Extend format arg help for simple tuple index access expression

compiler/rustc_parse_format/src/lib.rs

@@ -907,8 +907,8 @@ impl<'a> Parser<'a> {
             let byte_pos = self.to_span_index(end);
             let start = InnerOffset(byte_pos.0 + 1);
             let field = self.argument(start);
-            // We can only parse `foo.bar` field access, any deeper nesting,
-            // or another type of expression, like method calls, are not supported
+            // We can only parse simple `foo.bar` field access or `foo.0` tuple index access, any
+            // deeper nesting, or another type of expression, like method calls, are not supported
             if !self.consume('}') {
                 return;
             }
@@ -925,6 +925,18 @@ impl<'a> Parser<'a> {
                             suggestion: Suggestion::UsePositional,
                         },
                     );
+                } else if let ArgumentIs(_) = field.position {
+                    self.errors.insert(
+                        0,
+                        ParseError {
+                            description: "tuple index access isn't supported".to_string(),
+                            note: None,
+                            label: "not supported".to_string(),
+                            span: InnerSpan::new(arg.position_span.start, field.position_span.end),
+                            secondary_label: None,
+                            suggestion: Suggestion::UsePositional,
+                        },
+                    );
                 }
             }
         }

tests/ui/fmt/format-args-non-identifier-diagnostics.fixed

@@ -0,0 +1,7 @@
+//@ run-rustfix
+
+fn main() {
+    let x = (1,);
+    println!("{0}", x.0);
+    //~^ ERROR invalid format string
+}

tests/ui/fmt/format-args-non-identifier-diagnostics.rs

@@ -0,0 +1,7 @@
+//@ run-rustfix
+
+fn main() {
+    let x = (1,);
+    println!("{x.0}");
+    //~^ ERROR invalid format string
+}

tests/ui/fmt/format-args-non-identifier-diagnostics.stderr

@@ -0,0 +1,13 @@
+error: invalid format string: tuple index access isn't supported
+  --> $DIR/format-args-non-identifier-diagnostics.rs:5:16
+   |
+LL |     println!("{x.0}");
+   |                ^^^ not supported in format string
+   |
+help: consider using a positional formatting argument instead
+   |
+LL |     println!("{0}", x.0);
+   |                ~  +++++
+
+error: aborting due to 1 previous error
+