Recently I had a difficult time trying to patch a CVE in librsvg. The issue itself was simple to patch because Federico kindly backported the series of commits required to fix it to the branch we are using downstream. Problem was, one of the vendored deps in the old librsvg tarball did not build with our modern rustc, because the code contained a borrow error that was not caught by older versions of rustc. After finding the appropriate upstream fix, I tried naively patching the vendored dep, but that failed because cargo tries very hard to prevent you from patching its dependencies, and complains if the dependency does not match its checksum in Cargo.lock. I tried modifying the checksum in Cargo.lock, but then it complains that you modified the Cargo.lock. It seems cargo is designed to make patching dependencies as difficult as possible, and that not much thought was put into how cargo would be used from rpmbuild with no network access.
Anyway, it seems the kosher way to patch Rust dependencies is to add a [patch] section to librsvg’s Cargo.toml, but I could not figure out how to make that work. Eventually, I got some help: you can edit the .cargo-checksum.json of the vendored dependency and change “files” to an empty array, like so:
diff --git a/vendor/cssparser/.cargo-checksum.json b/vendor/cssparser/.cargo-checksum.json
index 246bb70..713372d 100644
--- a/vendor/cssparser/.cargo-checksum.json
+++ b/vendor/cssparser/.cargo-checksum.json
@@ -1 +1 @@
-{"files":{".cargo-ok":"e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855",".travis.yml":"f1fb4b65964c81bc1240544267ea334f554ca38ae7a74d57066f4d47d2b5d568","Cargo.toml":"7807f16d417eb1a6ede56cd4ba2da6c5c63e4530289b3f0848f4b154e18eba02","LICENSE":"fab3dd6bdab226f1c08630b1dd917e11fcb4ec5e1e020e2c16f83a0a13863e85","README.md":"c5781e673335f37ed3d7acb119f8ed33efdf6eb75a7094b7da2abe0c3230adb8","build.rs":"b29fc57747f79914d1c2fb541e2bb15a003028bb62751dcb901081ccc174b119","build/match_byte.rs":"2c84b8ca5884347d2007f49aecbd85b4c7582085526e2704399817249996e19b","docs/.nojekyll":"e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855","docs/404.html":"025861f76f8d1f6d67c20ab624c6e418f4f824385e2dd8ad8732c4ea563c6a2e","docs/index.html":"025861f76f8d1f6d67c20ab624c6e418f4f824385e2dd8ad8732c4ea563c6a2e","src/color.rs":"c60f1b0ab7a2a6213e434604ee33f78e7ef74347f325d86d0b9192d8225ae1cc","src/cow_rc_str.rs":"541216f8ef74ee3cc5cbbc1347e5f32ed66588c401851c9a7d68b867aede1de0","src/from_bytes.rs":"331fe63af2123ae3675b61928a69461b5ac77799fff3ce9978c55cf2c558f4ff","src/lib.rs":"46c377e0c9a75780d5cb0bcf4dfb960f0fb2a996a13e7349bb111b9082252233","src/macros.rs":"adb9773c157890381556ea83d7942dcc676f99eea71abbb6afeffee1e3f28960","src/nth.rs":"5c70fb542d1376cddab69922eeb4c05e4fcf8f413f27563a2af50f72a47c8f8c","src/parser.rs":"9ed4aec998221eb2d2ba99db2f9f82a02399fb0c3b8500627f68f5aab872adde","src/rules_and_declarations.rs":"be2c4f3f3bb673d866575b6cb6084f1879dff07356d583ca9a3595f63b7f916f","src/serializer.rs":"4ccfc9b4fe994aab3803662bbf31cc25052a6a39531073a867b14b224afe42dd","src/size_of_tests.rs":"e5f63c8c18721cc3ff7a5407e84f9889ffa10e66da96e8510a696c3e00ad72d5","src/tests.rs":"80b02c80ab0fd580dad9206615c918e0db7dff63dfed0feeedb66f317d24b24b","src/tokenizer.rs":"429b2cba419cf8b923fbcc32d3bd34c0b39284ebfcb9fc29b8eb8643d8d5f312","src/unicode_range.rs":"c1c4ed2493e09d248c526ce1ef8575a5f8258da3962b64ffc814ef3bdf9780d0"},"package":"8a807ac3ab7a217829c2a3b65732b926b2befe6a35f33b4bf8b503692430f223"} No newline at end of file
+{"files":{},"package":"8a807ac3ab7a217829c2a3b65732b926b2befe6a35f33b4bf8b503692430f223"}
Then cargo will stop complaining and you can patch the dependency. Success!
