Rework trait parameter patterns

This updates the description of trait parameter patterns, primarily to
shift it to the style where the main text describes the behavior in the
current edition, and changes in the edition are moved to separate
edition blocks.

The specific changes:

- Rework the edition presentation (as per our style guide).
- Try to use more consistent wording.
- Add examples.
- Rename some of the rules to switch to the updated edition style, and
  try to be a little clearer about their intent.
- Change the way it refers to "methods". The reference used to define
  methods as a separate kind of associated item, but we changed it so
  that functions allow an optional _SelfParam_, and methods aren't as
  special as they used to be. This text was still written assuming the
  previous presentation.

Author: ehuss

src/items/traits.md

@@ -287,44 +287,71 @@ The [trait implementation] must also begin with the `unsafe` keyword.
 r[items.traits.params]
 ## Parameter patterns
 
-r[items.traits.params.allowed-patterns]
-Function or method declarations without a body only allow [IDENTIFIER] or
-`_` [wild card][WildcardPattern] patterns. `mut` [IDENTIFIER] is currently
-allowed, but it is deprecated and will become a hard error in the future.
+r[items.traits.params.patterns-no-body]
+Parameters in associated functions without a body only allow [IDENTIFIER] or `_` [wild card][WildcardPattern] patterns, as well as the form allowed by [_SelfParam_]. `mut` [IDENTIFIER] is currently allowed, but it is deprecated and will become a hard error in the future.
 <!-- https://github.com/rust-lang/rust/issues/35203 -->
 
-r[items.traits.params.edition2015]
-In the 2015 edition, the pattern for a trait function or method parameter is
-optional:
-
-```rust,edition2015
-// 2015 Edition
+```rust
 trait T {
-    fn f(i32);  // Parameter identifiers are not required.
+    fn f1(&self);
+    fn f2(x: Self, _: i32);
 }
 ```
 
-r[items.traits.params.restriction]
-The kinds of patterns for parameters is limited to one of the following:
-
-* [IDENTIFIER]
-* `mut` [IDENTIFIER]
-* [`_`][WildcardPattern]
-* `&` [IDENTIFIER]
-* `&&` [IDENTIFIER]
+```rust,compile_fail,E0642
+trait T {
+    fn f2(&x: &i32); // ERROR: patterns aren't allowed in functions without bodies
+}
+```
 
-r[items.traits.params.restriction.edition2018]
-Beginning in the 2018 edition, function or method parameter patterns are no
-longer optional. Also, all irrefutable patterns are allowed as long as there
-is a body. Without a body, the limitations listed above are still in effect.
+r[items.traits.params.patterns-with-body]
+Parameters in associated functions with a body only allow irrefutable patterns.
 
 ```rust
 trait T {
-    fn f1((a, b): (i32, i32)) {}
-    fn f2(_: (i32, i32));  // Cannot use tuple pattern without a body.
+    fn f1((a, b): (i32, i32)) {} // OK: is irrefutable
 }
 ```
 
+```rust,compile_fail,E0005
+trait T {
+    fn f1(123: i32) {} // ERROR: pattern is refutable
+    fn f2(Some(x): Option<i32>) {} // ERROR: pattern is refutable
+}
+```
+
+r[items.traits.params.pattern-required.edition2018]
+> [!EDITION-2018]
+> Prior to the 2018 edition, the pattern for an associated function parameter is optional:
+>
+> ```rust,edition2015
+> // 2015 Edition
+> trait T {
+>     fn f(i32);  // Parameter identifiers are not required.
+> }
+> ```
+>
+> Beginning in the 2018 edition, patterns are no longer optional.
+
+r[items.traits.params.restriction-patterns.edition2018]
+> [!EDITION-2018]
+> Prior to the 2018 edition, parameters in associated functions with a body are limited to the following kinds of patterns:
+>
+> * [IDENTIFIER]
+> * `mut` [IDENTIFIER]
+> * [`_`][WildcardPattern]
+> * `&` [IDENTIFIER]
+> * `&&` [IDENTIFIER]
+>
+> ```rust,edition2015,compile_fail,E0642
+> // 2015 Edition
+> trait T {
+>     fn f1((a, b): (i32, i32)) {} // ERROR: pattern not allowed
+> }
+> ```
+>
+> Beginning in 2018, all irrefutable patterns are allowed as described in [items.traits.params.patterns-with-body].
+
 r[items.traits.associated-visibility]
 ## Item visibility
 
@@ -368,6 +395,7 @@ fn main() {
 [_AssociatedItem_]: associated-items.md
 [_GenericParams_]: generics.md
 [_InnerAttribute_]: ../attributes.md
+[_SelfParam_]: functions.md
 [_TypeParamBounds_]: ../trait-bounds.md
 [_Visibility_]: ../visibility-and-privacy.md
 [_WhereClause_]: generics.md#where-clauses