Keep build machine paths out of the wasm

Panic messages carry source paths, so the 7.5.20 wasm names the home
directory it was built in 54 times. Remap the sysroot, the cargo home
and the checkout to /rustc, /cargo and /build.

scripts/build-wasm.sh

@@ -16,6 +16,11 @@ DENO_ASM=$DENO_DIR/data.js
 # build new via wasm-pack
 echo "*** Building Rust sources"
 cd $PKG_NAME
+
+# Panic messages carry source paths. Without this the published wasm names the
+# home directory of whoever built it; with it the bytes no longer depend on
+# where the toolchain, the registry or the checkout live.
+export RUSTFLAGS="${RUSTFLAGS:-} --remap-path-prefix=$(rustc --print sysroot)=/rustc --remap-path-prefix=${CARGO_HOME:-$HOME/.cargo}=/cargo --remap-path-prefix=$PWD=/build"
 if [ "$RUST_VER" == "stable" ]; then
   RUSTC_BOOTSTRAP=1 cargo build --target wasm32-unknown-unknown --release -Z build-std=std,panic_abort
 else