Stale `check-cfg` warnings in cache (Rust 2024)

[BartMassey]

Note: This bug report was written by Claude Code based on a debugging session where Claude isolated this issue with my help; it was then edited by me. I have verified the report independently: to the best of my knowledge and ability it is correct, and clearer and more complete than what I would have written.

Summary

When using sccache with Rust 2024 edition projects, check-cfg warnings about unknown feature values are cached and persist even after:

• The Cargo.toml is properly configured with the features
• Running cargo clean
• Restarting the sccache server

The cached warnings only clear when either:

1. The sccache cache directory is manually deleted (rm -rf ~/.cache/sccache)
2. sccache is bypassed entirely (RUSTC_WRAPPER="" cargo check)
3. The package name is changed (forcing a new cache key)

This causes confusing behavior where cargo is passing the correct --check-cfg flags to rustc, but the compilation output still shows warnings about missing features.

Environment

• Rust version: 1.93.1 (01f6ddf75 2026-02-11)
• Cargo version: 1.93.1 (083ac5135 2025-12-15)
• sccache version: 0.13.0
• OS: Linux 6.18.9+deb14-amd64
• Edition: 2024

Reproduction Steps

1. Create a new library with Rust 2024 edition (WITHOUT features initially):

cargo new --lib test_sccache_bug
cd test_sccache_bug

2. Edit Cargo.toml to use edition 2024 but DON'T add features yet:

[package]
name = "test_sccache_bug"
version = "0.1.0"
edition = "2024"

3. Edit src/lib.rs to reference a feature that doesn't exist:

#![cfg_attr(not(feature = "std"), no_std)]

4. Build with sccache enabled to cache the warning:

cargo check --no-default-features

Expected: Warning about unknown std feature (this is correct behavior at this point)

Actual: You'll see the expected warning:

warning: unexpected `cfg` condition value: `std`
 --> src/lib.rs:1:17
  |
1 | #![cfg_attr(not(feature = "std"), no_std)]
  |                 ^^^^^^^^^^^^^^^ help: remove the condition
  |
  = note: no expected values for `feature`
  = help: consider adding `std` as a feature in `Cargo.toml`

This warning is now cached by sccache.

5. Fix the configuration by adding the feature to Cargo.toml:

[package]
name = "test_sccache_bug"
version = "0.1.0"
edition = "2024"

[features]
std = []
default = ["std"]

6. Build again - the warning should disappear but doesn't:

cargo check --no-default-features

Expected: No warnings (the std feature is now properly defined)

Actual: The same warning still appears! This is the bug.

7. Verify cargo is passing correct flags:

cargo clean
cargo check --no-default-features -vv 2>&1 | grep "check-cfg"

You'll see that cargo IS correctly passing:

--check-cfg 'cfg(feature, values("default", "std"))'

This proves cargo knows about the feature and is telling rustc, but sccache is returning the cached warning.

8. Confirm it's an sccache caching issue:

# Bypass sccache - warning disappears
cargo clean
RUSTC_WRAPPER="" cargo check --no-default-features
# No warning!

# Or clear sccache cache - warning disappears
rm -rf ~/.cache/sccache
cargo clean
cargo check --no-default-features
# No warning!

Root Cause Analysis

When cargo passes updated --check-cfg flags to rustc via sccache, sccache appears to return cached compilation results (including cached warnings) from a previous compilation where the flags were different or missing.

The cache key doesn't seem to properly account for changes in the --check-cfg flags, causing stale warnings to persist even when the underlying issue has been fixed in the project configuration.

Expected Behavior

sccache should either:

1. Include --check-cfg flags in its cache key calculation, or
2. Not cache warning/diagnostic output separately from the compilation itself, or
3. Invalidate cached results when --check-cfg flags change

Impact

This issue is particularly problematic because:

• It causes confusion during development (warnings that shouldn't exist)
• It makes it unclear whether project configuration is correct
• Standard debugging steps (cargo clean) don't resolve it
• It specifically affects Rust 2024 edition projects using the new check-cfg features
• New users may think there's a problem with their Cargo.toml when it's actually correct