Use implicit capture syntax in format_args

This updates the standard library's documentation to use the new syntax. The
documentation is worthwhile to update as it should be more idiomatic
(particularly for features like this, which are nice for users to get acquainted
with). The general codebase is likely more hassle than benefit to update: it'll
hurt git blame, and generally updates can be done by folks updating the code if
(and when) that makes things more readable with the new format.

A few places in the compiler and library code are updated (mostly just due to
already having been done when this commit was first authored).

library/std/src/env.rs

@@ -118,7 +118,7 @@ pub struct VarsOs {
 /// // We will iterate through the references to the element returned by
 /// // env::vars();
 /// for (key, value) in env::vars() {
-///     println!("{}: {}", key, value);
+///     println!("{key}: {value}");
 /// }
 /// ```
 ///
@@ -148,7 +148,7 @@ pub fn vars() -> Vars {
 /// // We will iterate through the references to the element returned by
 /// // env::vars_os();
 /// for (key, value) in env::vars_os() {
-///     println!("{:?}: {:?}", key, value);
+///     println!("{key:?}: {value:?}");
 /// }
 /// ```
 #[must_use]
@@ -212,8 +212,8 @@ impl fmt::Debug for VarsOs {
 ///
 /// let key = "HOME";
 /// match env::var(key) {
-///     Ok(val) => println!("{}: {:?}", key, val),
-///     Err(e) => println!("couldn't interpret {}: {}", key, e),
+///     Ok(val) => println!("{key}: {val:?}"),
+///     Err(e) => println!("couldn't interpret {key}: {e}"),
 /// }
 /// ```
 #[stable(feature = "env", since = "1.0.0")]
@@ -252,8 +252,8 @@ fn _var(key: &OsStr) -> Result<String, VarError> {
 ///
 /// let key = "HOME";
 /// match env::var_os(key) {
-///     Some(val) => println!("{}: {:?}", key, val),
-///     None => println!("{} is not defined in the environment.", key)
+///     Some(val) => println!("{key}: {val:?}"),
+///     None => println!("{key} is not defined in the environment.")
 /// }
 /// ```
 #[must_use]
@@ -343,7 +343,7 @@ pub fn set_var<K: AsRef<OsStr>, V: AsRef<OsStr>>(key: K, value: V) {
 
 fn _set_var(key: &OsStr, value: &OsStr) {
     os_imp::setenv(key, value).unwrap_or_else(|e| {
-        panic!("failed to set environment variable `{:?}` to `{:?}`: {}", key, value, e)
+        panic!("failed to set environment variable `{key:?}` to `{value:?}`: {e}")
     })
 }
 
@@ -385,7 +385,7 @@ pub fn remove_var<K: AsRef<OsStr>>(key: K) {
 
 fn _remove_var(key: &OsStr) {
     os_imp::unsetenv(key)
-        .unwrap_or_else(|e| panic!("failed to remove environment variable `{:?}`: {}", key, e))
+        .unwrap_or_else(|e| panic!("failed to remove environment variable `{key:?}`: {e}"))
 }
 
 /// An iterator that splits an environment variable into paths according to
@@ -421,7 +421,7 @@ pub struct SplitPaths<'a> {
 ///             println!("'{}'", path.display());
 ///         }
 ///     }
-///     None => println!("{} is not defined in the environment.", key)
+///     None => println!("{key} is not defined in the environment.")
 /// }
 /// ```
 #[stable(feature = "env", since = "1.0.0")]
@@ -684,7 +684,7 @@ pub fn temp_dir() -> PathBuf {
 /// match env::current_exe() {
 ///     Ok(exe_path) => println!("Path of this executable is: {}",
 ///                              exe_path.display()),
-///     Err(e) => println!("failed to get current exe path: {}", e),
+///     Err(e) => println!("failed to get current exe path: {e}"),
 /// };
 /// ```
 #[stable(feature = "env", since = "1.0.0")]
@@ -755,7 +755,7 @@ pub struct ArgsOs {
 ///
 /// // Prints each argument on a separate line
 /// for argument in env::args() {
-///     println!("{}", argument);
+///     println!("{argument}");
 /// }
 /// ```
 #[stable(feature = "env", since = "1.0.0")]
@@ -790,7 +790,7 @@ pub fn args() -> Args {
 ///
 /// // Prints each argument on a separate line
 /// for argument in env::args_os() {
-///     println!("{:?}", argument);
+///     println!("{argument:?}");
 /// }
 /// ```
 #[stable(feature = "env", since = "1.0.0")]