diff --git a/clippy_lints/src/print.rs b/clippy_lints/src/print.rs
index 6d9880e1335..a52d76e81da 100644
--- a/clippy_lints/src/print.rs
+++ b/clippy_lints/src/print.rs
@@ -78,12 +78,28 @@ declare_clippy_lint! {
"use of `Debug`-based formatting"
}
+/// **What it does:** This lint warns about the use of literals as `print!`/`println!` args.
+///
+/// **Why is this bad?** Using literals as `println!` args is inefficient
+/// (c.f., https://github.com/matthiaskrgr/rust-str-bench) and unnecessary
+/// (i.e., just put the literal in the format string)
+///
+/// **Example:**
+/// ```rust
+/// println!("{}", "foo");
+/// ```
+declare_lint! {
+ pub PRINT_LITERAL,
+ Allow,
+ "printing a literal with a format string"
+}
+
#[derive(Copy, Clone, Debug)]
pub struct Pass;
impl LintPass for Pass {
fn get_lints(&self) -> LintArray {
- lint_array!(PRINT_WITH_NEWLINE, PRINTLN_EMPTY_STRING, PRINT_STDOUT, USE_DEBUG)
+ lint_array!(PRINT_WITH_NEWLINE, PRINTLN_EMPTY_STRING, PRINT_STDOUT, USE_DEBUG, PRINT_LITERAL)
}
}
@@ -107,6 +123,10 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Pass {
span_lint(cx, PRINT_STDOUT, span, &format!("use of `{}!`", name));
+ // Check for literals in the print!/println! args
+ // Also, ensure the format string is `{}` with no special options, like `{:X}`
+ check_print_args_for_literal(cx, args);
+
if_chain! {
// ensure we're calling Arguments::new_v1
if args.len() == 1;
@@ -146,6 +166,49 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Pass {
}
}
+// Check for literals in print!/println! args
+// ensuring the format string for the literal is `DISPLAY_FMT_METHOD`
+// e.g., `println!("... {} ...", "foo")`
+// ^ literal in `println!`
+fn check_print_args_for_literal<'a, 'tcx>(
+ cx: &LateContext<'a, 'tcx>,
+ args: &HirVec<Expr>
+) {
+ if_chain! {
+ if args.len() == 1;
+ if let ExprCall(_, ref args_args) = args[0].node;
+ if args_args.len() > 1;
+ if let ExprAddrOf(_, ref match_expr) = args_args[1].node;
+ if let ExprMatch(ref matchee, ref arms, _) = match_expr.node;
+ if let ExprTup(ref tup) = matchee.node;
+ if arms.len() == 1;
+ if let ExprArray(ref arm_body_exprs) = arms[0].body.node;
+ then {
+ // it doesn't matter how many args there are in the `print!`/`println!`,
+ // if there's one literal, we should warn the user
+ for (idx, tup_arg) in tup.iter().enumerate() {
+ if_chain! {
+ // first, make sure we're dealing with a literal (i.e., an ExprLit)
+ if let ExprAddrOf(_, ref tup_val) = tup_arg.node;
+ if let ExprLit(_) = tup_val.node;
+
+ // next, check the corresponding match arm body to ensure
+ // this is "{}", or DISPLAY_FMT_METHOD
+ if let ExprCall(_, ref body_args) = arm_body_exprs[idx].node;
+ if body_args.len() == 2;
+ if let ExprPath(ref body_qpath) = body_args[1].node;
+ if let Some(fun_def_id) = opt_def_id(resolve_node(cx, body_qpath, body_args[1].hir_id));
+ if match_def_path(cx.tcx, fun_def_id, &paths::DISPLAY_FMT_METHOD) ||
+ match_def_path(cx.tcx, fun_def_id, &paths::DEBUG_FMT_METHOD);
+ then {
+ span_lint(cx, PRINT_LITERAL, tup_val.span, "printing a literal with an empty format string");
+ }
+ }
+ }
+ }
+ }
+}
+
// Check for print!("... \n", ...).
