[agent] Found by the scheduled Cargo bug-hunt routine (ledger #315).

Summary

When a second version of one crate is vendored, vendor decides whether to emit cargo_multi_version_old_cargo by reading the project's declared cargo floor (declared_cargo_minor). A workspace-root manifest whose [package] inherits the floor with rust-version.workspace = true (and [workspace.package] rust-version = "1.70") is treated as declaring nothing. The project gets the warning, and the warning says it "declares no rust-version or toolchain, so the cargo that builds it is unknown".

The cause: [package].rust-version is present but is a table ({ workspace = true }), not a string. .get("rust-version") returns Some(table), so the .or_else fallback to [workspace.package].rust-version never runs, and then .as_str() yields None.

Impact

Low severity. It's a false, misleading warning, not a broken build. The build is fine. Workspace inheritance needs cargo 1.64+, so any project using rust-version.workspace = true already can't be built by the pre-1.45 cargo the warning is about. Root packages that inherit from [workspace.package] are the standard modern workspace layout, so every multi-version vendor in such a project emits a spurious warning, which trains users to ignore it.

Repro

This used a scratch copy of crates/socket-patch-cli/tests/e2e_vendor_cargo_build.rs (stage_two_versions fixture: cfg-if 1.x plus cfg_if_old = { package = "cfg-if", version = "0.1" }, a marker patch per version, and the prebuilt_common artifact server). Only the root Cargo.toml was swapped, then I ran socket-patch vendor --json --offline:

[workspace]

[workspace.package]
rust-version = "1.70"

[package]
name = "consumer"
version = "0.1.0"
edition = "2018"
rust-version.workspace = true

[dependencies]
cfg-if = "1.0"
cfg_if_old = { package = "cfg-if", version = "0.1" }

I ran the scratch test twice with identical results. The warning text on the inherited shape:

cfg-if is now vendored at TWO versions (… beside …), which needs cargo 1.45 or newer — this project declares no rust-version or toolchain, so the cargo that builds it is unknown. …

Expected vs actual

• Expected. CLI_CONTRACT.md (cargo vendored row) says "a project that does not pin cargo ≥ 1.45 (rust-version or toolchain file) gets the cargo_multi_version_old_cargo warning", and docs/ecosystems.md says the warning is skipped when "the project's rust-version … promises cargo 1.45+". The doc comment on declared_cargo_minor reads "[package], else [workspace.package]". This project does pin 1.70, through cargo's documented inheritance, so it should get no warning.
• Actual. The warning fires and claims the project declares no rust-version.

OS × version

Main 045d7ec (CLI 4.0.0). I didn't bisect this: the multi-version warning was added with the v5 manifest wiring.

Suspect code

crates/socket-patch-core/src/vendor/cargo.rs:90-99 (declared_cargo_minor): doc.get("package").and_then(|p| p.get("rust-version")).or_else(...) short-circuits on the { workspace = true } table. Treat a non-string [package].rust-version (or rust-version.workspace = true) as "look in [workspace.package]". Optionally, treat any use of workspace inheritance as proof of cargo ≥ 1.64.

Backlog review — 2026-10-08

Priority: P2 → P3. Builds work; workspace rust-version inheritance only causes a spurious warning. Keep the Cargo reader work in #1110.