docs: convert AccessControl.rst to Markdown (swiftlang#37217)

Converts one of the doc files from reStructuredText to Markdown.

Continues work on SR-7454.

Author: MaxDesiatov

docs/AccessControl.md

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+# Access Control
+
+The general guiding principle of Swift access control:
+
+> **No entity can be defined in terms of another entity that has a lower
+> access level.**
+
+There are four levels of access: \"private\", \"fileprivate\",
+\"internal\", and \"public\". Private entities can only be accessed from
+within the lexical scope where they are defined. File-private entities
+can only be accessed from within the source file where they are defined.
+Internal entities can be accessed anywhere within the module they are
+defined. Public entities can be accessed from anywhere within the module
+and from any other context that imports the current module.
+
+The names `public` and `private` have precedent in many languages;
+`internal` comes from C\# and `fileprivate` from the Swift community. In
+the future, `public` may be used for both API and SPI, at which point we
+may design additional annotations to distinguish the two.
+
+By default, most entities in a source file have `internal` access. This
+optimizes for the most common case – a single-target application
+project – while not accidentally revealing entities to clients of a
+framework module.
+
+_**Warning**_
+
+This document has not yet been updated for
+[SE-0117](https://github.com/apple/swift-evolution/blob/main/proposals/0117-non-public-subclassable-by-default.md),
+which adds the \"open\" level of access.
+
+## Rules
+
+Access to a particular entity is considered relative to the current
+*access scope.* The access scope of an entity is its immediate lexical
+scope (if `private`), the current file (if `fileprivate`), the current
+module (if `internal`), or the current program (if `public`). A
+reference to an entity may only be written within the entity\'s access
+scope.
+
+If a particular entity is not accessible, it does not appear in name
+lookup, unlike in C++. However, access control does not restrict access
+to members via runtime reflection (where applicable), nor does it
+necessarily restrict visibility of symbols in a linked binary.
+
+### Globals and Members
+
+All globals and members have a default access level of `internal`,
+except within extensions (as described below).
+
+A declaration may have any access level less than or equal to the access
+level of its type. That is, a `private` constant can have `public` type,
+but not the other way around. It is legal for a member to have greater
+access than its enclosing type, but this has no effect.
+
+Accessors for variables have the same access level as their associated
+variable. The setter may be explicitly annotated with an access level
+less than or equal to the access level of the variable; this is written
+as `private(set)` or `internal(set)` before the `var` introducer.
+
+An initializer, method, subscript, or property may have any access level
+less than or equal to the access level of its type (including the
+implicit \'Self\' type), with a few additional rules:
+
+-   If a member is used to satisfy a protocol requirement, its access
+    level must be at least as high as the protocol conformance\'s; see
+    ["Protocols"](#protocols) below.
+-   If an initializer is `required` by a superclass, its access level
+    must be at least as high as the access level of the subclass itself.
+-   Accessors for subscripts follow the same rules as accessors for
+    variables.
+-   A member may be overridden whenever it is accessible.
+
+The implicit memberwise initializer for a struct has the minimum access
+level of all of the struct\'s stored properties, except that if all
+properties are `public` the initializer is `internal`. The implicit
+no-argument initializer for structs and classes follows the default
+access level for the type.
+
+Currently, enum cases always have the same access level as the enclosing
+enum.
+
+Deinitializers are only invoked by the runtime and do not nominally have
+access. Internally, the compiler represents them as having the same
+access level as the enclosing type.
+
+### Protocols
+
+A protocol may have any access level less than or equal to the access
+levels of the protocols it refines. That is, a `private` ExtendedWidget
+protocol can refine a `public` Widget protocol, but not the other way
+around.
+
+The access level of a requirement is the access level of the enclosing
+protocol, even when the protocol is `public`. Currently, requirements
+may not be given a lower access level than the enclosing protocol.
+
+Swift does not currently support private protocol conformances, so for
+runtime consistency, the access level of the conformance of type T to
+protocol P is equal to the minimum of T\'s access level and P\'s access
+level; that is, the conformance is accessible whenever both T and P are
+accessible. This does not change if the protocol is conformed to in an
+extension. (The access level of a conformance is not currently reflected
+in the source, but is a useful concept for applying restrictions
+consistently.)
+
+All members used to satisfy a conformance must have an access level at
+least as high as the conformance\'s. This ensures consistency between
+views of the type; if any member has a *lower* access level than the
+conformance, then the member could be accessed anyway through a generic
+function constrained by the protocol.
+
+_**Note**_
+
+This rule disallows an `internal` member of a protocol extension to
+satisfy a `public` requirement for a `public` type. Removing this
+limitation is not inherently unsafe, but (a) may be unexpected given the
+lack of explicit reference to the member, and (b) results in references
+to non-public symbols in the current representation.
+
