add unreachable_cfgs lint

This is emitted on branches of a `cfg_select!` that are statically known
to be unreachable.

Author: folkertdev

compiler/rustc_attr_parsing/src/attributes/cfg_select.rs

@@ -7,6 +7,8 @@ use rustc_hir::attrs::CfgEntry;
 use rustc_parse::exp;
 use rustc_parse::parser::Parser;
 use rustc_session::Session;
+use rustc_session::lint::BuiltinLintDiag;
+use rustc_session::lint::builtin::UNREACHABLE_CFGS;
 use rustc_span::{ErrorGuaranteed, Ident, Span};
 
 use crate::parser::MetaItemOrLitParser;
@@ -86,5 +88,16 @@ pub fn parse_cfg_select(
         }
     }
 
+    if let Some((underscore, _, _)) = branches.wildcard {
+        for (_, _, span) in &branches.unreachable {
+            p.psess.buffer_lint(
+                UNREACHABLE_CFGS,
+                *span,
+                lint_node_id,
+                BuiltinLintDiag::UnreachableCfg { span: *span, wildcard_span: underscore.span },
+            );
+        }
+    }
+
     Ok(branches)
 }

compiler/rustc_builtin_macros/messages.ftl

@@ -86,10 +86,6 @@ builtin_macros_cfg_accessible_unspecified_path = `cfg_accessible` path is not sp
 
 builtin_macros_cfg_select_no_matches = none of the predicates in this `cfg_select` evaluated to true
 
-builtin_macros_cfg_select_unreachable = unreachable predicate
-    .label = always matches
-    .label2 = this predicate is never reached
-
 builtin_macros_coerce_pointee_requires_maybe_sized = `derive(CoercePointee)` requires `{$name}` to be marked `?Sized`
 
 builtin_macros_coerce_pointee_requires_one_field = `CoercePointee` can only be derived on `struct`s with at least one field

compiler/rustc_builtin_macros/src/cfg_select.rs

@@ -1,12 +1,10 @@
 use rustc_ast::tokenstream::TokenStream;
 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, MacroExpanderResult};
 use rustc_span::{Ident, Span, sym};
 
-use crate::errors::{CfgSelectNoMatches, CfgSelectUnreachable};
+use crate::errors::CfgSelectNoMatches;
 
 /// Selects the first arm whose predicate evaluates to true.
 fn select_arm(ecx: &ExtCtxt<'_>, branches: CfgSelectBranches) -> Option<(TokenStream, Span)> {
@@ -32,18 +30,6 @@ pub(super) fn expand_cfg_select<'cx>(
             ecx.current_expansion.lint_node_id,
         ) {
             Ok(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((tts, arm_span)) = select_arm(ecx, branches) {
                     return ExpandResult::from_tts(
                         ecx,

compiler/rustc_builtin_macros/src/errors.rs

@@ -999,14 +999,3 @@ pub(crate) struct CfgSelectNoMatches {
     #[primary_span]
     pub span: Span,
 }
-
-#[derive(Diagnostic)]
-#[diag(builtin_macros_cfg_select_unreachable)]
-pub(crate) struct CfgSelectUnreachable {
-    #[primary_span]
-    #[label(builtin_macros_label2)]
-    pub span: Span,
-
-    #[label]
-    pub wildcard_span: Span,
-}

compiler/rustc_lint/messages.ftl

@@ -916,6 +916,10 @@ 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 = 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 +293,10 @@ pub fn decorate_builtin_lint(
             }
             .decorate_lint(diag);
         }
+        BuiltinLintDiag::UnreachableCfg { span, wildcard_span } => {
+            lints::UnreachableCfg { span, wildcard_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

@@ -295,6 +295,7 @@ fn register_builtins(store: &mut LintStore) {
         UNUSED_ASSIGNMENTS,
         DEAD_CODE,
         UNUSED_MUT,
+        UNREACHABLE_CFGS,
         UNREACHABLE_CODE,
         UNREACHABLE_PATTERNS,
         UNUSED_MUST_USE,

compiler/rustc_lint/src/lints.rs

@@ -3199,3 +3199,13 @@ pub(crate) struct UnusedVisibility {
     #[suggestion(style = "short", code = "", applicability = "machine-applicable")]
     pub span: Span,
 }
+
+#[derive(LintDiagnostic)]
+#[diag(lint_unreachable_cfg)]
+pub(crate) struct UnreachableCfg {
+    #[label(lint_label2)]
+    pub span: Span,
+
+    #[label]
+    pub wildcard_span: Span,
+}

compiler/rustc_lint_defs/src/builtin.rs

@@ -121,6 +121,7 @@ declare_lint_pass! {
         UNKNOWN_LINTS,
         UNNAMEABLE_TEST_ITEMS,
         UNNAMEABLE_TYPES,
+        UNREACHABLE_CFGS,
         UNREACHABLE_CODE,
         UNREACHABLE_PATTERNS,
         UNSAFE_ATTR_OUTSIDE_UNSAFE,
@@ -856,6 +857,32 @@ declare_lint! {
     "detects unreachable patterns"
 }
 
+declare_lint! {
+    /// The `unreachable_cfgs` lint detects unreachable configuration
+    /// predicates.
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// 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_CFGS,
+    Warn,
+    "detects unreachable configuration predicates"
+}
+
 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

@@ -696,6 +696,10 @@ pub enum BuiltinLintDiag {
     },
     UnusedVisibility(Span),
     AttributeLint(AttributeLintKind),
+    UnreachableCfg {
+        span: Span,
+        wildcard_span: Span,
+    },
 }
 
 #[derive(Debug, HashStable_Generic)]