Add short summaries to all 2021 edition changes.

src/rust-2021/IntoIterator-for-arrays.md

@@ -2,6 +2,11 @@
 
 ## Summary
 
+- Arrays implement `IntoIterator` in *all* editions.
+- The new `.into_iter()` is *hidden* in Rust 2015 and Rust 2018 in method call syntax.
+  So, `array.into_iter()` still resolves to `(&array).into_iter()` as before.
+- `array.into_iter()` changes meaning when switching to Rust 2021.
+
 ## Details
 
 Until Rust 1.53, only *references* to arrays implement `IntoIterator`.
@@ -46,4 +51,4 @@ there is no added complexity in the new edition.
 
 [25]: https://github.com/rust-lang/rust/issues/25725
 [20]: https://github.com/rust-lang/rust/pull/65819
-[22]: https://doc.rust-lang.org/book/ch05-03-method-syntax.html#wheres-the---operator
+[22]: https://doc.rust-lang.org/book/ch05-03-method-syntax.html#wheres-the---operator

src/rust-2021/default-cargo-resolver.md

@@ -2,6 +2,8 @@
 
 ## Summary
 
+- `edition = "2021"` implies `resolver = "2"` in `Cargo.toml`.
+
 ## Details
 
 Since Rust 1.51.0, Cargo has opt-in support for a [new feature resolver][4]
@@ -15,4 +17,4 @@ crates that are depended on in multiple ways.
 See [the announcement of Rust 1.51][5] for details.
 
 [4]: https://doc.rust-lang.org/cargo/reference/resolver.html#feature-resolver-version-2
-[5]: https://blog.rust-lang.org/2021/03/25/Rust-1.51.0.html#cargos-new-feature-resolver
+[5]: https://blog.rust-lang.org/2021/03/25/Rust-1.51.0.html#cargos-new-feature-resolver

src/rust-2021/disjoint-capture-in-closures.md

@@ -2,6 +2,9 @@
 
 ## Summary
 
+- `|| a.x + 1` now captures only `a.x` instead of `a`.
+- This can subtly change the drop order of things.
+
 ## Details
 
 [Closures](https://doc.rust-lang.org/book/ch13-01-closures.html)

src/rust-2021/or-patterns-macro-rules.md

@@ -2,6 +2,10 @@
 
 ## Summary
 
+- `$_:pat` in `macro_rules` now matches `|` too: e.g. `A | B`.
+- `$_:pat_param` behaves like `$_:pat` did before; it does not match (top level) `|`.
+- `$_:pat_param` is available in all editions.
+
 ## Details
 
 Starting in Rust 1.53.0, [patterns](https://doc.rust-lang.org/stable/reference/patterns.html)

src/rust-2021/panic-macro-consistency.md

@@ -2,8 +2,12 @@
 
 ## Summary
 
-## Details
+- `panic!(..)` now always use `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.
 
+## 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)
@@ -41,4 +45,4 @@ will be the only way to panic with something other than a formatted string.
 
 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 noticable when switching `#![no_std]` on or off.
+which can be noticable when switching `#![no_std]` on or off.

src/rust-2021/prelude.md

@@ -2,6 +2,9 @@
 
 ## Summary
 
+- `TryInto`, `TryFrom` and `FromIterator` are now part of the prelude.
+- This might change the meaning of e.g. `x.try_into()` depending on types and imports.
+
 ## Details
 
 The [prelude of the standard library](https://doc.rust-lang.org/stable/std/prelude/index.html)

src/rust-2021/reserving-syntax.md

@@ -2,6 +2,10 @@
 
 ## Summary
 
+- `any_prefix#..`, `any_prefix".."`, and `any_prefix'..'` are reserved syntax, and no longer tokenize.
+- These did not parse as valid Rust in any edition, except in arguments to macros.
+- Insert whitespace to avoid errors.
+
 ## Details
 
 To make space for some new syntax in the future,
@@ -37,4 +41,4 @@ These are some new prefixes you might see in the future:
   this prefix would've allowed us to accept `k#async` in edition 2015
   without having to wait for edition 2018 to reserve `async` as a keyword.
 
-[10]: https://github.com/rust-lang/rfcs/pull/3101
+[10]: https://github.com/rust-lang/rfcs/pull/3101

src/rust-2021/warnings-promoted-to-error.md

@@ -2,6 +2,8 @@
 
 ## Summary
 
+- `bare-trait-objects` and `ellipsis-inclusive-range-patters` are now hard errors.
+
 ## Details
 
 Two existing lints are becoming hard errors in Rust 2021.
@@ -14,4 +16,4 @@ These lints will remain warnings in older editions.
 * `ellipsis-inclusive-range-patterns`:
   The [deprecated `...` syntax](https://doc.rust-lang.org/stable/reference/patterns.html#range-patterns)
   for inclusive range patterns is no longer accepted in Rust 2021.
-  It has been superseded by `..=`, which is consistent with expressions.
+  It has been superseded by `..=`, which is consistent with expressions.