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: rust-lang/rust#144023

1 files changed, 1 insertions, 1 deletions

diff --git a/library/alloc/src/fmt.rs b/library/alloc/src/fmt.rs
index d0ba9c39886..82eaf7d8724 100644
--- a/library/alloc/src/fmt.rs
+++ b/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 '$'
 //! ```