Add -Zdead-fn-elimination: skip codegen of functions unreachable from the entry point

[yijunyu]

Implements the accepted MCP compiler-team#976: an unstable flag that skips codegen of functions a binary cannot reach from its entry point.

What it does

-Zdead-fn-elimination (off by default) adds, for binary crates only, a reachability analysis that runs after monomorphization collection and before partitioning. Functions unreachable from the entry point are dropped from the mono item set, so the backend never generates LLVM IR for them.

$ rustc -O -Zdead-fn-elimination main.rs
note: 107 unreachable functions excluded from codegen by -Z dead-fn-elimination

The flag is inert when unset.

How it works

In collect_and_partition_mono_items, after collect_crate_mono_items and before partition:

1. Build a call graph by walking each item's MIR (optimized_mir), recording direct calls, function references in constants/statics, address-taken functions, and inline-asm sym operands.
2. Seed a BFS with: reachable_set (covers exported/pub/lang-item/#[no_mangle]/#[used] items), the entry fn, and the methods of any dyn-constructed trait (dynamic dispatch is invisible to a call-graph walk, so vtable methods are seeded explicitly).
3. Run the BFS; any local fn-like item not in the result, and not otherwise required, is marked eliminable.
4. retain the mono item set, dropping only local MonoItem::Fns that are eliminable.

Conservatism

The analysis never eliminates:

• any item the monomorphization collector retained;
• lang items, exported symbols, the entry fn;
• drop glue, generic functions, async functions.

A #[cfg(debug_assertions)] invariant checks that reachable_set is a subset of the post-BFS reachable set.

Soundness testing

Verified on a stage1 build by compiling real binaries with and without the flag using the same compiler and diffing the output:

project	functions excluded	behavioral output
ripgrep	107	byte-identical to no-flag across 10 invocations (search, -n/-i/-c/-w/-v/-o, --json, recursive)
nushell	27 (6 crates)	identical across 8 pipelines (arithmetic, math sum, str ops, sort, record access, each closures)

Edge cases checked: flag-off and flag-on both produce reproducible binaries; library crates are inert (no entry seed); the #[test] harness still runs; panic=abort works; -C link-dead-code correctly defers (no elimination). Functions reached only through a closure (Iterator::map), a trait object (Box<dyn Fn>), or a function pointer are kept — only genuinely-unreachable functions are removed.
tests/codegen-units (46) and tests/ui/codegen (147) pass; tidy is clean.

Scope

This is the in-rustc flag only. It is self-contained and reviewable in isolation; there is no build-system or cross-crate coordination in this PR.

r? @oli-obk

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[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[bjorn3]

I still don't get what you are trying to do. If functions are unreachable from monomorphization roots (fn main(), #[no_mangle] or #[used] for binaries and a bunch more things for libraries), then the monomorphization collector should not collect them in the first place. Doing an analysis afterwards to throw away things that shouldn't have been collected in the first place seems to me like it would never eliminate anything.

Verified on a stage1 build by compiling real binaries with and without the flag using the same compiler and diffing the output:

Can you show a list of functions that are actually eliminated?

[yijunyu]

You're right, and thanks for pushing on this — I checked, and on a binary the analysis as written eliminates nothing the collector would have emitted.

I instrumented it to print, for every function the pass marks eliminable, whether the monomorphization collector had actually collected it. On ripgrep:

eliminated: 91
  already in all_mono_items (collected): 0
  not collected by the collector:        91

All 91 are items the lazy collector never collected in the first place — uninstantiated derive impls and helpers, e.g.:

<flags::lowargs::SpecialMode as Clone>::clone
<flags::lowargs::Mode as PartialEq>::eq
flags::hiargs::suggest_pcre2

So the retain drops things that were never in the codegen set. The "byte-identical output" I reported is real but for the wrong reason: the binary is identical because nothing that mattered was removed. For a binary crate, the collector's reachability already equals what an entry-point BFS would compute, so a post-collection filter is a no-op. That matches exactly what you said.

The only place this analysis removes anything the collector does emit is cross-crate: a library is compiled before its callers, so its collector must conservatively emit every pub non-generic function, even ones the final binary never calls. Eliminating those needs the binary's whole-program reachability fed back to the library's compilation — which a single rustc invocation doesn't have, and which is really a cargo/build-orchestration question (deferring library codegen, or a seed-exchange format) rather than something this flag can do on its own.

Given that, I think the useful question is whether the cross-crate version is something the team would want to pursue. The mechanism in this PR (the codegen-skipping hook + the pre-partition filter) is the plumbing a seed-consuming flag would need, but on its own — without a seed source telling a library which of its pub functions the final binary actually uses — it has no effect, so I don't want to push it as-is.

A couple of directions I could take it:

1. Develop the cross-crate version as a design proposal (it needs cargo coordination — deferring library codegen, or a seed-exchange format — so it's partly a build-system question).
2. Keep just the mechanism here as documented plumbing for that future flag, clearly noting it's inert without a seed source.

I'm happy to go whichever way the team thinks is most useful — including parking this until the cross-crate design is worked out. What would you prefer?

[rust-log-analyzer]

The job tidy failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

[TIMING:end] tool::ToolBuild { build_compiler: Compiler { stage: 0, host: x86_64-unknown-linux-gnu, forced_compiler: false }, target: x86_64-unknown-linux-gnu, tool: "tidy", path: "src/tools/tidy", mode: ToolBootstrap, source_type: InTree, extra_features: [], allow_features: "", cargo_args: [], artifact_kind: Binary } -- 12.382
[TIMING:end] tool::Tidy { compiler: Compiler { stage: 0, host: x86_64-unknown-linux-gnu, forced_compiler: false }, target: x86_64-unknown-linux-gnu } -- 0.000
fmt check
Diff in /checkout/compiler/rustc_monomorphize/src/used_set.rs:91:
 /// collector, which filters to local `DefId`s and so never lists non-generic extern fns.
 pub(crate) fn emit_used_sets(tcx: TyCtxt<'_>, dir: &Path) {
     if let Err(e) = std::fs::create_dir_all(dir) {
-        tcx.sess.dcx().warn(format!("-Z dead-fn-emit-used-set: cannot create {}: {e}", dir.display()));
+        tcx.sess
+            .dcx()
+            .warn(format!("-Z dead-fn-emit-used-set: cannot create {}: {e}", dir.display()));
         return;
     }
 
Diff in /checkout/compiler/rustc_monomorphize/src/used_set.rs:102:
         if !tcx.is_mir_available(def_id) {
             continue;
         }
-        let Ok(body) =
-            panic::catch_unwind(panic::AssertUnwindSafe(|| tcx.optimized_mir(def_id)))
+        let Ok(body) = panic::catch_unwind(panic::AssertUnwindSafe(|| tcx.optimized_mir(def_id)))
         else {
             continue;
         };
fmt: checked 7000 files
Bootstrap failed while executing `test src/tools/tidy tidyselftest --extra-checks=py,cpp,js,spellcheck`

[ds84182]

You're right, and thanks for pushing on this — I checked, and on a binary the analysis as written eliminates nothing the collector would have emitted.

Nice LLM-ism. I'm guessing your PR description, response, and code is AI generated?