+A protocol may be used as a type whenever it is accessible. A nominal
+can conform to a protocol whenever the protocol is accessible.
+
+### Structs, Enums, and Classes
+
+A struct, enum, or class may be used as a type whenever it is
+accessible. A struct, enum, or class may be extended whenever it is
+accessible.
+
+A class may be subclassed whenever it is accessible. A class may have
+any access level less than or equal to the access level of its
+superclass.
+
+Members within constrained extensions must have access less than or
+equal to the access level of the types used in the constraints.
+
+An extension may be marked with an explicit access modifier (e.g.
+`private extension`), in which case the default access level of members
+within the extension is changed to match. No member within such an
+extension may have broader access than the new default.
+
+Extensions with explicit access modifiers may not add new protocol
+conformances, since Swift does not support private protocol conformances
+(see ["Protocols"](#protocols) above).
+
+A type may conform to a protocol with lower access than the type itself.
+
+### Types
+
+A nominal type\'s access level is the same as the access level of the
+nominal declaration itself. A generic type\'s access level is the
+minimum of the access level of the base type and the access levels of
+all generic argument types.
+
+A tuple type\'s access level is the minimum of the access levels of its
+elements. A function type\'s access level is the minimum of the access
+levels of its input and return types.
+
+A typealias may have any access level up to the access level of the type
+it aliases. That is, a `private` typealias can refer to a `public` type,
+but not the other way around. This includes associated types used to
+satisfy protocol conformances.
+
+## Runtime Guarantees
+
+Non-`public` members of a class or extension will not be seen by
+subclasses or other extensions from outside the module. Therefore,
+members of a subclass or extension will not conflict with or
+inadvertently be considered to override non-accessible members of the
+superclass.
+
+Access levels lower than `public` increase opportunities for
+devirtualization, though it is still possible to put a subclass of a
+`private` class within the same scope.
+
+Most information about a non-`public` entity still has to be put into a
+module file for now, since we don\'t have resilience implemented. This
+can be improved later, and is no more revealing than the information
+currently available in the runtime for pure Objective-C classes.
+
+### Interaction with Objective-C
+
+If an entity is exposed to Objective-C, most of the runtime guarantees
+and optimization opportunities go out the window. We have to use a
+particular selector for members, everything can be inspected at runtime,
+and even a private member can cause selector conflicts. In this case,
+access control is only useful for discipline purposes.
+
+Members explicitly marked `private` or `fileprivate` are *not* exposed
+to Objective-C unless they are also marked `@objc` (or `@IBAction` or
+similar), even if declared within a class implicitly or explicitly
+marked `@objc`.
+
+Any `public` entities will be included in the generated header. In an
+application or unit test target, `internal` entities will be exposed as
+well.
+
+## Non-Goals: \"class-only\" and \"protected\"
+
+This proposal omits two forms of access control commonly found in other
+languages, a \"class-implementation-only\" access (often called
+\"private\"), and a \"class and any subclasses\" access (often called
+\"protected\"). We chose not to include these levels of access control
+because they do not add useful functionality beyond `private`,
+`fileprivate`, `internal`, and `public`.
+
+### "class-only"
+
+If \"class-only\" includes extensions of the class, it is clear that
+it provides no protection at all, since a class may be extended from
+any context where it is accessible. So a hypothetical \"class-only\"
+must already be limited with regards to extensions. Beyond that,
+however, a \"class-only\" limit forces code to be declared within
+the class that might otherwise naturally be a top-level helper or an
+extension method on another type.
+
+`private` and `fileprivate` serve the use case of limiting access to
+the implementation details of a class (even from the rest of the
+module!) while not tying access to the notion of type.
+
+### "protected"
+
+"protected\" access provides no guarantees of information hiding,
+since any subclass can now access the implementation details of its
+superclass\-\--and expose them publicly, if it so chooses. This
+interacts poorly with our future plans for resilient APIs.
+Additionally, it increases the complexity of the access control
+model for both the compiler and for developers, and like
+\"class-only\" it is not immediately clear how it interacts with
+extensions.
+
+Though it is not compiler-enforced, members that might be considered
+\"protected\" are effectively publicly accessible, and thus should
+be marked `public` in Swift. They can still be documented as
+intended for overriding rather than for subclassing, but the
+specific details of this are best dealt with on a case-by-case
+basis.
+
+## Potential Future Directions
+
+-   Allowing `private` or `internal` protocol conformances, which are
+    only accessible at compile-time from a particular access scope.
+-   Limiting particular capabilities, such as marking something
+    `final(public)` to restrict subclassing or overriding outside of the
+    current module.
+-   Allowing the Swift parts of a mixed-source framework to access
+    private headers.
+-   Revealing `internal` Swift API in a mixed-source framework in a
+    second generated header.
+-   Levels of `public`, for example `public("SPI")`.
+-   Enum cases less accessible than the enum.
+-   Protocol requirements less accessible than the protocol.