Rollup merge of #147136 - Jules-Bertholet:const-_-unused-vis, r=jdonszelmann

Add warn-by-default lint for visibility on `const _` declarations

Add a warn-by-default `unused_visibilities` lint for visibility qualifiers on `const _` declarations—e.g. `pub const _: () = ();`. Such qualifiers have no effect.

A [Sourcegraph search](https://sourcegraph.com/search?q=context:global+lang:Rust+pub%5Cs*%28%5C%28.*%5C%29%29%3F%5Cs*const%5Cs%2B_%5Cs*:&patternType=regexp&case=yes&sm=0) suggests that this pattern is relatively rare, and mostly found in tests (with only 3 exceptions). So perhaps this could become an FCW/hard error in the future.

`@rustbot` label T-lang A-lints A-visibility -T-clippy

Author: matthiaskrgr

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -33,7 +33,7 @@ use rustc_session::Session;
 use rustc_session::lint::BuiltinLintDiag;
 use rustc_session::lint::builtin::{
     DEPRECATED_WHERE_CLAUSE_LOCATION, MISSING_ABI, MISSING_UNSAFE_ON_EXTERN,
-    PATTERNS_IN_FNS_WITHOUT_BODY,
+    PATTERNS_IN_FNS_WITHOUT_BODY, UNUSED_VISIBILITIES,
 };
 use rustc_session::parse::feature_err;
 use rustc_span::{Ident, Span, kw, sym};
@@ -1339,14 +1339,26 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                     }
                 });
             }
-            ItemKind::Const(box ConstItem { defaultness, rhs, .. }) => {
+            ItemKind::Const(box ConstItem { defaultness, ident, rhs, .. }) => {
                 self.check_defaultness(item.span, *defaultness);
                 if rhs.is_none() {
                     self.dcx().emit_err(errors::ConstWithoutBody {
                         span: item.span,
                         replace_span: self.ending_semi_or_hi(item.span),
                     });
                 }
+                if ident.name == kw::Underscore
+                    && !matches!(item.vis.kind, VisibilityKind::Inherited)
+                    && ident.span.eq_ctxt(item.vis.span)
+                {
+                    self.lint_buffer.buffer_lint(
+                        UNUSED_VISIBILITIES,
+                        item.id,
+                        item.vis.span,
+                        BuiltinLintDiag::UnusedVisibility(item.vis.span),
+                    )
+                }
+
                 visit::walk_item(self, item);
             }
             ItemKind::Static(box StaticItem { expr, safety, .. }) => {

compiler/rustc_lint/messages.ftl

@@ -978,6 +978,10 @@ lint_unused_op = unused {$op} that must be used
 
 lint_unused_result = unused result of type `{$ty}`
 
+lint_unused_visibilities = visibility qualifiers have no effect on `const _` declarations
+    .note = `const _` does not declare a name, so there is nothing for the qualifier to apply to
+    .suggestion = remove the qualifier
+
 lint_use_let_underscore_ignore_suggestion = use `let _ = ...` to ignore the expression or result
 
 lint_useless_ptr_null_checks_fn_ptr = function pointers are not nullable, so checking them for null will always return false

compiler/rustc_lint/src/early/diagnostics.rs

@@ -302,6 +302,9 @@ pub fn decorate_builtin_lint(
         BuiltinLintDiag::UnusedCrateDependency { extern_crate, local_crate } => {
             lints::UnusedCrateDependency { extern_crate, local_crate }.decorate_lint(diag)
         }
+        BuiltinLintDiag::UnusedVisibility(span) => {
+            lints::UnusedVisibility { span }.decorate_lint(diag)
+        }
         BuiltinLintDiag::AttributeLint(kind) => decorate_attribute_lint(sess, tcx, &kind, diag),
     }
 }

compiler/rustc_lint/src/lib.rs

@@ -291,6 +291,7 @@ fn register_builtins(store: &mut LintStore) {
         "unused",
         UNUSED_IMPORTS,
         UNUSED_VARIABLES,
+        UNUSED_VISIBILITIES,
         UNUSED_ASSIGNMENTS,
         DEAD_CODE,
         UNUSED_MUT,

compiler/rustc_lint/src/lints.rs

@@ -3191,3 +3191,11 @@ pub(crate) struct UnsafeAttrOutsideUnsafeSuggestion {
     #[suggestion_part(code = ")")]
     pub right: Span,
 }
+
+#[derive(LintDiagnostic)]
+#[diag(lint_unused_visibilities)]
+#[note]
+pub(crate) struct UnusedVisibility {
+    #[suggestion(style = "short", code = "", applicability = "machine-applicable")]
+    pub span: Span,
+}

compiler/rustc_lint_defs/src/builtin.rs

@@ -143,6 +143,7 @@ declare_lint_pass! {
         UNUSED_QUALIFICATIONS,
         UNUSED_UNSAFE,
         UNUSED_VARIABLES,
+        UNUSED_VISIBILITIES,
         USELESS_DEPRECATED,
         VARARGS_WITHOUT_PATTERN,
         WARNINGS,
@@ -693,6 +694,26 @@ declare_lint! {
     "detect variables which are not used in any way"
 }
 
+declare_lint! {
+    /// The `unused_visibilities` lint detects visibility qualifiers (like `pub`)
+    /// on a `const _` item.
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// pub const _: () = {};
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// These qualifiers have no effect, as `const _` items are unnameable.
+    pub UNUSED_VISIBILITIES,
+    Warn,
+    "detect visibility qualifiers on `const _` items"
+}
+
 declare_lint! {
     /// The `unused_assignments` lint detects assignments that will never be read.
     ///

compiler/rustc_lint_defs/src/lib.rs

@@ -696,6 +696,7 @@ pub enum BuiltinLintDiag {
         extern_crate: Symbol,
         local_crate: Symbol,
     },
+    UnusedVisibility(Span),
     AttributeLint(AttributeLintKind),
 }
 

src/tools/clippy/tests/ui-toml/absolute_paths/absolute_paths.no_short.stderr

@@ -71,10 +71,10 @@ LL |         impl<T: core::cmp::Eq> core::fmt::Display for X<T>
    |                                ^^^^^^^^^^^^^^^^^^
 
 error: consider bringing this path into scope with the `use` keyword
-  --> tests/ui-toml/absolute_paths/absolute_paths.rs:113:14
+  --> tests/ui-toml/absolute_paths/absolute_paths.rs:113:10
    |
-LL | pub const _: crate::S = {
-   |              ^^^^^^^^
+LL | const _: crate::S = {
+   |          ^^^^^^^^
 
 error: consider bringing this path into scope with the `use` keyword
   --> tests/ui-toml/absolute_paths/absolute_paths.rs:114:9

src/tools/clippy/tests/ui-toml/absolute_paths/absolute_paths.rs

@@ -110,7 +110,7 @@ mod m1 {
 }
 
 //~[no_short]v absolute_paths
-pub const _: crate::S = {
+const _: crate::S = {
     let crate::S = m1::S; //~[no_short] absolute_paths
 
     crate::m1::S

tests/ui/consts/promoted_const_call3.rs

@@ -4,12 +4,12 @@ pub const C: () = {
     //~^ ERROR: destructor of `String` cannot be evaluated at compile-time
 };
 
-pub const _: () = {
+const _: () = {
     let _: &'static _ = &id(&String::new());
     //~^ ERROR: destructor of `String` cannot be evaluated at compile-time
 };
 
-pub const _: () = {
+const _: () = {
     let _: &'static _ = &std::mem::ManuallyDrop::new(String::new());
     //~^ ERROR: temporary value dropped while borrowed
 };