Suggest examples of format specifiers in error messages

1 files changed, 25 insertions, 2 deletions

diff --git a/compiler/rustc_builtin_macros/src/format.rs b/compiler/rustc_builtin_macros/src/format.rs
index 6415e55e0b0..ce9bda10721 100644
--- a/compiler/rustc_builtin_macros/src/format.rs
+++ b/compiler/rustc_builtin_macros/src/format.rs
@@ -758,8 +758,31 @@ fn report_missing_placeholders(
             check_foreign!(shell);
         }
     }
-    if !found_foreign && unused.len() == 1 {
-        diag.span_label(fmt_span, "formatting specifier missing");
+    if !found_foreign {
+        if unused.len() == 1 {
+            diag.span_label(fmt_span, "formatting specifier missing");
+        }
+        if used.iter().all(|used| !used) {
+            diag.note("format specifiers use curly braces: `{}`");
+        }
+
+        let mut suggest_fixed_fmt = format!("\"{}", &fmt_str[..fmt_str.len() - 1]);
+        for _ in &unused {
+            suggest_fixed_fmt.push_str("{}");
+        }
+        suggest_fixed_fmt.push('"');
+
+        let suggest_fmt_count = if unused.len() == 1 {
+            "consider adding format specifier".to_string()
+        } else {
+            format!("consider adding {} format specifiers", unused.len())
+        };
+        diag.span_suggestion_verbose(
+            fmt_span,
+            suggest_fmt_count,
+            suggest_fixed_fmt,
+            Applicability::MaybeIncorrect,
+        );
     }
 
     diag.emit();