rust: Add chainload cargo feature

Commit 590552a ("Makefile: unconditionally build rust stuff") started
building the rust chainloading code unconditionally. Add a cargo
"chainload" feature to build the code only if it is used as indicated by
the CHAINLOADING build config variable.
The dependency handling via `src/../build/build_cfg.h` is a quirk
coming from the C side.
Not all CFG values (like RELEASE) require a rebuild of the rust library
but creating an additional dependency file for cargo features seems
overkill.

Signed-off-by: Janne Grunau <[email protected]>

Author: jannau

Makefile

@@ -74,16 +74,17 @@ endif
 # Required for no_std + alloc for now
 export RUSTC_BOOTSTRAP=1
 RUST_LIB := librust.a
-ifeq ($(CHAINLOADING),1)
-CFG += CHAINLOADING
-endif
-
 ifeq ($(BUILDSTD),1)
 CARGO_FLAGS := -Z build-std=alloc,core
 else
 CARGO_FLAGS :=
 endif
 
+ifeq ($(CHAINLOADING),1)
+CFG += CHAINLOADING
+CARGO_FLAGS += --features chainload
+endif
+
 LDFLAGS := -EL -maarch64elf --no-undefined -X -Bsymbolic \
 	-z notext --no-apply-dynamic-relocs --orphan-handling=warn \
 	-z nocopyreloc --gc-sections -pie
@@ -208,7 +209,7 @@ rustfmt:
 rustfmt-check:
 	cd rust && cargo fmt --check
 
-build/$(RUST_LIB): rust/src/*.rs rust/src/gpu/*.rs rust/src/gpu/hw/*.rs rust/Cargo.toml rust/Cargo.lock
+build/$(RUST_LIB): src/../build/build_cfg.h rust/src/*.rs rust/src/gpu/*.rs rust/src/gpu/hw/*.rs rust/Cargo.toml rust/Cargo.lock
 	$(QUIET)echo "  RS    $@"
 	$(QUIET)mkdir -p $(DEPDIR)
 	$(QUIET)mkdir -p "$(dir $@)"

rust/Cargo.toml

@@ -11,6 +11,9 @@ crate-type = [ "staticlib" ]
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 [dependencies]
-fatfs = { path = "vendor/rust-fatfs", default-features = false, features = ["lfn", "alloc"] }
-uuid = { version = "1.7.0", default-features = false }
+fatfs = { path = "vendor/rust-fatfs", default-features = false, features = ["lfn", "alloc"], optional = true }
+uuid = { version = "1.7.0", default-features = false, optional = true }
 versions = { path = "./versions" }
+
+[features]
+chainload = ["dep:fatfs", "dep:uuid"]

rust/src/lib.rs

@@ -7,15 +7,19 @@
 #![feature(stmt_expr_attributes)]
 #![cfg_attr(all(version("1.82"), not(version("1.92"))), feature(new_zeroed_alloc))]
 
+#[allow(unused_imports)]
 #[macro_use]
 extern crate alloc;
 
 pub mod adt;
+#[cfg(feature = "chainload")]
 pub mod chainload;
 pub mod dlmalloc;
 pub mod float;
+#[cfg(feature = "chainload")]
 pub mod gpt;
 pub mod gpu;
+#[cfg(feature = "chainload")]
 pub mod nvme;
 pub mod print;