add unreachable_cfg_select_predicates lint

Cargo.lock

@@ -3539,6 +3539,7 @@ dependencies = [
  "rustc_abi",
  "rustc_ast",
  "rustc_ast_pretty",
+ "rustc_data_structures",
  "rustc_errors",
  "rustc_feature",
  "rustc_hir",

compiler/rustc_attr_parsing/Cargo.toml

@@ -8,6 +8,7 @@ edition = "2024"
 rustc_abi = { path = "../rustc_abi" }
 rustc_ast = { path = "../rustc_ast" }
 rustc_ast_pretty = { path = "../rustc_ast_pretty" }
+rustc_data_structures = { path = "../rustc_data_structures" }
 rustc_errors = { path = "../rustc_errors" }
 rustc_feature = { path = "../rustc_feature" }
 rustc_hir = { path = "../rustc_hir" }

compiler/rustc_attr_parsing/src/attributes/cfg_select.rs

@@ -1,22 +1,35 @@
 use rustc_ast::token::Token;
 use rustc_ast::tokenstream::TokenStream;
 use rustc_ast::{AttrStyle, NodeId, token};
+use rustc_data_structures::fx::FxHashMap;
 use rustc_feature::{AttributeTemplate, Features};
 use rustc_hir::attrs::CfgEntry;
 use rustc_hir::{AttrPath, Target};
 use rustc_parse::exp;
 use rustc_parse::parser::{Parser, Recovery};
 use rustc_session::Session;
-use rustc_span::{ErrorGuaranteed, Span, sym};
+use rustc_session::lint::BuiltinLintDiag;
+use rustc_session::lint::builtin::UNREACHABLE_CFG_SELECT_PREDICATES;
+use rustc_span::{ErrorGuaranteed, Span, Symbol, sym};
 
 use crate::parser::MetaItemOrLitParser;
 use crate::{AttributeParser, ParsedDescription, ShouldEmit, parse_cfg_entry};
 
+#[derive(Clone)]
 pub enum CfgSelectPredicate {
     Cfg(CfgEntry),
     Wildcard(Token),
 }
 
+impl CfgSelectPredicate {
+    fn span(&self) -> Span {
+        match self {
+            CfgSelectPredicate::Cfg(cfg_entry) => cfg_entry.span(),
+            CfgSelectPredicate::Wildcard(token) => token.span,
+        }
+    }
+}
+
 #[derive(Default)]
 pub struct CfgSelectBranches {
     /// All the conditional branches.
@@ -115,5 +128,102 @@ pub fn parse_cfg_select(
         }
     }
 
