Rollup merge of #146123 - IoaNNUwU:issue-68293, r=estebank

Suggest examples of format specifiers in error messages

Format macro now suggests adding `{}` if no formatting specifiers are present. It also gives an example:
```rust
LL |     println!("Hello", "World");
   |              -------  ^^^^^^^ argument never used
   |              |
   |              formatting specifier missing
   |
   = note: format specifiers use curly braces: `{}`
help: consider adding format specifier
   |
LL |     println!("Hello{}", "World");
   |                    ++
```
When one or more `{}` are present, it doesn't show 'format specifiers use curly braces: `{}`' and example, just small hint on how many you missing:
```rust
LL |     println!("list: {}", 1, 2, 3);
   |              ----------     ^  ^ argument never used
   |              |              |
   |              |              argument never used
   |              multiple missing formatting specifiers
   |
   = help: consider adding 2 format specifiers
```

Original issue: rust-lang/rust#68293
Based on discussion in this PR: rust-lang/rust#76443

Let me know if something is missing

1 files changed, 27 insertions, 0 deletions

diff --git a/compiler/rustc_builtin_macros/src/format.rs b/compiler/rustc_builtin_macros/src/format.rs
index 6415e55e0b0..a6c8e7d29cc 100644
--- a/compiler/rustc_builtin_macros/src/format.rs
+++ b/compiler/rustc_builtin_macros/src/format.rs
@@ -565,6 +565,7 @@ fn make_format_args(
             &used,
             &args,
             &pieces,
+            &invalid_refs,
             detect_foreign_fmt,
             str_style,
             fmt_str,
@@ -645,6 +646,7 @@ fn report_missing_placeholders(
     used: &[bool],
     args: &FormatArguments,
     pieces: &[parse::Piece<'_>],
+    invalid_refs: &[(usize, Option<Span>, PositionUsedAs, FormatArgPositionKind)],
     detect_foreign_fmt: bool,
     str_style: Option<usize>,
     fmt_str: &str,
@@ -762,6 +764,31 @@ fn report_missing_placeholders(
         diag.span_label(fmt_span, "formatting specifier missing");
     }
 
+    if !found_foreign && invalid_refs.is_empty() {
+        // Show example if user didn't use any format specifiers
+        let show_example = used.iter().all(|used| !used);
+
+        if !show_example {
+            if unused.len() > 1 {
+                diag.note(format!("consider adding {} format specifiers", unused.len()));
+            }
+        } else {
+            let original_fmt_str =
+                if fmt_str.len() >= 1 { &fmt_str[..fmt_str.len() - 1] } else { "" };
+
+            let msg = if unused.len() == 1 {
+                "a format specifier".to_string()
+            } else {
+                format!("{} format specifiers", unused.len())
+            };
+
+            let sugg = format!("\"{}{}\"", original_fmt_str, "{}".repeat(unused.len()));
+            let msg = format!("format specifiers use curly braces, consider adding {msg}");
+
+            diag.span_suggestion_verbose(fmt_span, msg, sugg, Applicability::MaybeIncorrect);
+        }
+    }
+
     diag.emit();
 }