[fud2] Make Empty Ops an Error (#2257)

Empty ops currently can be created in the new DSL. When exported, this
creates broken ninja code. This is an error of the user, not creating
the desired target, but this class of error can never create the desired
target. Therefore a helpful error was inserted when this happens.

fud2/fud-core/src/run.rs

@@ -383,7 +383,17 @@ impl<'a> Run<'a> {
                 }
             }) {
                 let stdout_files =
-                    std::fs::File::open(self.plan.workdir.join(filename))?;
+                    std::fs::File::open(self.plan.workdir.join(filename))
+                        // The output file we're emitting to stdout should exist. If it doesn't,
+                        // some op didn't produce the output file it claimed to.
+                        .map_err(|e| if let std::io::ErrorKind::NotFound = e.kind() {
+                            std::io::Error::new(
+                                e.kind(),
+                                format!("{}\nHint: Check ops actually generate all of their targets.", e)
+                            )
+                        } else {
+                            e
+                })?;
                 std::io::copy(
                     &mut std::io::BufReader::new(stdout_files),
                     &mut std::io::stdout(),

fud2/fud-core/src/script/error.rs

@@ -21,6 +21,7 @@ pub(super) enum RhaiSystemErrorKind {
     ExpectedString(String),
     ExpectedShell,
     ExpectedShellDeps,
+    EmptyOp,
 }
 
 impl RhaiSystemError {
@@ -90,6 +91,13 @@ impl RhaiSystemError {
         }
     }
 
+    pub(super) fn empty_op() -> Self {
+        Self {
+            kind: RhaiSystemErrorKind::EmptyOp,
+            position: rhai::Position::NONE,
+        }
+    }
+
     pub(super) fn with_pos(mut self, p: rhai::Position) -> Self {
         self.position = p;
         self
@@ -129,6 +137,9 @@ impl Display for RhaiSystemError {
             RhaiSystemErrorKind::ExpectedShellDeps => {
                 write!(f, "Expected `shell_deps`, got `shell`. Ops may contain only one of `shell` or `shell_deps` calls, not calls to both")
             }
+            RhaiSystemErrorKind::EmptyOp => {
+                write!(f, "Error: Op must contain at least one call to `shell` or `shell_deps`.")
+            }
         }
     }
 }

fud2/fud-core/src/script/plugin.rs

@@ -352,8 +352,16 @@ impl ScriptContext {
                         c
                     }
                     Some(ShellCommands::Cmds(c)) => c.clone(),
-                    None => vec![],
+
+                    None => {
+                        // If cmds is empty, then the op doesn't create any of it's targets and
+                        // should be considered erroneous.
+                        return Err(RhaiSystemError::empty_op()
+                            .with_pos(pos)
+                            .into());
+                    }
                 };
+
                 let op_name = name.clone();
                 let config_vars = config_vars.clone();
                 let op_emitter = crate::run::RulesOp {