Rollup of 8 pull requests

compiler/rustc_ast/src/ast.rs

@@ -100,7 +100,7 @@ pub struct Path {
 impl PartialEq<Symbol> for Path {
     #[inline]
     fn eq(&self, symbol: &Symbol) -> bool {
-        self.segments.len() == 1 && { self.segments[0].ident.name == *symbol }
+        matches!(&self.segments[..], [segment] if segment.ident.name == *symbol)
     }
 }
 
@@ -121,13 +121,13 @@ impl Path {
     }
 
     pub fn is_global(&self) -> bool {
-        !self.segments.is_empty() && self.segments[0].ident.name == kw::PathRoot
+        self.segments.first().is_some_and(|segment| segment.ident.name == kw::PathRoot)
     }
 
     /// If this path is a single identifier with no arguments, does not ensure
     /// that the path resolves to a const param, the caller should check this.
     pub fn is_potential_trivial_const_arg(&self) -> bool {
-        self.segments.len() == 1 && self.segments[0].args.is_none()
+        matches!(self.segments[..], [PathSegment { args: None, .. }])
     }
 }
 

compiler/rustc_ast/src/attr/mod.rs

@@ -302,7 +302,7 @@ impl AttrItem {
 impl MetaItem {
     /// For a single-segment meta item, returns its name; otherwise, returns `None`.
     pub fn ident(&self) -> Option<Ident> {
-        if self.path.segments.len() == 1 { Some(self.path.segments[0].ident) } else { None }
+        if let [PathSegment { ident, .. }] = self.path.segments[..] { Some(ident) } else { None }
     }
 
     pub fn name_or_empty(&self) -> Symbol {

compiler/rustc_ast/src/mut_visit.rs

@@ -1813,10 +1813,10 @@ pub fn walk_flat_map_stmt<T: MutVisitor>(
         .into_iter()
         .map(|kind| Stmt { id, kind, span })
         .collect();
-    match stmts.len() {
-        0 => {}
-        1 => vis.visit_span(&mut stmts[0].span),
-        2.. => panic!(
+    match &mut stmts[..] {
+        [] => {}
+        [stmt] => vis.visit_span(&mut stmt.span),
+        _ => panic!(
             "cloning statement `NodeId`s is prohibited by default, \
              the visitor should implement custom statement visiting"
         ),

compiler/rustc_ast/src/util/comments.rs

@@ -39,7 +39,7 @@ pub fn beautify_doc_string(data: Symbol, kind: CommentKind) -> Symbol {
         let mut i = 0;
         let mut j = lines.len();
         // first line of all-stars should be omitted
-        if !lines.is_empty() && lines[0].chars().all(|c| c == '*') {
+        if lines.first().is_some_and(|line| line.chars().all(|c| c == '*')) {
             i += 1;
         }
 
@@ -97,7 +97,7 @@ pub fn beautify_doc_string(data: Symbol, kind: CommentKind) -> Symbol {
                 return None;
             }
         }
-        if lines.is_empty() { None } else { Some(lines[0][..i].into()) }
+        Some(lines.first()?[..i].to_string())
     }
 
     let data_s = data.as_str();

compiler/rustc_const_eval/src/const_eval/error.rs

@@ -140,7 +140,7 @@ where
         // Don't emit a new diagnostic for these errors, they are already reported elsewhere or
         // should remain silent.
         err_inval!(AlreadyReported(info)) => ErrorHandled::Reported(info, span),
-        err_inval!(Layout(LayoutError::Unknown(_))) | err_inval!(TooGeneric) => {
+        err_inval!(Layout(LayoutError::TooGeneric(_))) | err_inval!(TooGeneric) => {
             ErrorHandled::TooGeneric(span)
         }
         err_inval!(Layout(LayoutError::ReferencesError(guar))) => {

compiler/rustc_hir_analysis/src/hir_ty_lowering/cmse.rs

@@ -201,7 +201,7 @@ fn should_emit_generic_error<'tcx>(abi: ExternAbi, layout_err: &'tcx LayoutError
     use LayoutError::*;
 
     match layout_err {
-        Unknown(ty) => {
+        TooGeneric(ty) => {
             match abi {
                 ExternAbi::CCmseNonSecureCall => {
                     // prevent double reporting of this error
@@ -211,7 +211,11 @@ fn should_emit_generic_error<'tcx>(abi: ExternAbi, layout_err: &'tcx LayoutError
                 _ => bug!("invalid ABI: {abi}"),
             }
         }
-        SizeOverflow(..) | NormalizationFailure(..) | ReferencesError(..) | Cycle(..) => {
+        Unknown(..)
+        | SizeOverflow(..)
+        | NormalizationFailure(..)
+        | ReferencesError(..)
+        | Cycle(..) => {
             false // not our job to report these
         }
     }

