chore: improve no_std maintainability:

Similar to how `rustls` does it:

https://github.com/rustls/rustls/blob/513e374b2e2ce9f1fb57ac78ab3ca053afc8f133/rustls/src/lib.rs#L353-L359

> ... `extern crate` plus the `#![no_std]` attribute changes the
default prelude from `std::prelude` to `core::prelude`. That forces
one to _explicitly_ import (`use`) everything that is in `std::prelude`
but not in `core::prelude`. This helps maintain no-std support as even
developers that are not interested in, or aware of, no-std support and /
or that never run `cargo build --no-default-features` locally will get
errors when they rely on `std::prelude` API.

prost-types/Cargo.toml

@@ -20,10 +20,10 @@ doctest = false
 
 [features]
 default = ["std"]
-std = ["prost/std"]
+std = ["prost/std", "proptest/std"]
 
 [dependencies]
 prost = { version = "0.12.4", path = "../prost", default-features = false, features = ["prost-derive"] }
 
 [dev-dependencies]
-proptest = "1"
+proptest = { version = "1", default-features = false }

prost-types/src/datetime.rs

@@ -1,6 +1,5 @@
 //! A date/time type which exists primarily to convert [`Timestamp`]s into an RFC 3339 formatted
 //! string.
-
 use core::fmt;
 
 use crate::Duration;
@@ -583,9 +582,12 @@ impl From<DateTime> for Timestamp {
 
 #[cfg(test)]
 mod tests {
-    use super::*;
+    use alloc::{format, string::ToString};
+
     use proptest::prelude::*;
 
+    use super::*;
+
     #[test]
     fn test_min_max() {
         assert_eq!(
@@ -604,8 +606,8 @@ mod tests {
         );
     }
 
-    #[cfg(feature = "std")]
     #[test]
+    #[cfg(feature = "std")]
     fn test_datetime_from_timestamp() {
         let case = |expected: &str, secs: i64, nanos: i32| {
             let timestamp = Timestamp {

prost-types/src/lib.rs

@@ -1,5 +1,3 @@
-#![doc(html_root_url = "https://docs.rs/prost-types/0.12.2")]
-
 //! Protocol Buffers well-known types.
 //!
 //! Note that the documentation for the types defined in this crate are generated from the Protobuf
@@ -12,7 +10,14 @@
 //!
 //! [1]: https://developers.google.com/protocol-buffers/docs/reference/google.protobuf
 
-#![cfg_attr(not(feature = "std"), no_std)]
+#![doc(html_root_url = "https://docs.rs/prost-types/0.12.2")]
+#![no_std]
+
+// See: https://github.com/tokio-rs/prost/pull/1047
+#[cfg(any(feature = "std", test))]
+extern crate std;
+
+extern crate alloc;
 
 #[rustfmt::skip]
 pub mod compiler;

prost/Cargo.toml

@@ -26,7 +26,7 @@ default = ["derive", "std"]
 derive = ["dep:prost-derive"]
 prost-derive = ["derive"]     # deprecated, please use derive feature instead
 no-recursion-limit = []
-std = []
+std = ["proptest/std"]
 
 [dependencies]
 bytes = { version = "1", default-features = false }
@@ -36,7 +36,7 @@ prost-derive = { version = "0.12.4", path = "../prost-derive", optional = true }
 criterion = { version = "0.4", default-features = false }
 env_logger = { version = "0.10", default-features = false }
 log = "0.4"
-proptest = "1"
+proptest = { version = "1", default-features = false }
 rand = "0.8"
 
 [[bench]]

prost/src/encoding.rs

@@ -1422,7 +1422,6 @@ pub mod btree_map {
 
 #[cfg(test)]
 mod test {
-    #[cfg(not(feature = "std"))]
     use alloc::string::ToString;
     use core::borrow::Borrow;
     use core::fmt::Debug;

prost/src/error.rs

@@ -1,9 +1,7 @@
 //! Protobuf encoding and decoding errors.
 
 use alloc::borrow::Cow;
-#[cfg(not(feature = "std"))]
 use alloc::boxed::Box;
-#[cfg(not(feature = "std"))]
 use alloc::vec::Vec;
 
 use core::fmt;

prost/src/lib.rs

@@ -1,6 +1,10 @@
 #![doc(html_root_url = "https://docs.rs/prost/0.12.2")]
-#![cfg_attr(not(feature = "std"), no_std)]
 #![doc = include_str!("../../README.md")]
+#![no_std]
+
+// See: https://github.com/tokio-rs/prost/pull/1047
+#[cfg(any(feature = "std", test))]
+extern crate std;
 
