refactor: one definition of "the model scanned a source file"

The empty-model condition was written three times: a scan.rs
diagnostic, a helper in main.rs for `build`'s warning, and the same
helper for `check`'s failure. They could drift. model.rs now owns
`has_source_files` (and `Building::has_source_files`), used by all
three. No behaviour change; the empty-scan CLI test still passes.

src/main.rs

@@ -199,7 +199,7 @@ fn run(cli: Cli) -> Result<()> {
                 json.display(),
                 html.display()
             );
-            if !has_source_files(&building) {
+            if !building.has_source_files() {
                 eprintln!(
                     "brick: warning: no supported source files were found; the model is empty and its score describes nothing"
                 );
@@ -252,7 +252,7 @@ fn run(cli: Cli) -> Result<()> {
                 );
             }
             let metrics = &recomputed.metrics;
-            if !has_source_files(&building) {
+            if !building.has_source_files() {
                 failures.push(
                     "the model contains no scanned source files; nothing was analysed".into(),
                 );
@@ -430,14 +430,6 @@ fn write_json(building: &Building, path: &PathBuf) -> Result<()> {
         .with_context(|| format!("cannot write graph {}", path.display()))
 }
 
-/// At least one source file was scanned into the model.
-fn has_source_files(building: &Building) -> bool {
-    building
-        .nodes
-        .iter()
-        .any(|node| node.kind == brick::model::NodeKind::File)
-}
-
 fn read_json(path: &PathBuf) -> Result<Building> {
     let raw = fs::read_to_string(path)
         .with_context(|| format!("cannot read graph {}", path.display()))?;

src/model.rs

@@ -20,6 +20,12 @@ pub struct Building {
 }
 
 impl Building {
+    /// At least one source file was scanned into the model. The one definition
+    /// `scan`, `build` and `check` share for "this analysed something".
+    pub fn has_source_files(&self) -> bool {
+        has_source_files(&self.nodes)
+    }
+
     pub fn validate_current_schema(&self) -> Result<(), CompatibilityError> {
         if self.schema_version != SCHEMA_VERSION {
             return Err(CompatibilityError::SchemaVersion {
@@ -601,6 +607,11 @@ fn compare_confidence(left: &Option<Confidence>, right: &Option<Confidence>) ->
     }
 }
 
+/// At least one of `nodes` is a scanned source file.
+pub fn has_source_files(nodes: &[Node]) -> bool {
+    nodes.iter().any(|node| node.kind == NodeKind::File)
+}
+
 #[cfg(test)]
 mod tests {
     use super::{

src/scan.rs

@@ -101,7 +101,7 @@ pub fn scan_with_options(root: &Path, options: &ScanOptions) -> Result<Building>
     state.resolve_relationships();
     state.annotate_indirect_uses();
     // An analysis of nothing must not read as a healthy repository.
-    if !state.nodes.iter().any(|node| node.kind == NodeKind::File) {
+    if !crate::model::has_source_files(&state.nodes) {
         state.diagnostics.push(Diagnostic {
             level: "warning".into(),
             message: "no supported source files were found; the model is empty".into(),