Improve error message for printf-style format strings

src/test/ui/fmt/issue-89173.rs

@@ -0,0 +1,14 @@
+// Regression test for #89173: Make sure a helpful note is issued for
+// printf-style format strings using `*` to specify the width.
+
+fn main() {
+    let num = 0x0abcde;
+    let width = 6;
+    print!("%0*x", width, num);
+    //~^ ERROR: multiple unused formatting arguments
+    //~| NOTE: multiple missing formatting specifiers
+    //~| NOTE: argument never used
+    //~| NOTE: argument never used
+    //~| NOTE: format specifiers use curly braces, and you have to use a positional or named parameter for the width
+    //~| NOTE: printf formatting not supported
+}

src/test/ui/fmt/issue-89173.stderr

@@ -0,0 +1,18 @@
+error: multiple unused formatting arguments
+  --> $DIR/issue-89173.rs:7:20
+   |
+LL |     print!("%0*x", width, num);
+   |            ------  ^^^^^  ^^^ argument never used
+   |            |       |
+   |            |       argument never used
+   |            multiple missing formatting specifiers
+   |
+note: format specifiers use curly braces, and you have to use a positional or named parameter for the width
+  --> $DIR/issue-89173.rs:7:13
+   |
+LL |     print!("%0*x", width, num);
+   |             ^^^^
+   = note: printf formatting not supported; see the documentation for `std::fmt`
+
+error: aborting due to previous error
+