Document available error codes

Author: dhardy

rand_core/CHANGELOG.md

@@ -8,9 +8,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Bump MSRV to 1.36, various code improvements (#1011)
 - Update to getrandom v0.2 (#1041)
-- Fix: `next_u32_via_fill` and `next_u64_via_fill` now use LE as documented (#?)
+- Fix: `next_u32_via_fill` and `next_u64_via_fill` now use LE as documented (#1061)
 - Reduce usage of `unsafe` (#962, #963, #1011)
 - Annotate feature-gates in documentation (#1019)
+- Document available error codes (#1061)
 - Various documentation tweaks
 - Fix some clippy warnings (#1036)
 - Apply rustfmt (#926)

rand_core/src/error.rs

@@ -29,6 +29,9 @@ impl Error {
     /// Codes at or above this point can be used by users to define their own
     /// custom errors.
     ///
+    /// This has a fixed value of `(1 << 31) + (1 << 30) = 0xC000_0000`,
+    /// therefore the number of values available for custom codes is `1 << 30`.
+    ///
     /// This is identical to [`getrandom::Error::CUSTOM_START`](https://docs.rs/getrandom/latest/getrandom/struct.Error.html#associatedconstant.CUSTOM_START).
     pub const CUSTOM_START: u32 = (1 << 31) + (1 << 30);
     /// Codes below this point represent OS Errors (i.e. positive i32 values).