Add a pure-Rust Zstandard backend behind a zstd-rust feature

.github/workflows/ci.yml

@@ -29,6 +29,8 @@ jobs:
           - ""
           - "--no-default-features --features deflate"
           - "--no-default-features --features zstd"
+          - "--no-default-features --features zstd-rust"
+          - "--features zstd-rust"
     steps:
       - uses: actions/checkout@v2
       - uses: actions-rs/toolchain@v1
@@ -52,7 +54,7 @@ jobs:
           default: true
       - name: assert deps don't exist with --no-default-features
         run: |
-          for DEP in libflate zstd; do
+          for DEP in libflate zstd structured-zstd; do
             if cargo tree --no-default-features --invert $DEP; then
               echo "Dependency $DEP should not exist with --no-default-features"
               exit 1
@@ -70,3 +72,11 @@ jobs:
             echo "Feature `zstd` should not create dep on `libflate`"
             exit 1
           fi
+      - name: assert `zstd-rust` feature doesn't create dep on the C zstd bindings
+        run: |
+          for DEP in zstd zstd-sys libflate; do
+            if cargo tree --no-default-features --features zstd-rust --invert $DEP; then
+              echo "Feature `zstd-rust` should not create dep on $DEP"
+              exit 1
+            fi
+          done

Cargo.toml

@@ -12,6 +12,7 @@ license = "Apache-2.0"
 [workspace.dependencies]
 libflate = "2.3.0"
 zstd = "0.14.0"
+structured-zstd = "0.0.59"
 
 [package]
 name = "include-flate"
@@ -34,4 +35,7 @@ include-flate-compress = { version = "0.3.4", path = "compress", default-feature
 default = ["deflate", "zstd"]
 deflate = ["include-flate-codegen/deflate", "include-flate-compress/deflate"]
 zstd = ["include-flate-codegen/zstd", "include-flate-compress/zstd"]
+# Zstandard through the pure-Rust `structured-zstd`, with no C toolchain,
+# selected by `with zstd_rust`. Frames are interchangeable with `with zstd`.
+zstd-rust = ["include-flate-codegen/zstd-rust", "include-flate-compress/zstd-rust"]
 no-compression-warnings = ["include-flate-codegen/no-compression-warnings"]

codegen/Cargo.toml

@@ -22,4 +22,5 @@ proc-macro-error3 = "3.0.2"
 default = ["deflate", "zstd"]
 deflate = ["include-flate-compress/deflate"]
 zstd = ["include-flate-compress/zstd"]
+zstd-rust = ["include-flate-compress/zstd-rust"]
 no-compression-warnings = []

codegen/src/lib.rs

@@ -72,6 +72,7 @@ pub fn deflate_utf8_file(ts: TokenStream) -> TokenStream {
 /// flate!(pub static DATA: [u8] from "assets/009f.dat"); // default, DEFLATE
 /// flate!(pub static DATA: [u8] from "assets/009f.dat" with zstd); // Use Zstd for this file spcifically
 /// flate!(pub static DATA: [u8] from "assets/009f.dat" with deflate); // Explicitly use DEFLATE.
+/// flate!(pub static DATA: [u8] from "assets/009f.dat" with zstd_rust); // Zstd without the C library
 /// ```
 struct FlateArgs {
     path: syn::LitStr,
@@ -105,6 +106,17 @@ impl syn::parse::Parse for FlateArgs {
                 }
                 #[cfg(not(feature = "zstd"))]
                 return Err(Error::new(input.span(), "Please enable the `zstd` feature"));
+            } else if lookahead.peek(kw::zstd_rust) {
+                #[cfg(feature = "zstd-rust")]
+                {
+                    input.parse::<kw::zstd_rust>()?;
+                    Some(CompressionMethodTy(CompressionMethod::ZstdRust))
+                }
+                #[cfg(not(feature = "zstd-rust"))]
+                return Err(Error::new(
+                    input.span(),
+                    "Please enable the `zstd-rust` feature",
+                ));
             } else {
                 return Err(lookahead.error());
             }
@@ -117,6 +129,7 @@ impl syn::parse::Parse for FlateArgs {
 mod kw {
     syn::custom_keyword!(deflate);
     syn::custom_keyword!(zstd);
+    syn::custom_keyword!(zstd_rust);
 }
 
 #[derive(Debug, Default)]

compress/Cargo.toml

@@ -11,8 +11,12 @@ description = "Compression algorithm provider"
 [dependencies]
 libflate = { workspace = true, optional = true }
 zstd = { workspace = true, optional = true }
+structured-zstd = { workspace = true, optional = true }
 
 [features]
 default = ["deflate", "zstd"]
 deflate = ["dep:libflate"]
 zstd = ["dep:zstd"]
+# Zstandard through the pure-Rust `structured-zstd`, as `CompressionMethod::ZstdRust`.
+# Same frame format as `CompressionMethod::Zstd`.
+zstd-rust = ["dep:structured-zstd"]