implement the basics of the lint static analysis

compiler/rustc_lint/src/lib.rs

@@ -83,6 +83,7 @@ mod passes;
 mod ptr_nulls;
 mod redundant_semicolon;
 mod reference_casting;
+mod span_use_eq_ctxt;
 mod traits;
 mod types;
 mod unused;

compiler/rustc_lint/src/span_use_eq_ctxt.rs

@@ -0,0 +1,38 @@
+use crate::{LateContext, LateLintPass};
+use rustc_hir::{BinOp, BinOpKind, Expr, ExprKind};
+use rustc_span::sym;
+
+declare_lint! {
+    pub SPAN_USE_EQ_CTXT,
+    Warn, // is this the right level?
+    "Use of `==` with `Span::ctxt` rather than `Span::eq_ctxt`"
+}
+
+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) {
+                todo!(); // emit lint
+            }
+        }
+    }
+}
+
+fn is_span_ctxt_call(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
+    match &expr.kind {
+        ExprKind::MethodCall(..) => {
+            // i gave a method a diagnostic item -- FIXME: switch to a diagnostic
+            // item for the Span type and check:
+            //   * method call path == "ctxt"
+            //   * receiver type matches Span diag item
+            // also FIXME(todo) remove old SpanCtxt diagnostic item
+            cx.typeck_results()
+                .type_dependent_def_id(expr.hir_id)
+                .is_some_and(|did| cx.tcx.is_diagnostic_item(sym::SpanCtxt, did))
+        }
+
+        _ => false,
+    }
+}

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`.
+    #[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,