fixup ErrorCode::Io variant for feature unsealed_read_write

Fixes #134
This partially reverts commit a8be10e.

Signed-off-by: Arthur Gautier <baloo@gandi.net>

Author: Arthur Gautier

.travis.yml

@@ -18,3 +18,4 @@ script:
   - cargo build --no-default-features --target thumbv7em-none-eabihf # Test we can build a platform that does not have std.
   - cargo test --no-default-features --lib --tests # Run no_std tests
   - cargo build --features unsealed_read_write # The crate should still build when the unsealed_read_write feature is enabled.
+  - cargo build --no-default-features --features unsealed_read_write # The crate should still build when the unsealed_read_write feature is enabled and std disabled.

src/error.rs

@@ -130,6 +130,8 @@ impl Error {
             ErrorCode::Message => Category::Data,
             #[cfg(feature = "std")]
             ErrorCode::Io(_) => Category::Io,
+            #[cfg(not(feature = "std"))]
+            ErrorCode::Io => Category::Io,
             ErrorCode::ScratchTooSmall => Category::Io,
             ErrorCode::EofWhileParsingValue
             | ErrorCode::EofWhileParsingArray
@@ -267,6 +269,9 @@ pub(crate) enum ErrorCode {
     Message,
     #[cfg(feature = "std")]
     Io(io::Error),
+    #[allow(unused)]
+    #[cfg(not(feature = "std"))]
+    Io,
     ScratchTooSmall,
     EofWhileParsingValue,
     EofWhileParsingArray,
@@ -292,6 +297,8 @@ impl fmt::Display for ErrorCode {
             ErrorCode::Message => f.write_str("Unknown error"),
             #[cfg(feature = "std")]
             ErrorCode::Io(ref err) => fmt::Display::fmt(err, f),
+            #[cfg(not(feature = "std"))]
+            ErrorCode::Io => f.write_str("Unknown I/O error"),
             ErrorCode::ScratchTooSmall => f.write_str("Scratch buffer too small"),
             ErrorCode::EofWhileParsingValue => f.write_str("EOF while parsing a value"),
             ErrorCode::EofWhileParsingArray => f.write_str("EOF while parsing an array"),