fix up build errors

nested/Cargo.toml

@@ -15,9 +15,9 @@ categories = ["encoding", "no-std"]
 features = ["std"]
 
 [features]
-default = ["alloc"]
 std = ["alloc", "sval/std", "sval_buffer/std"]
 alloc = ["sval/alloc", "sval_buffer/alloc"]
+no_debug_assertions = []
 
 [dependencies.sval]
 version = "2.10.2"

nested/src/error.rs

@@ -9,7 +9,14 @@ pub struct Error(ErrorKind);
 #[derive(Debug)]
 enum ErrorKind {
     Buffer(sval_buffer::Error),
-    InvalidValue { reason: &'static str },
+    InvalidValue {
+        reason: &'static str,
+    },
+    #[cfg(not(feature = "alloc"))]
+    #[allow(dead_code)]
+    NoAlloc {
+        method: &'static str,
+    },
 }
 
 impl fmt::Display for Error {
@@ -21,6 +28,8 @@ impl fmt::Display for Error {
             ErrorKind::InvalidValue { reason } => {
                 write!(f, "the value is invalid: {}", reason)
             }
+            #[cfg(not(feature = "alloc"))]
+            ErrorKind::NoAlloc { method } => write!(f, "cannot allocate for {}", method),
         }
     }
 }
@@ -36,6 +45,23 @@ impl Error {
     pub fn invalid_value(reason: &'static str) -> Self {
         Error(ErrorKind::InvalidValue { reason })
     }
+
+    #[cfg(not(feature = "alloc"))]
+    #[track_caller]
+    pub(crate) fn no_alloc(method: &'static str) -> Self {
+        /*
+        The pattern here is the same as what's used in `sval_buffer`.
+        */
+
+        #[cfg(all(debug_assertions, not(no_debug_assertions), not(test)))]
+        {
+            panic!("attempt to allocate for {} would fail; add the `alloc` feature of `sval_nested` or the depdendent `sval_*` library to support allocation. This call will error instead of panicking in release builds. Add the `no_debug_assertions` feature of `sval_nested` if this error is expected.", method);
+        }
+        #[cfg(not(all(debug_assertions, not(no_debug_assertions), not(test))))]
+        {
+            Error(ErrorKind::NoAlloc { method })
+        }
+    }
 }
 
 #[cfg(feature = "std")]
@@ -49,6 +75,8 @@ mod std_support {
             match self.0 {
                 ErrorKind::Buffer(ref err) => Some(err),
                 ErrorKind::InvalidValue { .. } => None,
+                #[cfg(not(feature = "alloc"))]
+                ErrorKind::NoAlloc { .. } => None,
             }
         }
     }

nested/src/lib.rs

@@ -1198,7 +1198,7 @@ pub mod default_stream {
 mod tests {
     use super::*;
 
-    use alloc::borrow::Cow;
+    use std::borrow::Cow;
 
     use sval_derive_macros::*;