docs: add long error explanation for error E0320

Author: Ezrashaw

compiler/rustc_error_codes/src/error_codes.rs

@@ -163,6 +163,7 @@ E0311: include_str!("./error_codes/E0311.md"),
 E0312: include_str!("./error_codes/E0312.md"),
 E0316: include_str!("./error_codes/E0316.md"),
 E0317: include_str!("./error_codes/E0317.md"),
+E0320: include_str!("./error_codes/E0320.md"),
 E0321: include_str!("./error_codes/E0321.md"),
 E0322: include_str!("./error_codes/E0322.md"),
 E0323: include_str!("./error_codes/E0323.md"),
@@ -575,7 +576,6 @@ E0791: include_str!("./error_codes/E0791.md"),
 //  E0314, // closure outlives stack frame
 //  E0315, // cannot invoke closure outside of its lifetime
 //  E0319, // trait impls for defaulted traits allowed just for structs/enums
-    E0320, // recursive overflow during dropck
 //  E0372, // coherence not object safe
     E0377, // the trait `CoerceUnsized` may only be implemented for a coercion
            // between structures with the same definition

compiler/rustc_error_codes/src/error_codes/E0320.md

@@ -0,0 +1,27 @@
+Recursion limit reached while creating drop-check rules.
+
+Example of erroneous code:
+
+```compile_fail,E0320
+enum A<T> {
+    B,
+    C(T, Box<A<(T, T)>>)
+}
+
+fn foo<T>() {
+    A::<T>::B; // error: overflow while adding drop-check rules for A<T>
+}
+```
+
+The Rust compiler must be able to reason about how a type is [`Drop`]ped, and
+by extension the types of its fields, to be able to generate the glue to
+properly drop a value. The code example above shows a type where this inference
+is impossible because it is recursive. Note that this is *not* the same as
+[E0072](E0072.html), where a type has an infinite size; the type here has a
+finite size but any attempt to `Drop` it would recurse infinitely. For more
+information, read [the `Drop` docs](../std/ops/trait.Drop.html).
+
+It is not possible to define a type with recursive drop-check rules. All such
+recursion must be removed.
+
+[`Drop`]: ../std/ops/trait.Drop.html

src/test/ui/dropck/dropck_no_diverge_on_nonregular_1.stderr

@@ -8,3 +8,4 @@ LL |     let ft =
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0320`.

src/test/ui/dropck/dropck_no_diverge_on_nonregular_2.stderr

@@ -8,3 +8,4 @@ LL |     let ft =
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0320`.

src/test/ui/dropck/dropck_no_diverge_on_nonregular_3.stderr

@@ -16,3 +16,4 @@ LL |         Some(Wrapper::Simple::<u32>);
 
 error: aborting due to 2 previous errors
 
+For more information about this error, try `rustc --explain E0320`.

src/test/ui/recursion/issue-38591-non-regular-dropck-recursion.stderr

@@ -8,3 +8,4 @@ LL | fn f(x: S<u32>) {}
 
 error: aborting due to previous error
 
+For more information about this error, try `rustc --explain E0320`.