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/*! A streamable value. */ mod impls; #[cfg(feature = "std")] pub(crate) mod owned; pub use crate::stream::RefMutStream as Stream; #[cfg(feature = "std")] pub use self::owned::OwnedValue; #[doc(inline)] pub use crate::Error; /** A value with a streamable structure. # Implementing `Value` Implementations of `Value` are expected to conform to the following model: ## Only a single primitive, map or sequence is streamed The following `Value` is valid: ``` # use sval::value::{self, Value}; # struct MyValue; impl Value for MyValue { fn stream(&self, stream: &mut value::Stream) -> value::Result { // VALID: The stream can take the primitive // value 42 stream.any(42) } } ``` The following `Value` is not valid: ``` # use sval::value::{self, Value}; # struct MyValue; impl Value for MyValue { fn stream(&self, stream: &mut value::Stream) -> value::Result { stream.any(42)?; // INVALID: The stream already received the // primitive value 42 stream.any(43) } } ``` ## All maps and sequences are completed, and in the right order The following `Value` is valid: ``` # use sval::value::{self, Value}; # struct MyValue; impl Value for MyValue { fn stream(&self, stream: &mut value::Stream) -> value::Result { stream.map_begin(None)?; stream.map_key("a")?; stream.map_value_begin()?.seq_begin(None)?; // VALID: The sequence is completed, then the map is completed stream.seq_end()?; stream.map_end() } } ``` The following `Value` is not valid: ``` # use sval::value::{self, Value}; # struct MyValue; impl Value for MyValue { fn stream(&self, stream: &mut value::Stream) -> value::Result { stream.map_begin(None)?; stream.map_key("a")?; stream.map_value_begin()?.seq_begin(None)?; // INVALID: The map is completed before the sequence, // even though the sequence was started last. stream.map_end()?; stream.seq_end() } } ``` The following `Value` is not valid: ``` # use sval::value::{self, Value}; # struct MyValue; impl Value for MyValue { fn stream(&self, stream: &mut value::Stream) -> value::Result { stream.map_begin(None)?; // INVALID: The map is never completed Ok(()) } } ``` ## Map keys and values are received before their corresponding structure The following `Value` is valid: ``` # use sval::value::{self, Value}; # struct MyValue; impl Value for MyValue { fn stream(&self, stream: &mut value::Stream) -> value::Result { stream.map_begin(None)?; // VALID: The `map_key` and `map_value` methods // always call the underlying stream correctly stream.map_key("a")?; stream.map_value("b")?; // VALID: `map_key` and `map_value` are called before // their actual values are given stream.map_key_begin()?.any("c")?; stream.map_value_begin()?.any("d")?; stream.map_end() } } ``` The following `Value` is not valid: ``` # use sval::value::{self, Value}; # struct MyValue; impl Value for MyValue { fn stream(&self, stream: &mut value::Stream) -> value::Result { stream.map_begin(None)?; // INVALID: The underlying `map_key` and `map_value` methods // aren't being called before their actual values are given stream.any("a")?; stream.any("b")?; stream.map_end() } } ``` ## Map keys are received before values The following `Value` is valid: ``` # use sval::value::{self, Value}; # struct MyValue; impl Value for MyValue { fn stream(&self, stream: &mut value::Stream) -> value::Result { stream.map_begin(None)?; // VALID: The key is streamed before the value stream.map_key("a")?; stream.map_value("b")?; stream.map_end() } } ``` The following `Value` is not valid: ``` # use sval::value::{self, Value}; # struct MyValue; impl Value for MyValue { fn stream(&self, stream: &mut value::Stream) -> value::Result { stream.map_begin(None)?; // INVALID: The value is streamed before the key stream.map_value("b")?; stream.map_key("a")?; stream.map_end() } } ``` ## Sequence elements are received before their corresponding structure The following `Value` is valid: ``` # use sval::value::{self, Value}; # struct MyValue; impl Value for MyValue { fn stream(&self, stream: &mut value::Stream) -> value::Result { stream.seq_begin(None)?; // VALID: The `seq_elem` method // always calls the underlying stream correctly stream.seq_elem("a")?; // VALID: `seq_elem` is called before // their actual values are given stream.seq_elem_begin()?.any("b")?; stream.seq_end() } } ``` The following `Value` is not valid: ``` # use sval::value::{self, Value}; # struct MyValue; impl Value for MyValue { fn stream(&self, stream: &mut value::Stream) -> value::Result { stream.seq_begin(None)?; // INVALID: The underlying `seq_elem` method // isn't being called before the actual value is given stream.any("a")?; stream.seq_end() } } ``` */ pub trait Value { /** Stream this value. # Examples Use a [`stream::OwnedStream`] to stream a value: ```no_run # #[cfg(not(feature = "std"))] # fn main() {} # #[cfg(feature = "std")] # fn main() -> Result<(), Box<dyn std::error::Error>> { use sval::stream::OwnedStream; let mut stream = OwnedStream::new(MyStream); stream.any(42)?; # Ok(()) # } # use sval::stream::{self, Stream}; # struct MyStream; # impl Stream for MyStream { # fn fmt(&mut self, _: stream::Arguments) -> stream::Result { unimplemented!() } # } ``` It's less convenient, but the `stream` method can be called directly instead of using `OwnedStream.any`: ```no_run # #[cfg(not(feature = "std"))] # fn main() {} # #[cfg(feature = "std")] # fn main() -> Result<(), Box<dyn std::error::Error>> { use sval::{ stream::OwnedStream, Value, }; let mut stream = OwnedStream::new(MyStream); 42.stream(&mut stream.borrow_mut())?; # Ok(()) # } # use sval::stream::{self, Stream}; # struct MyStream; # impl Stream for MyStream { # fn fmt(&mut self, _: stream::Arguments) -> stream::Result { unimplemented!() } # } ``` [`sval::stream`]: ../fn.stream.html [`stream::OwnedStream`]: ../stream/struct.OwnedStream.html */ fn stream(&self, stream: &mut Stream) -> Result; } impl<'a, T: ?Sized> Value for &'a T where T: Value, { #[inline] fn stream(&self, stream: &mut Stream) -> Result { (**self).stream(stream) } } /** The type returned by streaming methods. */ pub type Result = std::result::Result<(), Error>; #[cfg(test)] mod tests { use super::*; #[test] fn value_is_object_safe() { fn _safe(_: &dyn Value) {} } }