Document panic and format edition/verions, and improve xrefs

library/alloc/src/fmt.rs

@@ -84,6 +84,7 @@
 //!
 //! If a named parameter does not appear in the argument list, `format!` will
 //! reference a variable with that name in the current scope.
+//! (Rust 1.58 and later; in [`panic!`], requires Rust 2021.)
 //!
 //! ```
 //! let argument = 2 + 2;

library/alloc/src/macros.rs

@@ -73,10 +73,12 @@ macro_rules! vec {
 ///
 /// The first argument `format!` receives is a format string. This must be a string
 /// literal. The power of the formatting string is in the `{}`s contained.
-///
 /// Additional parameters passed to `format!` replace the `{}`s within the
 /// formatting string in the order given unless named or positional parameters
-/// are used; see [`std::fmt`] for more information.
+/// are used.
+///
+/// See [the formatting syntax documentation in `std::fmt`](../std/fmt/index.html)
+/// for details.
 ///
 /// A common use for `format!` is concatenation and interpolation of strings.
 /// The same convention is used with [`print!`] and [`write!`] macros,
@@ -85,7 +87,6 @@ macro_rules! vec {
 /// To convert a single value to a string, use the [`to_string`] method. This
 /// will use the [`Display`] formatting trait.
 ///
-/// [`std::fmt`]: ../std/fmt/index.html
 /// [`print!`]: ../std/macro.print.html
 /// [`write!`]: core::write
 /// [`to_string`]: crate::string::ToString

library/core/src/macros/mod.rs

@@ -849,7 +849,8 @@ pub(crate) mod builtin {
     /// assert_eq!(display, debug);
     /// ```
     ///
-    /// For more information, see the documentation in [`std::fmt`].
+    /// See [the formatting documentation in `std::fmt`](../std/fmt/index.html)
+    /// for details of the macro argument syntax, and further information.
     ///
     /// [`Display`]: crate::fmt::Display
     /// [`Debug`]: crate::fmt::Debug

library/core/src/macros/panic.md

@@ -9,7 +9,7 @@ tests. `panic!` is closely tied with the `unwrap` method of both
 `panic!` when they are set to [`None`] or [`Err`] variants.
 
 When using `panic!()` you can specify a string payload, that is built using
-the [`format!`] syntax. That payload is used when injecting the panic into
+the [`format!` syntax]. That payload is used when injecting the panic into
 the calling Rust thread, causing the thread to panic entirely.
 
 The behavior of the default `std` hook, i.e. the code that runs directly
@@ -55,7 +55,7 @@ For more detailed information about error handling check out the [book] or the
 [`panic_any`]: ../std/panic/fn.panic_any.html
 [`Box`]: ../std/boxed/struct.Box.html
 [`Any`]: crate::any::Any
-[`format!`]: ../std/macro.format.html
+[`format!` syntax]: ../std/fmt/index.html
 [book]: ../book/ch09-00-error-handling.html
 [`std::result`]: ../std/result/index.html
 
@@ -64,6 +64,25 @@ For more detailed information about error handling check out the [book] or the
 If the main thread panics it will terminate all your threads and end your
 program with code `101`.
 
+# Editions
+
+In Rust Editions prior to 2021, `std::panic!(x)` with a single
+argument is equivalent to
+[`std::panic::panic_any(x)`](../std/panic/fn.panic_any.html).
+This is true even if the argument is a string literal.
+
+For example, in Rust 2015 `panic!("problem: {reason}")` panics with a
+payload of literally `"problem: {reason}"` (a `&'static str`), which
+is probably not what was intended.  In current Rust this usage
+captures and formats a variable `reason` from the surrounding scope.
+
+In Rust editions prior to 2021, `core::panic!(x)` requires that
+`x` be `&str`, but does not require it to be a literal.  In Rust 2021,
+these cases must be written `panic!("{}", x)`.
+
+In Rust 2021 and later, `panic!` always requires a format string and
+the applicable format arguments, and is the same in `core` and `std`.
+
 # Examples
 
 ```should_panic

library/std/src/macros.rs

@@ -30,6 +30,9 @@ macro_rules! panic {
 /// Use `print!` only for the primary output of your program. Use
 /// [`eprint!`] instead to print error and progress messages.
 ///
+/// See [the formatting documentation in `std::fmt`](../std/fmt/index.html)
+/// for details of the macro argument syntax.
+///
 /// [flush]: crate::io::Write::flush
 ///
 /// # Panics
@@ -76,6 +79,9 @@ macro_rules! print {
 /// Use `println!` only for the primary output of your program. Use
 /// [`eprintln!`] instead to print error and progress messages.
 ///
+/// See [the formatting documentation in `std::fmt`](../std/fmt/index.html)
+/// for details of the macro argument syntax.
+///
 /// [`std::fmt`]: crate::fmt
 ///
 /// # Panics
@@ -116,6 +122,9 @@ macro_rules! println {
 /// [`io::stderr`]: crate::io::stderr
 /// [`io::stdout`]: crate::io::stdout
 ///
+/// See [the formatting documentation in `std::fmt`](../std/fmt/index.html)
+/// for details of the macro argument syntax.
+///
 /// # Panics
 ///
 /// Panics if writing to `io::stderr` fails.
@@ -144,6 +153,9 @@ macro_rules! eprint {
 /// Use `eprintln!` only for error and progress messages. Use `println!`
 /// instead for the primary output of your program.
 ///
+/// See [the formatting documentation in `std::fmt`](../std/fmt/index.html)
+/// for details of the macro argument syntax.
+///
 /// [`io::stderr`]: crate::io::stderr
 /// [`io::stdout`]: crate::io::stdout
 ///