Fix write_with_newline escaping false positive

Fixes #3514

clippy_lints/src/write.rs

@@ -206,10 +206,7 @@ impl EarlyLintPass for Pass {
         } else if mac.node.path == "print" {
             span_lint(cx, PRINT_STDOUT, mac.span, "use of `print!`");
             if let Some(fmtstr) = check_tts(cx, &mac.node.tts, false).0 {
-                if fmtstr.ends_with("\\n") &&
-                   // don't warn about strings with several `\n`s (#3126)
-                   fmtstr.matches("\\n").count() == 1
-                {
+                if check_newlines(&fmtstr) {
                     span_lint(
                         cx,
                         PRINT_WITH_NEWLINE,
@@ -221,10 +218,7 @@ impl EarlyLintPass for Pass {
             }
         } else if mac.node.path == "write" {
             if let Some(fmtstr) = check_tts(cx, &mac.node.tts, true).0 {
-                if fmtstr.ends_with("\\n") &&
-                   // don't warn about strings with several `\n`s (#3126)
-                   fmtstr.matches("\\n").count() == 1
-                {
+                if check_newlines(&fmtstr) {
                     span_lint(
                         cx,
                         WRITE_WITH_NEWLINE,
@@ -375,3 +369,29 @@ fn check_tts<'a>(cx: &EarlyContext<'a>, tts: &ThinTokenStream, is_write: bool) -
         }
     }
 }
+
+// Checks if `s` constains a single newline that terminates it
+fn check_newlines(s: &str) -> bool {
+    if s.len() < 2 {
+        return false;
+    }
+
+    let bytes = s.as_bytes();
+    if bytes[bytes.len() - 2] != b'\\' || bytes[bytes.len() - 1] != b'n' {
+        return false;
+    }
+
+    let mut escaping = false;
+    for (index, &byte) in bytes.iter().enumerate() {
+        if escaping {
+            if byte == b'n' {
+                return index == bytes.len() - 1;
+            }
+            escaping = false;
+        } else if byte == b'\\' {
+            escaping = true;
+        }
+    }
+
+    false
+}

tests/ui/write_with_newline.rs

@@ -38,4 +38,9 @@ fn main() {
     write!(&mut v, "Hello {} {}\n\n", "world", "#2");
     writeln!(&mut v, "\ndon't\nwarn\nfor\nmultiple\nnewlines\n"); // #3126
     writeln!(&mut v, "\nbla\n\n"); // #3126
+
+    // Escaping
+    write!(&mut v, "\\n"); // #3514
+    write!(&mut v, "\\\n");
+    write!(&mut v, "\\\\n");
 }

tests/ui/write_with_newline.stderr

@@ -24,5 +24,11 @@ error: using `write!()` with a format string that ends in a single newline, cons
 25 |     write!(&mut v, "{}/n", 1265);
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 4 previous errors
+error: using `write!()` with a format string that ends in a single newline, consider using `writeln!()` instead
+  --> $DIR/write_with_newline.rs:44:5
+   |
+44 |     write!(&mut v, "//n");
+   |     ^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 5 previous errors