add 'is_assign_rhs' to avoid weird suggesting 'let'

compiler/rustc_resolve/src/late.rs

@@ -527,6 +527,7 @@ struct DiagnosticMetadata<'ast> {
 
     /// Used to detect possible new binding written without `let` and to provide structured suggestion.
     in_assignment: Option<&'ast Expr>,
+    is_assign_rhs: bool,
 
     /// If we are currently in a trait object definition. Used to point at the bounds when
     /// encountering a struct or enum.
@@ -3963,10 +3964,15 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
                 self.resolve_expr(elem, Some(expr));
                 self.visit_expr(idx);
             }
-            ExprKind::Assign(..) => {
-                let old = self.diagnostic_metadata.in_assignment.replace(expr);
-                visit::walk_expr(self, expr);
-                self.diagnostic_metadata.in_assignment = old;
+            ExprKind::Assign(ref lhs, ref rhs, _) => {
+                if !self.diagnostic_metadata.is_assign_rhs {
+                    self.diagnostic_metadata.in_assignment = Some(expr);
+                }
+                self.visit_expr(lhs);
+                self.diagnostic_metadata.is_assign_rhs = true;
+                self.diagnostic_metadata.in_assignment = None;
+                self.visit_expr(rhs);
+                self.diagnostic_metadata.is_assign_rhs = false;
             }
             _ => {
                 visit::walk_expr(self, expr);

compiler/rustc_resolve/src/late/diagnostics.rs

@@ -1810,36 +1810,22 @@ impl<'a: 'ast, 'ast> LateResolutionVisitor<'a, '_, 'ast> {
         false
     }
 
+    // try to give a suggestion for this pattern: `name = blah`, which is common in other languages
+    // suggest `let name = blah` to introduce a new binding
     fn let_binding_suggestion(&mut self, err: &mut Diagnostic, ident_span: Span) -> bool {
-        // try to give a suggestion for this pattern: `name = 1`, which is common in other languages
-        let mut added_suggestion = false;
-        if let Some(Expr { kind: ExprKind::Assign(lhs, rhs, _), .. }) =
-            self.diagnostic_metadata.in_assignment
-        {
-            let is_rhs_assign = matches!(rhs.kind, ExprKind::Assign(..));
-            if let ast::ExprKind::Path(None, _) = lhs.kind && !is_rhs_assign {
-                let sm = self.r.session.source_map();
-                let line_span = sm.span_extend_to_line(ident_span);
-                let ident_name = sm.span_to_snippet(ident_span).unwrap();
-                // HACK(chenyukang): make sure ident_name is at the starting of the line to protect against macros,
-                // and avoid some special cases like `x = x = x`
-                if let Ok(line) = sm.span_to_snippet(line_span) &&
-                    let stripped = line.split_whitespace().collect::<String>() &&
-                    stripped.trim().starts_with(&ident_name) &&
-                    stripped.matches(&format!("{}=", &ident_name)).count() == 1
-                {
+        if let Some(Expr { kind: ExprKind::Assign(lhs, .. ), .. }) = self.diagnostic_metadata.in_assignment &&
+            let ast::ExprKind::Path(None, _) = lhs.kind {
+                if !ident_span.from_expansion() {
                     err.span_suggestion_verbose(
                         ident_span.shrink_to_lo(),
                         "you might have meant to introduce a new binding",
                         "let ".to_string(),
                         Applicability::MaybeIncorrect,
                     );
-                    added_suggestion = true;
+                    return true;
                 }
             }
-            self.diagnostic_metadata.in_assignment = None;
-        }
-        added_suggestion
+        false
     }
 
     fn find_module(&mut self, def_id: DefId) -> Option<(Module<'a>, ImportSuggestion)> {

src/test/ui/suggestions/issue-104086-suggest-let.stderr

@@ -2,7 +2,12 @@ error[E0425]: cannot find value `x` in this scope
   --> $DIR/issue-104086-suggest-let.rs:2:5
    |
 LL |     x = x = x;
-   |     ^ not found in this scope
+   |     ^
+   |
+help: you might have meant to introduce a new binding
+   |
+LL |     let x = x = x;
+   |     +++
 
 error[E0425]: cannot find value `x` in this scope
   --> $DIR/issue-104086-suggest-let.rs:2:9
@@ -20,7 +25,12 @@ error[E0425]: cannot find value `x` in this scope
   --> $DIR/issue-104086-suggest-let.rs:7:5
    |
 LL |     x = y = y = y;
-   |     ^ not found in this scope
+   |     ^
+   |
+help: you might have meant to introduce a new binding
+   |
+LL |     let x = y = y = y;
+   |     +++
 
 error[E0425]: cannot find value `y` in this scope
   --> $DIR/issue-104086-suggest-let.rs:7:9
@@ -44,7 +54,12 @@ error[E0425]: cannot find value `x` in this scope
   --> $DIR/issue-104086-suggest-let.rs:13:5
    |
 LL |     x = y = y;
-   |     ^ not found in this scope
+   |     ^
+   |
+help: you might have meant to introduce a new binding
+   |
+LL |     let x = y = y;
+   |     +++
 
 error[E0425]: cannot find value `y` in this scope
   --> $DIR/issue-104086-suggest-let.rs:13:9
@@ -62,7 +77,12 @@ error[E0425]: cannot find value `x` in this scope
   --> $DIR/issue-104086-suggest-let.rs:18:5
    |
 LL |     x = x = y;
-   |     ^ not found in this scope
+   |     ^
+   |
+help: you might have meant to introduce a new binding
+   |
+LL |     let x = x = y;
+   |     +++
 
 error[E0425]: cannot find value `x` in this scope
   --> $DIR/issue-104086-suggest-let.rs:18:9