cargo "could not compile X (lib) due to N previous errors" in CI

This is cargo's summary line, not the root cause. rustc emitted N errors while compiling the crate, and cargo reports the count after them. The actual errors (type mismatches, unresolved imports) are printed above this line.

What this error means

cargo build ends with "error: could not compile X (lib) due to N previous errors", preceded by one or more error[EXXXX]: diagnostics from rustc.

error[E0308]: mismatched types
  --> src/lib.rs:14:18
   |
14 |     let n: u32 = "x";
   |            ---   ^^^ expected `u32`, found `&str`

error: could not compile `app` (lib) due to 1 previous error

Common causes

One or more rustc errors above the summary

The summary only counts errors; the diagnostics with error[EXXXX] codes and source spans printed earlier are the real failures to fix.

A warning promoted to an error

With -D warnings (often via RUSTFLAGS in CI), a lint that passes locally becomes a hard error that stops compilation.

How to fix it

Scroll up and fix the first error

1. Find the first error[EXXXX]: block above the summary line.
2. Fix that error using the span and the suggestion rustc prints.
3. Rebuild; later errors often disappear once the first is resolved.

cargo build 2>&1 | grep -n 'error\[' | head

Reproduce CI lint settings locally

If CI denies warnings, set the same RUSTFLAGS locally so warnings surface as errors before you push.

RUSTFLAGS="-D warnings" cargo build

How to prevent it

• Run cargo build (and clippy) locally with the same flags CI uses.
• Treat the first error, not the summary, as the thing to fix.
• Keep RUSTFLAGS consistent between local and CI builds.

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