Clean up no_std and use of std in test code

This fixes a wedge of instances of:

```
warning: the item `String` is imported redundantly
  --> rustls/src/msgs/handshake.rs:27:5
   |
27 | use alloc::string::String;
   |     ^^^^^^^^^^^^^^^^^^^^^
```

Where `String` is present from the std prelude when built
for testing.  Like we just did in webpki, _always_ opt-in
to no_std, and then import the std prelude in tests where
necessary.

rustls/src/bs_debug.rs

@@ -42,6 +42,8 @@ impl<'a> fmt::Debug for BsDebug<'a> {
 #[cfg(test)]
 mod tests {
     use super::BsDebug;
+    use std::format;
+    use std::prelude::v1::*;
 
     #[test]
     fn debug() {

rustls/src/client/handy.rs

@@ -229,6 +229,7 @@ mod tests {
     use crate::msgs::persist::Tls13ClientSessionValue;
     use crate::suites::SupportedCipherSuite;
     use crate::test_provider::cipher_suite;
+    use alloc::vec::Vec;
 
     use pki_types::{ServerName, UnixTime};
 

rustls/src/crypto/ring/kx.rs

@@ -103,6 +103,8 @@ impl ActiveKeyExchange for KeyExchange {
 
 #[cfg(test)]
 mod tests {
+    use std::format;
+
     #[test]
     fn kxgroup_fmt_yields_name() {
         assert_eq!("X25519", format!("{:?}", super::X25519));

rustls/src/crypto/ring/quic.rs

@@ -187,6 +187,7 @@ mod tests {
     use crate::test_provider::tls13::{
         TLS13_AES_128_GCM_SHA256_INTERNAL, TLS13_CHACHA20_POLY1305_SHA256_INTERNAL,
     };
+    use std::dbg;
 
     fn test_short_packet(version: Version, expected: &[u8]) {
         const PN: u64 = 654360564;

rustls/src/crypto/tls13.rs

@@ -243,6 +243,7 @@ pub struct OutputLengthError;
 mod tests {
     use super::{expand, Hkdf, HkdfUsingHmac};
     use crate::test_provider::hmac;
+    use std::prelude::v1::*;
 
     struct ByteArray<const N: usize>([u8; N]);
 

rustls/src/error.rs

@@ -580,6 +580,8 @@ impl StdError for OtherError {
 mod tests {
     use super::{Error, InvalidMessage};
     use crate::error::{CertRevocationListError, OtherError};
+    use std::prelude::v1::*;
+    use std::{println, vec};
 
     #[test]
     fn certificate_error_equality() {

rustls/src/lib.rs

@@ -318,7 +318,7 @@
 #![cfg_attr(read_buf, feature(read_buf))]
 #![cfg_attr(read_buf, feature(core_io_borrowed_buf))]
 #![cfg_attr(bench, feature(test))]
-#![cfg_attr(not(test), no_std)]
+#![no_std]
 
 extern crate alloc;
 // This `extern crate` plus the `#![no_std]` attribute changes the default prelude from
@@ -326,7 +326,6 @@ extern crate alloc;
 // 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.
-#[cfg(not(test))]
 extern crate std;
 
 // Import `test` sysroot crate for `Bencher` definitions.

rustls/src/limited_cache.rs

@@ -117,6 +117,8 @@ where
 
 #[cfg(test)]
 mod tests {
+    use std::prelude::v1::*;
+
     type Test = super::LimitedCache<String, usize>;
 
     #[test]

rustls/src/msgs/codec.rs

@@ -317,6 +317,8 @@ impl<'a> Drop for LengthPrefixedBuffer<'a> {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use std::prelude::v1::*;
+    use std::vec;
 
     #[test]
     fn interrupted_length_prefixed_buffer_leaves_maximum_length() {

rustls/src/msgs/deframer.rs

@@ -577,7 +577,8 @@ const READ_SIZE: usize = 4096;
 
 #[cfg(test)]
 mod tests {
-    use std::io;
+    use std::prelude::v1::*;
+    use std::vec;
 
     use crate::msgs::message::Message;
 

rustls/src/msgs/enums.rs

@@ -339,7 +339,7 @@ pub(crate) mod tests {
     //! check panic-safety of relatively unused values.
 
     use super::*;
-    use crate::msgs::codec::Codec;
+    use std::prelude::v1::*;
 
     #[test]
     fn test_enums() {

rustls/src/msgs/fragmenter.rs

@@ -72,6 +72,7 @@ mod tests {
     use crate::enums::ProtocolVersion;
     use crate::msgs::base::Payload;
     use crate::msgs::message::{BorrowedPlainMessage, PlainMessage};
+    use std::prelude::v1::*;
 
     fn msg_eq(
         m: &BorrowedPlainMessage,

rustls/src/msgs/handshake_test.rs

@@ -21,6 +21,9 @@ use crate::verify::DigitallySignedStruct;
 
 use pki_types::{CertificateDer, DnsName};
 