compiler/rustc_hir_typeck/src/intrinsicck.rs

@@ -122,7 +122,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     format!("generic size {size}")
                 }
             }
-            Err(LayoutError::Unknown(bad)) => {
+            Err(LayoutError::TooGeneric(bad)) => {
                 if *bad == ty {
                     "this type does not have a fixed size".to_owned()
                 } else {

compiler/rustc_lint/messages.ftl

@@ -76,6 +76,11 @@ lint_builtin_deprecated_attr_link = use of deprecated attribute `{$name}`: {$rea
 lint_builtin_deref_nullptr = dereferencing a null pointer
     .label = this code causes undefined behavior when executed
 
+lint_builtin_double_negations = use of a double negation
+    .note = the prefix `--` could be misinterpreted as a decrement operator which exists in other languages
+    .note_decrement = use `-= 1` if you meant to decrement the value
+    .add_parens_suggestion = add parentheses for clarity
+
 lint_builtin_ellipsis_inclusive_range_patterns = `...` range patterns are deprecated
     .suggestion = use `..=` for an inclusive range
 

compiler/rustc_lint/src/builtin.rs

@@ -49,16 +49,16 @@ use rustc_trait_selection::traits::{self};
 use crate::errors::BuiltinEllipsisInclusiveRangePatterns;
 use crate::lints::{
     BuiltinAnonymousParams, BuiltinConstNoMangle, BuiltinDeprecatedAttrLink,
-    BuiltinDeprecatedAttrLinkSuggestion, BuiltinDerefNullptr,
-    BuiltinEllipsisInclusiveRangePatternsLint, BuiltinExplicitOutlives,
-    BuiltinExplicitOutlivesSuggestion, BuiltinFeatureIssueNote, BuiltinIncompleteFeatures,
-    BuiltinIncompleteFeaturesHelp, BuiltinInternalFeatures, BuiltinKeywordIdents,
-    BuiltinMissingCopyImpl, BuiltinMissingDebugImpl, BuiltinMissingDoc, BuiltinMutablesTransmutes,
-    BuiltinNoMangleGeneric, BuiltinNonShorthandFieldPatterns, BuiltinSpecialModuleNameUsed,
-    BuiltinTrivialBounds, BuiltinTypeAliasBounds, BuiltinUngatedAsyncFnTrackCaller,
-    BuiltinUnpermittedTypeInit, BuiltinUnpermittedTypeInitSub, BuiltinUnreachablePub,
-    BuiltinUnsafe, BuiltinUnstableFeatures, BuiltinUnusedDocComment, BuiltinUnusedDocCommentSub,
-    BuiltinWhileTrue, InvalidAsmLabel,
+    BuiltinDeprecatedAttrLinkSuggestion, BuiltinDerefNullptr, BuiltinDoubleNegations,
+    BuiltinDoubleNegationsAddParens, BuiltinEllipsisInclusiveRangePatternsLint,
+    BuiltinExplicitOutlives, BuiltinExplicitOutlivesSuggestion, BuiltinFeatureIssueNote,
+    BuiltinIncompleteFeatures, BuiltinIncompleteFeaturesHelp, BuiltinInternalFeatures,
+    BuiltinKeywordIdents, BuiltinMissingCopyImpl, BuiltinMissingDebugImpl, BuiltinMissingDoc,
+    BuiltinMutablesTransmutes, BuiltinNoMangleGeneric, BuiltinNonShorthandFieldPatterns,
+    BuiltinSpecialModuleNameUsed, BuiltinTrivialBounds, BuiltinTypeAliasBounds,
+    BuiltinUngatedAsyncFnTrackCaller, BuiltinUnpermittedTypeInit, BuiltinUnpermittedTypeInitSub,
+    BuiltinUnreachablePub, BuiltinUnsafe, BuiltinUnstableFeatures, BuiltinUnusedDocComment,
+    BuiltinUnusedDocCommentSub, BuiltinWhileTrue, InvalidAsmLabel,
 };
 use crate::nonstandard_style::{MethodLateContext, method_context};
 use crate::{
@@ -90,19 +90,11 @@ declare_lint! {
 
 declare_lint_pass!(WhileTrue => [WHILE_TRUE]);
 
