Support preserve_order in no_std builds

Author: razmser

.github/workflows/ci.yml

@@ -67,14 +67,18 @@ jobs:
       - run: cargo check --manifest-path tests/crate/Cargo.toml --no-default-features --features alloc
       - run: cargo check --manifest-path tests/crate/Cargo.toml --no-default-features --features alloc,arbitrary_precision
       - run: cargo check --manifest-path tests/crate/Cargo.toml --no-default-features --features alloc,raw_value
-      - run: cargo check --manifest-path tests/crate/Cargo.toml --features serde_json/preserve_order
+      - run: cargo check --manifest-path tests/crate/Cargo.toml --features preserve_order
         if: matrix.rust != '1.71.0'
-      - run: cargo check --manifest-path tests/crate/Cargo.toml --no-default-features --features alloc,serde_json/preserve_order
+      - run: cargo check --manifest-path tests/crate/Cargo.toml --no-default-features --features preserve_order
         if: matrix.rust != '1.71.0'
       - name: Build without std
         run: cargo check --manifest-path tests/crate/Cargo.toml --target
           ${{matrix.target}} --no-default-features --features alloc
         if: matrix.target
+      - name: Build without std with preserve_order
+        run: cargo check --manifest-path tests/crate/Cargo.toml --target
+          ${{matrix.target}} --no-default-features --features alloc,preserve_order
+        if: matrix.target
 
   minimal:
     name: Minimal versions

Cargo.toml

@@ -12,9 +12,10 @@ repository = "https://github.com/serde-rs/json"
 rust-version = "1.71"
 
 [dependencies]
-indexmap = { version = "2.2.3", optional = true }
+indexmap = { version = "2.2.3", optional = true, default-features = false }
 itoa = "1.0"
 memchr = { version = "2", default-features = false }
+rustc-hash = { version = "2", optional = true, default-features = false }
 serde_core = { version = "1.0.220", default-features = false }
 zmij = "1.0"
 
@@ -52,7 +53,7 @@ features = ["float_roundtrip", "raw_value", "unbounded_depth"]
 [features]
 default = ["std"]
 
-std = ["memchr/std", "serde_core/std"]
+std = ["memchr/std", "serde_core/std", "indexmap?/std"]
 
 # Provide integration for heap-allocated collections without depending on the
 # rest of the Rust standard library.
@@ -62,7 +63,7 @@ alloc = ["serde_core/alloc"]
 # Make serde_json::Map use a representation which maintains insertion order.
 # This allows data to be read into a Value and written back to a JSON string
 # while preserving the order of map keys in the input.
-preserve_order = ["indexmap", "std"]
+preserve_order = ["indexmap", "alloc", "dep:rustc-hash"]
 
 # Use sufficient precision when parsing fixed precision floats from JSON to
 # ensure that they maintain accuracy when round-tripped through JSON. This comes

src/map.rs

@@ -3,8 +3,14 @@
 //! By default the map is backed by a [`BTreeMap`]. Enable the `preserve_order`
 //! feature of serde_json to use [`IndexMap`] instead.
 //!
+//! In builds without the `std` feature, `preserve_order` hashes keys with a
+//! deterministic fixed-seed hasher, as no entropy source is available in
+//! core/alloc. Such builds are not resistant to HashDoS attacks. Builds with
+//! `std` use [`RandomState`] and are unaffected.
+//!
 //! [`BTreeMap`]: std::collections::BTreeMap
 //! [`IndexMap`]: indexmap::IndexMap
+//! [`RandomState`]: std::collections::hash_map::RandomState
 
 use crate::error::Error;
 use crate::value::Value;
@@ -32,15 +38,17 @@ pub struct Map<K, V> {
 
 #[cfg(not(feature = "preserve_order"))]
 type MapImpl<K, V> = BTreeMap<K, V>;
-#[cfg(feature = "preserve_order")]
+#[cfg(all(feature = "preserve_order", feature = "std"))]
 type MapImpl<K, V> = IndexMap<K, V>;
+#[cfg(all(feature = "preserve_order", not(feature = "std")))]
+type MapImpl<K, V> = IndexMap<K, V, rustc_hash::FxBuildHasher>;
 
 impl Map<String, Value> {
     /// Makes a new empty Map.
     #[inline]
     pub fn new() -> Self {
         Map {
-            map: MapImpl::new(),
+            map: MapImpl::default(),
         }
     }
 
@@ -54,8 +62,10 @@ impl Map<String, Value> {
                 let _ = capacity;
                 BTreeMap::new()
             },
-            #[cfg(feature = "preserve_order")]
+            #[cfg(all(feature = "preserve_order", feature = "std"))]
             map: IndexMap::with_capacity(capacity),
+            #[cfg(all(feature = "preserve_order", not(feature = "std")))]
+            map: IndexMap::with_capacity_and_hasher(capacity, rustc_hash::FxBuildHasher),
         }
     }
 
@@ -386,9 +396,7 @@ impl Map<String, Value> {
 impl Default for Map<String, Value> {
     #[inline]
     fn default() -> Self {
-        Map {
-            map: MapImpl::new(),
-        }
+        Map::new()
     }
 }
 

tests/crate/Cargo.toml

@@ -15,7 +15,7 @@ serde_json = { path = "../..", default-features = false }
 default = ["std"]
 std = ["serde_json/std"]
 alloc = ["serde_json/alloc"]
-#preserve_order = ["serde_json/preserve_order"]
+preserve_order = ["serde_json/preserve_order"]
 float_roundtrip = ["serde_json/float_roundtrip"]
 arbitrary_precision = ["serde_json/arbitrary_precision"]
 raw_value = ["serde_json/raw_value"]