build(fuzz): link migrated Rust archive into fuzz targets

The fuzz Makefile compiles libzstd sources directly, so C compatibility shims
previously left migrated symbols unresolved.  Build and link a configuration
matched Cargo archive after the C objects for every fuzz target.

Track forced HUF mode changes with a C-object stamp to prevent a stale C
decoder from being mixed with another Rust archive.  A -m32 invocation now
also selects Cargo's i686 target rather than attempting to link native Rust
objects into a 32-bit fuzzer.

Test Plan:
- build and run huf_round_trip and dictionary_round_trip on LICENSE
- transition default to forced X1 and back without cleaning, then run input
- build and run huf_round_trip with C/C++/linker -m32 flags
- cargo clippy, cargo clippy --benches, cargo clippy --tests, and nightly fmt

Refs: tests/fuzz/Makefile Rust archive configuration

tests/fuzz/Makefile

@@ -16,6 +16,7 @@ ASFLAGS ?=
 LDFLAGS ?=
 ARFLAGS ?=
 LIB_FUZZING_ENGINE ?= libregression.a
+FUZZING_ENGINE := $(LIB_FUZZING_ENGINE)
 PYTHON ?= python
 ifeq ($(shell sh -c 'MSYSTEM="MSYS" uname') , Darwin)
 	DOWNLOAD?=curl -L -o
@@ -33,6 +34,85 @@ default: all
 
 include $(LIBZSTD_MK_DIR)/libzstd.mk
 
+# The fuzzers compile the C source list directly. Keep their Rust archive
+# configuration in lockstep with the C HUF selection, and link it after all C
+# objects so migrated C shims resolve exactly as they do in the normal tests.
+CARGO ?= cargo
+RUST_DIR := ../../rust
+RUST_MANIFEST := $(RUST_DIR)/Cargo.toml
+RUST_SOURCES := $(RUST_MANIFEST) $(RUST_DIR)/Cargo.lock \
+                $(shell find $(RUST_DIR)/src -type f -name '*.rs' -print)
+
+RUST_HUF_FORCE_X1 := $(HUF_FORCE_DECOMPRESS_X1)
+RUST_HUF_FORCE_X2 := $(HUF_FORCE_DECOMPRESS_X2)
+ifneq ($(filter -DHUF_FORCE_DECOMPRESS_X1 -DHUF_FORCE_DECOMPRESS_X1=%,$(CPPFLAGS) $(CFLAGS) $(MOREFLAGS)),)
+RUST_HUF_FORCE_X1 := 1
+endif
+ifneq ($(filter -DHUF_FORCE_DECOMPRESS_X2 -DHUF_FORCE_DECOMPRESS_X2=%,$(CPPFLAGS) $(CFLAGS) $(MOREFLAGS)),)
+RUST_HUF_FORCE_X2 := 1
+endif
+ifneq ($(RUST_HUF_FORCE_X1),0)
+ifneq ($(RUST_HUF_FORCE_X2),0)
+$(error HUF_FORCE_DECOMPRESS_X1 and HUF_FORCE_DECOMPRESS_X2 are mutually exclusive)
+endif
+endif
+
+RUST_HUF_FEATURE :=
+RUST_BUILD_CONFIG := default
+ifneq ($(RUST_HUF_FORCE_X1),0)
+RUST_HUF_FEATURE := huf-force-decompress-x1
+RUST_BUILD_CONFIG := huf-force-decompress-x1
+endif
+ifneq ($(RUST_HUF_FORCE_X2),0)
+RUST_HUF_FEATURE := huf-force-decompress-x2
+RUST_BUILD_CONFIG := huf-force-decompress-x2
+endif
+
+RUST_TARGET_DIR := $(RUST_DIR)/target/fuzz/$(RUST_BUILD_CONFIG)
+RUST_CARGO_FLAGS := --manifest-path $(RUST_MANIFEST) --release \
+                    --target-dir $(RUST_TARGET_DIR)
+ifneq ($(RUST_HUF_FEATURE),)
+RUST_CARGO_FLAGS += --features $(RUST_HUF_FEATURE)
+endif
+
+# Keep a 32-bit C/C++ fuzz invocation from linking a native-width Cargo
+# archive.  Cross builds may select any supported Cargo target explicitly.
+RUST_TARGET ?=
+ifeq ($(strip $(RUST_TARGET)),)
+ifneq ($(filter -m32,$(CC) $(CXX) $(CPPFLAGS) $(CFLAGS) $(CXXFLAGS) $(LDFLAGS) $(MOREFLAGS)),)
+RUST_TARGET := i686-unknown-linux-gnu
+endif
+endif
+RUST_TARGET_COMPONENT :=
+ifneq ($(strip $(RUST_TARGET)),)
+RUST_CARGO_FLAGS += --target $(RUST_TARGET)
+RUST_TARGET_COMPONENT := /$(RUST_TARGET)
+endif
+RUST_STATICLIB := $(RUST_TARGET_DIR)$(RUST_TARGET_COMPONENT)/release/libzstd_rs.a
+
+$(RUST_STATICLIB): $(RUST_SOURCES)
+	$(CARGO) build $(RUST_CARGO_FLAGS)
+
+# `rt_` and `d_` object names are flat. Rebuild them whenever a forced HUF
+# mode changes instead of mixing a stale C decoder with another Rust archive.
+RUST_HUF_C_MODE_FILE := $(RUST_DIR)/target/.huf-c-mode-fuzz
+RUST_HUF_C_MODE_STAMP := $(RUST_DIR)/target/.huf-c-mode-fuzz.a
+.PHONY: FORCE_RUST_HUF_C_MODE
+FORCE_RUST_HUF_C_MODE:
+$(RUST_HUF_C_MODE_STAMP): FORCE_RUST_HUF_C_MODE
+	@mkdir -p $(@D)
+	@if test ! -r $(RUST_HUF_C_MODE_FILE) \
+		|| test "$$(cat $(RUST_HUF_C_MODE_FILE))" != "$(RUST_BUILD_CONFIG)" \
+		|| test ! -f $@; then \
+		printf '%s\n' "$(RUST_BUILD_CONFIG)" > $(RUST_HUF_C_MODE_FILE); \
+		$(AR) rcs $@; \
+		touch $@; \
+	fi
+
+# Recipes below already append LIB_FUZZING_ENGINE. Preserve user-provided
+# engines but place the Rust archive ahead of them and after C objects.
+LIB_FUZZING_ENGINE := $(RUST_STATICLIB) $(FUZZING_ENGINE)
+
 PRGDIR = ../../programs
 CONTRIBDIR = ../../contrib
 
