Tighten the 'introduce new binding' suggestion

compiler/rustc_resolve/src/late/diagnostics.rs

@@ -1810,18 +1810,23 @@ impl<'a: 'ast, 'ast> LateResolutionVisitor<'a, '_, 'ast> {
         false
     }
 
-    fn let_binding_suggestion(&self, err: &mut Diagnostic, ident_span: Span) -> bool {
+    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 ast::ExprKind::Path(None, _) = lhs.kind {
+        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
-                if sm
-                    .span_to_snippet(line_span)
-                    .map_or(false, |s| s.trim().starts_with(&ident_name))
+                // 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
                 {
                     err.span_suggestion_verbose(
                         ident_span.shrink_to_lo(),
@@ -1832,6 +1837,8 @@ impl<'a: 'ast, 'ast> LateResolutionVisitor<'a, '_, 'ast> {
                     added_suggestion = true;
                 }
             }
+            self.diagnostic_metadata.in_assignment = None;
+        }
         added_suggestion
     }
 

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

@@ -0,0 +1,30 @@
+fn main() {
+    x = x = x;
+    //~^ ERROR cannot find value `x` in this scope
+    //~| ERROR cannot find value `x` in this scope
+    //~| ERROR cannot find value `x` in this scope
+
+    x = y = y = y;
+    //~^ ERROR cannot find value `y` in this scope
+    //~| ERROR cannot find value `y` in this scope
+    //~| ERROR cannot find value `y` in this scope
+    //~| ERROR cannot find value `x` in this scope
+
+    x = y = y;
+    //~^ ERROR cannot find value `x` in this scope
+    //~| ERROR cannot find value `y` in this scope
+    //~| ERROR cannot find value `y` in this scope
+
+    x = x = y;
+    //~^ ERROR cannot find value `x` in this scope
+    //~| ERROR cannot find value `x` in this scope
+    //~| ERROR cannot find value `y` in this scope
+
+    x = x; // will suggest add `let`
+    //~^ ERROR cannot find value `x` in this scope
+    //~| ERROR cannot find value `x` in this scope
+
+    x = y // will suggest add `let`
+    //~^ ERROR cannot find value `x` in this scope
+    //~| ERROR cannot find value `y` in this scope
+}

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

@@ -0,0 +1,115 @@
+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
+
+error[E0425]: cannot find value `x` in this scope
+  --> $DIR/issue-104086-suggest-let.rs:2:9
+   |
+LL |     x = x = x;
+   |         ^ not found in this scope
+
+error[E0425]: cannot find value `x` in this scope
+  --> $DIR/issue-104086-suggest-let.rs:2:13
+   |
+LL |     x = x = x;
+   |             ^ not found in this scope
+
+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
+
+error[E0425]: cannot find value `y` in this scope
+  --> $DIR/issue-104086-suggest-let.rs:7:9
+   |
+LL |     x = y = y = y;
+   |         ^ not found in this scope
+
+error[E0425]: cannot find value `y` in this scope
+  --> $DIR/issue-104086-suggest-let.rs:7:13
+   |
+LL |     x = y = y = y;
+   |             ^ not found in this scope
+
+error[E0425]: cannot find value `y` in this scope
+  --> $DIR/issue-104086-suggest-let.rs:7:17
+   |
+LL |     x = y = y = y;
+   |                 ^ not found in this scope
+
+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
+
+error[E0425]: cannot find value `y` in this scope
+  --> $DIR/issue-104086-suggest-let.rs:13:9
+   |
+LL |     x = y = y;
+   |         ^ not found in this scope
+
+error[E0425]: cannot find value `y` in this scope
+  --> $DIR/issue-104086-suggest-let.rs:13:13
+   |
+LL |     x = y = y;
+   |             ^ not found in this scope
+
+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
+
+error[E0425]: cannot find value `x` in this scope
+  --> $DIR/issue-104086-suggest-let.rs:18:9
+   |
+LL |     x = x = y;
+   |         ^ not found in this scope
+
+error[E0425]: cannot find value `y` in this scope
+  --> $DIR/issue-104086-suggest-let.rs:18:13
+   |
+LL |     x = x = y;
+   |             ^ not found in this scope
+
+error[E0425]: cannot find value `x` in this scope
+  --> $DIR/issue-104086-suggest-let.rs:23:5
+   |
+LL |     x = x; // will suggest add `let`
+   |     ^
+   |
+help: you might have meant to introduce a new binding
+   |
+LL |     let x = x; // will suggest add `let`
+   |     +++
+
+error[E0425]: cannot find value `x` in this scope
+  --> $DIR/issue-104086-suggest-let.rs:23:9
+   |
+LL |     x = x; // will suggest add `let`
+   |         ^ not found in this scope
+
+error[E0425]: cannot find value `x` in this scope
+  --> $DIR/issue-104086-suggest-let.rs:27:5
+   |
+LL |     x = y // will suggest add `let`
+   |     ^
+   |
+help: you might have meant to introduce a new binding
+   |
+LL |     let x = y // will suggest add `let`
+   |     +++
+
+error[E0425]: cannot find value `y` in this scope
+  --> $DIR/issue-104086-suggest-let.rs:27:9
+   |
+LL |     x = y // will suggest add `let`
+   |         ^ not found in this scope
+
+error: aborting due to 17 previous errors
+
+For more information about this error, try `rustc --explain E0425`.