Auto merge of #43348 - kennytm:fix-24658-doc-every-platform, r=alexcrichton

Expose all OS-specific modules in libstd doc.

1. Uses the special `--cfg dox` configuration passed by rustbuild when running `rustdoc`. Changes the `#[cfg(platform)]` into `#[cfg(any(dox, platform))]` so that platform-specific API are visible to rustdoc.

2. Since platform-specific implementations often won't compile correctly on other platforms, `rustdoc` is changed to apply `everybody_loops` to the functions during documentation and doc-test harness.

3. Since platform-specific code are documented on all platforms now, it could confuse users who found a useful API but is non-portable. Also, their examples will be doc-tested, so must be excluded when not testing on the native platform. An undocumented attribute `#[doc(cfg(...))]` is introduced to serve the above purposed.

Fixes #24658 (Does _not_ fully implement #1998).

Author: bors

src/doc/unstable-book/src/language-features/doc-cfg.md

@@ -0,0 +1,42 @@
+# `doc_cfg`
+
+The tracking issue for this feature is: [#43781]
+
+------
+
+The `doc_cfg` feature allows an API be documented as only available in some specific platforms.
+This attribute has two effects:
+
+1. In the annotated item's documentation, there will be a message saying "This is supported on
+    (platform) only".
+
+2. The item's doc-tests will only run on the specific platform.
+
+This feature was introduced as part of PR [#43348] to allow the platform-specific parts of the
+standard library be documented.
+
+```rust
+#![feature(doc_cfg)]
+
+#[cfg(any(windows, feature = "documentation"))]
+#[doc(cfg(windows))]
+/// The application's icon in the notification area (a.k.a. system tray).
+///
+/// # Examples
+///
+/// ```no_run
+/// extern crate my_awesome_ui_library;
+/// use my_awesome_ui_library::current_app;
+/// use my_awesome_ui_library::windows::notification;
+///
+/// let icon = current_app().get::<notification::Icon>();
+/// icon.show();
+/// icon.show_message("Hello");
+/// ```
+pub struct Icon {
+    // ...
+}
+```
+
+[#43781]: https://github.com/rust-lang/rust/issues/43781
+[#43348]: https://github.com/rust-lang/rust/issues/43348

src/librustc_driver/pretty.rs

@@ -44,6 +44,7 @@ use std::io::{self, Write};
 use std::option;
 use std::path::Path;
 use std::str::FromStr;
+use std::mem;
 
 use rustc::hir::map as hir_map;
 use rustc::hir::map::blocks;
@@ -618,52 +619,53 @@ impl UserIdentifiedItem {
     }
 }
 
-struct ReplaceBodyWithLoop {
+// Note: Also used by librustdoc, see PR #43348. Consider moving this struct elsewhere.
+pub struct ReplaceBodyWithLoop {
     within_static_or_const: bool,
 }
 
 impl ReplaceBodyWithLoop {
-    fn new() -> ReplaceBodyWithLoop {
+    pub fn new() -> ReplaceBodyWithLoop {
         ReplaceBodyWithLoop { within_static_or_const: false }
     }
+
+    fn run<R, F: FnOnce(&mut Self) -> R>(&mut self, is_const: bool, action: F) -> R {
+        let old_const = mem::replace(&mut self.within_static_or_const, is_const);
+        let ret = action(self);
+        self.within_static_or_const = old_const;
+        ret
+    }
 }
 
 impl fold::Folder for ReplaceBodyWithLoop {
     fn fold_item_kind(&mut self, i: ast::ItemKind) -> ast::ItemKind {
-        match i {
-            ast::ItemKind::Static(..) |
-            ast::ItemKind::Const(..) => {
-                self.within_static_or_const = true;
-                let ret = fold::noop_fold_item_kind(i, self);
-                self.within_static_or_const = false;
-                return ret;
-            }
-            _ => fold::noop_fold_item_kind(i, self),
-        }
+        let is_const = match i {
+            ast::ItemKind::Static(..) | ast::ItemKind::Const(..) => true,
+            ast::ItemKind::Fn(_, _, ref constness, _, _, _) =>
+                constness.node == ast::Constness::Const,
+            _ => false,
+        };
+        self.run(is_const, |s| fold::noop_fold_item_kind(i, s))
     }
 
     fn fold_trait_item(&mut self, i: ast::TraitItem) -> SmallVector<ast::TraitItem> {
-        match i.node {
-            ast::TraitItemKind::Const(..) => {
-                self.within_static_or_const = true;
-                let ret = fold::noop_fold_trait_item(i, self);
-                self.within_static_or_const = false;
-                return ret;
-            }
-            _ => fold::noop_fold_trait_item(i, self),
-        }
+        let is_const = match i.node {
+            ast::TraitItemKind::Const(..) => true,
+            ast::TraitItemKind::Method(ast::MethodSig { ref constness, .. }, _) =>
+                constness.node == ast::Constness::Const,
+            _ => false,
+        };
+        self.run(is_const, |s| fold::noop_fold_trait_item(i, s))
     }
 
     fn fold_impl_item(&mut self, i: ast::ImplItem) -> SmallVector<ast::ImplItem> {
-        match i.node {
-            ast::ImplItemKind::Const(..) => {
-                self.within_static_or_const = true;
-                let ret = fold::noop_fold_impl_item(i, self);
-                self.within_static_or_const = false;
-                return ret;
-            }
-            _ => fold::noop_fold_impl_item(i, self),
-        }
+        let is_const = match i.node {
+            ast::ImplItemKind::Const(..) => true,
+            ast::ImplItemKind::Method(ast::MethodSig { ref constness, .. }, _) =>
+                constness.node == ast::Constness::Const,
+            _ => false,
+        };
+        self.run(is_const, |s| fold::noop_fold_impl_item(i, s))
     }
 
     fn fold_block(&mut self, b: P<ast::Block>) -> P<ast::Block> {