Add Explanation For Error E0772

2 files changed, 90 insertions, 1 deletions

diff --git a/compiler/rustc_error_codes/src/error_codes.rs b/compiler/rustc_error_codes/src/error_codes.rs
index 724e3f7fed3..5dafbb26b83 100644
--- a/compiler/rustc_error_codes/src/error_codes.rs
+++ b/compiler/rustc_error_codes/src/error_codes.rs
@@ -471,6 +471,7 @@ E0768: include_str!("./error_codes/E0768.md"),
 E0769: include_str!("./error_codes/E0769.md"),
 E0770: include_str!("./error_codes/E0770.md"),
 E0771: include_str!("./error_codes/E0771.md"),
+E0772: include_str!("./error_codes/E0772.md"),
 E0773: include_str!("./error_codes/E0773.md"),
 E0774: include_str!("./error_codes/E0774.md"),
 E0775: include_str!("./error_codes/E0775.md"),
@@ -641,5 +642,4 @@ E0785: include_str!("./error_codes/E0785.md"),
 //  E0723, unstable feature in `const` context
     E0726, // non-explicit (not `'_`) elided lifetime in unsupported position
 //  E0738, // Removed; errored on `#[track_caller] fn`s in `extern "Rust" { ... }`.
-    E0772, // `'static' obligation coming from `impl dyn Trait {}` or `impl Foo for dyn Bar {}`.
 }
diff --git a/compiler/rustc_error_codes/src/error_codes/E0772.md b/compiler/rustc_error_codes/src/error_codes/E0772.md
new file mode 100644
index 00000000000..262e52351ef
--- /dev/null
+++ b/compiler/rustc_error_codes/src/error_codes/E0772.md
@@ -0,0 +1,89 @@
+A trait object has some specific lifetime `'1`, but it was used in a way that
+requires it to have a `'static` lifetime.
+
+Example of erroneous code:
+
+```compile_fail,E0772
+trait BooleanLike {}
+trait Person {}
+
+impl BooleanLike for bool {}
+
+impl dyn Person {
+    fn is_cool(&self) -> bool {
+        // hey you, you're pretty cool
+        true
+    }
+}
+
+fn get_is_cool<'p>(person: &'p dyn Person) -> impl BooleanLike {
+    // error: `person` has an anonymous lifetime `'p` but calling
+    //        `print_cool_fn` introduces an implicit `'static` lifetime
+    //        requirement
+    person.is_cool()
+}
+```
+
+The trait object `person` in the function `get_is_cool`, while already being
+behind a reference with lifetime `'p`, also has it's own implicit lifetime,
+`'2`.
+
+Lifetime `'2` represents the data the trait object might hold inside, for
+example:
+
+```
+trait MyTrait {}
+
+struct MyStruct<'a>(&'a i32);
+
+impl<'a> MyTrait for MyStruct<'a> {}
+```
+
+With this scenario, if a trait object of `dyn MyTrait + '2` was made from
+`MyStruct<'a>`, `'a` must live as long, if not longer than `'2`. This allows the
+trait object's internal data to be accessed safely from any trait methods. This
+rule also goes for any lifetime any struct made into a trait object may have.
+
+In the implementation for `dyn Person`, the `'2` lifetime representing the
+internal data was ommitted, meaning that the compiler inferred the lifetime
+`'static`. As a result, the implementation's `is_cool` is inferred by the
+compiler to look like this:
+
+```
+# trait Person {}
+#
+# impl dyn Person {
+fn is_cool<'a>(self: &'a (dyn Person + 'static)) -> bool {unimplemented!()}
+# }
+```
+
+While the `get_is_cool` function is inferred to look like this:
+
+```
+# trait Person {}
+# trait BooleanLike {}
+#
+fn get_is_cool<'p, R: BooleanLike>(person: &'p (dyn Person + 'p)) -> R {
+    unimplemented!()
+}
+```
+
+Which brings us to the core of the problem; the assignment of type
+`&'_ (dyn Person + '_)` to type `&'_ (dyn Person + 'static)` is impossible.
+
+Fixing it is as simple as being generic over lifetime `'2`, as to prevent the
+compiler from inferring it as `'static`:
+
+```
+# trait Person {}
+#
+impl<'d> dyn Person + 'd {/* ... */}
+
+// This works too, and is more elegant:
+//impl dyn Person + '_ {/* ... */}
+```
+
+See the [Rust Reference on Trait Object Lifetime Bounds][trait-objects] for
+more information on trait object lifetimes.
+
+[trait-object-lifetime-bounds]: https://doc.rust-lang.org/reference/types/trait-object.html#trait-object-lifetime-bounds