build all files if none specified

I'm ambivalent about this behavior, but it's come up twice so I think
people depend on it.

Fixes #88 and #53.

src/graph.rs

@@ -317,6 +317,10 @@ impl GraphFiles {
             id
         })
     }
+
+    pub fn all_ids(&self) -> impl Iterator<Item = FileId> {
+        (0..self.by_id.next_id().0).map(|id| FileId(id))
+    }
 }
 
 /// MTime info gathered for a file.  This also models "file is absent".

src/run.rs

@@ -77,7 +77,9 @@ fn build(
             work.want_file(target)?;
         }
     } else {
-        anyhow::bail!("no path specified and no default");
+        work.progress
+            .log("no path specified and no default target; building everything");
+        work.want_every_file(build_file_target)?;
     }
 
     let tasks = trace::scope("work.run", || work.run())?;

src/work.rs

@@ -241,11 +241,11 @@ impl BuildStates {
             anyhow::bail!(err);
         }
 
-        stack.push(id);
         if let Some(bid) = graph.file(id).input {
+            stack.push(id);
             self.want_build(graph, stack, bid)?;
+            stack.pop();
         }
-        stack.pop();
         Ok(())
     }
 
@@ -309,7 +309,7 @@ pub struct Options {
 pub struct Work<'a> {
     graph: Graph,
     db: db::Writer,
-    progress: &'a mut dyn Progress,
+    pub progress: &'a mut dyn Progress,
     options: Options,
     file_state: FileState,
     last_hashes: Hashes,
@@ -347,6 +347,18 @@ impl<'a> Work<'a> {
         self.build_states.want_file(&self.graph, &mut stack, id)
     }
 
+    pub fn want_every_file(&mut self, exclude: Option<FileId>) -> anyhow::Result<()> {
+        for id in self.graph.files.all_ids() {
+            if let Some(exclude) = exclude {
+                if id == exclude {
+                    continue;
+                }
+            }
+            self.want_file(id)?;
+        }
+        Ok(())
+    }
+
     /// Check whether a given build is ready, generally after one of its inputs
     /// has been updated.
     fn recheck_ready(&self, id: BuildId) -> bool {

tests/e2e/basic.rs

@@ -7,7 +7,7 @@ fn empty_file() -> anyhow::Result<()> {
     let out = space.run(&mut n2_command(vec![]))?;
     assert_eq!(
         std::str::from_utf8(&out.stdout)?,
-        "n2: error: no path specified and no default\n"
+        "no path specified and no default target; building everything\n"
     );
     Ok(())
 }