shared: support runtime variable for schema lookup

[UebelAndre]

This change fixes a regression in being able to use version 0.2.75 for the Bazel wasm_bindgen rules. Bazel builds each crate in a separate sandbox and as a result, the CARGO_MANIFEST_DIR path is not consistent between use at runtime and compilation. To work around this, I'm looking for some way to control the lookup of the schema file when the build.rs script is run.

The change here allows for the wasm_bindgen_shared crate's build.rs script to allow CARGO_MANIFEST_DIR to be set at runtime to determine the location of the schema file.

[alexcrichton]

Is this an env var that Cargo sets for execution as well as compile time? In that can this change to simply env::var?

[UebelAndre]

Is this an env var that Cargo sets for execution as well as compile time? In that can this change to simply env::var?

Ah, I thought it wasn't but it appears it is:

wasm-bindgen % cargo build
   Compiling proc-macro2 v1.0.28
   Compiling unicode-xid v0.2.2
   Compiling log v0.4.14
   Compiling syn v1.0.74
   Compiling wasm-bindgen-shared v0.2.75 (/Users/user/wasm-bindgen/crates/shared)
   Compiling cfg-if v1.0.0
   Compiling bumpalo v3.7.0
   Compiling lazy_static v1.4.0
   Compiling wasm-bindgen v0.2.75 (/Users/user/wasm-bindgen)
   Compiling quote v1.0.9
   Compiling wasm-bindgen-backend v0.2.75 (/Users/user/wasm-bindgen/crates/backend)
   Compiling wasm-bindgen-macro-support v0.2.75 (/Users/user/wasm-bindgen/crates/macro-support)
   Compiling wasm-bindgen-macro v0.2.75 (/Users/user/wasm-bindgen/crates/macro)
    Finished dev [unoptimized + debuginfo] target(s) in 8.73s

(Tested with the following diff)

diff --git a/crates/shared/build.rs b/crates/shared/build.rs
index 8e6ea1b0..7de949af 100644
--- a/crates/shared/build.rs
+++ b/crates/shared/build.rs
@@ -22,6 +22,7 @@ fn main() {
 }

 fn set_schema_version_env_var() {
+    assert_eq!(env!("CARGO_MANIFEST_DIR").to_owned(), env::var("CARGO_MANIFEST_DIR").unwrap());
     let cargo_manifest_dir = env::var("CARGO_MANIFEST_DIR").expect(
         "The `CARGO_MANIFEST_DIR` environment variable is needed to locate the schema file",
     );

[alexcrichton]

Ok, thanks!

[UebelAndre]

Thanks @alexcrichton! What's the release cadence look like for wasm-bindgen? It'd be nice to get a 0.2.76 with this fix in 🙏

[alexcrichton]

I generally just release when requested, I'll try to get around to it this week.

[alexcrichton]

Ok I've published 0.2.76 now

[UebelAndre]

Thank you so much! And extra thanks for all the amazing work you've done. You're an inspiration to say the least 😄