fix: set CFLAGS to disable bulk-memory for wasm32 C deps

cargo-near-build/src/env_keys.rs

@@ -36,6 +36,11 @@ pub(crate) const CARGO_NEAR_VERSION: &str = "CARGO_NEAR_VERSION";
 #[cfg(feature = "build_internal")]
 pub(crate) const CARGO_NEAR_ABI_SCHEMA_VERSION: &str = "CARGO_NEAR_ABI_SCHEMA_VERSION";
 
+/// C compiler flags, respected by the `cc` crate when compiling C dependencies
+pub const CFLAGS_ENV: &str = "CFLAGS";
+/// C++ compiler flags, respected by the `cc` crate when compiling C++ dependencies
+pub const CXXFLAGS_ENV: &str = "CXXFLAGS";
+
 pub const COLOR_PREFERENCE_NO_COLOR: &str = "NO_COLOR";
 
 use std::collections::HashMap;

cargo-near-build/src/near/build/mod.rs

@@ -211,7 +211,18 @@ pub fn run(args: Opts) -> eyre::Result<CompilationArtifact> {
     let abi_path_env = buildtime_env::AbiPath::new(args.no_embed_abi, &min_abi_path);
 
     let build_env = {
-        let mut build_env = vec![(env_keys::RUSTFLAGS, "-C link-arg=-s")];
+        let mut build_env = vec![
+            (env_keys::RUSTFLAGS, "-C link-arg=-s"),
+            // Prevent C/C++ dependencies (compiled via the `cc` crate) from emitting
+            // post-MVP wasm instructions. LLVM 21+ enables `bulk-memory` and
+            // `bulk-memory-opt` by default for wasm32, which produces `memory.fill`/
+            // `memory.copy` instructions that NEAR's VM rejects.
+            // `-mcpu=mvp` restricts the C compiler to the MVP instruction set.
+            // Note: `-mno-bulk-memory` is insufficient as LLVM 21 ignores it for the
+            // separate `bulk-memory-opt` feature.
+            (env_keys::CFLAGS_ENV, "-mcpu=mvp"),
+            (env_keys::CXXFLAGS_ENV, "-mcpu=mvp"),
+        ];
         build_env.extend(
             args.env
                 .iter()