Convert some module-level // and /// comments to //!.

This makes their intent and expected location clearer. We see some
examples where these comments were not clearly separate from `use`
declarations, which made it hard to understand what the comment is
describing.

Author: nnethercote

alloc/src/boxed/thin.rs

@@ -1,6 +1,7 @@
-// Based on
-// https://github.com/matthieu-m/rfc2580/blob/b58d1d3cba0d4b5e859d3617ea2d0943aaa31329/examples/thin.rs
-// by matthieu-m
+//! Based on
+//! <https://github.com/matthieu-m/rfc2580/blob/b58d1d3cba0d4b5e859d3617ea2d0943aaa31329/examples/thin.rs>
+//! by matthieu-m
+
 use crate::alloc::{self, Layout, LayoutError};
 use core::error::Error;
 use core::fmt::{self, Debug, Display, Formatter};

portable-simd/crates/core_simd/examples/dot_product.rs

@@ -1,6 +1,5 @@
-// Code taken from the `packed_simd` crate
-// Run this code with `cargo test --example dot_product`
-//use std::iter::zip;
+//! Code taken from the `packed_simd` crate.
+//! Run this code with `cargo test --example dot_product`.
 
 #![feature(array_chunks)]
 #![feature(slice_as_chunks)]

std/src/io/buffered/bufreader/buffer.rs

@@ -1,13 +1,14 @@
-///! An encapsulation of `BufReader`'s buffer management logic.
-///
-/// This module factors out the basic functionality of `BufReader` in order to protect two core
-/// invariants:
-/// * `filled` bytes of `buf` are always initialized
-/// * `pos` is always <= `filled`
-/// Since this module encapsulates the buffer management logic, we can ensure that the range
-/// `pos..filled` is always a valid index into the initialized region of the buffer. This means
-/// that user code which wants to do reads from a `BufReader` via `buffer` + `consume` can do so
-/// without encountering any runtime bounds checks.
+//! An encapsulation of `BufReader`'s buffer management logic.
+//!
+//! This module factors out the basic functionality of `BufReader` in order to protect two core
+//! invariants:
+//! * `filled` bytes of `buf` are always initialized
+//! * `pos` is always <= `filled`
+//! Since this module encapsulates the buffer management logic, we can ensure that the range
+//! `pos..filled` is always a valid index into the initialized region of the buffer. This means
+//! that user code which wants to do reads from a `BufReader` via `buffer` + `consume` can do so
+//! without encountering any runtime bounds checks.
+
 use crate::cmp;
 use crate::io::{self, BorrowedBuf, Read};
 use crate::mem::MaybeUninit;

std/src/sys/os_str/wtf8.rs

@@ -1,5 +1,6 @@
-/// The underlying OsString/OsStr implementation on Windows is a
-/// wrapper around the "WTF-8" encoding; see the `wtf8` module for more.
+//! The underlying OsString/OsStr implementation on Windows is a
+//! wrapper around the "WTF-8" encoding; see the `wtf8` module for more.
+
 use crate::borrow::Cow;
 use crate::collections::TryReserveError;
 use crate::fmt;