-/// Traverse through any amount of parenthesis and return the first non-parens expression.
-fn pierce_parens(mut expr: &ast::Expr) -> &ast::Expr {
-    while let ast::ExprKind::Paren(sub) = &expr.kind {
-        expr = sub;
-    }
-    expr
-}
-
 impl EarlyLintPass for WhileTrue {
     #[inline]
     fn check_expr(&mut self, cx: &EarlyContext<'_>, e: &ast::Expr) {
         if let ast::ExprKind::While(cond, _, label) = &e.kind
-            && let ast::ExprKind::Lit(token_lit) = pierce_parens(cond).kind
+            && let ast::ExprKind::Lit(token_lit) = cond.peel_parens().kind
             && let token::Lit { kind: token::Bool, symbol: kw::True, .. } = token_lit
             && !cond.span.from_expansion()
         {
@@ -1576,6 +1568,58 @@ impl<'tcx> LateLintPass<'tcx> for TrivialConstraints {
     }
 }
 
+declare_lint! {
+    /// The `double_negations` lint detects expressions of the form `--x`.
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// fn main() {
+    ///     let x = 1;
+    ///     let _b = --x;
+    /// }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// Negating something twice is usually the same as not negating it at all.
+    /// However, a double negation in Rust can easily be confused with the
+    /// prefix decrement operator that exists in many languages derived from C.
+    /// Use `-(-x)` if you really wanted to negate the value twice.
+    ///
+    /// To decrement a value, use `x -= 1` instead.
+    pub DOUBLE_NEGATIONS,
+    Warn,
+    "detects expressions of the form `--x`"
+}
+
+declare_lint_pass!(
+    /// Lint for expressions of the form `--x` that can be confused with C's
+    /// prefix decrement operator.
+    DoubleNegations => [DOUBLE_NEGATIONS]
+);
+
+impl EarlyLintPass for DoubleNegations {
+    #[inline]
+    fn check_expr(&mut self, cx: &EarlyContext<'_>, expr: &ast::Expr) {
+        // only lint on the innermost `--` in a chain of `-` operators,
+        // even if there are 3 or more negations
+        if let ExprKind::Unary(UnOp::Neg, ref inner) = expr.kind
+            && let ExprKind::Unary(UnOp::Neg, ref inner2) = inner.kind
+            && !matches!(inner2.kind, ExprKind::Unary(UnOp::Neg, _))
+        {
+            cx.emit_span_lint(DOUBLE_NEGATIONS, expr.span, BuiltinDoubleNegations {
+                add_parens: BuiltinDoubleNegationsAddParens {
+                    start_span: inner.span.shrink_to_lo(),
+                    end_span: inner.span.shrink_to_hi(),
+                },
+            });
+        }
+    }
+}
+
 declare_lint_pass!(
     /// Does nothing as a lint pass, but registers some `Lint`s
     /// which are used by other parts of the compiler.
@@ -1594,7 +1638,8 @@ declare_lint_pass!(
         UNSTABLE_FEATURES,
         UNREACHABLE_PUB,
         TYPE_ALIAS_BOUNDS,
-        TRIVIAL_BOUNDS
+        TRIVIAL_BOUNDS,
+        DOUBLE_NEGATIONS
     ]
 );
 
@@ -2651,7 +2696,7 @@ impl<'tcx> LateLintPass<'tcx> for InvalidValue {
 }
 
 declare_lint! {
-    /// The `deref_nullptr` lint detects when an null pointer is dereferenced,
+    /// The `deref_nullptr` lint detects when a null pointer is dereferenced,
     /// which causes [undefined behavior].
     ///
     /// ### Example

compiler/rustc_lint/src/lib.rs

@@ -181,6 +181,7 @@ early_lint_methods!(
             UnusedDocComment: UnusedDocComment,
             Expr2024: Expr2024,
             Precedence: Precedence,
+            DoubleNegations: DoubleNegations,
         ]
     ]
 );

compiler/rustc_lint/src/lints.rs

@@ -331,6 +331,24 @@ pub(crate) struct BuiltinTrivialBounds<'a> {
     pub predicate: Clause<'a>,
 }
 
+#[derive(LintDiagnostic)]
+#[diag(lint_builtin_double_negations)]
+#[note(lint_note)]
+#[note(lint_note_decrement)]
+pub(crate) struct BuiltinDoubleNegations {
+    #[subdiagnostic]
+    pub add_parens: BuiltinDoubleNegationsAddParens,
+}
+
+#[derive(Subdiagnostic)]
+#[multipart_suggestion(lint_add_parens_suggestion, applicability = "maybe-incorrect")]
+pub(crate) struct BuiltinDoubleNegationsAddParens {
+    #[suggestion_part(code = "(")]
+    pub start_span: Span,
+    #[suggestion_part(code = ")")]
+    pub end_span: Span,
+}
+
 #[derive(LintDiagnostic)]
 pub(crate) enum BuiltinEllipsisInclusiveRangePatternsLint {
     #[diag(lint_builtin_ellipsis_inclusive_range_patterns)]

compiler/rustc_middle/messages.ftl

@@ -100,11 +100,14 @@ middle_strict_coherence_needs_negative_coherence =
     to use `strict_coherence` on this trait, the `with_negative_coherence` feature must be enabled
     .label = due to this attribute
 
