Implement an internal lint encouraging use of Span::eq_ctxt

compiler/rustc_hir_typeck/src/callee.rs

@@ -650,7 +650,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 .sess
                 .source_map()
                 .is_multiline(call_expr.span.with_lo(callee_expr.span.hi()))
-                && call_expr.span.ctxt() == callee_expr.span.ctxt();
+                && call_expr.span.eq_ctxt(callee_expr.span);
             if call_is_multiline {
                 err.span_suggestion(
                     callee_expr.span.shrink_to_hi(),

compiler/rustc_lint/messages.ftl

@@ -494,6 +494,8 @@ lint_renamed_lint = lint `{$name}` has been renamed to `{$replace}`
 
 lint_requested_level = requested on the command line with `{$level} {$lint_name}`
 
+lint_span_use_eq_ctxt = use `.eq_ctxt()` instead of `.ctxt() == .ctxt()`
+
 lint_supertrait_as_deref_target = `{$t}` implements `Deref` with supertrait `{$target_principal}` as target
     .label = target type is set here
 

compiler/rustc_lint/src/internal.rs

@@ -3,14 +3,14 @@
 
 use crate::lints::{
     BadOptAccessDiag, DefaultHashTypesDiag, DiagOutOfImpl, LintPassByHand, NonExistentDocKeyword,
-    QueryInstability, TyQualified, TykindDiag, TykindKind, UntranslatableDiag,
+    QueryInstability, SpanUseEqCtxtDiag, TyQualified, TykindDiag, TykindKind, UntranslatableDiag,
     UntranslatableDiagnosticTrivial,
 };
 use crate::{EarlyContext, EarlyLintPass, LateContext, LateLintPass, LintContext};
 use rustc_ast as ast;
 use rustc_hir::def::Res;
 use rustc_hir::{def_id::DefId, Expr, ExprKind, GenericArg, PatKind, Path, PathSegment, QPath};
-use rustc_hir::{HirId, Impl, Item, ItemKind, Node, Pat, Ty, TyKind};
+use rustc_hir::{BinOp, BinOpKind, HirId, Impl, Item, ItemKind, Node, Pat, Ty, TyKind};
 use rustc_middle::ty;
 use rustc_session::{declare_lint_pass, declare_tool_lint};
 use rustc_span::hygiene::{ExpnKind, MacroKind};
@@ -537,3 +537,33 @@ impl LateLintPass<'_> for BadOptAccess {
         }
     }
 }
+
+declare_tool_lint! {
+    pub rustc::SPAN_USE_EQ_CTXT,
+    Allow,
+    "forbid uses of `==` with `Span::ctxt`, suggest `Span::eq_ctxt` instead",
+    report_in_external_macro: true
+}
+
+declare_lint_pass!(SpanUseEqCtxt => [SPAN_USE_EQ_CTXT]);
+
+impl<'tcx> LateLintPass<'tcx> for SpanUseEqCtxt {
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &Expr<'_>) {
+        if let ExprKind::Binary(BinOp { node: BinOpKind::Eq, .. }, lhs, rhs) = expr.kind {
+            if is_span_ctxt_call(cx, lhs) && is_span_ctxt_call(cx, rhs) {
+                cx.emit_spanned_lint(SPAN_USE_EQ_CTXT, expr.span, SpanUseEqCtxtDiag);
+            }
+        }
+    }
+}
+
+fn is_span_ctxt_call(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
+    match &expr.kind {
+        ExprKind::MethodCall(..) => cx
+            .typeck_results()
+            .type_dependent_def_id(expr.hir_id)
+            .is_some_and(|call_did| cx.tcx.is_diagnostic_item(sym::SpanCtxt, call_did)),
+
+        _ => false,
+    }
+}

compiler/rustc_lint/src/lib.rs

@@ -531,6 +531,8 @@ fn register_internals(store: &mut LintStore) {
     store.register_late_mod_pass(|_| Box::new(BadOptAccess));
     store.register_lints(&PassByValue::get_lints());
     store.register_late_mod_pass(|_| Box::new(PassByValue));
+    store.register_lints(&SpanUseEqCtxt::get_lints());
+    store.register_late_mod_pass(|_| Box::new(SpanUseEqCtxt));
     // FIXME(davidtwco): deliberately do not include `UNTRANSLATABLE_DIAGNOSTIC` and
     // `DIAGNOSTIC_OUTSIDE_OF_IMPL` here because `-Wrustc::internal` is provided to every crate and
     // these lints will trigger all of the time - change this once migration to diagnostic structs
@@ -548,6 +550,7 @@ fn register_internals(store: &mut LintStore) {
             LintId::of(USAGE_OF_QUALIFIED_TY),
             LintId::of(EXISTING_DOC_KEYWORD),
             LintId::of(BAD_OPT_ACCESS),
+            LintId::of(SPAN_USE_EQ_CTXT),
         ],
     );
 }

