Fix format string grammar in docs and improve alignment error message

Author: nilotpal-n7

compiler/rustc_parse_format/src/lib.rs

@@ -858,8 +858,14 @@ impl<'input> Parser<'input> {
             self.errors.insert(
                 0,
                 ParseError {
-                    description: "expected format parameter to occur after `:`".to_owned(),
-                    note: None,
+                    description: format!(
+                        "expected `{}` (alignment specifier) after `:` in format string; example: `{{:>#X5}}`",
+                        alignment
+                    ),
+                    note: Some(
+                        "alignment must be one of `<` (left), `^` (center), or `>` (right)"
+                            .to_string(),
+                    ),
                     label: format!("expected `{}` to occur after `:`", alignment),
                     span: range,
                     secondary_label: None,

library/alloc/src/fmt.rs

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

tests/ui/fmt/format-alignment-hash.rs

@@ -0,0 +1,5 @@
+fn main() {
+    // INVALID: alignment (`>18`) comes after `#X`, should be `:>#18X`
+    println!("{0:#X>18}", 12345);
+    //~^ ERROR invalid format string: expected `>` (alignment specifier) after `:` in format string
+}

tests/ui/fmt/format-alignment-hash.stderr

@@ -0,0 +1,10 @@
+error: invalid format string: expected `>` (alignment specifier) after `:` in format string; example: `{:>#X5}`
+  --> $DIR/format-alignment-hash.rs:3:20
+   |
+LL |     println!("{0:#X>18}", 12345);
+   |                    ^ expected `>` to occur after `:` in format string
+   |
+   = note: alignment must be one of `<` (left), `^` (center), or `>` (right)
+
+error: aborting due to 1 previous error
+

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

@@ -13,9 +13,9 @@ 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: `{:>#X5}`
     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: `{:>#X5}`
     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: `{:>#X5}`
 }

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: `{:>#X5}`
   --> $DIR/format-string-wrong-order.rs:15:21
    |
 LL |     format!("Hello {<5:}!", "x");
    |                     ^ expected `<` to occur after `:` in format string
+   |
+   = note: alignment must be one of `<` (left), `^` (center), or `>` (right)
 
-error: invalid format string: expected format parameter to occur after `:`
+error: invalid format string: expected `^` (alignment specifier) after `:` in format string; example: `{:>#X5}`
   --> $DIR/format-string-wrong-order.rs:17:21
    |
 LL |     format!("Hello {^5:}!", "x");
    |                     ^ expected `^` to occur after `:` in format string
+   |
+   = note: alignment must be one of `<` (left), `^` (center), or `>` (right)
 
-error: invalid format string: expected format parameter to occur after `:`
+error: invalid format string: expected `>` (alignment specifier) after `:` in format string; example: `{:>#X5}`
   --> $DIR/format-string-wrong-order.rs:19:21
    |
 LL |     format!("Hello {>5:}!", "x");
    |                     ^ expected `>` to occur after `:` in format string
+   |
+   = note: alignment must be one of `<` (left), `^` (center), or `>` (right)
 
 error: aborting due to 9 previous errors