Establish toolchain and language versioning

docs/project/versioning.md

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+# Toolchain and language versioning
+
+<!--
+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/4105)
+
+<!-- toc -->
+
+## Table of contents
+
+-   [Overview](#overview)
+-   [Major version increments](#major-version-increments)
+    -   [Breaking changes](#breaking-changes)
+    -   [Exclusions to what constitutes a breaking change](#exclusions-to-what-constitutes-a-breaking-change)
+-   [Minor version increments](#minor-version-increments)
+-   [Patch version increments](#patch-version-increments)
+    -   [Examples:](#examples)
+-   [Pre-release versions](#pre-release-versions)
+    -   [Release qualification pre-releases: `MAJOR.MINOR.PATCH-rc.N`](#release-qualification-pre-releases-majorminorpatch-rcn)
+    -   [Incremental development versions: `MAJOR.MINOR.PATCH-0.{nightly,dev}.N`](#incremental-development-versions-majorminorpatch-0nightlydevn)
+        -   [Nightly pre-release versions](#nightly-pre-release-versions)
+        -   [Development pre-release versions](#development-pre-release-versions)
+-   [Relevant proposal](#relevant-proposal)
+
+<!-- tocstop -->
+
+## Overview
+
+Carbon uses a single versioning scheme across both the language itself and
+toolchain, including the standard library, compiler, linker, and all development
+tools released by the main Carbon project.
+
+The scheme conforms to and is closely based on Semantic Versioning (SemVer):
+
+-   Carbon versions: `MAJOR.MINOR.PATCH`
+-   `MAJOR` increments on backwards incompatible changes, including a
+    deprecation that might trigger a build-breaking warning.
+-   `MINOR` increments only expected during early development with a major
+    version of `0`.
+-   `PATCH` increments represent bug fixes only.
+-   Pre-release suffixes:
+    -   `MAJOR.MINOR.PATCH-rc.N`: The N-th potentially viable candidate for a
+        release.
+    -   `MAJOR.MINOR.PATCH-0.nightly.YYYY.MM.DD`: A nightly incremental
+        development build on a particular day during development of that
+        version.
+    -   `MAJOR.MINOR.PATCH-0.dev`: An interactive, incremental development build
+        on a particular day by some developer during development of that
+        version.
+
+See the sections below for the details of each aspect of these versions.
+
+## Major version increments
+
+Aligned with SemVer, the major version must increment for any _breaking change_
+to the language or any part of the toolchain.
+
+The first increment from 0 to 1 is expected to be based on achieving a desired
+milestone of feature completeness and quality to declare the language to have
+reached a stable version.
+
+Subsequent increments are expected to be done with a time-based release
+strategy. Features (or breaking changes) ready to ship will do so, and others
+will wait for the next major release. The exact cadence used is future work and
+should be determined based on discussions with customers in the ramp up to and
+after reaching the 1.0 milestone.
+
+However, just because we increment the major version for a major release and
+_can_ make breaking changes doesn't mean we _will_ or _should_. Breaking
+language changes are extraordinarily expensive for users due to the inherent
+scale of churn they impose. It is tempting to try to make non-breaking releases
+instead, but our experience with C++ language, compiler, and standard library
+updates is that truly making no breaking changes is extremely difficult and
+overly constraining. We expect most releases, especially in the early phases of
+the language to involve _some_ amount of breaking change and will simply work to
+make these as cheap to upgrade through and minimal as we can.
+
+At some future point, it is possible that Carbon will become so stable that it
+makes sense to consider using minor version increments for some releases. If and
+when this happens, we should revisit our versioning and release policies to
+establish a predictable and unsurprising structure.
+
+### Breaking changes
+
+Beyond traditional breaking API changes in either standard library APIs or tool
+APIs, Carbon also includes breaking changes in the language or toolchain.
+Language and toolchain breaking changes are any that cause correct, functioning,
+and non-reflective code to become invalid, rejected, incorrect, or silently
+change behavior.
