Rollup merge of #115310 - RalfJung:panic-and-format, r=scottmcm

Document panic behavior across editions, and improve xrefs

This revives (parts of) https://github.com/rust-lang/rust/pull/96518.
r? `@scottmcm`
Cc `@ijackson`

library/alloc/src/macros.rs

@@ -79,10 +79,12 @@ 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,
@@ -91,7 +93,6 @@ 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

@@ -8,8 +8,8 @@ tests. `panic!` is closely tied with the `unwrap` method of both
 [`Option`][ounwrap] and [`Result`][runwrap] enums. Both implementations call
 `panic!` when they are set to [`None`] or [`Err`] variants.
 
-When using `panic!()` you can specify a string payload, that is built using
+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
+[formatting 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
@@ -18,6 +18,7 @@ after the panic is invoked, is to print the message payload to
 call. You can override the panic hook using [`std::panic::set_hook()`].
 Inside the hook a panic can be accessed as a `&dyn Any + Send`,
 which contains either a `&str` or `String` for regular `panic!()` invocations.
+(Whether a particular invocation contains the payload at type `&str` or `String` is unspecified and can change.)
 To panic with a value of another other type, [`panic_any`] can be used.
 
 See also the macro [`compile_error!`], for raising errors during compilation.
@@ -55,7 +56,7 @@ 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 +65,29 @@ 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
+
+Behavior of the panic macros changed over editions.
+
+## 2021 and later
+
+In Rust 2021 and later, `panic!` always requires a format string and
+the applicable format arguments, and is the same in `core` and `std`.
+Use [`std::panic::panic_any(x)`](../std/panic/fn.panic_any.html) to
+panic with an arbitrary payload.
+
+## 2018 and 2015
+
+In Rust Editions prior to 2021, `std::panic!(x)` with a single
+argument directly uses that argument as a payload.
+This is true even if the argument is a string literal.
+For example, `panic!("problem: {reason}")` panics with a
+payload of literally `"problem: {reason}"` (a `&'static str`).
+
+`core::panic!(x)` with a single argument requires that `x` be `&str`,
+but otherwise behaves like `std::panic!`. In particular, the string
+need not be a literal, and is not interpreted as a format string.
+
 # Examples
 
 ```should_panic

library/std/src/macros.rs

@@ -41,6 +41,9 @@ 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
 /// [`println!`]: crate::println
 /// [`eprint!`]: crate::eprint
@@ -103,6 +106,9 @@ 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
 /// [`eprintln!`]: crate::eprintln
 /// [lock]: crate::io::Stdout
@@ -150,6 +156,9 @@ 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.
@@ -181,6 +190,9 @@ 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
 /// [`println!`]: crate::println