From ef4401d4acf2eb1e125b9dd01d1baef195e2f53b Mon Sep 17 00:00:00 2001
From: mcarton <cartonmartin+git@gmail.com>
Date: Sat, 20 Feb 2016 17:35:07 +0100
Subject: [PATCH] Lint about usage of `format!("string literal")`

---
 README.md                    |  3 ++-
 src/format.rs                | 40 ++++++++++++++++++++++++++++++++++++
 src/lib.rs                   |  2 ++
 tests/compile-fail/format.rs | 11 ++++++++++
 4 files changed, 55 insertions(+), 1 deletion(-)
 create mode 100644 src/format.rs
 create mode 100755 tests/compile-fail/format.rs

diff --git a/README.md b/README.md
index 7fa0a15715f4..62d6695b24d2 100644
--- a/README.md
+++ b/README.md
@@ -8,7 +8,7 @@ A collection of lints to catch common mistakes and improve your Rust code.
 [Jump to usage instructions](#usage)
 
 ##Lints
-There are 122 lints included in this crate:
+There are 123 lints included in this crate:
 
 name                                                                                                           | default | meaning
 ---------------------------------------------------------------------------------------------------------------|---------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
@@ -126,6 +126,7 @@ name
 [unused_lifetimes](https://github.com/Manishearth/rust-clippy/wiki#unused_lifetimes)                           | warn    | unused lifetimes in function definitions
 [use_debug](https://github.com/Manishearth/rust-clippy/wiki#use_debug)                                         | allow   | use `Debug`-based formatting
 [used_underscore_binding](https://github.com/Manishearth/rust-clippy/wiki#used_underscore_binding)             | warn    | using a binding which is prefixed with an underscore
+[useless_format](https://github.com/Manishearth/rust-clippy/wiki#useless_format)                               | warn    | useless use of `format!`
 [useless_transmute](https://github.com/Manishearth/rust-clippy/wiki#useless_transmute)                         | warn    | transmutes that have the same to and from types
 [useless_vec](https://github.com/Manishearth/rust-clippy/wiki#useless_vec)                                     | warn    | useless `vec!`
 [while_let_loop](https://github.com/Manishearth/rust-clippy/wiki#while_let_loop)                               | warn    | `loop { if let { ... } else break }` can be written as a `while let` loop
diff --git a/src/format.rs b/src/format.rs
new file mode 100644
index 000000000000..ad72b38e1107
--- /dev/null
+++ b/src/format.rs
@@ -0,0 +1,40 @@
+use rustc::lint::*;
+use rustc_front::hir::*;
+use utils::{is_expn_of, span_lint};
+
+/// **What it does:** This lints about use of `format!("string literal with no argument")`.
+///
+/// **Why is this bad?** There is no point of doing that. If you want a `String` you can use
+/// `to_owned` on the string literal. The even worst `&format!("foo")` is often encountered in the
+/// wild.
+///
+/// **Known problems:** None.
+///
+/// **Example:** `format!("foo")`
+declare_lint! {
+    pub USELESS_FORMAT,
+    Warn,
+    "useless use of `format!`"
+}
+
+#[derive(Copy, Clone, Debug)]
+pub struct FormatMacLint;
+
+impl LintPass for FormatMacLint {
+    fn get_lints(&self) -> LintArray {
+        lint_array![USELESS_FORMAT]
+    }
+}
+
+impl LateLintPass for FormatMacLint {
+    fn check_expr(&mut self, cx: &LateContext, expr: &Expr) {
+        // `format!("foo")` expansion contains `match () { () => [], }`
+        if let ExprMatch(ref matchee, _, _) = expr.node {
+            if let ExprTup(ref tup) = matchee.node {
+                if tup.is_empty() && is_expn_of(cx, expr.span, "format").is_some() {
+                    span_lint(cx, USELESS_FORMAT, expr.span, &"useless use of `format!`");
+                }
+            }
+        }
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
index 23accd2ebaac..b690a5cf45eb 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -163,6 +163,7 @@ pub fn plugin_registrar(reg: &mut Registry) {
     reg.register_late_lint_pass(box types::AbsurdExtremeComparisons);
     reg.register_late_lint_pass(box regex::RegexPass::default());
     reg.register_late_lint_pass(box copies::CopyAndPaste);
+    reg.register_late_lint_pass(box format::FormatMacLint);
 
     reg.register_lint_group("clippy_pedantic", vec![
         enum_glob_use::ENUM_GLOB_USE,
@@ -209,6 +210,7 @@ pub fn plugin_registrar(reg: &mut Registry) {
         eq_op::EQ_OP,
         escape::BOXED_LOCAL,
         eta_reduction::REDUNDANT_CLOSURE,
+        format::USELESS_FORMAT,
         identity_op::IDENTITY_OP,
         items_after_statements::ITEMS_AFTER_STATEMENTS,
         len_zero::LEN_WITHOUT_IS_EMPTY,
diff --git a/tests/compile-fail/format.rs b/tests/compile-fail/format.rs
new file mode 100755
index 000000000000..6cdceefd063e
--- /dev/null
+++ b/tests/compile-fail/format.rs
@@ -0,0 +1,11 @@
+#![feature(plugin)]
+#![plugin(clippy)]
+#![deny(useless_format)]
+
+fn main() {
+    format!("foo"); //~ERROR useless use of `format!`
+    format!("foo {}", 42);
+
+    println!("foo");
+    println!("foo {}", 42);
+}