Make useless_format recognize format!("")

Closes #7796

Author: aDotInTheVoid

clippy_lints/src/format.rs

@@ -50,15 +50,19 @@ impl<'tcx> LateLintPass<'tcx> for UselessFormat {
 
         let mut applicability = Applicability::MachineApplicable;
         if format_args.value_args.is_empty() {
-            if_chain! {
-                if let [e] = &*format_args.format_string_parts;
-                if let ExprKind::Lit(lit) = &e.kind;
-                if let Some(s_src) = snippet_opt(cx, lit.span);
-                then {
-                    // Simulate macro expansion, converting {{ and }} to { and }.
-                    let s_expand = s_src.replace("{{", "{").replace("}}", "}");
-                    let sugg = format!("{}.to_string()", s_expand);
-                    span_useless_format(cx, call_site, sugg, applicability);
+            if format_args.format_string_parts.is_empty() {
+                span_useless_format_empty(cx, call_site, "String::new()".to_owned(), applicability);
+            } else {
+                if_chain! {
+                    if let [e] = &*format_args.format_string_parts;
+                    if let ExprKind::Lit(lit) = &e.kind;
+                    if let Some(s_src) = snippet_opt(cx, lit.span);
+                    then {
+                        // Simulate macro expansion, converting {{ and }} to { and }.
+                        let s_expand = s_src.replace("{{", "{").replace("}}", "}");
+                        let sugg = format!("{}.to_string()", s_expand);
+                        span_useless_format(cx, call_site, sugg, applicability);
+                    }
                 }
             }
         } else if let [value] = *format_args.value_args {
@@ -90,6 +94,24 @@ impl<'tcx> LateLintPass<'tcx> for UselessFormat {
     }
 }
 
+
+fn span_useless_format_empty(cx: &LateContext<'_>, span: Span, mut sugg: String, mut applicability: Applicability) {
+    // The callsite span contains the statement semicolon for some reason.
+    if snippet_with_applicability(cx, span, "..", &mut applicability).ends_with(';') {
+        sugg.push(';');
+    }
+
+    span_lint_and_sugg(
+        cx,
+        USELESS_FORMAT,
+        span,
+        "useless use of `format!`",
+        "consider using `String::new()`",
+        sugg,
+        applicability,
+    );
+}
+
 fn span_useless_format(cx: &LateContext<'_>, span: Span, mut sugg: String, mut applicability: Applicability) {
     // The callsite span contains the statement semicolon for some reason.
     if snippet_with_applicability(cx, span, "..", &mut applicability).ends_with(';') {

tests/ui/format.fixed

@@ -16,6 +16,8 @@ fn main() {
     r##"foo {}
 " bar"##.to_string();
 
+    String::new();
+
     "foo".to_string();
     format!("{:?}", "foo"); // Don't warn about `Debug`.
     format!("{:8}", "foo");

tests/ui/format.rs

@@ -18,6 +18,8 @@ fn main() {
 " bar"##
     );
 
+    format!("");
+
     format!("{}", "foo");
     format!("{:?}", "foo"); // Don't warn about `Debug`.
     format!("{:8}", "foo");

tests/ui/format.stderr

@@ -36,62 +36,68 @@ LL + " bar"##.to_string();
 error: useless use of `format!`
   --> $DIR/format.rs:21:5
    |
+LL |     format!("");
+   |     ^^^^^^^^^^^^ help: consider using `String::new()`: `String::new();`
+
+error: useless use of `format!`
+  --> $DIR/format.rs:23:5
+   |
 LL |     format!("{}", "foo");
    |     ^^^^^^^^^^^^^^^^^^^^^ help: consider using `.to_string()`: `"foo".to_string();`
 
 error: useless use of `format!`
-  --> $DIR/format.rs:25:5
+  --> $DIR/format.rs:27:5
    |
 LL |     format!("{:+}", "foo"); // Warn when the format makes no difference.
    |     ^^^^^^^^^^^^^^^^^^^^^^^ help: consider using `.to_string()`: `"foo".to_string();`
 
 error: useless use of `format!`
-  --> $DIR/format.rs:26:5
+  --> $DIR/format.rs:28:5
    |
 LL |     format!("{:<}", "foo"); // Warn when the format makes no difference.
    |     ^^^^^^^^^^^^^^^^^^^^^^^ help: consider using `.to_string()`: `"foo".to_string();`
 
 error: useless use of `format!`
-  --> $DIR/format.rs:31:5
+  --> $DIR/format.rs:33:5
    |
 LL |     format!("{}", arg);
    |     ^^^^^^^^^^^^^^^^^^^ help: consider using `.to_string()`: `arg.to_string();`
 
 error: useless use of `format!`
-  --> $DIR/format.rs:35:5
+  --> $DIR/format.rs:37:5
    |
 LL |     format!("{:+}", arg); // Warn when the format makes no difference.
    |     ^^^^^^^^^^^^^^^^^^^^^ help: consider using `.to_string()`: `arg.to_string();`
 
 error: useless use of `format!`
-  --> $DIR/format.rs:36:5
+  --> $DIR/format.rs:38:5
    |
 LL |     format!("{:<}", arg); // Warn when the format makes no difference.
    |     ^^^^^^^^^^^^^^^^^^^^^ help: consider using `.to_string()`: `arg.to_string();`
 
 error: useless use of `format!`
-  --> $DIR/format.rs:63:5
+  --> $DIR/format.rs:65:5
    |
 LL |     format!("{}", 42.to_string());
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using `.to_string()`: `42.to_string();`
 
 error: useless use of `format!`
-  --> $DIR/format.rs:65:5
+  --> $DIR/format.rs:67:5
    |
 LL |     format!("{}", x.display().to_string());
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using `.to_string()`: `x.display().to_string();`
 
 error: useless use of `format!`
-  --> $DIR/format.rs:69:18
+  --> $DIR/format.rs:71:18
    |
 LL |     let _ = Some(format!("{}", a + "bar"));
    |                  ^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using `.to_string()`: `a + "bar"`
 
 error: useless use of `format!`
-  --> $DIR/format.rs:73:22
+  --> $DIR/format.rs:75:22
    |
 LL |     let _s: String = format!("{}", &*v.join("/n"));
    |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using `.to_string()`: `(&*v.join("/n")).to_string()`
 
-error: aborting due to 14 previous errors
+error: aborting due to 15 previous errors