fn check_print<'a, 'tcx>(
cx: &LateContext<'a, 'tcx>,
diff --git a/tests/ui/print_literal.rs b/tests/ui/print_literal.rs
new file mode 100644
index 00000000000..c803294ab0a
--- /dev/null
+++ b/tests/ui/print_literal.rs
@@ -0,0 +1,32 @@
+
+
+#![warn(print_literal)]
+
+fn main() {
+ // these should be fine
+ print!("Hello");
+ println!("Hello");
+ let world = "world";
+ println!("Hello {}", world);
+ println!("3 in hex is {:X}", 3);
+
+ // these should throw warnings
+ print!("Hello {}", "world");
+ println!("Hello {} {}", world, "world");
+ println!("Hello {}", "world");
+ println!("10 / 4 is {}", 2.5);
+ println!("2 + 1 = {}", 3);
+ println!("2 + 1 = {:.4}", 3);
+ println!("2 + 1 = {:5.4}", 3);
+ println!("Debug test {:?}", "hello, world");
+
+ // positional args don't change the fact
+ // that we're using a literal -- this should
+ // throw a warning
+ println!("{0} {1}", "hello", "world");
+ println!("{1} {0}", "hello", "world");
+
+ // named args shouldn't change anything either
+ println!("{foo} {bar}", foo="hello", bar="world");
+ println!("{bar} {foo}", foo="hello", bar="world");
+}
diff --git a/tests/ui/print_literal.stderr b/tests/ui/print_literal.stderr
new file mode 100644
index 00000000000..982be7dc537
--- /dev/null
+++ b/tests/ui/print_literal.stderr
@@ -0,0 +1,100 @@
+error: printing a literal with an empty format string
+ --> $DIR/print_literal.rs:14:24
+ |
+14 | print!("Hello {}", "world");
+ | ^^^^^^^
+ |
+ = note: `-D print-literal` implied by `-D warnings`
+
+error: printing a literal with an empty format string
+ --> $DIR/print_literal.rs:15:36
+ |
+15 | println!("Hello {} {}", world, "world");
+ | ^^^^^^^
+
+error: printing a literal with an empty format string
+ --> $DIR/print_literal.rs:16:26
+ |
+16 | println!("Hello {}", "world");
+ | ^^^^^^^
+
+error: printing a literal with an empty format string
+ --> $DIR/print_literal.rs:17:30
+ |
+17 | println!("10 / 4 is {}", 2.5);
+ | ^^^
+
+error: printing a literal with an empty format string
+ --> $DIR/print_literal.rs:18:28
+ |
+18 | println!("2 + 1 = {}", 3);
+ | ^
+
+error: printing a literal with an empty format string
+ --> $DIR/print_literal.rs:19:31
+ |
+19 | println!("2 + 1 = {:.4}", 3);
+ | ^
+
+error: printing a literal with an empty format string
+ --> $DIR/print_literal.rs:20:32
+ |
+20 | println!("2 + 1 = {:5.4}", 3);
+ | ^
+
+error: printing a literal with an empty format string
+ --> $DIR/print_literal.rs:21:33
+ |
+21 | println!("Debug test {:?}", "hello, world");
+ | ^^^^^^^^^^^^^^
+
+error: printing a literal with an empty format string
+ --> $DIR/print_literal.rs:26:25
+ |
+26 | println!("{0} {1}", "hello", "world");
+ | ^^^^^^^
+
+error: printing a literal with an empty format string
+ --> $DIR/print_literal.rs:26:34
+ |
+26 | println!("{0} {1}", "hello", "world");
+ | ^^^^^^^
+
+error: printing a literal with an empty format string
+ --> $DIR/print_literal.rs:27:25
+ |
+27 | println!("{1} {0}", "hello", "world");
+ | ^^^^^^^
+
+error: printing a literal with an empty format string
+ --> $DIR/print_literal.rs:27:34
+ |
+27 | println!("{1} {0}", "hello", "world");
+ | ^^^^^^^
+
+error: printing a literal with an empty format string
+ --> $DIR/print_literal.rs:30:33
+ |
+30 | println!("{foo} {bar}", foo="hello", bar="world");
+ | ^^^^^^^
+
+error: printing a literal with an empty format string
+ --> $DIR/print_literal.rs:30:46
+ |
+30 | println!("{foo} {bar}", foo="hello", bar="world");
+ | ^^^^^^^
+
+error: printing a literal with an empty format string
+ --> $DIR/print_literal.rs:31:33
+ |
+31 | println!("{bar} {foo}", foo="hello", bar="world");
+ | ^^^^^^^
+
+error: printing a literal with an empty format string
+ --> $DIR/print_literal.rs:31:46
+ |
+31 | println!("{bar} {foo}", foo="hello", bar="world");
+ | ^^^^^^^
+
+error: aborting due to 16 previous errors
+