+
+### Exclusions to what constitutes a breaking change
+
+Carbon excludes "reflective code" which is in some way detecting the Carbon
+version, or the presence or absence of features and as a consequence can be
+"broken" by detecting changes that are correctly designed to otherwise be
+non-breaking. We don't want adding features to be considered a breaking change
+and so exclude code that specifically detects such additions.
+
+Also excludes breaking changes to incorrect code unless it was accepted and
+functioning in some useful, and typically widespread, way despite its bugs.
+
+## Minor version increments
+
+Currently, Carbon plans to primarily use the minor version increments with a 0
+major version to track our progress towards our 1.0 milestone of a feature
+complete initial language. As a consequence we have defined 0.1 and 0.2
+milestones and may define more steps as needed.
+
+Beyond this and in a post-1.0 language world, we expect most significant
+features to also accompany some small and manageable breaking changes from
+deprecations. We may choose to revisit this in the future, but our current plan
+is not to make minor version releases post-1.0 and instead focus on our
+commitment to making those updates both easy and scalable for language users.
+
+## Patch version increments
+
+The patch version will increment when the change is fundamentally a bug fix to a
+previously released version. We expect the vast majority of these to be strictly
+backwards compatible bug fixes.
+
+Patch releases are expected to be driven by demand, and not necessarily present
+if unnecessary. However, the exact schedule and process will be determined as
+part of getting ready for the 1.0 milestone. Before that milestone we don't
+commit to any particular process or cadence for patch releases as no release
+before that point should be considered stable.
+
+Note that we still consider restoring the _intended_ "public API" of a release
+to be a bug fix. When these bug fixes theoretically break new code in a way that
+would typically require a major version increment, they may be made with merely
+a patch version increment when they are in fact restoring our intended behavior
+for that release. However, we take the SemVer guarantees very seriously as these
+fixes can still be disruptive and so we work to hold a high bar for them:
+
+-   They must be in some way fixing a _regressions_ for users from a previous
+    release, not merely a missing feature.
+    -   Can even be a regression the overall cohesion or reliability of the
+        language or tools, which a bug in a new feature might erode.
+    -   Key is that we motivate any patch fix through the lens of a regression
+        fix and stabilization rather than fixing forward.
+-   The scope of disruption from the fix is demonstrably small, due to some
+    combination of:
+    -   The short time duration of the release containing the regression.
+    -   The narrow scope of code constructs potentially impacted.
+-   The impact of the regression is large and cannot be easily worked around,
+    for example:
+    -   Undermining a core priority of the Carbon Language for a significant set
+        of users.
+    -   Making the engineering cost of adopting the release uneconomical for any
+        significant body of users.
+
+### Examples:
+
+-   We add a new feature that includes a bug which creates unsoundness and
+    allows incorrect code to be compiled that will crash or exhibit UB when
+    executed.
+    -   While this is a new feature and not a bug in an existing feature, it
+        would be a _serious_ regression to the reliability of the language as a
+        whole and the ability of users to reason about the correctness of
+        programs.
+    -   This would be a good candidate for a patch release to address unless it
+        is found very late (months) after the release _and_ the only ways to
+        address would have similarly bad effects on code written since the
+        initial release.
+    -   Even that level of user disruption could potentially be overcome if for
+        example the bug led to security vulnerabilities.
+-   We add a narrowly used new feature that includes a bug where some code
+    patterns that _should_ work with the feature are rejected at compile time or
+    crash reliably.
+    -   Unlikely this is worth a patch release to make an invasive change given
+        the narrow use case.
+    -   A good candidate to introduce a warning or error on using the feature in
+        the way that might lead to a crash, and possibly on using the feature at
+        all.
+-   We add a compiler opt-in feature behind a flag that doesn't work reliably.
+    -   Good candidate to have the flag disabled or trigger a warning message.
