rust: Add Follow with a different lifetime

This adds a FollowWith trait, which allows accessing a table type with a
different lifetime in generic code. This is necessary for writing
generic containers that own flatbuffers.

I'm using this (with a flatbuffers fork that already includes this
change) at
https://github.com/frc971/971-Robot-Code/blob/f4cae52cf7d990572e157ced324aee43cf89c523/aos/flatbuffers.rs.
I would be happy to move this to flatbuffers itself if there's
interested, but I think generating the small amount of code in this
commit to allow writing wrappers like that is independently valuable.

I think this is what was discussed in #5749, for
reference.

Author: bsilver8192

rust/flatbuffers/src/follow.rs

@@ -53,3 +53,7 @@ impl<'a, T: Follow<'a>> Follow<'a> for FollowStart<T> {
         T::follow(buf, loc)
     }
 }
+
+pub trait FollowWith<'a> {
+    type Inner: Follow<'a>;
+}

rust/flatbuffers/src/lib.rs

@@ -51,7 +51,7 @@ pub use crate::builder::FlatBufferBuilder;
 pub use crate::endian_scalar::{
     byte_swap_f32, byte_swap_f64, emplace_scalar, read_scalar, read_scalar_at, EndianScalar,
 };
-pub use crate::follow::{Follow, FollowStart};
+pub use crate::follow::{Follow, FollowStart, FollowWith};
 pub use crate::primitives::*;
 pub use crate::push::Push;
 pub use crate::table::{buffer_has_identifier, Table};

src/idl_gen_rust.cpp

@@ -1655,6 +1655,10 @@ class RustGenerator : public BaseGenerator {
     code_ += "  }";
     code_ += "}";
     code_ += "";
+    code_ += "impl<'a, 'b> flatbuffers::FollowWith<'a> for {{STRUCT_TY}}<'b> {";
+    code_ += "  type Inner = {{STRUCT_TY}}<'a>;";
+    code_ += "}";
+    code_ += "";
     code_ += "impl<'a> {{STRUCT_TY}}<'a> {";
 
     // Generate field id constants.