buildRustCrate: add lib output

This cuts down the dependency tree on some rust builds where a crate not
just exposes a binary but also a library. `$out/lib` contained a bunch
of extra support files that among other information carry linker flags
(including the full path to link-time dependencies). Worst case this led
to some binary outputs depending on the full build closure of rust
crates.

Moving all the `$out/lib` files to `$lib/lib` solves this nicely.

`lib` might be a bit weird here as they are most of the time just rlib
files (rust libraries). Those are essential only required during
compilation but they can also be shared objects (like with traditional
C-style packages). Which is why I went with `lib` for the new output.

One of the caveats we are running into here is that we do not (always)
know ahead of time of a crate produces just a library or just a binary.
Cargo allows for some ambiguity regarding whether or not a crate
provides one, two, … binaries and libraries as it's outputs. Ideally we
would be able to rely on the `crateType` entirely but so far that isn't
the case. More work on that area might show how difficult that actually
is.

3 files changed, 25 insertions, 20 deletions

diff --git a/pkgs/build-support/rust/build-rust-crate/build-crate.nix b/pkgs/build-support/rust/build-rust-crate/build-crate.nix
index e0a52e62561..2537b722cdc 100644
--- a/pkgs/build-support/rust/build-rust-crate/build-crate.nix
+++ b/pkgs/build-support/rust/build-rust-crate/build-crate.nix
@@ -91,18 +91,18 @@
 
     echo "$EXTRA_LINK_SEARCH" | while read i; do
        if [[ ! -z "$i" ]]; then
-         for lib in $i; do
-           echo "-L $lib" >> target/link
-           L=$(echo $lib | sed -e "s#$(pwd)/target/build#$out/lib#")
+         for library in $i; do
+           echo "-L $library" >> target/link
+           L=$(echo $library | sed -e "s#$(pwd)/target/build#$lib/lib#")
            echo "-L $L" >> target/link.final
          done
        fi
     done
     echo "$EXTRA_LINK" | while read i; do
        if [[ ! -z "$i" ]]; then
-         for lib in $i; do
-           echo "-l $lib" >> target/link
-           echo "-l $lib" >> target/link.final
+         for library in $i; do
+           echo "-l $library" >> target/link
+           echo "-l $library" >> target/link.final
          done
        fi
     done
diff --git a/pkgs/build-support/rust/build-rust-crate/default.nix b/pkgs/build-support/rust/build-rust-crate/default.nix
index 6534e21c0f0..381485e3752 100644
--- a/pkgs/build-support/rust/build-rust-crate/default.nix
+++ b/pkgs/build-support/rust/build-rust-crate/default.nix
@@ -22,9 +22,9 @@ let
           else
             extern;
         in (if lib.lists.any (x: x == "lib") dep.crateType then
-           " --extern ${name}=${dep.out}/lib/lib${extern}-${dep.metadata}.rlib"
+           " --extern ${name}=${dep.lib}/lib/lib${extern}-${dep.metadata}.rlib"
          else
-           " --extern ${name}=${dep.out}/lib/lib${extern}-${dep.metadata}${stdenv.hostPlatform.extensions.sharedLibrary}")
+           " --extern ${name}=${dep.lib}/lib/lib${extern}-${dep.metadata}${stdenv.hostPlatform.extensions.sharedLibrary}")
       ) dependencies);
 
     echo_build_heading = colors: ''
@@ -96,12 +96,12 @@ stdenv.mkDerivation (rec {
     buildInputs = (crate.buildInputs or []) ++ buildInputs_;
     dependencies =
       builtins.map
-        (dep: dep.override { rust = rust; release = release; verbose = verbose; crateOverrides = crateOverrides; })
+        (dep: lib.getLib (dep.override { rust = rust; release = release; verbose = verbose; crateOverrides = crateOverrides; }))
         dependencies_;
 
     buildDependencies =
       builtins.map
-        (dep: dep.override { rust = rust; release = release; verbose = verbose; crateOverrides = crateOverrides; })
+        (dep: lib.getLib (dep.override { rust = rust; release = release; verbose = verbose; crateOverrides = crateOverrides; }))
         buildDependencies_;
 
     completeDeps = lib.lists.unique (dependencies ++ lib.lists.concatMap (dep: dep.completeDeps) dependencies);
@@ -160,6 +160,9 @@ stdenv.mkDerivation (rec {
     };
     installPhase = installCrate crateName metadata;
 
+    outputs = [ "out" "lib" ];
+    outputDev = [ "lib" ];
+
 } // extraDerivationAttrs
 )) {
   rust = rustc;
diff --git a/pkgs/build-support/rust/build-rust-crate/install-crate.nix b/pkgs/build-support/rust/build-rust-crate/install-crate.nix
index 3b0282621ea..934c3a03176 100644
--- a/pkgs/build-support/rust/build-rust-crate/install-crate.nix
+++ b/pkgs/build-support/rust/build-rust-crate/install-crate.nix
@@ -1,24 +1,26 @@
 crateName: metadata:
 ''
   runHook preInstall
-  mkdir -p $out
+  # always create $out even if we do not have binaries. We are detecting binary targets during compilation, if those are missing there is no way to only have $lib
+  mkdir $out
   if [[ -s target/env ]]; then
-    cp target/env $out/env
+    mkdir -p $lib
+    cp target/env $lib/env
   fi
   if [[ -s target/link.final ]]; then
-    mkdir -p $out/lib
-    cp target/link.final $out/lib/link
+    mkdir -p $lib/lib
+    cp target/link.final $lib/lib/link
   fi
   if [[ "$(ls -A target/lib)" ]]; then
-    mkdir -p $out/lib
-    cp target/lib/* $out/lib #*/
-    for lib in $out/lib/*.so $out/lib/*.dylib; do #*/
-      ln -s $lib $(echo $lib | sed -e "s/-${metadata}//")
+    mkdir -p $lib/lib
+    cp target/lib/* $lib/lib #*/
+    for library in $lib/lib/*.so $lib/lib/*.dylib; do #*/
+      ln -s $library $(echo $library | sed -e "s/-${metadata}//")
     done
   fi
   if [[ "$(ls -A target/build)" ]]; then # */
-    mkdir -p $out/lib
-    cp -r target/build/* $out/lib # */
+    mkdir -p $lib/lib
+    cp -r target/build/* $lib/lib # */
   fi
   if [[ -d target/bin ]]; then
     if [[ "$(ls -A target/bin)" ]]; then