Coverage beyond blocks: M1 denominators, diagnostics, metadata tables, codegen (no rebuild)

[zmaril]

The parts of docs/coverage-plan.md that need no rebuild of the instrumented compiler: M1, M3, and the codegen half of M5. Each is a mirth-lab subcommand with a report and gap lists. rustc/coverage-report.sh runs them after the function and block report.

What's measured (2026-10-10, block corpus cov-blk plus the suites in cov-suites)

dimension	denominator	measured
branch arms (source level, M1)	115,951 arms in reachable functions (41,265 implicit across all functions)	67,444 measurable from block sites; 57,957 taken (85.9%); implicit arms wait for M2
configuration branches	1,114 (decision, kind) pairs: feature 321, option 334, target 357, edition 102	1,078 consulted; both arms seen at 168
feature gates	262: 136 checked by accessor calls, 109 only through generic code, 17 not named in the compiler	all 136 consulted
delayed-bug sites	238 in reachable functions	177 reached (74.4%)
keyed engine sites	330 queries × 1,018 engine sites	307,444 possible pairs (M4 measures them)
error codes	387 live (131 retired)	365 (94.3%); 4 retired codes still emitted
lints	253	225 (88.9%)
diagnostic / subdiagnostic messages	1,900 / 705 measurable	1,308 (68.8%) / 599 (85.0%)
metadata tables	82	67 encoded, 62 decoded in the blessed records; no dylib/staticlib/cdylib record
LLVM intrinsics	76	59 in the IR of the 3,493 runnable tests (77.6%)
Rust intrinsic codegen arms	189	143 (75.7%)
calling-convention lowering	4,432 block sites in 26 files	3,705 (83.6%)

The plan's budget, corrected

• Branch arms: M1 counts 0.16 source-level arms per reachable site. The plan estimated 0.5. Expect about +100k to +200k arm sites, which M2's MIR pass will settle.
• Keyed engine sites: 330 queries × 1,018 engine sites. The plan estimated about 700 × 150.

Caveats

• Estimates. Numbers that come from block sites are estimates: a span counts as having run when a block site inside it was reached.
• Moved source. Files whose source has moved since the coverage build are detected and skipped.
• Corpus. The diagnostics corpus is the standalone UI tests at --emit=metadata, so crate-loading errors, auxiliary-crate lints and codegen/link lints can't appear.

🤖 Generated with Claude Code

https://claude.ai/code/session_01QXiEXbESemwqMLYKaWLDbT