compiler/rustc_lint/src/lints.rs

@@ -900,6 +900,10 @@ pub struct QueryInstability {
     pub query: Symbol,
 }
 
+#[derive(LintDiagnostic)]
+#[diag(lint_span_use_eq_ctxt)]
+pub struct SpanUseEqCtxtDiag;
+
 #[derive(LintDiagnostic)]
 #[diag(lint_tykind_kind)]
 pub struct TykindKind {

compiler/rustc_mir_transform/src/coverage/spans.rs

@@ -404,7 +404,7 @@ impl<'a> CoverageSpansGenerator<'a> {
 
         let Some(visible_macro) = curr.visible_macro(self.body_span) else { return };
         if let Some(prev) = &self.some_prev
-            && prev.expn_span.ctxt() == curr.expn_span.ctxt()
+            && prev.expn_span.eq_ctxt(curr.expn_span)
         {
             return;
         }

compiler/rustc_passes/src/loops.rs

@@ -231,7 +231,7 @@ impl<'a, 'hir> CheckLoopVisitor<'a, 'hir> {
             AsyncClosure(closure_span) => {
                 self.sess.emit_err(BreakInsideAsyncBlock { span, closure_span, name });
             }
-            UnlabeledBlock(block_span) if is_break && block_span.ctxt() == break_span.ctxt() => {
+            UnlabeledBlock(block_span) if is_break && block_span.eq_ctxt(break_span) => {
                 let suggestion = Some(OutsideLoopSuggestion { block_span, break_span });
                 self.sess.emit_err(OutsideLoop { span, name, is_break, suggestion });
             }

compiler/rustc_span/src/span_encoding.rs

@@ -212,6 +212,7 @@ impl Span {
 
     /// This function is used as a fast path when decoding the full `SpanData` is not necessary.
     /// It's a cut-down version of `data_untracked`.
+    #[cfg_attr(not(test), rustc_diagnostic_item = "SpanCtxt")]
     #[inline]
     pub fn ctxt(self) -> SyntaxContext {
         if self.len_with_tag_or_marker != BASE_LEN_INTERNED_MARKER {

compiler/rustc_span/src/symbol.rs

@@ -303,6 +303,7 @@ symbols! {
         SliceIndex,
         SliceIter,
         Some,
+        SpanCtxt,
         String,
         StructuralEq,
         StructuralPartialEq,

src/tools/clippy/clippy_lints/src/dereference.rs

@@ -701,7 +701,7 @@ fn deref_method_same_type<'tcx>(result_ty: Ty<'tcx>, arg_ty: Ty<'tcx>) -> bool {
 
 fn in_postfix_position<'tcx>(cx: &LateContext<'tcx>, e: &'tcx Expr<'tcx>) -> bool {
     if let Some(parent) = get_parent_expr(cx, e)
-        && parent.span.ctxt() == e.span.ctxt()
+        && parent.span.eq_ctxt(e.span)
     {
         match parent.kind {
             ExprKind::Call(child, _) | ExprKind::MethodCall(_, child, _, _) | ExprKind::Index(child, _, _)

src/tools/clippy/clippy_lints/src/entry.rs

@@ -241,7 +241,7 @@ fn try_parse_contains<'tcx>(cx: &LateContext<'_>, expr: &'tcx Expr<'_>) -> Optio
                 },
             ],
             _,
-        ) if key_span.ctxt() == expr.span.ctxt() => {
+        ) if key_span.eq_ctxt(expr.span) => {
             let id = cx.typeck_results().type_dependent_def_id(expr.hir_id)?;
             let expr = ContainsExpr {
                 negated,

src/tools/clippy/clippy_lints/src/formatting.rs

@@ -274,7 +274,7 @@ fn check_array(cx: &EarlyContext<'_>, expr: &Expr) {
         for element in array {
             if_chain! {
                 if let ExprKind::Binary(ref op, ref lhs, _) = element.kind;
-                if has_unary_equivalent(op.node) && lhs.span.ctxt() == op.span.ctxt();
+                if has_unary_equivalent(op.node) && lhs.span.eq_ctxt(op.span);
                 let space_span = lhs.span.between(op.span);
                 if let Some(space_snippet) = snippet_opt(cx, space_span);
                 let lint_span = lhs.span.with_lo(lhs.span.hi());

