misc fixes and nits from clippy

[asmello]

• Make diagnose_with lazy, as it was intended to be
• Clippy nits related to lifetimes
• remove leftover test

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 95.7%. Comparing base (c0201c7) to head (fe11cdc).

Files with missing lines	Patch %	Lines
src/diagnostic.rs	0.0%	1 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
src/pointer.rs	95.5% <ø> (-0.2%)	⬇️
src/diagnostic.rs	74.4% <0.0%> (ø)

[asmello]

Going to merge since I think this is fairly uncontroversial, but worth a look for awareness @chanced .