 // Re-export the alloc crate for use within derived code.
 #[doc(hidden)]

prost/src/message.rs

@@ -1,6 +1,4 @@
-#[cfg(not(feature = "std"))]
 use alloc::boxed::Box;
-#[cfg(not(feature = "std"))]
 use alloc::vec::Vec;
 
 use core::fmt::Debug;

prost/src/name.rs

@@ -2,7 +2,6 @@
 
 use crate::Message;
 
-#[cfg(not(feature = "std"))]
 use alloc::{format, string::String};
 
 /// Associate a type name with a [`Message`] type.

prost/src/types.rs

@@ -5,9 +5,7 @@
 //! the `prost-types` crate in order to avoid a cyclic dependency between `prost` and
 //! `prost-build`.
 
-#[cfg(not(feature = "std"))]
 use alloc::string::String;
-#[cfg(not(feature = "std"))]
 use alloc::vec::Vec;
 
 use ::bytes::{Buf, BufMut, Bytes};

tests/src/bootstrap.rs

@@ -7,6 +7,7 @@ use std::io::Read;
 use std::io::Write;
 use std::path::Path;
 use std::path::PathBuf;
+use std::string::String;
 
 /// Test which bootstraps protobuf.rs and compiler.rs from the .proto definitions in the Protobuf
 /// repo. Ensures that the checked-in compiled versions are up-to-date.

tests/src/debug.rs

@@ -3,9 +3,8 @@
 //! The tests check against expected output. This may be a bit fragile, but it is likely OK for
 //! actual use.
 
-use prost::alloc::format;
-#[cfg(not(feature = "std"))]
-use prost::alloc::string::String;
+use ::alloc::format;
+use ::alloc::string::String;
 
 // Borrow some types from other places.
 #[cfg(feature = "std")]
@@ -16,6 +15,8 @@ use crate::message_encoding::BasicEnumeration;
 #[test]
 #[cfg(feature = "std")]
 fn basic() {
+    use std::string::ToString;
+
     let mut basic = Basic::default();
     assert_eq!(
         format!("{:?}", basic),

tests/src/deprecated_field.rs

@@ -1,4 +1,3 @@
-#[cfg(not(feature = "std"))]
 use alloc::string::ToString;
 
 mod deprecated_field {

tests/src/lib.rs

@@ -3,7 +3,11 @@
     clippy::module_inception,
     clippy::unreadable_literal
 )]
-#![cfg_attr(not(feature = "std"), no_std)]
+#![no_std]
+
+// See: https://github.com/tokio-rs/prost/pull/1036
+#[cfg(any(feature = "std", test))]
+extern crate std;
 
 #[macro_use]
 extern crate cfg_if;
@@ -14,7 +18,6 @@ cfg_if! {
     if #[cfg(feature = "edition-2015")] {
         extern crate anyhow;
         extern crate bytes;
-        extern crate core;
         extern crate prost;
         extern crate prost_types;
         extern crate protobuf;
@@ -140,7 +143,6 @@ pub mod default_string_escape {
     include!(concat!(env!("OUT_DIR"), "/default_string_escape.rs"));
 }
 
-#[cfg(not(feature = "std"))]
 use alloc::vec::Vec;
 
 use anyhow::anyhow;
@@ -280,10 +282,8 @@ where
 
 #[cfg(test)]
 mod tests {
-
     use alloc::collections::{BTreeMap, BTreeSet};
     use alloc::vec;
-    #[cfg(not(feature = "std"))]
     use alloc::{borrow::ToOwned, boxed::Box, string::ToString};
 
     use super::*;

tests/src/message_encoding.rs

@@ -1,6 +1,5 @@
-use prost::alloc::vec;
-#[cfg(not(feature = "std"))]
-use prost::alloc::{borrow::ToOwned, string::String, vec::Vec};
+use alloc::vec;
+use alloc::{borrow::ToOwned, string::String, vec::Vec};
 
 use prost::bytes::Bytes;
 use prost::{Enumeration, Message, Oneof};

tests/src/skip_debug.rs

@@ -1,10 +1,7 @@
 //! Tests for skipping the default Debug implementation.
 
-use std::fmt;
-
-use prost::alloc::format;
-#[cfg(not(feature = "std"))]
-use prost::alloc::string::String;
+use alloc::string::{String, ToString};
+use alloc::{fmt, format};
 
 use crate::custom_debug::{msg, AnEnum, Msg};
 use crate::message_encoding::BasicEnumeration;