src/tools/clippy/clippy_lints/src/let_with_type_underscore.rs

@@ -31,7 +31,7 @@ impl LateLintPass<'_> for UnderscoreTyped {
             if !in_external_macro(cx.tcx.sess, local.span);
             if let Some(ty) = local.ty; // Ensure that it has a type defined
             if let TyKind::Infer = &ty.kind; // that type is '_'
-            if local.span.ctxt() == ty.span.ctxt();
+            if local.span.eq_ctxt(ty.span);
             then {
                 // NOTE: Using `is_from_proc_macro` on `init` will require that it's initialized,
                 // this doesn't. Alternatively, `WithSearchPat` can be implemented for `Ty`

src/tools/clippy/clippy_lints/src/manual_let_else.rs

@@ -59,7 +59,7 @@ impl<'tcx> QuestionMark {
             let Some(init) = local.init &&
             local.els.is_none() &&
             local.ty.is_none() &&
-            init.span.ctxt() == stmt.span.ctxt() &&
+            init.span.eq_ctxt(stmt.span) &&
             let Some(if_let_or_match) = IfLetOrMatch::parse(cx, init)
         {
             match if_let_or_match {

src/tools/clippy/clippy_lints/src/matches/collapsible_match.rs

@@ -57,7 +57,7 @@ fn check_arm<'tcx>(
                 }
             },
         };
-        if outer_pat.span.ctxt() == inner_scrutinee.span.ctxt();
+        if outer_pat.span.eq_ctxt(inner_scrutinee.span);
         // match expression must be a local binding
         // match <local> { .. }
         if let Some(binding_id) = path_to_local(peel_ref_operators(cx, inner_scrutinee));

src/tools/clippy/clippy_lints/src/matches/manual_utils.rs

@@ -119,7 +119,7 @@ where
     // it's being passed by value.
     let scrutinee = peel_hir_expr_refs(scrutinee).0;
     let (scrutinee_str, _) = snippet_with_context(cx, scrutinee.span, expr_ctxt, "..", &mut app);
-    let scrutinee_str = if scrutinee.span.ctxt() == expr.span.ctxt() && scrutinee.precedence().order() < PREC_POSTFIX {
+    let scrutinee_str = if scrutinee.span.eq_ctxt(expr.span) && scrutinee.precedence().order() < PREC_POSTFIX {
         format!("({scrutinee_str})")
     } else {
         scrutinee_str.into()
@@ -130,7 +130,7 @@ where
         if_chain! {
             if !some_expr.needs_unsafe_block;
             if let Some(func) = can_pass_as_func(cx, id, some_expr.expr);
-            if func.span.ctxt() == some_expr.expr.span.ctxt();
+            if func.span.eq_ctxt(some_expr.expr.span);
             then {
                 snippet_with_applicability(cx, func.span, "..", &mut app).into_owned()
             } else {

src/tools/clippy/clippy_lints/src/methods/map_unwrap_or.rs

@@ -56,7 +56,7 @@ pub(super) fn check<'tcx>(
         // lint, with note if neither arg is > 1 line and both map() and
         // unwrap_or_else() have the same span
         let multiline = map_snippet.lines().count() > 1 || unwrap_snippet.lines().count() > 1;
-        let same_span = map_arg.span.ctxt() == unwrap_arg.span.ctxt();
+        let same_span = map_arg.span.eq_ctxt(unwrap_arg.span);
         if same_span && !multiline {
             let var_snippet = snippet(cx, recv.span, "..");
             span_lint_and_sugg(

src/tools/clippy/clippy_lints/src/needless_question_mark.rs

@@ -125,7 +125,7 @@ fn check(cx: &LateContext<'_>, expr: &Expr<'_>) {
         if let ExprKind::Match(inner_expr_with_q, _, MatchSource::TryDesugar(_)) = &arg.kind;
         if let ExprKind::Call(called, [inner_expr]) = &inner_expr_with_q.kind;
         if let ExprKind::Path(QPath::LangItem(LangItem::TryTraitBranch, ..)) = &called.kind;
-        if expr.span.ctxt() == inner_expr.span.ctxt();
+        if expr.span.eq_ctxt(inner_expr.span);
         let expr_ty = cx.typeck_results().expr_ty(expr);
         let inner_ty = cx.typeck_results().expr_ty(inner_expr);
         if expr_ty == inner_ty;

src/tools/clippy/clippy_lints/src/non_octal_unix_permissions.rs

@@ -51,7 +51,7 @@ impl<'tcx> LateLintPass<'tcx> for NonOctalUnixPermissions {
                         || (path.ident.name == sym!(set_mode)
                             && cx.tcx.is_diagnostic_item(sym::FsPermissions, adt.did()));
                     if let ExprKind::Lit(_) = param.kind;
-                    if param.span.ctxt() == expr.span.ctxt();
+                    if param.span.eq_ctxt(expr.span);
 
