fix escaping problem in write_literal and print_literal lint

tests/ui/print_literal.fixed

@@ -66,3 +66,17 @@ fn main() {
 
     println!("mixed: {{hello}} {world}");
 }
+
+fn issue_13959() {
+    println!("\"");
+    println!(
+        "
+        foo
+        \\
+        \\\\
+        \"
+        \\\"
+        bar
+"
+    );
+}

tests/ui/print_literal.rs

@@ -66,3 +66,18 @@ fn main() {
 
     println!("mixed: {} {world}", "{hello}");
 }
+
+fn issue_13959() {
+    println!("{}", r#"""#);
+    println!(
+        "{}",
+        r#"
+        foo
+        \
+        \\
+        "
+        \"
+        bar
+"#
+    );
+}

tests/ui/print_literal.stderr

@@ -192,5 +192,41 @@ LL -     println!("mixed: {} {world}", "{hello}");
 LL +     println!("mixed: {{hello}} {world}");
    |
 
-error: aborting due to 16 previous errors
+error: literal with an empty format string
+  --> tests/ui/print_literal.rs:71:20
+   |
+LL |     println!("{}", r#"""#);
+   |                    ^^^^^^
+   |
+help: try
+   |
+LL -     println!("{}", r#"""#);
+LL +     println!("\"");
+   |
+
+error: literal with an empty format string
+  --> tests/ui/print_literal.rs:74:9
+   |
+LL | /         r#"
+LL | |         foo
+LL | |         \
+LL | |         \\
+...  |
+LL | |         bar
+LL | | "#
+   | |__^
+   |
+help: try
+   |
+LL ~         "
+LL +         foo
+LL +         \\
+LL +         \\\\
+LL +         \"
+LL +         \\\"
+LL +         bar
+LL ~ "
+   |
+
+error: aborting due to 18 previous errors
 

tests/ui/write_literal.fixed

@@ -62,3 +62,19 @@ fn main() {
     writeln!(v, "hello {0} {1}, world {2}", 2, 3, 4);
     //~^ ERROR: literal with an empty format string
 }
+
+fn issue_13959() {
+    let mut v = Vec::new();
+    writeln!(v, "\"");
+    writeln!(
+        v,
+        "
+        foo
+        \\
+        \\\\
+        \"
+        \\\"
+        bar
+"
+    );
+}

tests/ui/write_literal.rs

@@ -62,3 +62,20 @@ fn main() {
     writeln!(v, "{0} {1} {2}, {3} {4}", "hello", 2, 3, "world", 4);
     //~^ ERROR: literal with an empty format string
 }
+
+fn issue_13959() {
+    let mut v = Vec::new();
+    writeln!(v, "{}", r#"""#);
+    writeln!(
+        v,
+        "{}",
+        r#"
+        foo
+        \
+        \\
+        "
+        \"
+        bar
+"#
+    );
+}

tests/ui/write_literal.stderr

@@ -144,5 +144,41 @@ LL -     writeln!(v, "{0} {1} {2}, {3} {4}", "hello", 2, 3, "world", 4);
 LL +     writeln!(v, "hello {0} {1}, world {2}", 2, 3, 4);
    |
 
-error: aborting due to 12 previous errors
+error: literal with an empty format string
+  --> tests/ui/write_literal.rs:68:23
+   |
+LL |     writeln!(v, "{}", r#"""#);
+   |                       ^^^^^^
+   |
+help: try
+   |
+LL -     writeln!(v, "{}", r#"""#);
+LL +     writeln!(v, "\"");
+   |
+
+error: literal with an empty format string
+  --> tests/ui/write_literal.rs:72:9
+   |
+LL | /         r#"
+LL | |         foo
+LL | |         \
+LL | |         \\
+...  |
+LL | |         bar
+LL | | "#
+   | |__^
+   |
+help: try
+   |
+LL ~         "
+LL +         foo
+LL +         \\
+LL +         \\\\
+LL +         \"
+LL +         \\\"
+LL +         bar
+LL ~ "
+   |
+
+error: aborting due to 14 previous errors