rustc: improve error messages from wrong --pretty flowgraph use.

This defers to .fatal and .span_fatal for errors (rather than `fail!`
which prints the ICE message). It also adds the span lookup when an id
doesn't correspond to a block, to show what it is pointing at.

It also makes the argument parser slightly looser, so that passing
`--pretty flowgraph` recognises the `flowgraph` part and suggests to use
an integer.

Author: huonw

src/librustc/driver/driver.rs

@@ -653,11 +653,22 @@ pub fn pretty_print_input(sess: Session,
         PpmFlowGraph(nodeid) => {
             let ast_map = ast_map.expect("--pretty flowgraph missing ast_map");
             let node = ast_map.find(nodeid).unwrap_or_else(|| {
-                fail!("--pretty flowgraph=id couldn't find id: {}", id)
+                sess.fatal(format_strbuf!("--pretty flowgraph couldn't find id: {}",
+                                          nodeid).as_slice())
             });
             let block = match node {
                 syntax::ast_map::NodeBlock(block) => block,
-                _ => fail!("--pretty=flowgraph needs block, got {:?}", node)
+                _ => {
+                    let message = format_strbuf!("--pretty=flowgraph needs block, got {:?}",
+                                                 node);
+
+                    // point to what was found, if there's an
+                    // accessible span.
+                    match ast_map.opt_span(nodeid) {
+                        Some(sp) => sess.span_fatal(sp, message.as_slice()),
+                        None => sess.fatal(message.as_slice())
+                    }
+                }
             };
             let analysis = phase_3_run_analysis_passes(sess, &krate, ast_map);
             print_flowgraph(analysis, block, out)

src/librustc/driver/mod.rs

@@ -302,11 +302,12 @@ pub fn parse_pretty(sess: &Session, name: &str) -> PpMode {
         (None, "typed")        => PpmTyped,
         (None, "expanded,identified") => PpmExpandedIdentified,
         (None, "identified")   => PpmIdentified,
-        (Some(s), "flowgraph") => {
-             match from_str(s) {
+        (arg, "flowgraph") => {
+             match arg.and_then(from_str) {
                  Some(id) => PpmFlowGraph(id),
-                 None => sess.fatal(format!("`pretty flowgraph=<nodeid>` needs \
-                                             an integer <nodeid>; got {}", s))
+                 None => sess.fatal(format_strbuf!("`pretty flowgraph=<nodeid>` needs \
+                                                     an integer <nodeid>; got {}",
+                                                   arg.unwrap_or("nothing")).as_slice())
              }
         }
         _ => {

src/libsyntax/ast_map.rs

@@ -388,8 +388,8 @@ impl Map {
         f(attrs)
     }
 
-    pub fn span(&self, id: NodeId) -> Span {
-        match self.find(id) {
+    pub fn opt_span(&self, id: NodeId) -> Option<Span> {
+        let sp = match self.find(id) {
             Some(NodeItem(item)) => item.span,
             Some(NodeForeignItem(foreign_item)) => foreign_item.span,
             Some(NodeTraitMethod(trait_method)) => {
@@ -406,8 +406,14 @@ impl Map {
             Some(NodePat(pat)) => pat.span,
             Some(NodeBlock(block)) => block.span,
             Some(NodeStructCtor(_)) => self.expect_item(self.get_parent(id)).span,
-            _ => fail!("node_span: could not find span for id {}", id),
-        }
+            _ => return None,
+        };
+        Some(sp)
+    }
+
+    pub fn span(&self, id: NodeId) -> Span {
+        self.opt_span(id)
+            .unwrap_or_else(|| fail!("AstMap.span: could not find span for id {}", id))
     }
 
     pub fn node_to_str(&self, id: NodeId) -> StrBuf {