Rollup merge of #72127 - jademcgough:long-error-explanation-E0228, r=petrochenkov

add long error explanation for E0228

Add long explanation for the E0228 error code
Part of #61137

Let me know if this is wrong at all (or can be written more clearly), I'm still learning Rust.

Author: Dylan-DPC

src/librustc_error_codes/error_codes.rs

@@ -120,6 +120,7 @@ E0223: include_str!("./error_codes/E0223.md"),
 E0224: include_str!("./error_codes/E0224.md"),
 E0225: include_str!("./error_codes/E0225.md"),
 E0226: include_str!("./error_codes/E0226.md"),
+E0228: include_str!("./error_codes/E0228.md"),
 E0229: include_str!("./error_codes/E0229.md"),
 E0230: include_str!("./error_codes/E0230.md"),
 E0231: include_str!("./error_codes/E0231.md"),
@@ -482,7 +483,6 @@ E0753: include_str!("./error_codes/E0753.md"),
 //  E0218, // no associated type defined
 //  E0219, // associated type defined in higher-ranked supertrait
     E0227, // ambiguous lifetime bound, explicit lifetime bound required
-    E0228, // explicit lifetime bound required
 //  E0233,
 //  E0234,
 //  E0235, // structure constructor specifies a structure of type but

src/librustc_error_codes/error_codes/E0228.md

@@ -0,0 +1,40 @@
+The lifetime bound for this object type cannot be deduced from context and must
+be specified.
+
+Erroneous code example:
+
+```compile_fail,E0228
+trait Trait { }
+
+struct TwoBounds<'a, 'b, T: Sized + 'a + 'b> {
+    x: &'a i32,
+    y: &'b i32,
+    z: T,
+}
+
+type Foo<'a, 'b> = TwoBounds<'a, 'b, dyn Trait>;
+```
+
+When a trait object is used as a type argument of a generic type, Rust will try
+to infer its lifetime if unspecified. However, this isn't possible when the
+containing type has more than one lifetime bound.
+
+The above example can be resolved by either reducing the number of lifetime
+bounds to one or by making the trait object lifetime explicit, like so:
+
+```
+trait Trait { }
+
+struct TwoBounds<'a, 'b, T: Sized + 'a + 'b> {
+    x: &'a i32,
+    y: &'b i32,
+    z: T,
+}
+
+type Foo<'a, 'b> = TwoBounds<'a, 'b, dyn Trait + 'b>;
+```
+
+For more information, see [RFC 599] and its amendment [RFC 1156].
+
+[RFC 599]: https://github.com/rust-lang/rfcs/blob/master/text/0599-default-object-bound.md
+[RFC 1156]: https://github.com/rust-lang/rfcs/blob/master/text/1156-adjust-default-object-bounds.md

src/test/ui/object-lifetime/object-lifetime-default-ambiguous.stderr

@@ -18,3 +18,4 @@ LL | fn f(t: &Ref2<dyn Test>) {
 
 error: aborting due to 3 previous errors
 
+For more information about this error, try `rustc --explain E0228`.

src/test/ui/object-lifetime/object-lifetime-default-dyn-binding-nonstatic1.stderr

@@ -6,3 +6,4 @@ LL | fn bar<'a>(x: &'a str) -> &'a dyn Foo<'a, Item = dyn Bar> { &() }
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0228`.

src/test/ui/object-lifetime/object-lifetime-default-dyn-binding-nonstatic2.stderr

@@ -6,3 +6,4 @@ LL | fn bar<'a>(x: &'a str) -> &'a dyn Foo<'a, Item = dyn Bar> { &() }
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0228`.

src/test/ui/object-lifetime/object-lifetime-default-dyn-binding-nonstatic3.stderr

@@ -6,3 +6,4 @@ LL | fn bar(x: &str) -> &dyn Foo<Item = dyn Bar> { &() }
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0228`.

src/test/ui/suggestions/missing-lifetime-specifier.stderr

@@ -252,5 +252,5 @@ LL |     static f: RefCell<HashMap<i32, Vec<Vec<&Tar<'static, i32>>>>> = RefCell
 
 error: aborting due to 28 previous errors
 
-Some errors have detailed explanations: E0106, E0107.
+Some errors have detailed explanations: E0106, E0107, E0228.
 For more information about an error, try `rustc --explain E0106`.

src/test/ui/underscore-lifetime/dyn-trait-underscore-in-struct.stderr

@@ -18,4 +18,5 @@ LL |     x: Box<dyn Debug + '_>,
 
 error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0106`.
+Some errors have detailed explanations: E0106, E0228.
+For more information about an error, try `rustc --explain E0106`.