rust: Pick up std feature by default for x509 and tls crate

It could be a major footgun to commit to no_std by default on these
crates, given that callbacks could still panic-unwind into C frames.

- If a user wants to use these crates in `no_std`, it would be better for
them to explicitly opt-out and install their own panic handler.
- It is also very rare to have any use for these crates in `no_std`.

Signed-off-by: Xiangfei Ding <xfding@google.com>
Change-Id: Id05c90822774da711dbfe9649fbb51026a6a6964
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/90747
Reviewed-by: Adam Langley <agl@google.com>
Reviewed-by: Rudolf Polzer <rpolzer@google.com>

rust/bssl-tls/Cargo.toml

diff --git a/rust/bssl-tls/Cargo.toml b/rust/bssl-tls/Cargo.toml
index 5271a13..7482a7f 100644
--- a/rust/bssl-tls/Cargo.toml
+++ b/rust/bssl-tls/Cargo.toml
@@ -16,7 +16,7 @@
 tracing-subscriber = "0.3"
 
 [features]
-default = []
+default = ["std"]
 # `std` depends on the Rust `std` crate, but adds some useful trait impls if
 # available.
 std = ["bssl-crypto/std"]

rust/bssl-x509/Cargo.toml

diff --git a/rust/bssl-x509/Cargo.toml b/rust/bssl-x509/Cargo.toml
index 85a0166..a84c9e3 100644
--- a/rust/bssl-x509/Cargo.toml
+++ b/rust/bssl-x509/Cargo.toml
@@ -16,7 +16,7 @@
 path = "../bssl-sys"
 
 [features]
-default = []
+default = ["std"]
 # `std` depends on the Rust `std` crate, but adds some useful trait impls if
 # available.
 std = ["bssl-crypto/std"]