Suggest swapping the equality in user-written code

Signed-off-by: xizheyin <xizheyin@smail.nju.edu.cn>

Author: xizheyin

compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

@@ -3539,15 +3539,25 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 .must_apply_modulo_regions()
         {
             let sm = self.tcx.sess.source_map();
-            if let Ok(rhs_snippet) = sm.span_to_snippet(rhs_expr.span)
-                && let Ok(lhs_snippet) = sm.span_to_snippet(lhs_expr.span)
+            // If the span of rhs_expr or lhs_expr is in an external macro,
+            // we should find the user-written code span. See issue #139050
+            if let Some(rhs_span) = rhs_expr.span.find_ancestor_not_from_extern_macro(sm)
+                && let Some(lhs_span) = lhs_expr.span.find_ancestor_not_from_extern_macro(sm)
+                && let Ok(rhs_snippet) = sm.span_to_snippet(rhs_span)
+                && let Ok(lhs_snippet) = sm.span_to_snippet(lhs_span)
             {
                 err.note(format!("`{rhs_ty}` implements `PartialEq<{lhs_ty}>`"));
-                err.multipart_suggestion(
-                    "consider swapping the equality",
-                    vec![(lhs_expr.span, rhs_snippet), (rhs_expr.span, lhs_snippet)],
-                    Applicability::MaybeIncorrect,
-                );
+                if rhs_span != lhs_span {
+                    err.multipart_suggestion(
+                        "consider swapping the equality",
+                        vec![(lhs_span, rhs_snippet), (rhs_span, lhs_snippet)],
+                        Applicability::MaybeIncorrect,
+                    );
+                } else {
+                    // rhs_span and lhs_span are the same because it from extern macro.
+                    // we should suggest to swap two arguments of the equality
+                    err.span_help(rhs_span, "consider swapping two arguments of the equality");
+                }
             }
         }
     }

tests/ui/typeck/auxiliary/extern-macro-issue-139050.rs

@@ -1,5 +1,5 @@
 #[macro_export]
-macro_rules! eq {    
+macro_rules! eq {
     (assert $a:expr, $b:expr) => {
         match (&$a, &$b) {
             (left_val, right_val) => {
@@ -12,4 +12,4 @@ macro_rules! eq {
             }
         }
     };
-}
+}

tests/ui/typeck/sugg-swap-equality-in-macro-issue-139050.rs

@@ -8,7 +8,7 @@ extern crate ext;
 
 use std::fmt::Debug;
 
-macro_rules! eq_local {    
+macro_rules! eq_local {
     (assert $a:expr, $b:expr) => {
         match (&$a, &$b) {
             (left_val, right_val) => {
@@ -28,7 +28,6 @@ where
     Item: Eq + Debug, //~ ERROR cannot find type `Item` in this scope [E0412]
 {
     ext::eq!(assert iter.next(), Some(value)); //~ ERROR  mismatched types [E0308]
-    eq_local!(assert iter.next(), Some(value)); 
+    eq_local!(assert iter.next(), Some(value));
 }
 fn main() {}
-

tests/ui/typeck/sugg-swap-equality-in-macro-issue-139050.stderr

@@ -13,13 +13,12 @@ LL |     ext::eq!(assert iter.next(), Some(value));
    = note: expected enum `Option<_>`
               found enum `Option<&_>`
    = note: `Option<&<I as Iterator>::Item>` implements `PartialEq<Option<<I as Iterator>::Item>>`
-   = note: this error originates in the macro `ext::eq` (in Nightly builds, run with -Z macro-backtrace for more info)
-help: consider swapping the equality
-  --> $DIR/auxiliary/extern-macro-issue-139050.rs:6:22
-   |
-LL -                 if !(*left_val == *right_val) {
-LL +                 if !(*right_val == *left_val) {
+help: consider swapping two arguments of the equality
+  --> $DIR/sugg-swap-equality-in-macro-issue-139050.rs:30:5
    |
+LL |     ext::eq!(assert iter.next(), Some(value));
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: this error originates in the macro `ext::eq` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error[E0308]: mismatched types
   --> $DIR/sugg-swap-equality-in-macro-issue-139050.rs:15:35