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).

compiler/rustc_errors/src/lib.rs

@@ -1087,12 +1087,12 @@ impl HandlerInner {
         let warnings = match self.deduplicated_warn_count {
             0 => String::new(),
             1 => "1 warning emitted".to_string(),
-            count => format!("{} warnings emitted", count),
+            count => format!("{count} warnings emitted"),
         };
         let errors = match self.deduplicated_err_count {
             0 => String::new(),
             1 => "aborting due to previous error".to_string(),
-            count => format!("aborting due to {} previous errors", count),
+            count => format!("aborting due to {count} previous errors"),
         };
         if self.treat_err_as_bug() {
             return;

compiler/rustc_lint_defs/src/builtin.rs

@@ -3399,7 +3399,7 @@ declare_lint! {
     ///     //                  ^^^^^^^^
     ///     // This call to try_into matches both Foo:try_into and TryInto::try_into as
     ///     // `TryInto` has been added to the Rust prelude in 2021 edition.
-    ///     println!("{}", x);
+    ///     println!("{x}");
     /// }
     /// ```
     ///

compiler/rustc_middle/src/ty/closure.rs

@@ -281,7 +281,7 @@ pub struct CaptureInfo {
     /// let mut t = (0,1);
     ///
     /// let c = || {
-    ///     println!("{}",t); // L1
+    ///     println!("{t}"); // L1
     ///     t.1 = 4; // L2
     /// };
     /// ```

compiler/rustc_typeck/src/check/upvar.rs

@@ -458,10 +458,10 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// let s: String;  // hir_id_s
     /// let mut p: Point; // his_id_p
     /// let c = || {
-    ///        println!("{}", s);  // L1
+    ///        println!("{s}");  // L1
     ///        p.x += 10;  // L2
-    ///        println!("{}" , p.y) // L3
-    ///        println!("{}", p) // L4
+    ///        println!("{}" , p.y); // L3
+    ///        println!("{p}"); // L4
     ///        drop(s);   // L5
     /// };
     /// ```

library/alloc/src/alloc.rs

@@ -397,7 +397,7 @@ pub mod __alloc_error_handler {
     // if there is no `#[alloc_error_handler]`
     #[rustc_std_internal_symbol]
     pub unsafe extern "C-unwind" fn __rdl_oom(size: usize, _align: usize) -> ! {
-        panic!("memory allocation of {} bytes failed", size)
+        panic!("memory allocation of {size} bytes failed")
     }
 
     // if there is an `#[alloc_error_handler]`

library/alloc/src/borrow.rs

@@ -161,7 +161,7 @@ where
 /// let readonly = [1, 2];
 /// let borrowed = Items::new((&readonly[..]).into());
 /// match borrowed {
-///     Items { values: Cow::Borrowed(b) } => println!("borrowed {:?}", b),
+///     Items { values: Cow::Borrowed(b) } => println!("borrowed {b:?}"),
 ///     _ => panic!("expect borrowed value"),
 /// }
 ///

library/alloc/src/boxed.rs

@@ -31,7 +31,7 @@
 //! }
 //!
 //! let list: List<i32> = List::Cons(1, Box::new(List::Cons(2, Box::new(List::Nil))));
-//! println!("{:?}", list);
+//! println!("{list:?}");
 //! ```
 //!
 //! This will print `Cons(1, Cons(2, Nil))`.
@@ -1408,7 +1408,7 @@ impl<T: Copy> From<&[T]> for Box<[T]> {
     /// let slice: &[u8] = &[104, 101, 108, 108, 111];
     /// let boxed_slice: Box<[u8]> = Box::from(slice);
     ///
-    /// println!("{:?}", boxed_slice);
+    /// println!("{boxed_slice:?}");
     /// ```
     fn from(slice: &[T]) -> Box<[T]> {
         let len = slice.len();
@@ -1450,7 +1450,7 @@ impl From<&str> for Box<str> {
     ///
     /// ```rust
     /// let boxed: Box<str> = Box::from("hello");
-    /// println!("{}", boxed);
+    /// println!("{boxed}");
     /// ```
     #[inline]
     fn from(s: &str) -> Box<str> {
@@ -1475,14 +1475,14 @@ impl From<Cow<'_, str>> for Box<str> {
     ///
     /// let unboxed = Cow::Borrowed("hello");
     /// let boxed: Box<str> = Box::from(unboxed);
-    /// println!("{}", boxed);
+    /// println!("{boxed}");
     /// ```
     ///
     /// ```rust
     /// # use std::borrow::Cow;
     /// let unboxed = Cow::Owned("hello".to_string());
     /// let boxed: Box<str> = Box::from(unboxed);
-    /// println!("{}", boxed);
+    /// println!("{boxed}");
     /// ```
     #[inline]
     fn from(cow: Cow<'_, str>) -> Box<str> {
@@ -1529,7 +1529,7 @@ impl<T, const N: usize> From<[T; N]> for Box<[T]> {
     ///
     /// ```rust
     /// let boxed: Box<[u8]> = Box::from([4, 2]);
-    /// println!("{:?}", boxed);
+    /// println!("{boxed:?}");
     /// ```
     fn from(array: [T; N]) -> Box<[T]> {
         box array