+    -   Not a good candidate to try to fix the feature forward.
+-   We add a compiler opt-out feature that doesn't work reliably.
+    -   What to do likely depends on the scope of users impacted. If _very_ few
+        users impacted, possibly just document how to opt-out.
+    -   If enough are impacted to be a nuisance and a regression in experience
+        in general, likely worth attempting a patch release that narrowly fixes
+        or mitigates the issue.
+
+## Pre-release versions
+
+Beyond the major, minor, and patch versions of an actual release, SemVer
+provides a foundation for pre-release version management. However, it is a very
+open-ended foundation with a wide range of possibilities and no particular
+semantic model provided. Some of this is to support the wide variety of
+different needs across different projects. It also reflects the fundamentally
+less crisp and well defined criteria needed for pre-releases.
+
+That said, Carbon should try to have a small and well articulated scheme of
+pre-release versions to help communicate what these releases do and don't
+constitute and how they should be interpreted. These are selected to provide a
+framework that allows pre-release versions to order in a reasonably cohesive way
+when compared using SemVer.
+
+In descending order, Carbon pre-releases may use:
+
+-   `MAJOR.MINOR.PATCH-rc.N`
+-   `MAJOR.MINOR.PATCH-0.nightly.YYYY.MM.DD`
+-   `MAJOR.MINOR.PATCH-0.dev`
+
+We expand on each of these below to provide criteria and details.
+
+### Release qualification pre-releases: `MAJOR.MINOR.PATCH-rc.N`
+
+We create release candidate or "rc" pre-releases when we believe that version to
+be complete and ready to release and want to collect feedback. There should not
+be interesting or significant gaps, even known ones, from the intended release.
+The expectation should always be that unless some feedback arrives to the
+contrary, the release candidate could simply become the release.
+
+For each pre-release category, we always suffix with a sequential count `N` of
+the pre-releases at that version. We must start with a `.0` in order to have
+subsequent iterations of the same version and category of pre-release to sort
+after the first.
+
+### Incremental development versions: `MAJOR.MINOR.PATCH-0.{nightly,dev}.N`
+
+These are versions that are not in any way associated with the actual expected
+release, but are periodically produced as an incremental tracking of development
+progress. Because they are not expected to be part of qualifying a specific
+release, they're not defined by any particular criteria of completeness or
+readiness. In practice, these will occur at every stage between one release and
+the next.
+
+#### Nightly pre-release versions
+
+These are automated incremental development versions built each night when the
+automated testing passes. There is _no_ attempt to provide any higher quality
+bar or refinement than whatever was in the tree at the time the automation runs,
+and whatever automated tests are present pass.
+
+It is important to emphasize that the primary use case of these pre-release
+versions is not to evaluate a potential release but for the developers and
+contributors to Carbon itself to track incremental development progress. That
+development-oriented goal drives how they are built and what they do and don't
+provide.
+
+Mechanically, we prefix the `nightly` pre-release version component with a `0`
+component to ensure these versions sort before any and all release qualification
+pre-release versions. We also add a date-derived suffix to provide a rough
+chronological ordering of nightly builds with otherwise identical versions.
+
+#### Development pre-release versions
+
+During development, interactive builds of Carbon need to be versioned in an
+unambiguous way, and we do that with the `dev` pre-release version. Much like
+nightly versions, these are only produced as artifacts of development activities
+and are never part of any actual release process. These versions are further not
+built expected to be automated or necessarily repeatable. They may contain
+in-flight edits and all manner of other variations.
+
+Mechanically, we prefix the `dev` pre-release version component with `0` in the
+same way as `nightly` is prefixed to ensure effective ordering. We don't add any
+additional information to keep the mechanics of development builds simple --
+there is no easy way to extract the date of the build . The exact timestamp of
+the build may be available but doesn't participate for simplicity and minimizing
+cache impact.
+
+## Relevant proposal
+
+-   [Proposal p4105](/proposals/p4105.md)