Auto merge of rust-lang#94298 - Urgau:rustbuild-check-cfg, r=Mark-Simulacrum

Enable conditional compilation checking on the Rust codebase

This pull-request enable conditional compilation checking on every rust project build by the `bootstrap` tool.

To be more specific, this PR only enable well known names checking + extra names (bootstrap, parallel_compiler, ...).

r? `@Mark-Simulacrum`

Author: bors

core/src/mem/maybe_uninit.rs

@@ -817,6 +817,7 @@ impl<T> MaybeUninit<T> {
     /// ### Correct usage of this method:
     ///
     /// ```rust
+    /// # #![allow(unexpected_cfgs)]
     /// use std::mem::MaybeUninit;
     ///
     /// # unsafe extern "C" fn initialize_buffer(buf: *mut [u8; 1024]) { *buf = [0; 1024] }

std/src/os/windows/fs.rs

@@ -158,6 +158,7 @@ pub trait OpenOptionsExt {
     /// # Examples
     ///
     /// ```no_run
+    /// # #![allow(unexpected_cfgs)]
     /// # #[cfg(for_demonstration_only)]
     /// extern crate winapi;
     /// # mod winapi { pub const FILE_FLAG_DELETE_ON_CLOSE: u32 = 0x04000000; }
@@ -195,6 +196,7 @@ pub trait OpenOptionsExt {
     /// # Examples
     ///
     /// ```no_run
+    /// # #![allow(unexpected_cfgs)]
     /// # #[cfg(for_demonstration_only)]
     /// extern crate winapi;
     /// # mod winapi { pub const FILE_ATTRIBUTE_HIDDEN: u32 = 2; }
@@ -236,6 +238,7 @@ pub trait OpenOptionsExt {
     /// # Examples
     ///
     /// ```no_run
+    /// # #![allow(unexpected_cfgs)]
     /// # #[cfg(for_demonstration_only)]
     /// extern crate winapi;
     /// # mod winapi { pub const SECURITY_IDENTIFICATION: u32 = 0; }