Use checksums for recreating crates.io's directory

Closes #138.

build.nix

@@ -404,7 +404,37 @@ let
     ''
       mkdir -p $out
       tar -xzf ${crate} -C $out
-      echo '{"package":"${sha256}","files":{}}' > $out/${name}-${version}/.cargo-checksum.json
+
+      # Most filesystems have a maximum filename length of 255
+      dest="$out/$(echo "${name}-${version}-${sha256}" | head -c 255)"
+
+      # Unpacked crates contain a directory named after package's name and its
+      # version - here we're renaming that directory to contain the package's
+      # checksum as well, to avoid clashes in edge cases like:
+      #
+      # ```
+      # [dependencies]
+      # rand_core = "0.6.1"
+      # rand = { git = "https://github.com/rust-random/rand.git", rev = "...", package = "rand_core" }
+      # ```
+      #
+      # ... which might end up having similar entries in the Cargo.lock file:
+      #
+      # ```
+      # [[package]]
+      # name = "rand_core"
+      # version = "0.6.1"
+      # source = "registry+https://github.com/rust-lang/crates.io-index"
+      # checksum = "..."
+      #
+      # [[package]]
+      # name = "rand_core"
+      # version = "0.6.1"
+      # source = "git+https://github.com/rust-random/rand.git?rev=..."
+      # ```
+      mv "$out/${name}-${version}" "$dest"
+
+      echo '{"package":"${sha256}","files":{}}' > "$dest/.cargo-checksum.json"
     '';
 
   unpackGitDependency = { checkout, key, name, url, ... }:

test/fast/default.nix

@@ -2,6 +2,7 @@ args: {
   cargo-wildcard = import ./cargo-wildcard args;
   default-run = import ./default-run args;
   dummyfication = import ./dummyfication args;
+  duplicated-cargo-lock-items = import ./duplicated-cargo-lock-items args;
   git-dep = import ./git-dep args;
   git-dep-by-branch = import ./git-dep-by-branch args;
   git-dep-by-branch-with-slash = import ./git-dep-by-branch-with-slash args;

test/fast/duplicated-cargo-lock-items/default.nix

@@ -0,0 +1,6 @@
+{ naersk, ... }:
+
+naersk.buildPackage {
+  src = ./fixtures;
+  doCheck = true;
+}

test/fast/duplicated-cargo-lock-items/fixtures/Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+name = "app"
+version = "0.1.0"
+edition = "2018"
+
+[dependencies]
+rand = { git = "https://github.com/rust-random/rand.git", rev = "bda997404d9ab8a89d590734d8b0bb72aed10758", package = "rand_core" }
+rand_core = "0.6.1"

test/fast/duplicated-cargo-lock-items/fixtures/src/main.rs

@@ -0,0 +1,3 @@
+fn main() {
+    println!("Hello, world!");
+}