                     then {
                         let Some(snip) = snippet_opt(cx, param.span) else {
@@ -70,7 +70,7 @@ impl<'tcx> LateLintPass<'tcx> for NonOctalUnixPermissions {
                     if let Some(def_id) = cx.qpath_res(path, func.hir_id).opt_def_id();
                     if match_def_path(cx, def_id, &paths::PERMISSIONS_FROM_MODE);
                     if let ExprKind::Lit(_) = param.kind;
-                    if param.span.ctxt() == expr.span.ctxt();
+                    if param.span.eq_ctxt(expr.span);
                     if let Some(snip) = snippet_opt(cx, param.span);
                     if !snip.starts_with("0o");
                     then {

src/tools/clippy/clippy_lints/src/redundant_async_block.rs

@@ -48,7 +48,7 @@ impl<'tcx> LateLintPass<'tcx> for RedundantAsyncBlock {
             let Some(body_expr) = desugar_async_block(cx, expr) &&
             let Some(expr) = desugar_await(peel_blocks(body_expr)) &&
             // The await prefix must not come from a macro as its content could change in the future.
-            expr.span.ctxt() == body_expr.span.ctxt() &&
+            expr.span.eq_ctxt(body_expr.span) &&
             // An async block does not have immediate side-effects from a `.await` point-of-view.
             (!expr.can_have_side_effects() || desugar_async_block(cx, expr).is_some()) &&
             let Some(shortened_span) = walk_span_to_context(expr.span, span.ctxt())

src/tools/clippy/clippy_lints/src/reference.rs

@@ -50,7 +50,7 @@ impl EarlyLintPass for DerefAddrOf {
         if_chain! {
             if let ExprKind::Unary(UnOp::Deref, ref deref_target) = e.kind;
             if let ExprKind::AddrOf(_, ref mutability, ref addrof_target) = without_parens(deref_target).kind;
-            if deref_target.span.ctxt() == e.span.ctxt();
+            if deref_target.span.eq_ctxt(e.span);
             if !addrof_target.span.from_expansion();
             then {
                 let mut applicability = Applicability::MachineApplicable;

src/tools/clippy/clippy_lints/src/suspicious_xor_used_as_pow.rs

@@ -33,7 +33,7 @@ impl LateLintPass<'_> for ConfusingXorAndPow {
         if !in_external_macro(cx.sess(), expr.span)
             && let ExprKind::Binary(op, left, right) = &expr.kind
             && op.node == BinOpKind::BitXor
-            && left.span.ctxt() == right.span.ctxt()
+            && left.span.eq_ctxt(right.span)
             && let ExprKind::Lit(lit_left) = &left.kind
             && let ExprKind::Lit(lit_right) = &right.kind
             && matches!(lit_right.node, LitKind::Int(..) | LitKind::Float(..))

src/tools/clippy/clippy_utils/src/macros.rs

@@ -245,7 +245,7 @@ impl<'a> PanicExpn<'a> {
             return None;
         };
         let result = match name {
-            "panic" if arg.span.ctxt() == expr.span.ctxt() => Self::Empty,
+            "panic" if arg.span.eq_ctxt(expr.span) => Self::Empty,
             "panic" | "panic_str" => Self::Str(arg),
             "panic_display" | "panic_cold_display" => {
                 let ExprKind::AddrOf(_, _, e) = &arg.kind else {

tests/ui-fulldeps/internal-lints/span_use_eq_ctxt.rs

@@ -0,0 +1,13 @@
+// Test the `rustc::span_use_eq_ctxt` internal lint
+// compile-flags: -Z unstable-options
+
+#![feature(rustc_private)]
+#![deny(rustc::span_use_eq_ctxt)]
+#![crate_type = "lib"]
+
+extern crate rustc_span;
+use rustc_span::Span;
+
+pub fn f(s: Span, t: Span) -> bool {
+    s.ctxt() == t.ctxt() //~ ERROR use `.eq_ctxt()` instead of `.ctxt() == .ctxt()`
+}

tests/ui-fulldeps/internal-lints/span_use_eq_ctxt.stderr

@@ -0,0 +1,14 @@
+error: use `.eq_ctxt()` instead of `.ctxt() == .ctxt()`
+  --> $DIR/span_use_eq_ctxt.rs:12:5
+   |
+LL |     s.ctxt() == t.ctxt()
+   |     ^^^^^^^^^^^^^^^^^^^^
+   |
+note: the lint level is defined here
+  --> $DIR/span_use_eq_ctxt.rs:5:9
+   |
+LL | #![deny(rustc::span_use_eq_ctxt)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+