[fud2] add flag to print commands instead of running them (#2259)

docs/running-calyx/fud2/index.md

@@ -79,6 +79,7 @@ Here are some options you might need:
     * `emit`: Just print the Ninja build file to stdout. The `gen` mode is therefore approximately `fud2 -m emit > .fud2/build.ninja`.
     * `plan`: Print a brief description of the plan, i.e., the sequence of operations that the build would run.
     * `dot`: Print a [GraphViz][] depiction of the plan. Try `fud2 -m dot | dot -Tpdf > graph.pdf` and take a look.
+    * `cmds`: Print the commands Ninja would run when executing the plan, but do not execute them.
 
 There are also some subcommands for doing things other than building stuff:
 

fud2/fud-core/src/cli.rs

@@ -13,6 +13,7 @@ enum Mode {
     ShowDot,
     Generate,
     Run,
+    Cmds,
 }
 
 impl FromStr for Mode {
@@ -25,6 +26,7 @@ impl FromStr for Mode {
             "gen" => Ok(Mode::Generate),
             "run" => Ok(Mode::Run),
             "dot" => Ok(Mode::ShowDot),
+            "cmds" => Ok(Mode::Cmds),
             _ => Err("unknown mode".to_string()),
         }
     }
@@ -38,6 +40,7 @@ impl Display for Mode {
             Mode::Generate => write!(f, "gen"),
             Mode::Run => write!(f, "run"),
             Mode::ShowDot => write!(f, "dot"),
+            Mode::Cmds => write!(f, "cmds"),
         }
     }
 }
@@ -380,7 +383,8 @@ pub fn cli(driver: &Driver, config: &figment::Figment) -> anyhow::Result<()> {
         Mode::ShowDot => run.show_dot(),
         Mode::EmitNinja => run.emit_to_stdout()?,
         Mode::Generate => run.emit_to_dir(&workdir)?.keep(),
-        Mode::Run => run.emit_and_run(&workdir)?,
+        Mode::Run => run.emit_and_run(&workdir, false)?,
+        Mode::Cmds => run.emit_and_run(&workdir, true)?,
     }
 
     Ok(())

fud2/fud-core/src/run.rs

@@ -336,8 +336,10 @@ impl<'a> Run<'a> {
         Ok(dir)
     }
 
-    /// Emit `build.ninja` to a temporary directory and then actually execute ninja.
-    pub fn emit_and_run(&self, dir: &Utf8Path) -> EmitResult {
+    /// Emit `build.ninja` to a temporary directory and then actually execute Ninja.
+    ///
+    /// If `print_cmds` is true, Ninja will print commands it is to run instead of executing them.
+    pub fn emit_and_run(&self, dir: &Utf8Path, print_cmds: bool) -> EmitResult {
         // Emit the Ninja file.
         let dir = self.emit_to_dir(dir)?;
 
@@ -369,6 +371,10 @@ impl<'a> Run<'a> {
             cmd.arg("--verbose");
         }
 
+        if print_cmds {
+            cmd.arg("-tcommands");
+        }
+
         cmd.stdout(std::io::stderr()); // Send Ninja's stdout to our stderr.
         let status = cmd.status().map_err(ninja_cmd_io_error)?;