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+# Panic macro consistency
+
+## Summary
+
+- `panic!(..)` now always uses `format_args!(..)`, just like `println!()`.
+- `panic!("{")` is no longer accepted, without escaping the `{` as `{{`.
+- `panic!(x)` is no longer accepted if `x` is not a string literal.
+  - Use `std::panic::panic_any(x)` to panic with a non-string payload.
+  - Or use `panic!("{}", x)` to use `x`'s `Display` implementation.
+- The same applies to `assert!(expr, ..)`.
+
+## Details
+
+The `panic!()` macro is one of Rust's most well known macros.
+However, it has [some subtle surprises](https://github.com/rust-lang/rfcs/blob/master/text/3007-panic-plan.md)
+that we can't just change due to backwards compatibility.
+
+```rust,ignore
+// Rust 2018
+panic!("{}", 1); // Ok, panics with the message "1"
+panic!("{}"); // Ok, panics with the message "{}"
+```
+
+The `panic!()` macro only uses string formatting when it's invoked with more than one argument.
+When invoked with a single argument, it doesn't even look at that argument.
+
+```rust,ignore
+// Rust 2018
+let a = "{";
+println!(a); // Error: First argument must be a format string literal
+panic!(a); // Ok: The panic macro doesn't care
+```
+
+It even accepts non-strings such as `panic!(123)`, which is uncommon and rarely useful since it
+produces a surprisingly unhelpful message: `panicked at 'Box<Any>'`.
+
+This will especially be a problem once
+[implicit format arguments](https://rust-lang.github.io/rfcs/2795-format-args-implicit-identifiers.html)
+are stabilized.
+That feature will make `println!("hello {name}")` a short-hand for `println!("hello {}", name)`.
+However, `panic!("hello {name}")` would not work as expected,
+since `panic!()` doesn't process a single argument as format string.
+
+To avoid that confusing situation, Rust 2021 features a more consistent `panic!()` macro.
+The new `panic!()` macro will no longer accept arbitrary expressions as the only argument.
+It will, just like `println!()`, always process the first argument as format string.
+Since `panic!()` will no longer accept arbitrary payloads,
+[`panic_any()`](https://doc.rust-lang.org/stable/std/panic/fn.panic_any.html)
+will be the only way to panic with something other than a formatted string.
+
+```rust,ignore
+// Rust 2021
+panic!("{}", 1); // Ok, panics with the message "1"
+panic!("{}"); // Error, missing argument
+panic!(a); // Error, must be a string literal
+```
+
+In addition, `core::panic!()` and `std::panic!()` will be identical in Rust 2021.
+Currently, there are some historical differences between those two,
+which can be noticeable when switching `#![no_std]` on or off.
+
+## Migration
+
+A lint, `non_fmt_panics`, gets triggered whenever there is some call to `panic` that uses some 
+deprecated behavior that will error in Rust 2021. The `non_fmt_panics` lint has already been a warning 
+by default on all editions since the 1.50 release (with several enhancements made in later releases). 
+If your code is already warning free, then it should already be ready to go for Rust 2021!
+
+You can automatically migrate your code to be Rust 2021 Edition compatible or ensure it is already compatible by
+running:
+
+```sh
+cargo fix --edition
+```
+
+Should you choose or need to manually migrate, you'll need to update all panic invocations to either use the same 
+formatting as `println` or use `std::panic::panic_any` to panic with non-string data.
+
+For example, in the case of `panic!(MyStruct)`, you'll need to convert to using `std::panic::panic_any` (note
+that this is a function not a macro): `std::panic::panic_any(MyStruct)`.
+
+In the case of panic messages that include curly braces but the wrong number of arguments (e.g., `panic!("Some curlies: {}")`), 
+you can panic with the string literal by either using the same syntax as `println!` (i.e., `panic!("{}", "Some curlies: {}")`) 
+or by escaping the curly braces (i.e., `panic!("Some curlies: {{}}")`).
+\ No newline at end of file