+    if let Some(features) = features
+        && features.enabled(sym::cfg_select)
+    {
+        let it = branches
+            .reachable
+            .iter()
+            .map(|(entry, _, _)| CfgSelectPredicate::Cfg(entry.clone()))
+            .chain(branches.wildcard.as_ref().map(|(t, _, _)| CfgSelectPredicate::Wildcard(*t)))
+            .chain(
+                branches.unreachable.iter().map(|(entry, _, _)| CfgSelectPredicate::clone(entry)),
+            );
+
+        lint_unreachable(p, it, lint_node_id);
+    }
+
     Ok(branches)
 }
+
+fn lint_unreachable(
+    p: &mut Parser<'_>,
+    predicates: impl Iterator<Item = CfgSelectPredicate>,
+    lint_node_id: NodeId,
+) {
+    // Symbols that have a known value.
+    let mut known = FxHashMap::<Symbol, bool>::default();
+    let mut wildcard_span = None;
+    let mut it = predicates;
+
+    let branch_is_unreachable = |predicate: CfgSelectPredicate, wildcard_span| {
+        let span = predicate.span();
+        p.psess.buffer_lint(
+            UNREACHABLE_CFG_SELECT_PREDICATES,
+            span,
+            lint_node_id,
+            BuiltinLintDiag::UnreachableCfg { span, wildcard_span },
+        );
+    };
+
+    for predicate in &mut it {
+        let CfgSelectPredicate::Cfg(ref cfg_entry) = predicate else {
+            wildcard_span = Some(predicate.span());
+            break;
+        };
+
+        match cfg_entry {
+            CfgEntry::Bool(true, _) => {
+                wildcard_span = Some(predicate.span());
+                break;
+            }
+            CfgEntry::Bool(false, _) => continue,
+            CfgEntry::NameValue { name, value, .. } => match value {
+                None => {
+                    // `name` will be false in all subsequent branches.
+                    let current = known.insert(*name, false);
+
+                    match current {
+                        None => continue,
+                        Some(false) => {
+                            branch_is_unreachable(predicate, None);
+                            break;
+                        }
+                        Some(true) => {
+                            // this branch will be taken, so all subsequent branches are unreachable.
+                            break;
+                        }
+                    }
+                }
+                Some(_) => { /* for now we don't bother solving these */ }
+            },
+            CfgEntry::Not(inner, _) => match &**inner {
+                CfgEntry::NameValue { name, value: None, .. } => {
+                    // `name` will be true in all subsequent branches.
+                    let current = known.insert(*name, true);
+
+                    match current {
+                        None => continue,
+                        Some(true) => {
+                            branch_is_unreachable(predicate, None);
+                            break;
+                        }
+                        Some(false) => {
+                            // this branch will be taken, so all subsequent branches are unreachable.
+                            break;
+                        }
+                    }
+                }
+                _ => { /* for now we don't bother solving these */ }
+            },
+            CfgEntry::All(_, _) | CfgEntry::Any(_, _) => {
+                /* for now we don't bother solving these */
+            }
+            CfgEntry::Version(..) => { /* don't bother solving these */ }
+        }
+    }
+
+    for predicate in it {
+        branch_is_unreachable(predicate, wildcard_span)
+    }
+}

compiler/rustc_builtin_macros/src/cfg_select.rs

@@ -1,14 +1,12 @@
 use rustc_ast::tokenstream::TokenStream;
 use rustc_ast::{Expr, ast};
 use rustc_attr_parsing as attr;
-use rustc_attr_parsing::{
-    CfgSelectBranches, CfgSelectPredicate, EvalConfigResult, parse_cfg_select,
-};
+use rustc_attr_parsing::{CfgSelectBranches, EvalConfigResult, parse_cfg_select};
 use rustc_expand::base::{DummyResult, ExpandResult, ExtCtxt, MacResult, MacroExpanderResult};
 use rustc_span::{Ident, Span, sym};
 use smallvec::SmallVec;
 
-use crate::errors::{CfgSelectNoMatches, CfgSelectUnreachable};
+use crate::errors::CfgSelectNoMatches;
 
 /// This intermediate structure is used to emit parse errors for the branches that are not chosen.
 /// The `MacResult` instance below parses all branches, emitting any errors it encounters, but only
@@ -75,18 +73,6 @@ pub(super) fn expand_cfg_select<'cx>(
             ecx.current_expansion.lint_node_id,
         ) {
             Ok(mut branches) => {
-                if let Some((underscore, _, _)) = branches.wildcard {
-                    // Warn for every unreachable predicate. We store the fully parsed branch for rustfmt.
-                    for (predicate, _, _) in &branches.unreachable {
-                        let span = match predicate {
-                            CfgSelectPredicate::Wildcard(underscore) => underscore.span,
-                            CfgSelectPredicate::Cfg(cfg) => cfg.span(),
-                        };
-                        let err = CfgSelectUnreachable { span, wildcard_span: underscore.span };
-                        ecx.dcx().emit_warn(err);
-                    }
-                }
-
                 if let Some((selected_tts, selected_span)) = branches.pop_first_match(|cfg| {
                     matches!(attr::eval_config_entry(&ecx.sess, cfg), EvalConfigResult::True)
                 }) {

compiler/rustc_builtin_macros/src/errors.rs

@@ -1086,17 +1086,6 @@ pub(crate) struct CfgSelectNoMatches {
     pub span: Span,
 }
 
