Document how non_exhaustive interacts with tuple and unit-like structs.

Author: kpreid

src/attributes/type_system.md

@@ -20,6 +20,12 @@ pub struct Config {
     pub window_height: u16,
 }
 
+#[non_exhaustive]
+pub struct Token;
+
+#[non_exhaustive]
+pub struct Id(pub u64);
+
 #[non_exhaustive]
 pub enum Error {
     Message(String),
@@ -34,11 +40,13 @@ pub enum Message {
 
 // Non-exhaustive structs can be constructed as normal within the defining crate.
 let config = Config { window_width: 640, window_height: 480 };
+let token = Token;
+let id = Id(4);
 
 // Non-exhaustive structs can be matched on exhaustively within the defining crate.
-if let Config { window_width, window_height } = config {
-    // ...
-}
+let Config { window_width, window_height } = config;
+let Token = token;
+let Id(id_number) = id;
 
 let error = Error::Other;
 let message = Message::Reaction(3);
@@ -64,30 +72,47 @@ Non-exhaustive types cannot be constructed outside of the defining crate:
 
 - Non-exhaustive variants ([`struct`][struct] or [`enum` variant][enum]) cannot be constructed
   with a [_StructExpression_] \(including with [functional update syntax]).
+- The visibility of the same-named constant of a [unit-like struct][struct]
+  is lowered to `min($vis, pub(crate))`.
+- The visibility of the same-named constructor function of a [tuple struct][struct]
+  is lowered to `min($vis, pub(crate))`.
 - [`enum`][enum] instances can be constructed.
 
+The following examples of construction do not compile when outside the defining crate:
+
 <!-- ignore: requires external crates -->
 ```rust,ignore
-// `Config`, `Error`, and `Message` are types defined in an upstream crate that have been
-// annotated as `#[non_exhaustive]`.
-use upstream::{Config, Error, Message};
+// These are types defined in an upstream crate that have been annotated as
+// `#[non_exhaustive]`.
+use upstream::{Config, Token, Id, Error, Message};
 
-// Cannot construct an instance of `Config`, if new fields were added in
+// Cannot construct an instance of `Config`; if new fields were added in
 // a new version of `upstream` then this would fail to compile, so it is
 // disallowed.
 let config = Config { window_width: 640, window_height: 480 };
 
-// Can construct an instance of `Error`, new variants being introduced would
+// Cannot construct an instance of `Token`; if new fields were added, then
+// it would not be a unit-like struct any more, so the same-named constant
+// created by it being a unit-like struct is not public outside the crate;
+// this code fails to compile.
+let token = Token;
+
+// Cannot construct an instance of `Id`; if new fields were added, then
+// its constructor function signature would change, so its constructor
+// function is not public outside the crate; this code fails to compile.
+let id = Id(5);
+
+// Can construct an instance of `Error`; new variants being introduced would
 // not result in this failing to compile.
 let error = Error::Message("foo".to_string());
 
-// Cannot construct an instance of `Message::Send` or `Message::Reaction`,
+// Cannot construct an instance of `Message::Send` or `Message::Reaction`;
 // if new fields were added in a new version of `upstream` then this would
 // fail to compile, so it is disallowed.
 let message = Message::Send { from: 0, to: 1, contents: "foo".to_string(), };
 let message = Message::Reaction(0);
 
-// Cannot construct an instance of `Message::Quit`, if this were converted to
+// Cannot construct an instance of `Message::Quit`; if this were converted to
 // a tuple-variant `upstream` then this would fail to compile.
 let message = Message::Quit;
 ```
@@ -100,11 +125,13 @@ There are limitations when matching on non-exhaustive types outside of the defin
 - When pattern matching on a non-exhaustive [`enum`][enum], matching on a variant does not
   contribute towards the exhaustiveness of the arms.
 
+The following examples of matching do not compile when outside the defining crate:
+
 <!-- ignore: requires external crates -->
 ```rust, ignore
-// `Config`, `Error`, and `Message` are types defined in an upstream crate that have been
-// annotated as `#[non_exhaustive]`.
-use upstream::{Config, Error, Message};
+// These are types defined in an upstream crate that have been annotated as
+// `#[non_exhaustive]`.
+use upstream::{Config, Token, Id, Error, Message};
 
 // Cannot match on a non-exhaustive enum without including a wildcard arm.
 match error {
@@ -118,6 +145,13 @@ if let Ok(Config { window_width, window_height }) = config {
     // would compile with: `..`
 }
 
+// Cannot match a non-exhaustive unit-like or tuple struct except by using
+// braced struct syntax with a wildcard.
+// This would compile as `let Token { .. } = token;`
+let Token = token;
+// This would compile as `let Id { 0: id_number, .. } = id;`
+let Id(id_number) = id;
+
 match message {
   // Cannot match on a non-exhaustive struct enum variant without including a wildcard.
   Message::Send { from, to, contents } => { },