Auto merge of #149509 - mu001999-contrib:flag/pub-as-pub-crate, r=jdonszelmann

Lint unused pub items in binary crates



~~This PR adds a new unstable flag -Ztreat-pub-as-pub-crate as [@Kobzol](https://github.com/Kobzol) suggested.~~
~~When compiling binary crates with this flag, the seed worklist will only contain the entry fn and won't contain other reachable items. Then we can do the dead code analysis for pub items just like they are pub(crate).~~

Related zulip thread [#general > pub/pub(crate) within a binary is a footgun](https://rust-lang.zulipchat.com/#narrow/channel/122651-general/topic/pub.2Fpub.28crate.29.20within.20a.20binary.20is.20a.20footgun/with/558931034).

---

Updated:

Adds a new lint `unused_pub_items_in_binary` (crate-level, default allow for now) instead of the previous unstable flag to lint unused `pub` items for binary crates.

See more details of implementation in #149509 (comment).

This lint is allowed by default, but I believe this has been better than the unstable flag. Making it warn-by-default will lead to a lot of noise for this PR (like bless many tests). So I'd like to make it warn-by-default in a separate PR in the future.

Author: bors

compiler/rustc_lint_defs/src/builtin.rs

@@ -34,6 +34,7 @@ declare_lint_pass! {
         CONST_EVALUATABLE_UNCHECKED,
         CONST_ITEM_MUTATION,
         DEAD_CODE,
+        DEAD_CODE_PUB_IN_BINARY,
         DEPENDENCY_ON_UNIT_NEVER_TYPE_FALLBACK,
         DEPRECATED,
         DEPRECATED_IN_FUTURE,
@@ -789,6 +790,37 @@ declare_lint! {
     "detect unused, unexported items"
 }
 
+declare_lint! {
+    /// The `dead_code_pub_in_binary` lint detects unused `pub` items in
+    /// executable crates.
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// #![deny(dead_code_pub_in_binary)]
+    ///
+    /// pub fn unused_pub_fn() {}
+    ///
+    /// fn main() {}
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// In executable crates, `pub` items are often implementation details
+    /// rather than part of an external API. This lint helps find those items
+    /// when they are never used.
+    ///
+    /// This lint only applies to executable crates. In library crates, public
+    /// items are considered part of the crate's API and are not reported by
+    /// this lint.
+    pub DEAD_CODE_PUB_IN_BINARY,
+    Allow,
+    "detect public items in executable crates that are never used",
+    crate_level_only
+}
+
 declare_lint! {
     /// The `unused_attributes` lint detects attributes that were not used by
     /// the compiler.

compiler/rustc_middle/src/middle/dead_code.rs

@@ -0,0 +1,23 @@
+use rustc_data_structures::fx::FxIndexSet;
+use rustc_hir::def_id::{DefId, LocalDefIdMap, LocalDefIdSet};
+use rustc_macros::StableHash;
+
+/// A single snapshot of dead-code liveness analysis state.
+#[derive(Clone, Debug, StableHash)]
+pub struct DeadCodeLivenessSnapshot {
+    pub live_symbols: LocalDefIdSet,
+    /// Maps each ADT to derived traits (for example `Debug` and `Clone`) that should be ignored
+    /// when checking for dead code diagnostics.
+    pub ignored_derived_traits: LocalDefIdMap<FxIndexSet<DefId>>,
+}
+
+/// Dead-code liveness data for both analysis phases.
+///
+/// `pre_deferred_seeding` is computed before reachable-public and `#[allow(dead_code)]` seeding,
+/// and is used for lint `dead_code_pub_in_binary`.
+/// `final_result` is the final liveness snapshot used for lint `dead_code`.
+#[derive(Clone, Debug, StableHash)]
+pub struct DeadCodeLivenessSummary {
+    pub pre_deferred_seeding: DeadCodeLivenessSnapshot,
+    pub final_result: DeadCodeLivenessSnapshot,
+}

compiler/rustc_middle/src/middle/mod.rs

@@ -1,4 +1,5 @@
 pub mod codegen_fn_attrs;
+pub mod dead_code;
 pub mod debugger_visualizer;
 pub mod deduced_param_attrs;
 pub mod dependency_format;

compiler/rustc_middle/src/queries.rs

@@ -64,9 +64,7 @@ use rustc_errors::{ErrorGuaranteed, catch_fatal_errors};
 use rustc_hir as hir;
 use rustc_hir::attrs::{EiiDecl, EiiImpl, StrippedCfgItem};
 use rustc_hir::def::{DefKind, DocLinkResMap};
-use rustc_hir::def_id::{
-    CrateNum, DefId, DefIdMap, LocalDefId, LocalDefIdMap, LocalDefIdSet, LocalModDefId,
-};
+use rustc_hir::def_id::{CrateNum, DefId, DefIdMap, LocalDefId, LocalDefIdSet, LocalModDefId};
 use rustc_hir::lang_items::{LangItem, LanguageItems};
 use rustc_hir::{ItemLocalId, ItemLocalMap, PreciseCapturingArgKind, TraitCandidate};
 use rustc_index::IndexVec;
@@ -87,6 +85,7 @@ use crate::infer::canonical::{self, Canonical};
 use crate::lint::LintExpectation;
 use crate::metadata::ModChild;
 use crate::middle::codegen_fn_attrs::{CodegenFnAttrs, SanitizerFnAttrs};
+use crate::middle::dead_code::DeadCodeLivenessSummary;
 use crate::middle::debugger_visualizer::DebuggerVisualizerFile;
 use crate::middle::deduced_param_attrs::DeducedParamAttrs;
 use crate::middle::exported_symbols::{ExportedSymbol, SymbolExportInfo};
@@ -1203,13 +1202,8 @@ rustc_queries! {
         desc { "checking liveness of variables in `{}`", tcx.def_path_str(key.to_def_id()) }
     }
 
-    /// Return the live symbols in the crate for dead code check.
-    ///
-    /// The second return value maps from ADTs to ignored derived traits (e.g. Debug and Clone).
-    query live_symbols_and_ignored_derived_traits(_: ()) -> Result<&'tcx (
-        LocalDefIdSet,
-        LocalDefIdMap<FxIndexSet<DefId>>,
-    ), ErrorGuaranteed> {
+    /// Return dead-code liveness summary for the crate.
+    query live_symbols_and_ignored_derived_traits(_: ()) -> Result<&'tcx DeadCodeLivenessSummary, ErrorGuaranteed> {
         arena_cache
         desc { "finding live symbols in crate" }
     }

compiler/rustc_passes/src/check_attr.rs

@@ -1526,6 +1526,15 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
                         return;
                     }
                 }
+            } else if hir_id == CRATE_HIR_ID
+                && attr.has_any_name(&[sym::allow, sym::warn, sym::deny, sym::forbid, sym::expect])
+                && let Some(meta) = attr.meta_item_list()
+                && meta.iter().any(|meta| {
+                    meta.meta_item().is_some_and(|item| item.path == sym::dead_code_pub_in_binary)
+                })
+                && !self.tcx.crate_types().contains(&CrateType::Executable)
+            {
+                errors::UnusedNote::NoEffectDeadCodePubInBinary
             } else if attr.has_name(sym::default_method_body_is_const) {
                 errors::UnusedNote::DefaultMethodBodyConst
             } else {