Rollup merge of #145592 - nilotpal-n7:fix-format-alignment, r=lcnr

Fix format string grammar in docs and improve alignment error message for #144023

This PR improves error messages and documentation for format strings involving alignment and formatting traits.

Highlights:

- Clearer error messages for invalid alignment specifiers (e.g., `{0:#X>18}`), showing the expected `<`, `^`, or `>` and a working example:

    println!("{0:>#18X}", value);

- Updated UI test `format-alignment-hash.rs` to reflect the improved error output.
- Documentation clarification: ensures examples correctly show how width, alignment, and traits like `x`, `X`, `#` combine.

Motivation:
Previously, using `#` with alignment and width produced confusing errors. This PR guides users on the correct syntax and provides actionable examples.

Testing:
- Built the compiler (`./x build`)
- Blessed and ran UI tests (`./x. test src/test/ui/fmt/format-alignment-hash.rs --bless`)
- Verified full test suite passes (`./x test`)

Issue: #144023

Author: matthiaskrgr

compiler/rustc_parse_format/src/lib.rs

@@ -858,7 +858,9 @@ impl<'input> Parser<'input> {
             self.errors.insert(
                 0,
                 ParseError {
-                    description: "expected format parameter to occur after `:`".to_owned(),
+                    description:
+                        "expected alignment specifier after `:` in format string; example: `{:>?}`"
+                            .to_owned(),
                     note: None,
                     label: format!("expected `{}` to occur after `:`", alignment),
                     span: range,

library/alloc/src/fmt.rs

@@ -354,7 +354,7 @@
 //! sign := '+' | '-'
 //! width := count
 //! precision := count | '*'
-//! type := '?' | 'x?' | 'X?' | identifier
+//! type := '?' | 'x?' | 'X?' | 'o' | 'x' | 'X' | 'p' | 'b' | 'e' | 'E'
 //! count := parameter | integer
 //! parameter := argument '$'
 //! ```

tests/ui/fmt/format-string-wrong-order.rs

@@ -13,9 +13,11 @@ fn main() {
     format!("{?:#?}", bar);
     //~^ ERROR invalid format string: expected format parameter to occur after `:`
     format!("Hello {<5:}!", "x");
-    //~^ ERROR invalid format string: expected format parameter to occur after `:`
+    //~^ ERROR invalid format string: expected alignment specifier after `:` in format string; example: `{:>?}`
     format!("Hello {^5:}!", "x");
-    //~^ ERROR invalid format string: expected format parameter to occur after `:`
+    //~^ ERROR invalid format string: expected alignment specifier after `:` in format string; example: `{:>?}`
     format!("Hello {>5:}!", "x");
-    //~^ ERROR invalid format string: expected format parameter to occur after `:`
+    //~^ ERROR invalid format string: expected alignment specifier after `:` in format string; example: `{:>?}`
+    println!("{0:#X>18}", 12345);
+    //~^ ERROR invalid format string: expected alignment specifier after `:` in format string; example: `{:>?}`
 }

tests/ui/fmt/format-string-wrong-order.stderr

@@ -50,23 +50,29 @@ LL |     format!("{?:#?}", bar);
    |
    = note: `?` comes after `:`, try `:?` instead
 
-error: invalid format string: expected format parameter to occur after `:`
+error: invalid format string: expected alignment specifier after `:` in format string; example: `{:>?}`
   --> $DIR/format-string-wrong-order.rs:15:21
    |
 LL |     format!("Hello {<5:}!", "x");
    |                     ^ expected `<` to occur after `:` in format string
 
-error: invalid format string: expected format parameter to occur after `:`
+error: invalid format string: expected alignment specifier after `:` in format string; example: `{:>?}`
   --> $DIR/format-string-wrong-order.rs:17:21
    |
 LL |     format!("Hello {^5:}!", "x");
    |                     ^ expected `^` to occur after `:` in format string
 
-error: invalid format string: expected format parameter to occur after `:`
+error: invalid format string: expected alignment specifier after `:` in format string; example: `{:>?}`
   --> $DIR/format-string-wrong-order.rs:19:21
    |
 LL |     format!("Hello {>5:}!", "x");
    |                     ^ expected `>` to occur after `:` in format string
 
-error: aborting due to 9 previous errors
+error: invalid format string: expected alignment specifier after `:` in format string; example: `{:>?}`
+  --> $DIR/format-string-wrong-order.rs:21:20
+   |
+LL |     println!("{0:#X>18}", 12345);
+   |                    ^ expected `>` to occur after `:` in format string
+
+error: aborting due to 10 previous errors