Add no_std crate support

.github/workflows/ci.yml

@@ -18,3 +18,7 @@ jobs:
       - run: cargo clippy --all-features -- --deny warnings
       - run: cargo test --all-features
       - run: cargo bench --all-features
+      # For no_std we use check instead of test because
+      # std is required for the test suite to run.
+      - run: cargo clippy --no-default-features -- --deny warnings
+      - run: cargo check --no-default-features

Cargo.toml

@@ -9,12 +9,33 @@ readme = "README.md"
 repository = "https://github.com/jcreekmore/pem-rs.git"
 version = "1.1.2-alpha.0"
 categories = [ "cryptography" ]
+keywords = [
+    "no-std",
+    "no_std",
+    "pem",
+]
 edition = "2021"
 rust-version = "1.67.0"
 
-[dependencies]
-base64 = "0.21.0"
-serde = { version = "1", optional = true, features = ["serde_derive"] }
+[features]
+default = ["std"]
+std = [
+    "base64/std",
+    # enable serde's std feature iff the serde and std features are both activated
+    "serde?/std",
+]
+serde = ["dep:serde"]
+
+[dependencies.base64]
+version = "0.21.0"
+default-features = false
+features = ["alloc"]
+
+[dependencies.serde]
+version = "1"
+optional = true
+default-features = false
+features = ["serde_derive"]
 
 [dev-dependencies]
 criterion = "0.3.0"

src/errors.rs

@@ -3,8 +3,13 @@
 // Licensed under the MIT license <LICENSE.md or
 // http://opensource.org/licenses/MIT>. This file may not be
 // copied, modified, or distributed except according to those terms.
+use core::fmt;
+
+#[cfg(any(feature = "std", test))]
 use std::error::Error;
-use std::fmt;
+
+#[cfg(not(any(feature = "std", test)))]
+use alloc::string::String;
 
 /// The `pem` error type.
 #[derive(Debug, Eq, PartialEq)]
@@ -16,7 +21,7 @@ pub enum PemError {
     MissingEndTag,
     MissingData,
     InvalidData(::base64::DecodeError),
-    NotUtf8(::std::str::Utf8Error),
+    NotUtf8(::core::str::Utf8Error),
 }
 
 impl fmt::Display for PemError {
@@ -35,6 +40,7 @@ impl fmt::Display for PemError {
     }
 }
 
+#[cfg(any(feature = "std", test))]
 impl Error for PemError {
     fn source(&self) -> Option<&(dyn Error + 'static)> {
         match self {
@@ -48,4 +54,4 @@ impl Error for PemError {
 }
 
 /// The `pem` result type.
-pub type Result<T> = ::std::result::Result<T, PemError>;
+pub type Result<T> = ::core::result::Result<T, PemError>;

src/lib.rs

@@ -92,6 +92,17 @@
 //!  assert_eq!(pems[0].tag(), "INTERMEDIATE CERT");
 //!  assert_eq!(pems[1].tag(), "CERTIFICATE");
 //! ```
+//!
+//! # Features
+//!
+//! This crate supports two features: `std` and `serde`.
+//!
+//! The `std` feature is enabled by default. If you specify
+//! `default-features = false` to disable `std`, be aware that
+//! this crate still needs an allocator.
+//!
+//! The `serde` feature implements `serde::{Deserialize, Serialize}`
+//! for this crate's `Pem` struct.
 
 #![recursion_limit = "1024"]
 #![deny(
@@ -105,15 +116,24 @@
     unused_import_braces,
     unused_qualifications
 )]
+#![cfg_attr(not(feature = "std"), no_std)]
+
+#[cfg(not(any(feature = "std", test)))]
+extern crate alloc;
+#[cfg(not(any(feature = "std", test)))]
+use alloc::{
+    format,
+    string::{String, ToString},
+    vec::Vec,
+};
 
 mod errors;
 mod parser;
 use parser::{parse_captures, parse_captures_iter, Captures};
 
 pub use crate::errors::{PemError, Result};
 use base64::Engine as _;
-use std::fmt;
-use std::str;
+use core::{fmt, str};
 
 /// The line length for PEM encoding
 const LINE_WRAP: usize = 64;