Improve error message when closing bracket interpreted as formatting fill character

compiler/rustc_parse_format/src/lib.rs

@@ -109,6 +109,8 @@ pub struct Argument<'a> {
 pub struct FormatSpec<'a> {
     /// Optionally specified character to fill alignment with.
     pub fill: Option<char>,
+    /// Span of the optionally specified fill character.
+    pub fill_span: Option<InnerSpan>,
     /// Optionally specified alignment.
     pub align: Alignment,
     /// The `+` or `-` flag.
@@ -264,7 +266,7 @@ impl<'a> Iterator for Parser<'a> {
                         Some(String(self.string(pos + 1)))
                     } else {
                         let arg = self.argument(lbrace_end);
-                        if let Some(rbrace_pos) = self.must_consume('}') {
+                        if let Some(rbrace_pos) = self.consume_closing_brace(&arg) {
                             if self.is_source_literal {
                                 let lbrace_byte_pos = self.to_span_index(pos);
                                 let rbrace_byte_pos = self.to_span_index(rbrace_pos);
@@ -450,69 +452,51 @@ impl<'a> Parser<'a> {
 
     /// Forces consumption of the specified character. If the character is not
     /// found, an error is emitted.
-    fn must_consume(&mut self, c: char) -> Option<usize> {
+    fn consume_closing_brace(&mut self, arg: &Argument<'_>) -> Option<usize> {
         self.ws();
 
-        if let Some(&(pos, maybe)) = self.cur.peek() {
+        let pos;
-            if c == maybe {
+        let description;
+
+        if let Some(&(peek_pos, maybe)) = self.cur.peek() {
+            if maybe == '}' {
                 self.cur.next();
-                Some(pos)
+                return Some(peek_pos);
-            } else {
-                let pos = self.to_span_index(pos);
-                let description = format!("expected `'}}'`, found `{maybe:?}`");
-                let label = "expected `'}'`".to_owned();
-                let (note, secondary_label) = if c == '}' {
-                    (
-                        Some(
-                            "if you intended to print `{`, you can escape it using `{{`".to_owned(),
-                        ),
-                        self.last_opening_brace
-                            .map(|sp| ("because of this opening brace".to_owned(), sp)),
-                    )
-                } else {
-                    (None, None)
-                };
-                self.errors.push(ParseError {
-                    description,
-                    note,
-                    label,
-                    span: pos.to(pos),
-                    secondary_label,
-                    should_be_replaced_with_positional_argument: false,
-                });
-                None
             }
+
+            pos = peek_pos;
+            description = format!("expected `'}}'`, found `{maybe:?}`");
         } else {
-            let description = format!("expected `{c:?}` but string was terminated");
+            description = "expected `'}'` but string was terminated".to_owned();
             // point at closing `"`
-            let pos = self.input.len() - if self.append_newline { 1 } else { 0 };
+            pos = self.input.len() - if self.append_newline { 1 } else { 0 };
-            let pos = self.to_span_index(pos);
-            if c == '}' {
-                let label = format!("expected `{c:?}`");
-                let (note, secondary_label) = if c == '}' {
-                    (
-                        Some(
-                            "if you intended to print `{`, you can escape it using `{{`".to_owned(),
-                        ),
-                        self.last_opening_brace
-                            .map(|sp| ("because of this opening brace".to_owned(), sp)),
-                    )
-                } else {
-                    (None, None)
-                };
-                self.errors.push(ParseError {
-                    description,
-                    note,
-                    label,
-                    span: pos.to(pos),
-                    secondary_label,
-                    should_be_replaced_with_positional_argument: false,
-                });
-            } else {
-                self.err(description, format!("expected `{c:?}`"), pos.to(pos));
-            }
-            None
         }
+
+        let pos = self.to_span_index(pos);
+
+        let label = "expected `'}'`".to_owned();
+        let (note, secondary_label) = if arg.format.fill == Some('}') {
+            (
+                Some("the character `'}'` is interpreted as a fill character because of the `:` that precedes it".to_owned()),
+                arg.format.fill_span.map(|sp| ("this is not interpreted as a formatting closing brace".to_owned(), sp)),
+            )
+        } else {
+            (
+                Some("if you intended to print `{`, you can escape it using `{{`".to_owned()),
+                self.last_opening_brace.map(|sp| ("because of this opening brace".to_owned(), sp)),
+            )
+        };
+
+        self.errors.push(ParseError {
+            description,
+            note,
+            label,
+            span: pos.to(pos),
+            secondary_label,
+            should_be_replaced_with_positional_argument: false,
+        });
+
+        None
     }
 
     /// Consumes all whitespace characters until the first non-whitespace character
@@ -608,6 +592,7 @@ impl<'a> Parser<'a> {
     fn format(&mut self) -> FormatSpec<'a> {
         let mut spec = FormatSpec {
             fill: None,
+            fill_span: None,
             align: AlignUnknown,
             sign: None,
             alternate: false,
@@ -625,9 +610,10 @@ impl<'a> Parser<'a> {
         }
 
         // fill character
-        if let Some(&(_, c)) = self.cur.peek() {
+        if let Some(&(idx, c)) = self.cur.peek() {
             if let Some((_, '>' | '<' | '^')) = self.cur.clone().nth(1) {
                 spec.fill = Some(c);
+                spec.fill_span = Some(self.span(idx, idx + 1));
                 self.cur.next();
             }
         }
@@ -722,6 +708,7 @@ impl<'a> Parser<'a> {
     fn inline_asm(&mut self) -> FormatSpec<'a> {
         let mut spec = FormatSpec {
             fill: None,
+            fill_span: None,
             align: AlignUnknown,
             sign: None,
             alternate: false,

tests/ui/fmt/closing-brace-as-fill.rs

@@ -0,0 +1,8 @@
+// issue: 112732
+
+// `}` is typoed since it is interpreted as a fill character rather than a closing bracket
+
+fn main() {
+    println!("Hello, world! {0:}<3", 2);
+    //~^ ERROR invalid format string: expected `'}'` but string was terminated
+}

tests/ui/fmt/closing-brace-as-fill.stderr

@@ -0,0 +1,12 @@
+error: invalid format string: expected `'}'` but string was terminated
+  --> $DIR/closing-brace-as-fill.rs:6:35
+   |
+LL |     println!("Hello, world! {0:}<3", 2);
+   |                                -  ^ expected `'}'` in format string
+   |                                |
+   |                                this is not interpreted as a formatting closing brace
+   |
+   = note: the character `'}'` is interpreted as a fill character because of the `:` that precedes it
+
+error: aborting due to previous error
+