-#[derive(Diagnostic)]
-#[diag("unreachable predicate")]
-pub(crate) struct CfgSelectUnreachable {
-    #[primary_span]
-    #[label("this predicate is never reached")]
-    pub span: Span,
-
-    #[label("always matches")]
-    pub wildcard_span: Span,
-}
-
 #[derive(Diagnostic)]
 #[diag("`#[eii_declaration(...)]` is only valid on macros")]
 pub(crate) struct EiiExternTargetExpectedMacro {

compiler/rustc_feature/src/unstable.rs

@@ -392,6 +392,8 @@ declare_features! (
     (unstable, cfg_sanitize, "1.41.0", Some(39699)),
     /// Allows `cfg(sanitizer_cfi_generalize_pointers)` and `cfg(sanitizer_cfi_normalize_integers)`.
     (unstable, cfg_sanitizer_cfi, "1.77.0", Some(89653)),
+    /// Provides a native way to easily manage multiple conditional flags without having to rewrite each clause multiple times.
+    (unstable, cfg_select, "CURRENT_RUSTC_VERSION", Some(115585)),
     /// Allows `cfg(target(abi = "..."))`.
     (unstable, cfg_target_compact, "1.63.0", Some(96901)),
     /// Allows `cfg(target_has_atomic_load_store = "...")`.

compiler/rustc_lint/messages.ftl

@@ -998,6 +998,13 @@ lint_unpredictable_fn_pointer_comparisons = function pointer comparisons do not
 
 lint_unqualified_local_imports = `use` of a local item without leading `self::`, `super::`, or `crate::`
 
+lint_unreachable_cfg_select_predicate = unreachable configuration predicate
+    .label = this configuration predicate is never reached
+
+lint_unreachable_cfg_select_predicate_wildcard = unreachable configuration predicate
+    .label = always matches
+    .label2 = this configuration predicate is never reached
+
 lint_unsafe_attr_outside_unsafe = unsafe attribute used without unsafe
     .label = usage of unsafe attribute
 lint_unsafe_attr_outside_unsafe_suggestion = wrap the attribute in `unsafe(...)`

compiler/rustc_lint/src/early/diagnostics.rs

@@ -293,6 +293,14 @@ pub fn decorate_builtin_lint(
             }
             .decorate_lint(diag);
         }
+        BuiltinLintDiag::UnreachableCfg { span, wildcard_span } => match wildcard_span {
+            Some(wildcard_span) => {
+                lints::UnreachableCfgSelectPredicateWildcard { span, wildcard_span }
+                    .decorate_lint(diag)
+            }
+            None => lints::UnreachableCfgSelectPredicate { span }.decorate_lint(diag),
+        },
+
         BuiltinLintDiag::UnusedCrateDependency { extern_crate, local_crate } => {
             lints::UnusedCrateDependency { extern_crate, local_crate }.decorate_lint(diag)
         }

compiler/rustc_lint/src/lib.rs

@@ -297,6 +297,9 @@ fn register_builtins(store: &mut LintStore) {
         UNUSED_ASSIGNMENTS,
         DEAD_CODE,
         UNUSED_MUT,
+        // FIXME: add this lint when it becomes stable,
+        // see https://github.com/rust-lang/rust/issues/115585.
+        // UNREACHABLE_CFG_SELECT_PREDICATES,
         UNREACHABLE_CODE,
         UNREACHABLE_PATTERNS,
         UNUSED_MUST_USE,

compiler/rustc_lint/src/lints.rs

@@ -3340,3 +3340,20 @@ pub(crate) struct UnknownCrateTypesSuggestion {
     pub span: Span,
     pub snippet: Symbol,
 }
+
+#[derive(LintDiagnostic)]
+#[diag(lint_unreachable_cfg_select_predicate)]
+pub(crate) struct UnreachableCfgSelectPredicate {
+    #[label]
+    pub span: Span,
+}
+
+#[derive(LintDiagnostic)]
+#[diag(lint_unreachable_cfg_select_predicate_wildcard)]
+pub(crate) struct UnreachableCfgSelectPredicateWildcard {
+    #[label(lint_label2)]
+    pub span: Span,
+
+    #[label]
+    pub wildcard_span: Span,
+}

