From fdbaaac245673d8dbf7bcc21ebedc833693434f6 Mon Sep 17 00:00:00 2001
From: Nilotpal Gupta <nilotpalgupta0701@gmail.com>
Date: Tue, 19 Aug 2025 11:10:32 +0530
Subject: [PATCH] Fix format string grammar in docs and improve alignment error
 message

---
 compiler/rustc_parse_format/src/lib.rs        |  4 +++-
 library/alloc/src/fmt.rs                      |  2 +-
 tests/ui/fmt/format-string-wrong-order.rs     |  8 +++++---
 tests/ui/fmt/format-string-wrong-order.stderr | 14 ++++++++++----
 4 files changed, 19 insertions(+), 9 deletions(-)

diff --git a/compiler/rustc_parse_format/src/lib.rs b/compiler/rustc_parse_format/src/lib.rs
index 8e4da7923fcb5..5cda0b813d233 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,
diff --git a/library/alloc/src/fmt.rs b/library/alloc/src/fmt.rs
index d0ba9c398864f..82eaf7d87244d 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 '$'
 //! ```
diff --git a/tests/ui/fmt/format-string-wrong-order.rs b/tests/ui/fmt/format-string-wrong-order.rs
index 891279b97e4d3..4d4e04ecd042d 100644
--- a/tests/ui/fmt/format-string-wrong-order.rs
+++ b/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: `{:>?}`
 }
diff --git a/tests/ui/fmt/format-string-wrong-order.stderr b/tests/ui/fmt/format-string-wrong-order.stderr
index 7f017511761f5..441ae6d2e505f 100644
--- a/tests/ui/fmt/format-string-wrong-order.stderr
+++ b/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