Add errors for data/text conversions from slices

recapn/src/data.rs

@@ -1,5 +1,6 @@
 //! A fixed size blob of bytes contained in a Cap'n Proto message
 
+use core::fmt;
 use core::ops::{Deref, DerefMut};
 
 use crate::internal::Sealed;
@@ -10,6 +11,18 @@ pub mod ptr {
     pub use crate::ptr::{BlobBuilder as Builder, BlobReader as Reader};
 }
 
+/// An error returned when trying to set a data field to a slice value if the value is too large.
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+pub struct TryFromSliceError(pub(crate) ());
+
+impl fmt::Display for TryFromSliceError {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.write_str("attempted to create a data blob from too large a slice")
+    }
+}
+
+impl core::error::Error for TryFromSliceError {}
+
 #[derive(Clone, Copy)]
 pub struct Data<T = Family>(T);
 
@@ -57,6 +70,14 @@ impl<'a> Reader<'a> {
         Self(r)
     }
 
+    #[inline]
+    pub const fn try_from_slice(slice: &'a [u8]) -> Result<Self, TryFromSliceError> {
+        match ptr::Reader::new(slice) {
+            Some(b) => Ok(Self(b)),
+            None => Err(TryFromSliceError(())),
+        }
+    }
+
     #[inline]
     pub const fn len(&self) -> u32 {
         self.0.len().get()

recapn/src/text.rs

@@ -29,6 +29,18 @@ macro_rules! text {
 
 const EMPTY_SLICE: &[u8] = &[0];
 
+/// An error returned when trying to set a text field to a string value if the value is too large.
+#[derive(Debug, Copy, Clone, PartialEq, Eq)]
+pub struct TryFromStrError(pub(crate) ());
+
+impl fmt::Display for TryFromStrError {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.write_str("attempted to create a text blob from too large a string")
+    }
+}
+
+impl core::error::Error for TryFromStrError {}
+
 #[derive(Clone, Copy)]
 pub struct Text<T = Family>(T);