However the [problems](#problem) discussed above result from this situation. We
+can gain the benefit of access to the codebase while reducing its impact on
+developers and users by moving it into a separate git repository.
+
+[^5]:
+    https://discord.com/channels/655572317891461132/998959756045713438/1225116234199203860
+
+[^6]:
+    https://discord.com/channels/655572317891461132/998959756045713438/1237143981150830673
+
+[^7]:
+    https://discord.com/channels/655572317891461132/709488742942900284/1250577021474443376
+
+[^8]:
+    https://discord.com/channels/655572317891461132/748959784815951963/1255669935439482993
+
+[^9]:
+    https://discord.com/channels/655572317891461132/655578254970716160/1302033729761443963
+
+[^10]:
+    https://discord.com/channels/655572317891461132/941071822756143115/1349523309682753606
+
+## Alternatives considered
+
+### Use a branch to refer to the explorer codebase
+
+We could create a branch in the `carbon-lang` repository that contains explorer,
+but delete it from `trunk`. This would also allow the code to be found and
+linked to, however GitHub search does not support searching in a branch. The
+main purpose of the explorer code while archived is for reading the
+implementation, and search is an important part of that.
+
+### Do nothing
+
+In the future, we may want to restart development of the explorer codebase. At
+that time we would benefit from keeping the codebase building as we upgrade
+Bazel, Clang, Clang-tidy, etc. However we don't currently have the resources to
+build two implementations of the language, and there's no plan in our roadmaps
+to restart explorer development. So the cost of letting the explorer codebase
+become stale is outweighed by the costs of keeping it active.