[ma2bd]

Motivation

CI on testnet_conway is red since ad6ceee1fb ("prepare release of SDK v0.15.23", #6816), in
three jobs that all fail the same way:

Workflow / job	Failing test
Rust / default-features-and-witty-integration-test	local::test_project_new
Rust / storage-service-tests	test_project_publish::storage_service_grpc
ScyllaDB tests / test	test_project_publish::scylladb_grpc

error: rustc 1.86.0 is not supported by the following packages:
  encoding_rs@0.8.41 requires rustc 1.88

These tests build a project generated from linera-service/template/Cargo.toml.template, which has
no lockfile, so Cargo resolves each dependency to its newest version. encoding_rs 0.8.40
(2026-09-07) and 0.8.41 (2026-09-09) raised the minimum Rust version to 1.88, while this branch is
pinned to 1.86.0. The last green run predates 0.8.40 by about four hours. The workspace's own
Cargo.lock still has 0.8.35, so nothing else is affected.

Since the template ships inside the linera binary, linera project new also fails for users of
the freshly released SDK v0.15.23.

Proposal

Cap encoding_rs below 0.8.40 in the template, next to the other versions pinned for Rust 1.86.0
under TODO(#4742), as in #6707 and #6356.

Test Plan

CI, plus a local reproduction: instantiating the template by hand with linera-sdk as a path
dependency and running the same command the test runs,
cargo build --release --target wasm32-unknown-unknown on Rust 1.86.0.

• without the pin: fails with the error above, identical to CI;
• with the pin: resolves encoding_rs to 0.8.35 and builds cleanly, with no other crate reporting
  an incompatible minimum Rust version.

Release Plan

This PR targets testnet_conway directly and should go into the next SDK release from this
branch. main is on Rust 1.95 and is not affected.

Links

• reviewer checklist
• [testnet] Keep the project template on icu versions supporting Rust 1.86 #6707, [testnet] template: pin allocative_derive < 0.3.5 #6356: earlier pins of the same kind
• Applications don't load with Rust 1.87 or later #4742: tracking issue for removing these pins