Handle implicit named arguments in useless_format

clippy_lints/src/format.rs

@@ -9,7 +9,7 @@
 use rustc_middle::ty;
 use rustc_session::{declare_lint_pass, declare_tool_lint};
 use rustc_span::symbol::kw;
-use rustc_span::{sym, Span};
+use rustc_span::{sym, BytePos, Span};
 
 declare_clippy_lint! {
     /// ### What it does
@@ -84,7 +84,22 @@ fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
                         ExprKind::MethodCall(path, ..) => path.ident.name.as_str() == "to_string",
                         _ => false,
                     };
-                    let sugg = if is_new_string {
+                    let sugg = if format_args.format_string_span.contains(value.span) {
+                        // Implicit argument. e.g. `format!("{x}")` span points to `{x}`
+                        let spdata = value.span.data();
+                        let span = Span::new(
+                            spdata.lo + BytePos(1),
+                            spdata.hi - BytePos(1),
+                            spdata.ctxt,
+                            spdata.parent
+                        );
+                        let snip = snippet_with_applicability(cx, span, "..", &mut applicability);
+                        if is_new_string {
+                            snip.into()
+                        } else {
+                            format!("{snip}.to_string()")
+                        }
+                    } else if is_new_string {
                         snippet_with_applicability(cx, value.span, "..", &mut applicability).into_owned()
                     } else {
                         let sugg = Sugg::hir_with_applicability(cx, value, "<arg>", &mut applicability);

tests/ui/format.fixed

@@ -73,4 +73,10 @@ fn main() {
     let _s: String = (&*v.join("\n")).to_string();
 
     format!("prepend {:+}", "s");
+
+    // Issue #8290
+    let x = "foo";
+    let _ = x.to_string();
+    let _ = format!("{x:?}"); // Don't lint on debug
+    let _ = x.to_string();
 }

tests/ui/format.rs

@@ -75,4 +75,10 @@ fn main() {
     let _s: String = format!("{}", &*v.join("\n"));
 
     format!("prepend {:+}", "s");
+
+    // Issue #8290
+    let x = "foo";
+    let _ = format!("{x}");
+    let _ = format!("{x:?}"); // Don't lint on debug
+    let _ = format!("{y}", y = x);
 }

tests/ui/format.stderr

@@ -99,5 +99,17 @@ error: useless use of `format!`
 LL |     let _s: String = format!("{}", &*v.join("/n"));
    |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using `.to_string()`: `(&*v.join("/n")).to_string()`
 
-error: aborting due to 15 previous errors
+error: useless use of `format!`
+  --> $DIR/format.rs:81:13
+   |
+LL |     let _ = format!("{x}");
+   |             ^^^^^^^^^^^^^^ help: consider using `.to_string()`: `x.to_string()`
+
+error: useless use of `format!`
+  --> $DIR/format.rs:83:13
+   |
+LL |     let _ = format!("{y}", y = x);
+   |             ^^^^^^^^^^^^^^^^^^^^^ help: consider using `.to_string()`: `x.to_string()`
+
+error: aborting due to 17 previous errors