compiler/rustc_lint_defs/src/builtin.rs

@@ -123,6 +123,7 @@ declare_lint_pass! {
         UNKNOWN_LINTS,
         UNNAMEABLE_TEST_ITEMS,
         UNNAMEABLE_TYPES,
+        UNREACHABLE_CFG_SELECT_PREDICATES,
         UNREACHABLE_CODE,
         UNREACHABLE_PATTERNS,
         UNSAFE_ATTR_OUTSIDE_UNSAFE,
@@ -855,6 +856,34 @@ declare_lint! {
     "detects unreachable patterns"
 }
 
+declare_lint! {
+    /// The `unreachable_cfg_select_predicates` lint detects unreachable configuration
+    /// predicates in the `cfg_select!` macro.
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// #![feature(cfg_select)]
+    /// cfg_select! {
+    ///     _ => (),
+    ///     windows => (),
+    /// }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// This usually indicates a mistake in how the predicates are specified or
+    /// ordered. In this example, the `_` predicate will always match, so the
+    /// `windows` is impossible to reach. Remember, arms match in order, you
+    /// probably wanted to put the `windows` case above the `_` case.
+    pub UNREACHABLE_CFG_SELECT_PREDICATES,
+    Warn,
+    "detects unreachable configuration predicates in the cfg_select macro",
+    @feature_gate = cfg_select;
+}
+
 declare_lint! {
     /// The `overlapping_range_endpoints` lint detects `match` arms that have [range patterns] that
     /// overlap on their endpoints.

compiler/rustc_lint_defs/src/lib.rs

@@ -748,6 +748,10 @@ pub enum BuiltinLintDiag {
     },
     UnusedVisibility(Span),
     AttributeLint(AttributeLintKind),
+    UnreachableCfg {
+        span: Span,
+        wildcard_span: Option<Span>,
+    },
 }
 
 #[derive(Debug, HashStable_Generic)]

tests/ui/check-cfg/cfg-select.rs

@@ -4,7 +4,7 @@
 #![crate_type = "lib"]
 
 cfg_select! {
-    true => {}
+    false => {}
     invalid_cfg1 => {}
     //~^ WARN unexpected `cfg` condition name
     _ => {}
@@ -13,6 +13,6 @@ cfg_select! {
 cfg_select! {
     invalid_cfg2 => {}
     //~^ WARN unexpected `cfg` condition name
-    true => {}
+    false => {}
     _ => {}
 }

tests/ui/feature-gates/feature-gate-cfg-select.rs

@@ -0,0 +1,11 @@
+#![warn(unreachable_cfg_select_predicates)]
+//~^ WARN unknown lint: `unreachable_cfg_select_predicates`
+
+cfg_select! {
+    //~^ ERROR use of unstable library feature `cfg_select`
+    _ => {}
+    // With the feature enabled, this branch would trip the unreachable_cfg_select_predicate lint.
+    true => {}
+}
+
+fn main() {}

tests/ui/feature-gates/feature-gate-cfg-select.stderr

@@ -0,0 +1,25 @@
+error[E0658]: use of unstable library feature `cfg_select`
+  --> $DIR/feature-gate-cfg-select.rs:4:1
+   |
+LL | cfg_select! {
+   | ^^^^^^^^^^
+   |
+   = note: see issue #115585 <https://github.com/rust-lang/rust/issues/115585> for more information
+   = help: add `#![feature(cfg_select)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+warning: unknown lint: `unreachable_cfg_select_predicates`
+  --> $DIR/feature-gate-cfg-select.rs:1:9
+   |
+LL | #![warn(unreachable_cfg_select_predicates)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: the `unreachable_cfg_select_predicates` lint is unstable
+   = note: see issue #115585 <https://github.com/rust-lang/rust/issues/115585> for more information
+   = help: add `#![feature(cfg_select)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+   = note: `#[warn(unknown_lints)]` on by default
+
+error: aborting due to 1 previous error; 1 warning emitted
+
+For more information about this error, try `rustc --explain E0658`.