Rust workflow recompiles the full dependency tree once per function in a Cargo workspace (build time scales linearly with function count)

[bnusunny]

Description

This is a performance follow-up to #634. That issue fixed correctness for Cargo-workspace projects (binaries are now found), but the underlying build strategy still compiles each function in isolation, so shared dependencies are recompiled once per function.

For a workspace with N Rust functions that share heavy dependencies (e.g. the AWS SDK), sam build compiles the shared dependency tree N times. In a real project with 16 functions sharing the AWS SDK, this makes sam build take over an hour.

Root cause

In cargo_lambda.py (v1.65.0):

if not os.getenv("CARGO_TARGET_DIR"):
    # This results in the "target" dir being created under the member dir of a cargo workspace
    os.environ["CARGO_TARGET_DIR"] = "target"

CARGO_TARGET_DIR is set to the relative path target, and the build runs with cwd=self._source_dir (each function's own member directory — actions.py#L97). So each function gets an isolated <member>/target/, and cargo cannot reuse any compiled dependency across functions. The dependency graph is rebuilt from scratch for every function.

Expected behavior

For a workspace, compile once and package many. A single cargo lambda build --release [--arm64] at the workspace root builds every member binary in one pass, compiling shared deps only once.

Local measurement on the same 16-function workspace:

• Current (per-function): > 1 hour
• Single workspace build: ~10m cold, ~2m19s warm

Possible fix

The builder already supports selecting a single binary — build_command appends --bin <handler> (actions.py#L90). A workspace-aware path could:

1. Detect the Cargo workspace root and run one cargo lambda build there (shared target/, deps compiled once).
2. Resolve each function's binary from the shared target/lambda/<name>/ output by package/bin name, and copy it into that function's ARTIFACTS_DIR — instead of shelling out per CodeUri.

This keeps BuildMethod: rust-cargolambda transparent to users: no per-function Makefiles and no manual binary distribution. (cargo-lambda-cdk already handles the workspace case, as noted in #634.)

Current workaround

Switch each function to BuildMethod: makefile and drive a single workspace cargo lambda build yourself, distributing each bootstrap into its function dir before sam build packages it. Works, but pushes SAM's build responsibility into user-maintained Makefiles.

[bnusunny]

Fix in #901