RFC: Private fields by default

active/0000-private-fields.md

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+# Summary
+
+This is an RFC to make all struct fields private by default. This includes both
+tuple structs and structural structs.
+
+# Motivation
+
+Reasons for default private visibility
+
+* Visibility is often how soundness is achieved for many types in rust. These
+  types are normally wrapping unsafe behavior of an FFI type or some other
+  rust-specific behavior under the hood (such as the standard `Vec` type).
+  Requiring these types to opt-in to being sound is unfortunate.
+
+* Forcing tuple struct fields to have non-overridable public visibility greatly
+  reduces the utility of such types. Tuple structs cannot be used to create
+  abstraction barriers as they can always be easily destructed.
+
+* Private-by-default is more consistent with the rest of the Rust language. All
+  other aspects of privacy are private-by-default except for enum variants. Enum
+  variants, however, are a special case in that they are inserted into the
+  parent namespace, and hence naturally inherit privacy.
+
+* Public fields of a `struct` must be considered as part of the API of the type.
+  This means that the exact definition of all structs is *by default* the API of
+  the type. Structs must opt-out of this behavior if the `priv` keyword is
+  required. By requiring the `pub` keyword, structs must opt-in to exposing more
+  surface area to their API.
+
+Reasons for inherited visibility (today's design)
+
+* Public definitions like `pub struct Point { x: int, y: int }` are concise and
+  easy to read.
+* Private definitions certainly want private fields (to hide implementation
+  details).
+
+# Detailed design
+
+Currently, rustc has two policies for dealing with the privacy of struct fields:
+
+* Tuple structs have public fields by default (including "newtype structs")
+* Fields of structural structs (`struct Foo { ... }`) inherit the same privacy
+  of the enclosing struct.
+
+This RFC is a proposal to unify the privacy of struct fields with the rest of
+the language by making them private by default. This means that both tuple
+variants and structural variants of structs would have private fields by
+default. For example, the program below is accepted today, but would be rejected
+with this RFC.
+
+```rust
+mod inner {
+    pub struct Foo(u64);
+    pub struct Bar { field: u64 }
+}
+
+fn main() {
+    inner::Foo(10);
+    inner::Bar { field: 10 };
+}
+```
+
+### Refinements to structural structs
+
+Public fields are quite a useful feature of the language, so syntax is required
+to opt out of the private-by-default semantics. Structural structs already allow
+visibility qualifiers on fields, and the `pub` qualifier would make the field
+public instead of private.
+
+Additionally, the `priv` visibility will no longer be allowed to modify struct
+fields. Similarly to how a `priv fn` is a compiler error, a `priv` field will
+become a compiler error.
+
+### Refinements on tuple structs
+
+As with their structural cousins, it's useful to have tuple structs with public
+fields. This RFC will modify the tuple struct grammar to:
+
+```ebnf
+tuple_struct := 'struct' ident '(' fields ')' ';'
+fields := field | field ',' fields
+field := type | visibility type
+```
+
+For example, these definitions will be added to the language:
+
+```rust
+// a "newtype wrapper" struct with a private field
+struct Foo(u64);
+
+// a "newtype wrapper" struct with a public field
+struct Bar(pub u64);
+
+// a tuple struct with many fields, only the first and last of which are public
+struct Baz(pub u64, u32, f32, pub int);
+```
+
+Public fields on tuple structs will maintain the semantics that they currently
+have today. The structs can be constructed, destructed, and participate in
+pattern matches.
+
+Private fields on tuple structs will prevent the following behaviors:
+
+* Private fields cannot be bound in patterns (both in irrefutable and refutable
+  contexts, i.e. `let` and `match` statements).
+* Private fields cannot be specified outside of the defining module when
+  constructing a tuple struct.
+
+These semantics are intended to closely mirror the behavior of private fields
+for structural structs.
+
+### Statistics gathered
+
+A brief survey was performed over the entire `mozilla/rust` repository to gather
+these statistics. While not representative of all projects, this repository
+should give a good indication of what most structs look like in the real world.
+The repository has both libraries (`libstd`) as well as libraries which don't
+care much about privacy (`librustc`).
+
+These numbers tally up all structs from all locations, and only take into
+account structural structs, not tuple structs.
+
+|                       | Inherited privacy | Private-by-default |
+|-----------------------|------------------:|-------------------:|
+| Private fields        |              1418 |               1852 |
+| Public fields         |              2036 |               1602 |
+| All-private structs   |      551 (52.23%) |       671 (63.60%) |
+| All-public structs    |      468 (44.36%) |       352 (33.36%) |
+| Mixed privacy structs |       36 ( 3.41%) |        32 ( 3.03%) |
+
+The numbers clearly show that the predominant pattern is to have all-private
+structs, and that there are many public fields today which can be private (and
+perhaps should!). Additionally, there is on the order of 1418 instances of the
+word `priv` today, when in theory there should be around `1852`. With this RFC,
+there would need to be `1602` instances of the word `pub`. A very large portion
+of structs requiring `pub` fields are FFI structs defined in the `libc`
+module.
+
+### Impact on enums
+
+This RFC does not impact enum variants in any way. All enum variants will
+continue to inherit privacy from the outer enum type. This includes both the
+fields of tuple variants as well as fields of struct variants in enums.
+
+# Alternatives
+
+The main alternative to this design is what is currently implemented today,
+where fields inherit the privacy of the outer structure. The pros and cons of
+this strategy are discussed above.
+
+# Unresolved questions
+
+As the above statistics show, almost all structures are either all public or all
+private. This RFC provides an easy method to make struct fields all private, but
+it explicitly does not provide a method to make struct fields all public. The
+statistics show that `pub` will be written less often than `priv` is today, and
+it's always possible to add a method to specify a struct as all-public in the
+future in a backwards-compatible fashion.
+
+That being said, it's an open question whether syntax for an "all public struct"
+is necessary at this time.