build_web: use rustup's cargo even when another Rust comes first on PATH

A standalone "Rust stable GNU" install ahead of ~/.cargo/bin made cargo
fail with E0463 (no wasm32 std). The script now puts CARGO_HOME/bin first,
checks for rustup and the wasm32 target, and says what to install.

tools/build_web.py

@@ -2,10 +2,11 @@
 
     python tools/build_web.py [--out dist/web] [--skip-cargo]
 
-Needs the wasm32-unknown-unknown target (rustup target add wasm32-unknown-unknown)
-and the wasm-bindgen CLI at the version in Cargo.lock. wasm-opt (binaryen) is
-used when it is on PATH. The output folder is static files: serve it from any
-web server (it needs http(s), not file://).
+Needs rustup with the wasm32-unknown-unknown target (rustup target add
+wasm32-unknown-unknown) and the wasm-bindgen CLI at the version in Cargo.lock.
+rustup's cargo is used even when another Rust install comes first on PATH.
+wasm-opt (binaryen) is used when it is on PATH. The output folder is static
+files: serve it from any web server (it needs http(s), not file://).
 """
 import argparse
 import hashlib
@@ -19,11 +20,44 @@ from pathlib import Path
 
 ROOT = Path(__file__).resolve().parent.parent
 SKIP_DIRS = {"dist", "target", "node_modules"}
+TARGET = "wasm32-unknown-unknown"
+
+# rustup's bin folder first: a standalone Rust install earlier on PATH (one
+# without the wasm target, e.g. "Rust stable GNU") must not win
+CARGO_BIN = Path(os.environ.get("CARGO_HOME") or Path.home() / ".cargo") / "bin"
+ENV = dict(os.environ)
+if CARGO_BIN.is_dir():
+    ENV["PATH"] = str(CARGO_BIN) + os.pathsep + ENV.get("PATH", "")
+
+
+def tool(name):
+    return shutil.which(name, path=ENV["PATH"])
 
 
 def run(cmd, **kw):
     print("+", " ".join(str(c) for c in cmd), flush=True)
-    subprocess.run([str(c) for c in cmd], check=True, cwd=ROOT, **kw)
+    subprocess.run([str(c) for c in cmd], check=True, cwd=ROOT, env=ENV, **kw)
+
+
+def check_toolchain():
+    """rustup with the wasm target, and wasm-bindgen at the locked version."""
+    rustup = tool("rustup")
+    if not rustup:
+        sys.exit("rustup not found (https://rustup.rs): the WASM build needs rustup's toolchain")
+    installed = subprocess.run([rustup, "target", "list", "--installed"], capture_output=True, text=True,
+                               cwd=ROOT, env=ENV).stdout.split()
+    if TARGET not in installed:
+        sys.exit(f"the {TARGET} target is not installed: rustup target add {TARGET}")
+    cargo = tool("cargo")
+    if not cargo or Path(cargo).parent.resolve() != CARGO_BIN.resolve():
+        sys.exit(f"cargo resolves to {cargo}, not rustup's in {CARGO_BIN}")
+    want = lock_version("wasm-bindgen")
+    exe = tool("wasm-bindgen")
+    if not exe:
+        sys.exit(f"wasm-bindgen CLI not found: cargo install wasm-bindgen-cli --version {want}")
+    have = subprocess.run([exe, "--version"], capture_output=True, text=True, env=ENV).stdout.split()[-1]
+    if have != want:
+        sys.exit(f"wasm-bindgen CLI is {have}, Cargo.lock has {want}: cargo install wasm-bindgen-cli --version {want}")
 
 
 def lock_version(name):
@@ -63,25 +97,21 @@ def main():
     wasm = ROOT / "target" / "wasm32-unknown-unknown" / "web" / "isokit_wasm.wasm"
 
     if not a.skip_cargo:
-        want = lock_version("wasm-bindgen")
-        try:
-            have = subprocess.run(["wasm-bindgen", "--version"], capture_output=True, text=True, check=True).stdout.split()[-1]
-        except (OSError, subprocess.CalledProcessError):
-            sys.exit(f"wasm-bindgen CLI not found: cargo install wasm-bindgen-cli --version {want}")
-        if have != want:
-            sys.exit(f"wasm-bindgen CLI is {have}, Cargo.lock has {want}: cargo install wasm-bindgen-cli --version {want}")
-        run(["cargo", "build", "-p", "isokit-wasm", "--target", "wasm32-unknown-unknown", "--profile", "web"])
+        check_toolchain()
+        run([tool("cargo"), "build", "-p", "isokit-wasm", "--target", TARGET, "--profile", "web"])
+    elif not tool("wasm-bindgen"):
+        sys.exit("wasm-bindgen CLI not found")
 
     if out.exists():
         shutil.rmtree(out)
     web.mkdir(parents=True)
     if not wasm.exists():
         sys.exit(f"{wasm} missing: run without --skip-cargo")
-    run(["wasm-bindgen", "--target", "web", "--no-typescript", "--remove-name-section",
+    run([tool("wasm-bindgen"), "--target", "web", "--no-typescript", "--remove-name-section",
          "--remove-producers-section", "--out-dir", web.as_posix(), wasm.as_posix()])
     bg = web / "isokit_wasm_bg.wasm"
-    if shutil.which("wasm-opt"):
-        run(["wasm-opt", "-Oz", "--all-features", bg.as_posix(), "-o", bg.as_posix()])
+    if tool("wasm-opt"):
+        run([tool("wasm-opt"), "-Oz", "--all-features", bg.as_posix(), "-o", bg.as_posix()])
     else:
         print("  (wasm-opt not found: the .wasm stays unoptimised by binaryen, about 15% larger)")