makes slash operator default lhs value to an empty string

README.md

@@ -924,6 +924,17 @@ $ just --evaluate bar
 a//b
 ```
 
+Absolute paths can also be constructed:
+
+```make
+foo := / "b"
+```
+
+```
+$ just --evaluate foo
+/b
+```
+
 The `/` operator uses the `/` character, even on Windows. Thus, using the `/` operator should be avoided with paths that use universal naming convention (UNC), i.e., those that start with `\?`, since forward slashes are not supported with UNC paths.
 
 #### Escaping `{{`

src/assignment_resolver.rs

@@ -101,10 +101,16 @@ impl<'src: 'run, 'run> AssignmentResolver<'src, 'run> {
           self.resolve_expression(c)
         }
       },
-      Expression::Concatenation { lhs, rhs } | Expression::Join { lhs, rhs } => {
+      Expression::Concatenation { lhs, rhs } => {
         self.resolve_expression(lhs)?;
         self.resolve_expression(rhs)
       }
+      Expression::Join { lhs, rhs } => {
+        if let Some(lhs) = lhs {
+          self.resolve_expression(lhs)?;
+        }
+        self.resolve_expression(rhs)
+      }
       Expression::Conditional {
         lhs,
         rhs,

src/evaluator.rs

@@ -176,9 +176,11 @@ impl<'src, 'run> Evaluator<'src, 'run> {
         }
       }
       Expression::Group { contents } => self.evaluate_expression(contents),
-      Expression::Join { lhs, rhs } => {
-        Ok(self.evaluate_expression(lhs)? + "/" + &self.evaluate_expression(rhs)?)
-      }
+      Expression::Join { lhs: None, rhs } => Ok("/".to_string() + &self.evaluate_expression(rhs)?),
+      Expression::Join {
+        lhs: Some(lhs),
+        rhs,
+      } => Ok(self.evaluate_expression(lhs)? + "/" + &self.evaluate_expression(rhs)?),
     }
   }
 

src/expression.rs

@@ -32,7 +32,7 @@ pub(crate) enum Expression<'src> {
   Group { contents: Box<Expression<'src>> },
   /// `lhs / rhs`
   Join {
-    lhs: Box<Expression<'src>>,
+    lhs: Option<Box<Expression<'src>>>,
     rhs: Box<Expression<'src>>,
   },
   /// `"string_literal"` or `'string_literal'`
@@ -51,7 +51,11 @@ impl<'src> Display for Expression<'src> {
   fn fmt(&self, f: &mut Formatter) -> Result<(), fmt::Error> {
     match self {
       Expression::Backtick { token, .. } => write!(f, "{}", token.lexeme()),
-      Expression::Join { lhs, rhs } => write!(f, "{} / {}", lhs, rhs),
+      Expression::Join { lhs: None, rhs } => write!(f, "/ {}", rhs),
+      Expression::Join {
+        lhs: Some(lhs),
+        rhs,
+      } => write!(f, "{} / {}", lhs, rhs),
       Expression::Concatenation { lhs, rhs } => write!(f, "{} + {}", lhs, rhs),
       Expression::Conditional {
         lhs,

src/node.rs

@@ -118,7 +118,11 @@ impl<'src> Node<'src> for Expression<'src> {
       } => Tree::string(cooked),
       Expression::Backtick { contents, .. } => Tree::atom("backtick").push(Tree::string(contents)),
       Expression::Group { contents } => Tree::List(vec![contents.tree()]),
-      Expression::Join { lhs, rhs } => Tree::atom("/").push(lhs.tree()).push(rhs.tree()),
+      Expression::Join { lhs: None, rhs } => Tree::atom("/").push(rhs.tree()),
+      Expression::Join {
+        lhs: Some(lhs),
+        rhs,
+      } => Tree::atom("/").push(lhs.tree()).push(rhs.tree()),
     }
   }
 }

src/parser.rs

@@ -406,11 +406,15 @@ impl<'tokens, 'src> Parser<'tokens, 'src> {
 
     let expression = if self.accepted_keyword(Keyword::If)? {
       self.parse_conditional()?
