Rollup merge of #116787 - a-lafrance:span-internal-lint, r=oli-obk

Implement an internal lint encouraging use of `Span::eq_ctxt`

Adds a new Rustc internal lint that forbids use of `.ctxt() == .ctxt()` for spans, encouraging use of `.eq_ctxt()` instead (see https://github.com/rust-lang/rust/issues/49509).

Also fixed a few violations of the lint in the Rustc codebase (a fun additional way I could test my code). Edit: MIR opt folks I believe that's why you're CC'ed, just a heads up.

Two things I'm not sure about:
1. The way I chose to detect calls to `Span::ctxt`. I know adding diagnostic items to methods is generally discouraged, but after some searching and experimenting I couldn't find anything else that worked, so I went with it. That said, I'm happy to implement something different if there's a better way out there. (For what it's worth, if there is a better way, it might be worth documenting in the rustc-dev-guide, which I'm happy to take care of)
2. The error message for the lint. Ideally it would probably be good to give some context as to why the suggestion is made (e.g. `rustc::default_hash_types` tells the user that it's because of performance), but I don't have that context so I couldn't put it in the error message. Happy to iterate on the error message based on feedback during review.

r? ``@oli-obk``

25 files changed, 89 insertions, 21 deletions

diff --git a/compiler/rustc_hir_typeck/src/callee.rs b/compiler/rustc_hir_typeck/src/callee.rs
index 512d73fc103..1c23ccd1579 100644
--- a/compiler/rustc_hir_typeck/src/callee.rs
+++ b/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(),
diff --git a/compiler/rustc_lint/messages.ftl b/compiler/rustc_lint/messages.ftl
index 197fe6552d7..4c4d2933bf4 100644
--- a/compiler/rustc_lint/messages.ftl
+++ b/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
 
diff --git a/compiler/rustc_lint/src/internal.rs b/compiler/rustc_lint/src/internal.rs
index fc2d3d0a254..2d86129c480 100644
--- a/compiler/rustc_lint/src/internal.rs
+++ b/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,
+    }
+}
diff --git a/compiler/rustc_lint/src/lib.rs b/compiler/rustc_lint/src/lib.rs
index b9e455e6c2a..d61c59af1e0 100644
--- a/compiler/rustc_lint/src/lib.rs
+++ b/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),
         ],
     );
 }
diff --git a/compiler/rustc_lint/src/lints.rs b/compiler/rustc_lint/src/lints.rs
index 594ef97b3ff..4eaf8bbf5de 100644
--- a/compiler/rustc_lint/src/lints.rs
+++ b/compiler/rustc_lint/src/lints.rs
@@ -901,6 +901,10 @@ pub struct QueryInstability {
 }
 
