Rollup merge of #89340 - FabianWolff:issue-89173, r=petrochenkov

Improve error message for `printf`-style format strings

Fixes #89173. The following is actually supported today:
```rust
fn main() {
    let num = 5;
    let width = 20;
    print!("%*2$x", num, width);
}
```
```
error: multiple unused formatting arguments
 --> src/main.rs:4:21
  |
4 |     print!("%*2$x", num, width);
  |            -------  ^^^  ^^^^^ argument never used
  |            ||       |
  |            ||       argument never used
  |            |help: format specifiers use curly braces: `{:1$x}`
  |            multiple missing formatting specifiers
  |
  = note: printf formatting not supported; see the documentation for `std::fmt`
```
However, as noted in #89173, something like
```rust
    print!("%0*x", width, num);
```
does not give a helpful suggestion. I think this is partly intended, because there actually _is_ no Rust equivalent to this; you always have to use a positional or named argument to specify the width (instead of just using the "next" argument, as `printf` or even `.*` as a precision specifier in Rust would). Therefore, I have added a note:
```
[...]
note: format specifiers use curly braces, and you have to use a positional or named parameter for the width
 --> t2.rs:4:13
  |
4 |     print!("%0*x", width, num);
  |             ^^^^
  = note: printf formatting not supported; see the documentation for `std::fmt`
```
This is not perfect, but it should at least point the user in the right direction, instead of issuing no explanation at all.

cc ```@lcnr```

2 files changed, 32 insertions, 0 deletions

diff --git a/src/test/ui/fmt/issue-89173.rs b/src/test/ui/fmt/issue-89173.rs
new file mode 100644
index 00000000000..96277d4d0d9
--- /dev/null
+++ b/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
+}
diff --git a/src/test/ui/fmt/issue-89173.stderr b/src/test/ui/fmt/issue-89173.stderr
new file mode 100644
index 00000000000..7b21e0a4fc8
--- /dev/null
+++ b/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
+