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, 3 insertions, 1 deletions

diff --git a/compiler/rustc_parse_format/src/lib.rs b/compiler/rustc_parse_format/src/lib.rs
index 8e4da7923fc..5cda0b813d2 100644
--- a/compiler/rustc_parse_format/src/lib.rs
+++ b/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,