Changing workspace Cargo.toml doesn't invalidate build cache for workspace members

[illicitonion]

Problem

The environment variables Cargo sets when building crates don't appear to get updated if the crate's Cargo.toml inherits values from the workspace, and the only change between builds is the value in the workspace's Cargo.toml.

Steps

% cat >Cargo.toml <<EOF
[workspace]
members = ["lib"]
package.version = "0.2.0"
package.readme = "beep.md"
EOF
% mkdir -p lib/src
% cat >lib/Cargo.toml <<EOF
[package]
name = "lib"
version = "0.1.0"
edition = "2021"
readme = { workspace = true }
EOF
% cat >lib/src/main.rs <<EOF
fn main() {
    println!("Readme: {}", env!("CARGO_PKG_README"));
}
EOF
% cargo run -p lib 2>/dev/null
Readme: ../beep.md
% cat >Cargo.toml <<EOF
[workspace]
members = ["lib"]
package.version = "0.2.0"
package.readme = true
EOF
% # Next command should print ../README.md because the workspace value changed:
% cargo run -p lib 2>/dev/null
Readme: ../beep.md
% # Clearing the cache forces a rebuild, which gives the correct value:
% rm -rf target
% cargo run -p lib 2>/dev/null
Readme: ../README.md

Possible Solution(s)

No response

Notes

No response

Version

cargo 1.79.0 (ffa9cf99a 2024-06-03)
release: 1.79.0
commit-hash: ffa9cf99a594e59032757403d4c780b46dc2c43a
commit-date: 2024-06-03
host: aarch64-apple-darwin
libgit2: 1.7.2 (sys:0.18.3 vendored)
libcurl: 8.1.2 (sys:0.4.72+curl-8.6.0 system ssl:(SecureTransport) LibreSSL/3.3.6)
ssl: OpenSSL 1.1.1w  11 Sep 2023
os: Mac OS 13.5.2 [64-bit]

[epage]

Just reproduced this without workspace inheritance involved.

[epage]

Not seeing an existing issue for cargo-generated env variables but this seems similar in internals to #13280 and #10358.

[weihanglo]

Actually this doesn't need workspace to involve. Simply with a single package change package.readme = "foo" to package.readme = "bar", and observe it doesn't rebuild.

I believe the bug shares the same root cause from #13280 (comment), where CARGO_PKG_README was removed.

[weihanglo]

I'll go ahead a close this in favor of #13280. Thanks for the report and reproduction!

[weihanglo]

Reopen since #13280 could have a partial fix not covering internal environment variables like CARGO_PKG_README.