+    } else if self.accepted(Slash)? {
+      let lhs = None;
+      let rhs = Box::new(self.parse_expression()?);
+      Expression::Join { lhs, rhs }
     } else {
       let value = self.parse_value()?;
 
       if self.accepted(Slash)? {
-        let lhs = Box::new(value);
+        let lhs = Some(Box::new(value));
         let rhs = Box::new(self.parse_expression()?);
         Expression::Join { lhs, rhs }
       } else if self.accepted(Plus)? {
@@ -1991,6 +1995,7 @@ mod tests {
         Identifier,
         ParenL,
         ParenR,
+        Slash,
         StringToken,
       ],
       found: Eof,
@@ -2010,6 +2015,7 @@ mod tests {
         Identifier,
         ParenL,
         ParenR,
+        Slash,
         StringToken,
       ],
       found: InterpolationEnd,

src/summary.rs

@@ -197,7 +197,7 @@ pub enum Expression {
     operator: ConditionalOperator,
   },
   Join {
-    lhs: Box<Expression>,
+    lhs: Option<Box<Expression>>,
     rhs: Box<Expression>,
   },
   String {
@@ -258,7 +258,7 @@ impl Expression {
         rhs: Box::new(Expression::new(rhs)),
       },
       Join { lhs, rhs } => Expression::Join {
-        lhs: Box::new(Expression::new(lhs)),
+        lhs: lhs.as_ref().map(|lhs| Box::new(Expression::new(lhs))),
         rhs: Box::new(Expression::new(rhs)),
       },
       Conditional {

src/variables.rs

@@ -53,10 +53,16 @@ impl<'expression, 'src> Iterator for Variables<'expression, 'src> {
           self.stack.push(lhs);
         }
         Expression::Variable { name, .. } => return Some(name.token()),
-        Expression::Concatenation { lhs, rhs } | Expression::Join { lhs, rhs } => {
+        Expression::Concatenation { lhs, rhs } => {
           self.stack.push(rhs);
           self.stack.push(lhs);
         }
+        Expression::Join { lhs, rhs } => {
+          self.stack.push(rhs);
+          if let Some(lhs) = lhs {
+            self.stack.push(lhs);
+          }
+        }
         Expression::Group { contents } => {
           self.stack.push(contents);
         }

tests/slash_operator.rs

@@ -18,6 +18,45 @@ fn twice() {
     .run();
 }
 
+#[test]
+fn no_lhs_once() {
+  Test::new()
+    .justfile("x := / 'a'")
+    .args(&["--evaluate", "x"])
+    .stdout("/a")
+    .run();
+}
+
+#[test]
+fn no_lhs_twice() {
+  Test::new()
+    .justfile("x := / 'a' / 'b'")
+    .args(&["--evaluate", "x"])
+    .stdout("/a/b")
+    .run();
+  Test::new()
+    .justfile("x := // 'a'")
+    .args(&["--evaluate", "x"])
+    .stdout("//a")
+    .run();
+}
+
+#[test]
+fn no_rhs_once() {
+  Test::new()
+    .justfile("x := 'a' /")
+    .stderr(
+      "
+      error: Expected backtick, identifier, '(', '/', or string, but found end of file
+        |
+      1 | x := 'a' /
+        |           ^
+    ",
+    )
+    .status(EXIT_FAILURE)
+    .run();
+}
+
 #[test]
 fn default_un_parenthesized() {
   Test::new()
@@ -39,6 +78,27 @@ fn default_un_parenthesized() {
     .run();
 }
 
+#[test]
+fn no_lhs_un_parenthesized() {
+  Test::new()
+    .justfile(
+      "
+      foo x=/ 'a' / 'b':
+        echo {{x}}
+    ",
+    )
+    .stderr(
+      "
+      error: Expected backtick, identifier, '(', or string, but found '/'
+        |
+      1 | foo x=/ 'a' / 'b':
+        |       ^
+    ",
+    )
+    .status(EXIT_FAILURE)
+    .run();
+}
+
 #[test]
 fn default_parenthesized() {
   Test::new()
@@ -52,3 +112,17 @@ fn default_parenthesized() {
     .stdout("a/b\n")
     .run();
 }
+
+#[test]
+fn no_lhs_parenthesized() {
+  Test::new()
+    .justfile(
+      "
+      foo x=(/ 'a' / 'b'):
+        echo {{x}}
+    ",
+    )
+    .stderr("echo /a/b\n")
+    .stdout("/a/b\n")
+    .run();
+}