From a11bc4acbfee5816d7e2cc7364217b98c75838ed Mon Sep 17 00:00:00 2001
From: Gaurav Chaudhary <chaudharygaurav2004@gmail.com>
Date: Tue, 7 Jul 2026 16:57:35 +0530
Subject: [PATCH] fix(core,chain): disable serde default features for no_std

Serde's default `std` feature was leaking into downstream no_std builds
via Cargo feature unification when `bdk_chain/serde` was enabled. Gate
`serde/std` behind each crate's `std` feature instead, add the `alloc`
feature (matching the `bitcoin` crate), and guard against regressions in
CI with a feature-tree assertion plus a bare-metal target check.

Fixes #2208
---
 .github/workflows/cont_integration.yml | 21 +++++++++++++++++++++
 crates/chain/Cargo.toml                |  4 ++--
 crates/core/Cargo.toml                 |  4 ++--
 3 files changed, 25 insertions(+), 4 deletions(-)

diff --git a/.github/workflows/cont_integration.yml b/.github/workflows/cont_integration.yml
index 1a5c38444..9e44f2542 100644
--- a/.github/workflows/cont_integration.yml
+++ b/.github/workflows/cont_integration.yml
@@ -72,6 +72,27 @@ jobs:
         working-directory: ./crates/chain
         # TODO "--target thumbv6m-none-eabi" should work but currently does not
         run: cargo check --no-default-features --features hashbrown
+      - name: Check serde does not enable serde/std in no_std builds
+        run: |
+          set -euo pipefail
+          check_no_serde_std() {
+            local pkg="$1" features="$2"
+            if cargo tree -p "$pkg" -e features \
+                --no-default-features --features "$features" --no-dev-dependencies \
+                | grep -q 'serde feature "std"'; then
+              echo "error: serde/std enabled for $pkg with features=$features"
+              exit 1
+            fi
+          }
+          check_no_serde_std bdk_core serde
+          check_no_serde_std bdk_chain serde
+      - name: Install ARM cross-compiler
+        run: sudo apt-get update && sudo apt-get install -y gcc-arm-none-eabi
+      - name: Add bare-metal target
+        run: rustup target add thumbv7em-none-eabihf
+      - name: Check bdk_chain serde on bare-metal target
+        working-directory: ./crates/chain
+        run: cargo check --no-default-features --features serde --target thumbv7em-none-eabihf
       - name: Check esplora
         working-directory: ./crates/esplora
         # TODO "--target thumbv6m-none-eabi" should work but currently does not
diff --git a/crates/chain/Cargo.toml b/crates/chain/Cargo.toml
index 949f177c8..8a65e7d1c 100644
--- a/crates/chain/Cargo.toml
+++ b/crates/chain/Cargo.toml
@@ -18,7 +18,7 @@ workspace = true
 [dependencies]
 bitcoin = { version = "0.32.0", default-features = false }
 bdk_core = { path = "../core", version = "0.6.2", default-features = false }
-serde = { version = "1", optional = true, features = ["derive", "rc"] }
+serde = { version = "1", optional = true, default-features = false, features = ["derive", "rc", "alloc"] }
 miniscript = { version = "13.0.0", optional = true, default-features = false }
 
 # Feature dependencies
@@ -32,7 +32,7 @@ criterion = { version = "0.7" }
 
 [features]
 default = ["std", "miniscript"]
-std = ["bitcoin/std", "miniscript?/std", "bdk_core/std"]
+std = ["bitcoin/std", "miniscript?/std", "bdk_core/std", "serde?/std"]
 serde = ["dep:serde", "bitcoin/serde", "miniscript?/serde", "bdk_core/serde"]
 hashbrown = ["bdk_core/hashbrown"]
 rusqlite = ["std", "dep:rusqlite", "serde"]
diff --git a/crates/core/Cargo.toml b/crates/core/Cargo.toml
index ed313f3ae..29f988cb9 100644
--- a/crates/core/Cargo.toml
+++ b/crates/core/Cargo.toml
@@ -12,12 +12,12 @@ readme = "README.md"
 
 [dependencies]
 bitcoin = { version = "0.32", default-features = false }
-serde = { version = "1", optional = true, features = ["derive", "rc"] }
+serde = { version = "1", optional = true, default-features = false, features = ["derive", "rc", "alloc"] }
 hashbrown = { version = "0.14.5", optional = true, default-features = false, features = ["ahash", "inline-more"] }
 
 [features]
 default = ["std"]
-std = ["bitcoin/std"]
+std = ["bitcoin/std", "serde?/std"]
 serde = ["dep:serde", "bitcoin/serde", "hashbrown?/serde"]
 
 [dev-dependencies]