library/alloc/src/collections/binary_heap.rs

@@ -194,7 +194,7 @@ use super::SpecExtend;
 /// // We can iterate over the items in the heap, although they are returned in
 /// // a random order.
 /// for x in &heap {
-///     println!("{}", x);
+///     println!("{x}");
 /// }
 ///
 /// // If we instead pop these scores, they should come back in order.
@@ -830,7 +830,7 @@ impl<T> BinaryHeap<T> {
     ///
     /// // Print 1, 2, 3, 4 in arbitrary order
     /// for x in heap.iter() {
-    ///     println!("{}", x);
+    ///     println!("{x}");
     /// }
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
@@ -1110,7 +1110,7 @@ impl<T> BinaryHeap<T> {
     ///
     /// // Will print in some order
     /// for x in vec {
-    ///     println!("{}", x);
+    ///     println!("{x}");
     /// }
     /// ```
     #[must_use = "`self` will be dropped if the result is not used"]
@@ -1179,7 +1179,7 @@ impl<T> BinaryHeap<T> {
     /// assert!(!heap.is_empty());
     ///
     /// for x in heap.drain() {
-    ///     println!("{}", x);
+    ///     println!("{x}");
     /// }
     ///
     /// assert!(heap.is_empty());
@@ -1624,7 +1624,7 @@ impl<T> IntoIterator for BinaryHeap<T> {
     /// // Print 1, 2, 3, 4 in arbitrary order
     /// for x in heap.into_iter() {
     ///     // x has type i32, not &i32
-    ///     println!("{}", x);
+    ///     println!("{x}");
     /// }
     /// ```
     fn into_iter(self) -> IntoIter<T> {

library/alloc/src/collections/btree/map.rs

@@ -104,8 +104,8 @@ pub(super) const MIN_LEN: usize = node::MIN_LEN_AFTER_SPLIT;
 /// let to_find = ["Up!", "Office Space"];
 /// for movie in &to_find {
 ///     match movie_reviews.get(movie) {
-///        Some(review) => println!("{}: {}", movie, review),
-///        None => println!("{} is unreviewed.", movie)
+///        Some(review) => println!("{movie}: {review}"),
+///        None => println!("{movie} is unreviewed.")
 ///     }
 /// }
 ///
@@ -114,7 +114,7 @@ pub(super) const MIN_LEN: usize = node::MIN_LEN_AFTER_SPLIT;
 ///
 /// // iterate over everything.
 /// for (movie, review) in &movie_reviews {
-///     println!("{}: \"{}\"", movie, review);
+///     println!("{movie}: \"{review}\"");
 /// }
 /// ```
 ///
@@ -1061,7 +1061,7 @@ impl<K, V> BTreeMap<K, V> {
     /// map.insert(5, "b");
     /// map.insert(8, "c");
     /// for (&key, &value) in map.range((Included(&4), Included(&8))) {
-    ///     println!("{}: {}", key, value);
+    ///     println!("{key}: {value}");
     /// }
     /// assert_eq!(Some((&5, &"b")), map.range(4..).next());
     /// ```
@@ -1104,7 +1104,7 @@ impl<K, V> BTreeMap<K, V> {
     ///     *balance += 100;
     /// }
     /// for (name, balance) in &map {
-    ///     println!("{} => {}", name, balance);
+    ///     println!("{name} => {balance}");
     /// }
     /// ```
     #[stable(feature = "btree_range", since = "1.17.0")]
@@ -2088,7 +2088,7 @@ impl<K, V> BTreeMap<K, V> {
     /// map.insert(1, "a");
     ///
     /// for (key, value) in map.iter() {
-    ///     println!("{}: {}", key, value);
+    ///     println!("{key}: {value}");
     /// }
     ///
     /// let (first_key, first_value) = map.iter().next().unwrap();

