Improve invalid escape sequence error messages (#328)

The invalid escape sequence error message is delimited with backticks
and isn't used as input to other programs. This diff tweaks the escaping rules
slightly when printing invalid escape sequences. In particular, `, \, ',
and " are now not be escaped.

src/compilation_error.rs

@@ -67,9 +67,16 @@ impl<'a> Display for CompilationError<'a> {
                       variable, circle.join(" -> "))?;
         }
       }
+
       InvalidEscapeSequence{character} => {
-        writeln!(f, "`\\{}` is not a valid escape sequence",
-                    character.escape_default().collect::<String>())?;
+        let representation = match character {
+          '`'  => r"\`".to_string(),
+          '\\' => r"\".to_string(),
+          '\'' => r"'".to_string(),
+          '"' => r#"""#.to_string(),
+          _ => character.escape_default().collect(),
+        };
+        writeln!(f, "`\\{}` is not a valid escape sequence", representation)?;
       }
       DuplicateParameter{recipe, parameter} => {
         writeln!(f, "Recipe `{}` has duplicate parameter `{}`", recipe, parameter)?;

tests/integration.rs

@@ -1800,3 +1800,18 @@ echo:
    stderr:   "echo dotenv-value\necho dotenv-value\n",
    status:   EXIT_SUCCESS,
 }
+
+integration_test! {
+   name:     invalid_escape_sequence_message,
+   justfile: r#"
+X = "\'"
+"#,
+   args:     (),
+   stdout:   "",
+   stderr:   r#"error: `\'` is not a valid escape sequence
+  |
+2 | X = "\'"
+  |     ^^^^
+"#,
+   status:   EXIT_FAILURE,
+}