@@ -104,6 +184,8 @@ FUZZ_RT_OBJ9 := $(FUZZ_RT_OBJ8:.c=.o)
 FUZZ_RT_OBJ10 := $(THIRD_PARTY_SEQ_PROD_OBJ) $(FUZZ_RT_OBJ9)
 FUZZ_ROUND_TRIP_OBJ := $(FUZZ_RT_OBJ10:.S=.o)
 
+$(FUZZ_DECOMPRESS_OBJ) $(FUZZ_ROUND_TRIP_OBJ): $(RUST_HUF_C_MODE_STAMP)
+
 FUZZ_TARGETS :=       \
 	simple_round_trip \
 	stream_round_trip \
@@ -130,6 +212,11 @@ FUZZ_TARGETS :=       \
 .PHONY: all clean cleanall
 all: libregression.a $(FUZZ_TARGETS)
 
+$(FUZZ_TARGETS): $(RUST_STATICLIB) $(RUST_HUF_C_MODE_STAMP)
+ifneq ($(filter libregression.a,$(FUZZING_ENGINE)),)
+$(FUZZ_TARGETS): libregression.a
+endif
+
 rt_lib_common_%.o: $(LIB_SRCDIR)/common/%.c
 	$(CC) $(FUZZ_CPPFLAGS) $(FUZZ_CFLAGS) $(FUZZ_ROUND_TRIP_FLAGS) $< -c -o $@
 
@@ -269,7 +356,7 @@ regressiontest: corpora
 	$(PYTHON) ./fuzz.py regression $(REGRESSION_TARGET)
 
 clean:
-	@$(RM) *.a *.o $(FUZZ_TARGETS)
+	@$(RM) *.a *.o $(FUZZ_TARGETS) $(RUST_HUF_C_MODE_FILE) $(RUST_HUF_C_MODE_STAMP)
 	@echo Cleaning completed
 
 cleanall: