Simplify imports by importing fewer items directly (#2404)

When reading the code without an IDE, I find it useful to use explicit
module names, especially when items come from the standard library.

So `io::Error` instead of just `Error`, especially when people have
just been told about `std::error::Error` as well.

I also omitted most single-use items: I find it has less cognitive
overhead to say “we import `fmt`” and then later use `fmt::Display`
and `fmt::Formatter` in the code. It’s clear from the name that these
two things have something to do with formatting.

Finally, I made a few usages more consistent so that we refer to each
item in the same way within a single codeblock.

src/concurrency/async-control-flow/channels.md

@@ -7,9 +7,9 @@ minutes: 8
 Several crates have support for asynchronous channels. For instance `tokio`:
 
 ```rust,editable,compile_fail
-use tokio::sync::mpsc::{self, Receiver};
+use tokio::sync::mpsc;
 
-async fn ping_handler(mut input: Receiver<()>) {
+async fn ping_handler(mut input: mpsc::Receiver<()>) {
     let mut count: usize = 0;
 
     while let Some(_) = input.recv().await {

src/concurrency/async-exercises/dining-philosophers.rs

@@ -15,8 +15,7 @@
 // ANCHOR: solution
 // ANCHOR: Philosopher
 use std::sync::Arc;
-use tokio::sync::mpsc::{self, Sender};
-use tokio::sync::Mutex;
+use tokio::sync::{mpsc, Mutex};
 use tokio::time;
 
 struct Fork;
@@ -26,7 +25,7 @@ struct Philosopher {
     // ANCHOR_END: Philosopher
     left_fork: Arc<Mutex<Fork>>,
     right_fork: Arc<Mutex<Fork>>,
-    thoughts: Sender<String>,
+    thoughts: mpsc::Sender<String>,
 }
 
 // ANCHOR: Philosopher-think

src/concurrency/async-pitfalls/cancellation.md

@@ -10,7 +10,7 @@ the system works correctly even when futures are cancelled. For example, it
 shouldn't deadlock or lose data.
 
 ```rust,editable,compile_fail
-use std::io::{self, ErrorKind};
+use std::io;
 use std::time::Duration;
 use tokio::io::{AsyncReadExt, AsyncWriteExt, DuplexStream};
 
@@ -36,12 +36,12 @@ impl LinesReader {
             return Ok(None);
         }
         let s = String::from_utf8(bytes)
-            .map_err(|_| io::Error::new(ErrorKind::InvalidData, "not UTF-8"))?;
+            .map_err(|_| io::Error::new(io::ErrorKind::InvalidData, "not UTF-8"))?;
         Ok(Some(s))
     }
 }
 
-async fn slow_copy(source: String, mut dest: DuplexStream) -> std::io::Result<()> {
+async fn slow_copy(source: String, mut dest: DuplexStream) -> io::Result<()> {
     for b in source.bytes() {
         dest.write_u8(b).await?;
         tokio::time::sleep(Duration::from_millis(10)).await
@@ -50,7 +50,7 @@ async fn slow_copy(source: String, mut dest: DuplexStream) -> std::io::Result<()
 }
 
 #[tokio::main]
-async fn main() -> std::io::Result<()> {
+async fn main() -> io::Result<()> {
     let (client, server) = tokio::io::duplex(5);
     let handle = tokio::spawn(slow_copy("hi\nthere\n".to_owned(), client));
 
@@ -102,7 +102,7 @@ async fn main() -> std::io::Result<()> {
             // ...
             let raw = std::mem::take(&mut self.bytes);
             let s = String::from_utf8(raw)
-                .map_err(|_| io::Error::new(ErrorKind::InvalidData, "not UTF-8"))?;
+                .map_err(|_| io::Error::new(io::ErrorKind::InvalidData, "not UTF-8"))?;
             // ...
         }
     }

src/error-handling/thiserror.md

@@ -9,8 +9,8 @@ avoid boilerplate when defining error types. It provides derive macros that
 assist in implementing `From<T>`, `Display`, and the `Error` trait.
 
 ```rust,editable,compile_fail
-use std::fs;
-use std::io::{self, Read};
+use std::io::Read;
+use std::{fs, io};
 use thiserror::Error;
 
 #[derive(Debug, Error)]

src/error-handling/try-conversions.md

@@ -28,9 +28,8 @@ higher-level errors.
 
 ```rust,editable
 use std::error::Error;
-use std::fmt::{self, Display, Formatter};
-use std::fs::File;
-use std::io::{self, Read};
+use std::io::Read;
+use std::{fmt, fs, io};
 
 #[derive(Debug)]
 enum ReadUsernameError {
@@ -40,8 +39,8 @@ enum ReadUsernameError {
 
 impl Error for ReadUsernameError {}
 
-impl Display for ReadUsernameError {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+impl fmt::Display for ReadUsernameError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         match self {
             Self::IoError(e) => write!(f, "I/O error: {e}"),
             Self::EmptyUsername(path) => write!(f, "Found no username in {path}"),
@@ -57,7 +56,7 @@ impl From<io::Error> for ReadUsernameError {
 
 fn read_username(path: &str) -> Result<String, ReadUsernameError> {
     let mut username = String::with_capacity(100);
-    File::open(path)?.read_to_string(&mut username)?;
+    fs::File::open(path)?.read_to_string(&mut username)?;
     if username.is_empty() {
         return Err(ReadUsernameError::EmptyUsername(String::from(path)));
     }