 #[derive(LintDiagnostic)]
+#[diag(lint_span_use_eq_ctxt)]
+pub struct SpanUseEqCtxtDiag;
+
+#[derive(LintDiagnostic)]
 #[diag(lint_tykind_kind)]
 pub struct TykindKind {
     #[suggestion(code = "ty", applicability = "maybe-incorrect")]
diff --git a/compiler/rustc_mir_transform/src/coverage/spans.rs b/compiler/rustc_mir_transform/src/coverage/spans.rs
index 1d1be8f2492..f1a0f762041 100644
--- a/compiler/rustc_mir_transform/src/coverage/spans.rs
+++ b/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;
         }
diff --git a/compiler/rustc_passes/src/loops.rs b/compiler/rustc_passes/src/loops.rs
index 4590ab9e4f5..25e131d7477 100644
--- a/compiler/rustc_passes/src/loops.rs
+++ b/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 });
             }
diff --git a/compiler/rustc_span/src/span_encoding.rs b/compiler/rustc_span/src/span_encoding.rs
index bfc9e125362..f7d17a267d6 100644
--- a/compiler/rustc_span/src/span_encoding.rs
+++ b/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 {
diff --git a/compiler/rustc_span/src/symbol.rs b/compiler/rustc_span/src/symbol.rs
index ea261923c65..be8c65862dc 100644
--- a/compiler/rustc_span/src/symbol.rs
+++ b/compiler/rustc_span/src/symbol.rs
@@ -303,6 +303,7 @@ symbols! {
         SliceIndex,
         SliceIter,
         Some,
+        SpanCtxt,
         String,
         StructuralEq,
         StructuralPartialEq,
diff --git a/src/tools/clippy/clippy_lints/src/dereference.rs b/src/tools/clippy/clippy_lints/src/dereference.rs
index 14877385646..5134cf66050 100644
--- a/src/tools/clippy/clippy_lints/src/dereference.rs
+++ b/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, _, _)
diff --git a/src/tools/clippy/clippy_lints/src/entry.rs b/src/tools/clippy/clippy_lints/src/entry.rs
index 6197b5b19eb..70a467dde61 100644
--- a/src/tools/clippy/clippy_lints/src/entry.rs
+++ b/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,
diff --git a/src/tools/clippy/clippy_lints/src/formatting.rs b/src/tools/clippy/clippy_lints/src/formatting.rs
index d03480c2108..4ebf0e9667d 100644
--- a/src/tools/clippy/clippy_lints/src/formatting.rs
+++ b/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());
diff --git a/src/tools/clippy/clippy_lints/src/let_with_type_underscore.rs b/src/tools/clippy/clippy_lints/src/let_with_type_underscore.rs
index 4e9d77ea156..79d728a021c 100644
--- a/src/tools/clippy/clippy_lints/src/let_with_type_underscore.rs
+++ b/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`
diff --git a/src/tools/clippy/clippy_lints/src/manual_let_else.rs b/src/tools/clippy/clippy_lints/src/manual_let_else.rs
index 2117308cd40..86bbdb4ea19 100644
--- a/src/tools/clippy/clippy_lints/src/manual_let_else.rs
+++ b/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 {
diff --git a/src/tools/clippy/clippy_lints/src/matches/collapsible_match.rs b/src/tools/clippy/clippy_lints/src/matches/collapsible_match.rs
index 33a052c41a3..29b935fb61a 100644
--- a/src/tools/clippy/clippy_lints/src/matches/collapsible_match.rs
+++ b/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));
diff --git a/src/tools/clippy/clippy_lints/src/matches/manual_utils.rs b/src/tools/clippy/clippy_lints/src/matches/manual_utils.rs
index 6b611f567ae..781ee138c76 100644
--- a/src/tools/clippy/clippy_lints/src/matches/manual_utils.rs
+++ b/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 {
diff --git a/src/tools/clippy/clippy_lints/src/methods/map_unwrap_or.rs b/src/tools/clippy/clippy_lints/src/methods/map_unwrap_or.rs
index 5464e455dea..e70a1bc9879 100644
--- a/src/tools/clippy/clippy_lints/src/methods/map_unwrap_or.rs
+++ b/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(
diff --git a/src/tools/clippy/clippy_lints/src/needless_question_mark.rs b/src/tools/clippy/clippy_lints/src/needless_question_mark.rs
index 7b0f7eaf1f0..0e834fb3ac7 100644
--- a/src/tools/clippy/clippy_lints/src/needless_question_mark.rs
+++ b/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;
diff --git a/src/tools/clippy/clippy_lints/src/non_octal_unix_permissions.rs b/src/tools/clippy/clippy_lints/src/non_octal_unix_permissions.rs
index d47728f190a..e94e4589966 100644
--- a/src/tools/clippy/clippy_lints/src/non_octal_unix_permissions.rs
+++ b/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 {
diff --git a/src/tools/clippy/clippy_lints/src/redundant_async_block.rs b/src/tools/clippy/clippy_lints/src/redundant_async_block.rs
index 534b2762ba7..8193057a6eb 100644
--- a/src/tools/clippy/clippy_lints/src/redundant_async_block.rs
+++ b/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())
diff --git a/src/tools/clippy/clippy_lints/src/reference.rs b/src/tools/clippy/clippy_lints/src/reference.rs
index db870ec4c5b..12da29f1108 100644
--- a/src/tools/clippy/clippy_lints/src/reference.rs
+++ b/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;
diff --git a/src/tools/clippy/clippy_lints/src/suspicious_xor_used_as_pow.rs b/src/tools/clippy/clippy_lints/src/suspicious_xor_used_as_pow.rs
index 8e156b8829b..39cd289b67a 100644
--- a/src/tools/clippy/clippy_lints/src/suspicious_xor_used_as_pow.rs
+++ b/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(..))
diff --git a/src/tools/clippy/clippy_utils/src/macros.rs b/src/tools/clippy/clippy_utils/src/macros.rs
index eaf590f6ad7..46ce4ffdce5 100644
--- a/src/tools/clippy/clippy_utils/src/macros.rs
+++ b/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 {
diff --git a/tests/ui-fulldeps/internal-lints/span_use_eq_ctxt.rs b/tests/ui-fulldeps/internal-lints/span_use_eq_ctxt.rs
new file mode 100644
index 00000000000..39980ee7c67
--- /dev/null
+++ b/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()`
+}
diff --git a/tests/ui-fulldeps/internal-lints/span_use_eq_ctxt.stderr b/tests/ui-fulldeps/internal-lints/span_use_eq_ctxt.stderr
new file mode 100644
index 00000000000..b33f6212545
--- /dev/null
+++ b/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
+