docs(error-codes): Add long error explanation for E0227

compiler/rustc_error_codes/src/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"),
+E0227: include_str!("./error_codes/E0227.md"),
 E0228: include_str!("./error_codes/E0228.md"),
 E0229: include_str!("./error_codes/E0229.md"),
 E0230: include_str!("./error_codes/E0230.md"),
@@ -530,7 +531,6 @@ E0786: include_str!("./error_codes/E0786.md"),
 //  E0217, // ambiguous associated type, defined in multiple supertraits
 //  E0218, // no associated type defined
 //  E0219, // associated type defined in higher-ranked supertrait
-    E0227, // ambiguous lifetime bound, explicit lifetime bound required
 //  E0233,
 //  E0234,
 //  E0235, // structure constructor specifies a structure of type but

compiler/rustc_error_codes/src/error_codes/E0227.md

@@ -0,0 +1,33 @@
+This error indicates that the compiler is unable to determine whether there is
+exactly one unique region in the set of derived region bounds.
+
+Example of erroneous code:
+
+```compile_fail,E0227
+trait Foo<'foo>: 'foo {}
+trait Bar<'bar>: 'bar {}
+
+trait FooBar<'foo, 'bar>: Foo<'foo> + Bar<'bar> {}
+
+struct Baz<'foo, 'bar> {
+    baz: dyn FooBar<'foo, 'bar>,
+}
+```
+
+Here, `baz` can have either `'foo` or `'bar` lifetimes.
+
+To resolve this error, provide an explicit lifetime:
+
+```rust
+trait Foo<'foo>: 'foo {}
+trait Bar<'bar>: 'bar {}
+
+trait FooBar<'foo, 'bar>: Foo<'foo> + Bar<'bar> {}
+
+struct Baz<'foo, 'bar, 'baz>
+where
+    'baz: 'foo + 'bar,
+{
+    obj: dyn FooBar<'foo, 'bar> + 'baz,
+}
+```

src/test/ui/error-codes/E0227.rs

@@ -0,0 +1,12 @@
+trait Foo<'foo>: 'foo {}
+trait Bar<'bar>: 'bar {}
+
+trait FooBar<'foo, 'bar>: Foo<'foo> + Bar<'bar> {}
+
+struct Baz<'foo, 'bar> {
+    baz: dyn FooBar<'foo, 'bar>,
+    //~^ ERROR ambiguous lifetime bound, explicit lifetime bound required
+}
+
+fn main() {
+}

src/test/ui/error-codes/E0227.stderr

@@ -0,0 +1,9 @@
+error[E0227]: ambiguous lifetime bound, explicit lifetime bound required
+  --> $DIR/E0227.rs:7:10
+   |
+LL |     baz: dyn FooBar<'foo, 'bar>,
+   |          ^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0227`.

src/tools/tidy/src/error_codes_check.rs

@@ -10,8 +10,8 @@ use regex::Regex;
 
 // A few of those error codes can't be tested but all the others can and *should* be tested!
 const EXEMPTED_FROM_TEST: &[&str] = &[
-    "E0227", "E0279", "E0280", "E0313", "E0377", "E0461", "E0462", "E0465", "E0476", "E0514",
-    "E0519", "E0523", "E0554", "E0640", "E0717", "E0729",
+    "E0279", "E0280", "E0313", "E0377", "E0461", "E0462", "E0465", "E0476", "E0514", "E0519",
+    "E0523", "E0554", "E0640", "E0717", "E0729",
 ];
 
 // Some error codes don't have any tests apparently...