library/alloc/src/collections/btree/map/tests.rs

@@ -1758,7 +1758,7 @@ fn test_ord_absence() {
     }
 
     fn map_debug<K: Debug>(mut map: BTreeMap<K, ()>) {
-        format!("{:?}", map);
+        format!("{map:?}");
         format!("{:?}", map.iter());
         format!("{:?}", map.iter_mut());
         format!("{:?}", map.keys());

library/alloc/src/collections/btree/set.rs

@@ -60,7 +60,7 @@ use super::Recover;
 ///
 /// // Iterate over everything.
 /// for book in &books {
-///     println!("{}", book);
+///     println!("{book}");
 /// }
 /// ```
 ///
@@ -284,7 +284,7 @@ impl<T> BTreeSet<T> {
     /// set.insert(5);
     /// set.insert(8);
     /// for &elem in set.range((Included(&4), Included(&8))) {
-    ///     println!("{}", elem);
+    ///     println!("{elem}");
     /// }
     /// assert_eq!(Some(&5), set.range(4..).next());
     /// ```

library/alloc/src/collections/btree/set/tests.rs

@@ -431,10 +431,10 @@ fn test_show() {
     set.insert(1);
     set.insert(2);
 
-    let set_str = format!("{:?}", set);
+    let set_str = format!("{set:?}");
 
     assert_eq!(set_str, "{1, 2}");
-    assert_eq!(format!("{:?}", empty), "{}");
+    assert_eq!(format!("{empty:?}"), "{}");
 }
 
 #[test]
@@ -649,7 +649,7 @@ fn test_ord_absence() {
     }
 
     fn set_debug<K: Debug>(set: BTreeSet<K>) {
-        format!("{:?}", set);
+        format!("{set:?}");
         format!("{:?}", set.iter());
         format!("{:?}", set.into_iter());
     }

library/alloc/src/fmt.rs

@@ -416,9 +416,9 @@
 //! fn main() {
 //!     let myvector = Vector2D { x: 3, y: 4 };
 //!
-//!     println!("{}", myvector);       // => "(3, 4)"
-//!     println!("{:?}", myvector);     // => "Vector2D {x: 3, y:4}"
-//!     println!("{:10.3b}", myvector); // => "     5.000"
+//!     println!("{myvector}");       // => "(3, 4)"
+//!     println!("{myvector:?}");     // => "Vector2D {x: 3, y:4}"
+//!     println!("{myvector:10.3b}"); // => "     5.000"
 //! }
 //! ```
 //!

library/alloc/src/rc/tests.rs

@@ -309,7 +309,7 @@ fn test_cowrc_clone_weak() {
 #[test]
 fn test_show() {
     let foo = Rc::new(75);
-    assert_eq!(format!("{:?}", foo), "75");
+    assert_eq!(format!("{foo:?}"), "75");
 }
 
 #[test]
@@ -324,7 +324,7 @@ fn test_maybe_thin_unsized() {
     use std::ffi::{CStr, CString};
 
     let x: Rc<CStr> = Rc::from(CString::new("swordfish").unwrap().into_boxed_c_str());
-    assert_eq!(format!("{:?}", x), "\"swordfish\"");
+    assert_eq!(format!("{x:?}"), "\"swordfish\"");
     let y: Weak<CStr> = Rc::downgrade(&x);
     drop(x);
 