+use std::prelude::v1::*;
+use std::{format, println, vec};
+
 #[test]
 fn rejects_short_random() {
     let bytes = [0x01; 31];

rustls/src/msgs/message_test.rs

@@ -6,9 +6,10 @@ use super::codec::Reader;
 use super::enums::AlertLevel;
 use super::message::{Message, OpaqueMessage, PlainMessage};
 
-use std::fs;
 use std::io::Read;
 use std::path::{Path, PathBuf};
+use std::prelude::v1::*;
+use std::{format, fs, println, vec};
 
 #[test]
 fn test_read_fuzz_corpus() {

rustls/src/msgs/persist.rs

@@ -411,6 +411,7 @@ mod tests {
 
     #[test]
     fn serversessionvalue_is_debug() {
+        use std::{println, vec};
         let ssv = ServerSessionValue::new(
             None,
             ProtocolVersion::TLSv1_3,

rustls/src/quic.rs

@@ -897,9 +897,9 @@ impl Default for Version {
 
 #[cfg(test)]
 mod tests {
-    use crate::quic::HeaderProtectionKey;
-
     use super::PacketKey;
+    use crate::quic::HeaderProtectionKey;
+    use std::prelude::v1::*;
 
     #[test]
     fn auto_traits() {

rustls/src/record_layer.rs

@@ -234,6 +234,7 @@ pub(crate) struct Decrypted {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use std::vec;
 
     #[test]
     fn test_has_decrypted() {

rustls/src/server/handy.rs

@@ -207,6 +207,7 @@ mod tests {
     use crate::server::ProducesTickets;
     use crate::server::ResolvesServerCert;
     use crate::server::StoresServerSessions;
+    use std::vec;
 
     #[test]
     fn test_noserversessionstorage_drops_put() {

rustls/src/server/server_conn.rs

@@ -866,6 +866,7 @@ impl crate::conn::SideData for ServerConnectionData {}
 #[cfg(test)]
 mod tests {
     use super::*;
+    use std::format;
 
     // these branches not reachable externally, unless something else goes wrong.
     #[test]

rustls/src/suites.rs

@@ -252,6 +252,7 @@ mod tests {
     use super::*;
     use crate::enums::CipherSuite;
     use crate::test_provider::tls13::*;
+    use std::{println, vec};
 
     #[test]
     fn test_client_pref() {

rustls/src/tls13/key_schedule.rs

@@ -831,6 +831,8 @@ where
 #[cfg(all(test, any(feature = "ring", feature = "aws_lc_rs")))]
 mod tests {
     use core::fmt::Debug;
+    use std::prelude::v1::*;
+    use std::vec;
 
     use super::{derive_traffic_iv, derive_traffic_key, KeySchedule, SecretKind};
     use crate::test_provider::ring_like::aead;

rustls/src/verify.rs

@@ -337,6 +337,8 @@ impl Codec for DigitallySignedStruct {
 
 #[test]
 fn assertions_are_debug() {
+    use std::format;
+
     assert_eq!(
         format!("{:?}", ClientCertVerified::assertion()),
         "ClientCertVerified(())"

rustls/src/verifybench.rs

@@ -7,6 +7,8 @@
 #![cfg(feature = "ring")]
 
 use core::time::Duration;
+use std::prelude::v1::*;
+use std::println;
 use std::time::Instant;
 
 use crate::crypto::ring;

rustls/src/webpki/client_verifier.rs

@@ -413,7 +413,9 @@ mod tests {
 
     use pki_types::{CertificateDer, CertificateRevocationListDer};
 
+    use std::prelude::v1::*;
     use std::sync::Arc;
+    use std::{format, println, vec};
 
     fn load_crls(crls_der: &[&[u8]]) -> Vec<CertificateRevocationListDer<'static>> {
         crls_der

rustls/src/webpki/server_verifier.rs

@@ -282,7 +282,9 @@ impl ServerCertVerifier for WebPkiServerVerifier {
 
 #[cfg(all(test, any(feature = "ring", feature = "aws_lc_rs")))]
 mod tests {
+    use std::prelude::v1::*;
     use std::sync::Arc;
+    use std::{println, vec};
 
     use pki_types::{CertificateDer, CertificateRevocationListDer};
 

rustls/src/webpki/verify.rs

@@ -220,6 +220,7 @@ pub(crate) fn verify_server_cert_signed_by_trust_anchor_impl(
 #[cfg(test)]
 mod tests {
     use super::*;
+    use std::format;
 
     #[test]
     fn certificate_debug() {

rustls/src/x509.rs

@@ -43,6 +43,7 @@ const DER_SEQUENCE_TAG: u8 = 0x30;
 #[cfg(test)]
 mod tests {
     use super::*;
+    use std::vec;
 
     #[test]
     fn test_empty() {