+middle_too_generic = `{$ty}` does not have a fixed size
+
 middle_type_length_limit = reached the type-length limit while instantiating `{$shrunk}`
 
 middle_unknown_layout =
     the type `{$ty}` has an unknown layout
 
 middle_values_too_big =
     values of the type `{$ty}` are too big for the target architecture
+
 middle_written_to_path = the full type name has been written to '{$path}'

compiler/rustc_middle/src/error.rs

@@ -129,6 +129,9 @@ pub enum LayoutError<'tcx> {
     #[diag(middle_unknown_layout)]
     Unknown { ty: Ty<'tcx> },
 
+    #[diag(middle_too_generic)]
+    TooGeneric { ty: Ty<'tcx> },
+
     #[diag(middle_values_too_big)]
     Overflow { ty: Ty<'tcx> },
 

compiler/rustc_middle/src/ty/layout.rs

@@ -231,6 +231,7 @@ impl fmt::Display for ValidityRequirement {
 pub enum LayoutError<'tcx> {
     Unknown(Ty<'tcx>),
     SizeOverflow(Ty<'tcx>),
+    TooGeneric(Ty<'tcx>),
     NormalizationFailure(Ty<'tcx>, NormalizationError<'tcx>),
     ReferencesError(ErrorGuaranteed),
     Cycle(ErrorGuaranteed),
@@ -244,6 +245,7 @@ impl<'tcx> LayoutError<'tcx> {
         match self {
             Unknown(_) => middle_unknown_layout,
             SizeOverflow(_) => middle_values_too_big,
+            TooGeneric(_) => middle_too_generic,
             NormalizationFailure(_, _) => middle_cannot_be_normalized,
             Cycle(_) => middle_cycle,
             ReferencesError(_) => middle_layout_references_error,
@@ -257,6 +259,7 @@ impl<'tcx> LayoutError<'tcx> {
         match self {
             Unknown(ty) => E::Unknown { ty },
             SizeOverflow(ty) => E::Overflow { ty },
+            TooGeneric(ty) => E::TooGeneric { ty },
             NormalizationFailure(ty, e) => {
                 E::NormalizationFailure { ty, failure_ty: e.get_type_for_failure() }
             }
@@ -272,6 +275,9 @@ impl<'tcx> fmt::Display for LayoutError<'tcx> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match *self {
             LayoutError::Unknown(ty) => write!(f, "the type `{ty}` has an unknown layout"),
+            LayoutError::TooGeneric(ty) => {
+                write!(f, "`{ty}` does not have a fixed size")
+            }
             LayoutError::SizeOverflow(ty) => {
                 write!(f, "values of the type `{ty}` are too big for the target architecture")
             }
@@ -350,10 +356,11 @@ impl<'tcx> SizeSkeleton<'tcx> {
                     return Err(tcx.arena.alloc(LayoutError::Unknown(ty)));
                 }
             }
-            Err(err @ LayoutError::Unknown(_)) => err,
+            Err(err @ LayoutError::TooGeneric(_)) => err,
             // We can't extract SizeSkeleton info from other layout errors
             Err(
                 e @ LayoutError::Cycle(_)
+                | e @ LayoutError::Unknown(_)
                 | e @ LayoutError::SizeOverflow(_)
                 | e @ LayoutError::NormalizationFailure(..)
                 | e @ LayoutError::ReferencesError(_),
@@ -413,10 +420,9 @@ impl<'tcx> SizeSkeleton<'tcx> {
                             // Alignment is unchanged by arrays.
                             return Ok(SizeSkeleton::Known(Size::from_bytes(size), a));
                         }
-                        Err(tcx.arena.alloc(LayoutError::Unknown(ty)))
+                        Err(err)
                     }
-                    SizeSkeleton::Pointer { .. } => Err(err),
-                    SizeSkeleton::Generic(_) => Err(tcx.arena.alloc(LayoutError::Unknown(ty))),
+                    SizeSkeleton::Pointer { .. } | SizeSkeleton::Generic(_) => Err(err),
                 }
             }
 

compiler/rustc_transmute/src/layout/tree.rs

@@ -197,6 +197,7 @@ pub(crate) mod rustc {
             match err {
                 LayoutError::Unknown(..)
                 | LayoutError::ReferencesError(..)
+                | LayoutError::TooGeneric(..)
                 | LayoutError::NormalizationFailure(..) => Self::UnknownLayout,
                 LayoutError::SizeOverflow(..) => Self::SizeOverflow,
                 LayoutError::Cycle(err) => Self::TypeError(*err),