port compiletest to use JSON output

This uncovered a lot of bugs in compiletest and also some shortcomings
of our existing JSON output. We had to add information to the JSON
output, such as suggested text and macro backtraces. We also had to fix
various bugs in the existing tests.

Joint work with jntrnr.

src/libsyntax/codemap.rs

@@ -1394,6 +1394,56 @@ impl CodeMap {
     pub fn count_lines(&self) -> usize {
         self.files.borrow().iter().fold(0, |a, f| a + f.count_lines())
     }
+
+    pub fn macro_backtrace(&self, span: Span) -> Vec<MacroBacktrace> {
+        let mut last_span = DUMMY_SP;
+        let mut span = span;
+        let mut result = vec![];
+        loop {
+            let span_name_span = self.with_expn_info(span.expn_id, |expn_info| {
+                expn_info.map(|ei| {
+                    let (pre, post) = match ei.callee.format {
+                        MacroAttribute(..) => ("#[", "]"),
+                        MacroBang(..) => ("", "!"),
+                    };
+                    let macro_decl_name = format!("{}{}{}",
+                                                  pre,
+                                                  ei.callee.name(),
+                                                  post);
+                    let def_site_span = ei.callee.span;
+                    (ei.call_site, macro_decl_name, def_site_span)
+                })
+            });
+
+            match span_name_span {
+                None => break,
+                Some((call_site, macro_decl_name, def_site_span)) => {
+                    // Don't print recursive invocations
+                    if !call_site.source_equal(&last_span) {
+                        result.push(MacroBacktrace {
+                            call_site: call_site,
+                            macro_decl_name: macro_decl_name,
+                            def_site_span: def_site_span,
+                        });
+                    }
+                    last_span = span;
+                    span = call_site;
+                }
+            }
+        }
+        result
+    }
+}
+
+pub struct MacroBacktrace {
+    /// span where macro was applied to generate this code
+    pub call_site: Span,
+
+    /// name of macro that was applied (e.g., "foo!" or "#[derive(Eq)]")
+    pub macro_decl_name: String,
+
+    /// span where macro was defined (if known)
+    pub def_site_span: Option<Span>,
 }
 
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