Avoid comments that describe multiple use items.

There are some comments describing multiple subsequent `use` items. When
the big `use` reformatting happens some of these `use` items will be
reordered, possibly moving them away from the comment. With this
additional level of formatting it's not really feasible to have comments
of this type. This commit removes them in various ways:

- merging separate `use` items when appropriate;

- inserting blank lines between the comment and the first `use` item;

- outright deletion (for comments that are relatively low-value);

- adding a separate "top-level" comment.

We also entirely skip formatting for four library files that contain
nothing but `pub use` re-exports, where reordering would be painful.

library/std/src/lib.rs

@@ -469,7 +469,6 @@ pub mod rt;
 // The Rust prelude
 pub mod prelude;
 
-// Public module declarations and re-exports
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use alloc_crate::borrow;
 #[stable(feature = "rust1", since = "1.0.0")]

library/std/src/os/fortanix_sgx/mod.rs

@@ -28,7 +28,6 @@ pub mod usercalls {
         pub use crate::sys::abi::usercalls::raw::{do_usercall, Usercalls as UsercallNrs};
         pub use crate::sys::abi::usercalls::raw::{Register, RegisterArgument, ReturnValue};
 
-        // fortanix-sgx-abi re-exports
         pub use crate::sys::abi::usercalls::raw::Error;
         pub use crate::sys::abi::usercalls::raw::{
             ByteBuffer, Cancel, FifoDescriptor, Return, Usercall,

library/std/src/prelude/common.rs

@@ -2,6 +2,9 @@
 //!
 //! See the [module-level documentation](super) for more.
 
+// No formatting: this file is nothing but re-exports, and their order is worth preserving.
+#![cfg_attr(rustfmt, rustfmt::skip)]
+
 // Re-exported core operators
 #[stable(feature = "rust1", since = "1.0.0")]
 #[doc(no_inline)]

library/std/src/prelude/mod.rs

@@ -95,6 +95,9 @@
 //! [book-enums]: ../../book/ch06-01-defining-an-enum.html
 //! [book-iter]: ../../book/ch13-02-iterators.html
 
+// No formatting: this file is nothing but re-exports, and their order is worth preserving.
+#![cfg_attr(rustfmt, rustfmt::skip)]
+
 #![stable(feature = "rust1", since = "1.0.0")]
 
 mod common;

library/std/src/rt.rs

@@ -16,7 +16,6 @@
 #![deny(unsafe_op_in_unsafe_fn)]
 #![allow(unused_macros)]
 
-// Re-export some of our utilities which are expected by other crates.
 pub use crate::panicking::{begin_panic, panic_count};
 pub use core::panicking::{panic_display, panic_fmt};
 

library/std/src/sys/pal/wasi/mod.rs

@@ -39,12 +39,15 @@ pub mod time;
 #[deny(unsafe_op_in_unsafe_fn)]
 #[allow(unused)]
 mod common;
+
 pub use common::*;
 
 mod helpers;
-// These exports are listed individually to work around Rust's glob import
-// conflict rules. If we glob export `helpers` and `common` together, then
-// the compiler complains about conflicts.
+
+// The following exports are listed individually to work around Rust's glob
+// import conflict rules. If we glob export `helpers` and `common` together,
+// then the compiler complains about conflicts.
+
 pub use helpers::abort_internal;
 pub use helpers::decode_error_kind;
 use helpers::err2io;

library/std/src/sys/pal/wasip2/mod.rs

@@ -41,13 +41,16 @@ pub mod time;
 #[deny(unsafe_op_in_unsafe_fn)]
 #[allow(unused)]
 mod common;
+
 pub use common::*;
 
 #[path = "../wasi/helpers.rs"]
 mod helpers;
-// These exports are listed individually to work around Rust's glob import
-// conflict rules. If we glob export `helpers` and `common` together, then
-// the compiler complains about conflicts.
+
+// The following exports are listed individually to work around Rust's glob
+// import conflict rules. If we glob export `helpers` and `common` together,
+// then the compiler complains about conflicts.
+
 pub use helpers::abort_internal;
 pub use helpers::decode_error_kind;
 use helpers::err2io;