@@ -451,7 +451,7 @@ fn test_from_box_trait_zero_sized() {
     let b: Box<dyn Debug> = box ();
     let r: Rc<dyn Debug> = Rc::from(b);
 
-    assert_eq!(format!("{:?}", r), "()");
+    assert_eq!(format!("{r:?}"), "()");
 }
 
 #[test]

library/alloc/src/slice.rs

@@ -48,7 +48,7 @@
 //! ```
 //! let numbers = &[0, 1, 2];
 //! for n in numbers {
-//!     println!("{} is a number!", n);
+//!     println!("{n} is a number!");
 //! }
 //! ```
 //!

library/alloc/src/string.rs

@@ -2718,7 +2718,7 @@ impl From<String> for Vec<u8> {
     /// let v1 = Vec::from(s1);
     ///
     /// for b in v1 {
-    ///     println!("{}", b);
+    ///     println!("{b}");
     /// }
     /// ```
     fn from(string: String) -> Vec<u8> {

library/alloc/src/sync.rs

@@ -200,7 +200,7 @@ macro_rules! acquire {
 ///     let five = Arc::clone(&five);
 ///
 ///     thread::spawn(move || {
-///         println!("{:?}", five);
+///         println!("{five:?}");
 ///     });
 /// }
 /// ```
@@ -221,7 +221,7 @@ macro_rules! acquire {
 ///
 ///     thread::spawn(move || {
 ///         let v = val.fetch_add(1, Ordering::SeqCst);
-///         println!("{:?}", v);
+///         println!("{v:?}");
 ///     });
 /// }
 /// ```

library/alloc/src/sync/tests.rs

@@ -335,7 +335,7 @@ fn test_weak_count() {
 #[test]
 fn show_arc() {
     let a = Arc::new(5);
-    assert_eq!(format!("{:?}", a), "5");
+    assert_eq!(format!("{a:?}"), "5");
 }
 
 // Make sure deriving works with Arc<T>
@@ -347,7 +347,7 @@ struct Foo {
 #[test]
 fn test_unsized() {
     let x: Arc<[i32]> = Arc::new([1, 2, 3]);
-    assert_eq!(format!("{:?}", x), "[1, 2, 3]");
+    assert_eq!(format!("{x:?}"), "[1, 2, 3]");
     let y = Arc::downgrade(&x.clone());
     drop(x);
     assert!(y.upgrade().is_none());
@@ -359,7 +359,7 @@ fn test_maybe_thin_unsized() {
     use std::ffi::{CStr, CString};
 
     let x: Arc<CStr> = Arc::from(CString::new("swordfish").unwrap().into_boxed_c_str());
-    assert_eq!(format!("{:?}", x), "\"swordfish\"");
+    assert_eq!(format!("{x:?}"), "\"swordfish\"");
     let y: Weak<CStr> = Arc::downgrade(&x);
     drop(x);
 
@@ -509,7 +509,7 @@ fn test_from_box_trait_zero_sized() {
     let b: Box<dyn Debug> = box ();
     let r: Arc<dyn Debug> = Arc::from(b);
 
-    assert_eq!(format!("{:?}", r), "()");
+    assert_eq!(format!("{r:?}"), "()");
 }
 
 #[test]

library/alloc/src/tests.rs

@@ -47,18 +47,18 @@ fn any_move() {
 fn test_show() {
     let a = Box::new(8) as Box<dyn Any>;
     let b = Box::new(Test) as Box<dyn Any>;
-    let a_str = format!("{:?}", a);
-    let b_str = format!("{:?}", b);
+    let a_str = format!("{a:?}");
+    let b_str = format!("{b:?}");
     assert_eq!(a_str, "Any { .. }");
     assert_eq!(b_str, "Any { .. }");
 
     static EIGHT: usize = 8;
     static TEST: Test = Test;
     let a = &EIGHT as &dyn Any;
     let b = &TEST as &dyn Any;
-    let s = format!("{:?}", a);
+    let s = format!("{a:?}");
     assert_eq!(s, "Any { .. }");
-    let s = format!("{:?}", b);
+    let s = format!("{b:?}");
     assert_eq!(s, "Any { .. }");
 }