Rust: remove extern crate declaration for std from generated code

Starting from the 2018 edition of Rust, paths starting with `::` must refer to
a crate, so when we refer to `::std` there is no risk of accidental collision
with another identifier.

Using `::std` instead of `::__std` is useful because it works even when the
generated code is in a module rather than directly in the crate root. This
allows us to remove one of the post-processing steps from the codegen crate.

PiperOrigin-RevId: 702858213

rust/release_crates/protobuf_codegen/src/lib.rs

@@ -144,8 +144,7 @@ impl CodeGen {
             .unwrap()
             .replace("crate::", "")
             .replace("protobuf_upb", "protobuf")
-            .replace("::__pb", "__pb")
-            .replace("::__std", "__std");
+            .replace("::__pb", "__pb");
         let mut file = OpenOptions::new().write(true).truncate(true).open(path).unwrap();
         file.write(contents.as_bytes()).unwrap();
     }

src/google/protobuf/compiler/rust/generator.cc

@@ -147,19 +147,18 @@ bool RustGenerator::Generate(const FileDescriptor* file,
 
   // Convenience shorthands for common symbols.
   auto v = ctx.printer().WithVars({
-      {"std", "::__std"},
+      {"std", "::std"},
       {"pb", "::__pb"},
       {"pbi", "::__pb::__internal"},
       {"pbr", "::__pb::__runtime"},
-      {"NonNull", "::__std::ptr::NonNull"},
-      {"Phantom", "::__std::marker::PhantomData"},
-      {"Result", "::__std::result::Result"},
-      {"Option", "::__std::option::Option"},
+      {"NonNull", "::std::ptr::NonNull"},
+      {"Phantom", "::std::marker::PhantomData"},
+      {"Result", "::std::result::Result"},
+      {"Option", "::std::option::Option"},
   });
 
   ctx.Emit({{"kernel", KernelRsName(ctx.opts().kernel)}}, R"rs(
     extern crate protobuf_$kernel$ as __pb;
-    extern crate std as __std;
 
   )rs");