Solving complex build.rs issues

[pnathan]

Hi,

I'm experimenting with converting my cargo project to buck and I found that I have mime-guess as a dependency about 3 levels down. Its build.rs is really quite special! https://github.com/abonander/mime_guess/blob/master/build.rs

How should I address this in the fixup?

[jsgf]

That looks like a fairly normal gen_srcs fixup with env to set MIME_TYPES_GENERATED_PATH.

[Lev1ty]

Hacked it into working for now. The issue is that the env option sets the variable for both build script and library. So had to find a way to manually resolve the path. Please let me know if there is a better alternative!

# fixup.toml

version = "=2.0.5"

[env]
# HACK: In place of $(location :mime_guess-2.0.5-build-script-run[out_dir])/mime_types_generated.rs
# The correct method causes cyclic dependency because the variable is set for both:
# :mime_guess-2.0.5 [library]
# :mime_guess-2.0.5-build-script-build [binary]
# the latter of which is a dependency of :mime_guess-2.0.5-build-script-run[out_dir].
# This only works because the OUT_DIR (set by buildscript.gen_srcs) depends on :mime_guess-2.0.5-build-script-run[out_dir].
MIME_TYPES_GENERATED_PATH = "$(location //third-party:third-party)/../../../rust/__mime_guess-2.0.5-build-script-run__/OUT_DIR/mime_types_generated.rs"

[[buildscript]]
[buildscript.gen_srcs]

# third-party BUCK

genrule(name = "third-party", cmd = "echo third-party> $OUT", out = "third-party", visibility = ["PUBLIC"])

# generated BUCK

cargo.rust_library(
    name = "mime_guess-2.0.5",
    srcs = [":mime_guess-2.0.5.crate"],
    crate = "mime_guess",
    crate_root = "mime_guess-2.0.5.crate/src/lib.rs",
    edition = "2015",
    env = {
        "MIME_TYPES_GENERATED_PATH": "$(location //third-party:third-party)/../../../rust/__mime_guess-2.0.5-build-script-run__/OUT_DIR/mime_types_generated.rs",
        "OUT_DIR": "$(location :mime_guess-2.0.5-build-script-run[out_dir])",
    },
    rustc_flags = ["@$(location :mime_guess-2.0.5-build-script-run[rustc_flags])"],
    visibility = [],
    deps = [
        ":mime-0.3.17",
        ":unicase-2.7.0",
    ],
)

cargo.rust_binary(
    name = "mime_guess-2.0.5-build-script-build",
    srcs = [":mime_guess-2.0.5.crate"],
    crate = "build_script_build",
    crate_root = "mime_guess-2.0.5.crate/build.rs",
    edition = "2015",
    env = {
        "MIME_TYPES_GENERATED_PATH": "$(location //third-party:third-party)/../../../rust/__mime_guess-2.0.5-build-script-run__/OUT_DIR/mime_types_generated.rs",
    },
    visibility = [],
    deps = [":unicase-2.7.0"],
)

[cormacrelf]

Usually in this situation I would just submit a PR to the crate in question that makes the build script easier to adapt. In this case by not writing an additional environment variable, rather just writing

include!(concat!(env!("OUT_DIR"), "/mime_types_generated.rs"));

This is the standard way to do it.

---

But if this pops up again, there is also rust-lang/rust#80792

which would work with this change to the prelude:

--- a/prelude/rust/tools/buildscript_run.py
+++ b/prelude/rust/tools/buildscript_run.py
@@ -214,11 +214,14 @@
     script_output = run_buildscript(args.buildscript, env=env, cwd=cwd)

     cargo_rustc_cfg_pattern = re.compile("^cargo:rustc-cfg=(.*)")
+    cargo_rustc_env_pattern = re.compile("^cargo:rustc-env=(.*)")
     flags = ""
     for line in script_output.split("\n"):
-        cargo_rustc_cfg_match = cargo_rustc_cfg_pattern.match(line)
-        if cargo_rustc_cfg_match:
-            flags += "--cfg={}\n".format(cargo_rustc_cfg_match.group(1))
+        if match := cargo_rustc_cfg_pattern.match(line):
+            flags += "--cfg={}\n".format(match.group(1))
+        elif match := cargo_rustc_env_pattern.match(line):
+            # Only works on nightly for now  https://github.com/rust-lang/rust/issues/80792
+            flags += "--env-set={}\n".format(match.group(1))
         else:
             print(line, end="\n")
     args.outfile.write(flags)