Rollup merge of #157012 - Kokoro2336:fix/issue-156868, r=mejrs

Fix missing note of escaping `{` for braces including whitespaces

Fixes #156868

Add note of escaping `{` for braces including whitespaces

Author: JonathanBrouwer

compiler/rustc_builtin_macros/src/format.rs

@@ -987,6 +987,7 @@ fn report_invalid_references(
         // Collect all the implicit positions:
         let mut spans = Vec::new();
         let mut num_placeholders = 0;
+        let mut has_white_space_only_missing_arg = false;
         for piece in template {
             let mut placeholder = None;
             // `{arg:.*}`
@@ -1009,13 +1010,21 @@ fn report_invalid_references(
             }
             // `{}`
             if let FormatArgsPiece::Placeholder(FormatPlaceholder {
-                argument: FormatArgPosition { kind: FormatArgPositionKind::Implicit, .. },
+                argument: FormatArgPosition { kind: FormatArgPositionKind::Implicit, index, .. },
                 span,
                 ..
             }) = piece
             {
                 placeholder = *span;
                 num_placeholders += 1;
+                //  Check whether there's any non-space whitespace in the placeholder. If so, we should emit a note suggesting an escaping `{`.
+                if index.is_err()
+                    && let Some(span) = span
+                    && let Ok(snippet) = ecx.source_map().span_to_snippet(*span)
+                    && snippet.chars().any(|c| c.is_whitespace() && c != ' ')
+                {
+                    has_white_space_only_missing_arg = true;
+                }
             }
             // For `{:.*}`, we only push one span.
             spans.extend(placeholder);
@@ -1071,6 +1080,9 @@ fn report_invalid_references(
         if has_precision_star {
             e.note("positional arguments are zero-based");
         }
+        if has_white_space_only_missing_arg {
+            e.note("if you intended to print `{`, you can escape it with `{{`");
+        }
     } else {
         let mut indexes: Vec<_> = invalid_refs.iter().map(|&(index, _, _, _)| index).collect();
         // Avoid `invalid reference to positional arguments 7 and 7 (there is 1 argument)`

tests/ui/fmt/format-string-error-2.rs

@@ -91,4 +91,15 @@ raw  { \n
 
     println!("{x=}");
     //~^ ERROR invalid format string: python's f-string debug `=` is not supported in rust, use `dbg(x)` instead
+
+    println!(
+        "fn main() {\n\
+            \n\
+        }"
+        //~^^^ ERROR 1 positional argument in format string
+    );
+
+    // Don't emit note suggesting an escaping `{` for `{ }`.
+    println!("{ }");
+    //~^ ERROR 1 positional argument in format string
 }

tests/ui/fmt/format-string-error-2.stderr

@@ -208,5 +208,22 @@ LL |     println!("{x=}");
    |
    = note: to print `{`, you can escape it using `{{`
 
-error: aborting due to 21 previous errors
+error: 1 positional argument in format string, but no arguments were given
+  --> $DIR/format-string-error-2.rs:96:20
+   |
+LL |           "fn main() {\n\
+   |  ____________________^
+LL | |             \n\
+LL | |         }"
+   | |_________^
+   |
+   = note: if you intended to print `{`, you can escape it with `{{`
+
+error: 1 positional argument in format string, but no arguments were given
+  --> $DIR/format-string-error-2.rs:103:15
+   |
+LL |     println!("{ }");
+   |               ^^^
+
+error: aborting due to 23 previous errors