Check {print,write}_with_newline for literal newline

Both regular strings and raw strings can contain literal newlines. This commit
extends the lint to also warn about terminating strings with these.

Behaviour handling for raw strings is also moved into `check_newlines` by
passing in the `is_raw` boolean from `check_tts` as
[suggested](rust-lang/rust-clippy#3781 (review))

Author: uniphil

clippy_lints/src/write.rs

@@ -201,7 +201,7 @@ impl EarlyLintPass for Pass {
         } else if mac.node.path == "print" {
             span_lint(cx, PRINT_STDOUT, mac.span, "use of `print!`");
             if let (Some(fmtstr), _, is_raw) = check_tts(cx, &mac.node.tts, false) {
-                if !is_raw && check_newlines(&fmtstr) {
+                if check_newlines(&fmtstr, is_raw) {
                     span_lint(
                         cx,
                         PRINT_WITH_NEWLINE,
@@ -213,7 +213,7 @@ impl EarlyLintPass for Pass {
             }
         } else if mac.node.path == "write" {
             if let (Some(fmtstr), _, is_raw) = check_tts(cx, &mac.node.tts, true) {
-                if !is_raw && check_newlines(&fmtstr) {
+                if check_newlines(&fmtstr, is_raw) {
                     span_lint(
                         cx,
                         WRITE_WITH_NEWLINE,
@@ -382,7 +382,14 @@ fn check_tts<'a>(cx: &EarlyContext<'a>, tts: &TokenStream, is_write: bool) -> (O
 }
 
 // Checks if `s` constains a single newline that terminates it
-fn check_newlines(s: &str) -> bool {
+// Literal and escaped newlines are both checked (only literal for raw strings)
+fn check_newlines(s: &str, is_raw: bool) -> bool {
+    if s.ends_with('\n') {
+        return true;
+    } else if is_raw {
+        return false;
+    }
+
     if s.len() < 2 {
         return false;
     }

tests/ui/print_with_newline.rs

@@ -29,4 +29,14 @@ fn main() {
 
     // Raw strings
     print!(r"\n"); // #3778
+
+    // Literal newlines should also fail
+    print!(
+        "
+"
+    );
+    print!(
+        r"
+"
+    );
 }

tests/ui/print_with_newline.stderr

@@ -30,5 +30,23 @@ error: using `print!()` with a format string that ends in a single newline, cons
 LL |     print!("//n"); // should fail
    |     ^^^^^^^^^^^^^^
 
-error: aborting due to 5 previous errors
+error: using `print!()` with a format string that ends in a single newline, consider using `println!()` instead
+  --> $DIR/print_with_newline.rs:34:5
+   |
+LL | /     print!(
+LL | |         "
+LL | | "
+LL | |     );
+   | |_____^
+
+error: using `print!()` with a format string that ends in a single newline, consider using `println!()` instead
+  --> $DIR/print_with_newline.rs:38:5
+   |
+LL | /     print!(
+LL | |         r"
+LL | | "
+LL | |     );
+   | |_____^
+
+error: aborting due to 7 previous errors
 

tests/ui/write_with_newline.rs

@@ -34,4 +34,16 @@ fn main() {
 
     // Raw strings
     write!(&mut v, r"\n"); // #3778
+
+    // Literal newlines should also fail
+    write!(
+        &mut v,
+        "
+"
+    );
+    write!(
+        &mut v,
+        r"
+"
+    );
 }

tests/ui/write_with_newline.stderr

@@ -30,5 +30,25 @@ error: using `write!()` with a format string that ends in a single newline, cons
 LL |     write!(&mut v, "//n"); // should fail
    |     ^^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 5 previous errors
+error: using `write!()` with a format string that ends in a single newline, consider using `writeln!()` instead
+  --> $DIR/write_with_newline.rs:39:5
+   |
+LL | /     write!(
+LL | |         &mut v,
+LL | |         "
+LL | | "
+LL | |     );
+   | |_____^
+
+error: using `write!()` with a format string that ends in a single newline, consider using `writeln!()` instead
+  --> $DIR/write_with_newline.rs:44:5
+   |
+LL | /     write!(
+LL | |         &mut v,
+LL | |         r"
+LL | | "
+LL | |     );
+   | |_____^
+
+error: aborting due to 7 previous errors