Add just_executable() function (#775)

The `just_executable()` function returns the absolute path of the
currently running `just` executable.

README.adoc

@@ -622,6 +622,22 @@ script:
   ./{{justfile_directory()}}/scripts/some_script
 ```
 
+==== Just Executable
+
+- `just_executable()` - Absolute path to the just executable.
+
+For example:
+
+```make
+executable:
+    @echo The executable is at: {{just_executable()}}
+```
+
+```
+$ just
+The executable is at: /bin/just
+```
+
 ==== Dotenv Integration
 
 `just` will load environment variables from a file named `.env`. This file can be located in the same directory as your justfile or in a parent directory. These variables are environment variables, not `just` variables, and so must be accessed using `$VARIABLE_NAME` in recipes and backticks.

src/function.rs

@@ -18,6 +18,7 @@ lazy_static! {
     ("invocation_directory", Nullary(invocation_directory)),
     ("env_var", Unary(env_var)),
     ("env_var_or_default", Binary(env_var_or_default)),
+    ("just_executable", Nullary(just_executable)),
   ]
   .into_iter()
   .collect();
@@ -123,3 +124,15 @@ fn env_var_or_default(
     Ok(value) => Ok(value),
   }
 }
+
+fn just_executable(_context: &FunctionContext) -> Result<String, String> {
+  let exe_path =
+    std::env::current_exe().map_err(|e| format!("Error getting current executable: {}", e))?;
+
+  exe_path.to_str().map(str::to_owned).ok_or_else(|| {
+    format!(
+      "Executable path is not valid unicode: {}",
+      exe_path.to_string_lossy()
+    )
+  })
+}

tests/misc.rs

@@ -1203,6 +1203,18 @@ test! {
   status:   EXIT_FAILURE,
 }
 
+test! {
+  name:     test_just_executable_function,
+  justfile: "
+    a:
+      @printf 'Executable path is: %s\\n' '{{ just_executable() }}'
+  ",
+  args:     ("a"),
+  stdout:   format!("Executable path is: {}\n", executable_path("just").to_str().unwrap()).as_str(),
+  stderr:   "",
+  status:   EXIT_SUCCESS,
+}
+
 test! {
   name: infallable_command,
   justfile: r#"