Add a new unstable flag -Ztreat-pub-as-pub-crate

compiler/rustc_passes/src/dead.rs

@@ -19,6 +19,7 @@ use rustc_middle::middle::privacy::Level;
 use rustc_middle::query::Providers;
 use rustc_middle::ty::{self, AssocTag, TyCtxt};
 use rustc_middle::{bug, span_bug};
+use rustc_session::config::CrateType;
 use rustc_session::lint::builtin::DEAD_CODE;
 use rustc_session::lint::{self, LintExpectationId};
 use rustc_span::{Symbol, kw, sym};
@@ -813,22 +814,24 @@ fn maybe_record_as_seed<'tcx>(
 fn create_and_seed_worklist(
     tcx: TyCtxt<'_>,
 ) -> (Vec<(LocalDefId, ComesFromAllowExpect)>, Vec<LocalDefId>) {
-    let effective_visibilities = &tcx.effective_visibilities(());
     let mut unsolved_impl_item = Vec::new();
-    let mut worklist = effective_visibilities
-        .iter()
-        .filter_map(|(&id, effective_vis)| {
-            effective_vis
-                .is_public_at_level(Level::Reachable)
-                .then_some(id)
-                .map(|id| (id, ComesFromAllowExpect::No))
-        })
-        // Seed entry point
-        .chain(
-            tcx.entry_fn(())
-                .and_then(|(def_id, _)| def_id.as_local().map(|id| (id, ComesFromAllowExpect::No))),
-        )
-        .collect::<Vec<_>>();
+    let mut worklist = Vec::new();
+
+    if let Some((def_id, _)) = tcx.entry_fn(())
+        && let Some(local_def_id) = def_id.as_local()
+    {
+        worklist.push((local_def_id, ComesFromAllowExpect::No));
+    }
+
+    if !tcx.sess.opts.unstable_opts.treat_pub_as_pub_crate
+        || !tcx.crate_types().contains(&CrateType::Executable)
+    {
+        for (id, effective_vis) in tcx.effective_visibilities(()).iter() {
+            if effective_vis.is_public_at_level(Level::Reachable) {
+                worklist.push((*id, ComesFromAllowExpect::No));
+            }
+        }
+    }
 
     let crate_items = tcx.hir_crate_items(());
     for id in crate_items.owners() {

compiler/rustc_session/src/options.rs

@@ -2727,6 +2727,8 @@ written to standard error output)"),
     treat_err_as_bug: Option<NonZero<usize>> = (None, parse_treat_err_as_bug, [TRACKED],
         "treat the `val`th error that occurs as bug (default if not specified: 0 - don't treat errors as bugs. \
         default if specified without a value: 1 - treat the first error as bug)"),
+    treat_pub_as_pub_crate: bool = (false, parse_bool, [UNTRACKED],
+        "treat `pub` items as `pub(crate)` items for binary crates (default: no)"),
     trim_diagnostic_paths: bool = (true, parse_bool, [UNTRACKED],
         "in diagnostics, use heuristics to shorten paths referring to items"),
     tune_cpu: Option<String> = (None, parse_opt_string, [TRACKED],

src/doc/unstable-book/src/compiler-flags/treat-pub-as-pub-crate.md

@@ -0,0 +1,31 @@
+# `treat-pub-as-pub-crate`
+
+The `-Ztreat-pub-as-pub-crate` flag causes the compiler to treat all `pub` items in the crate as if they were `pub(crate)`. This is useful for finding dead code in binary crates where public items are not intended to be exported to other crates.
+
+When this flag is enabled, the dead code lint will warn about public items that are not used within the crate (unless they are the entry point, like `main`).
+
+## Example
+
+Consider the following code in a binary crate:
+
+```rust
+pub fn unused_pub_fn() {}
+
+fn main() {
+    println!("Hello, world!");
+}
+```
+
+By default, `rustc` assumes that `unused_pub_fn` might be used by other crates linking to this binary, so it does not warn about it being unused.
+
+With `-Ztreat-pub-as-pub-crate`, `rustc` will emit a warning:
+
+```text
+warning: function `unused_pub_fn` is never used
+ --> src/main.rs:1:8
+  |
+1 | pub fn unused_pub_fn() {}
+  |        ^^^^^^^^^^^^^
+  |
+  = note: `#[warn(dead_code)]` on by default
+```

tests/ui/lint/dead-code/treat-pub-as-pub-crate-lib.rs

@@ -0,0 +1,8 @@
+//@ compile-flags: -Ztreat-pub-as-pub-crate
+#![feature(rustc_attrs)]
+#![deny(dead_code)]
+#![crate_type = "lib"]
+
+pub fn unused_pub_fn() {} // Should NOT error because it's a library crate
+
+fn unused_priv_fn() {} //~ ERROR function `unused_priv_fn` is never used

tests/ui/lint/dead-code/treat-pub-as-pub-crate-lib.stderr

@@ -0,0 +1,14 @@
+error: function `unused_priv_fn` is never used
+  --> $DIR/treat-pub-as-pub-crate-lib.rs:8:4
+   |
+LL | fn unused_priv_fn() {}
+   |    ^^^^^^^^^^^^^^
+   |
+note: the lint level is defined here
+  --> $DIR/treat-pub-as-pub-crate-lib.rs:3:9
+   |
+LL | #![deny(dead_code)]
+   |         ^^^^^^^^^
+
+error: aborting due to 1 previous error
+

tests/ui/lint/dead-code/treat-pub-as-pub-crate.rs

@@ -0,0 +1,11 @@
+//@ compile-flags: -Ztreat-pub-as-pub-crate
+#![feature(rustc_attrs)]
+#![deny(dead_code)]
+
+pub fn unused_pub_fn() {} //~ ERROR function `unused_pub_fn` is never used
+
+pub fn used_pub_fn() {}
+
+fn main() {
+    used_pub_fn();
+}

tests/ui/lint/dead-code/treat-pub-as-pub-crate.stderr

@@ -0,0 +1,14 @@
+error: function `unused_pub_fn` is never used
+  --> $DIR/treat-pub-as-pub-crate.rs:5:8
+   |
+LL | pub fn unused_pub_fn() {}
+   |        ^^^^^^^^^^^^^
+   |
+note: the lint level is defined here
+  --> $DIR/treat-pub-as-pub-crate.rs:3:9
+   |
+LL | #![deny(dead_code)]
+   |         ^^^^^^^^^
+
+error: aborting due to 1 previous error
+