Prefix verifier errors with ; (#585)

* Prefix verifier errors with ;

extract arrow drawing code into helper
print ; at beginning of line followed by arrow and error body

lib/codegen/src/print_errors.rs

@@ -84,11 +84,7 @@ fn pretty_instruction_error(
                     printed_instr = true;
                 }
 
-                write!(w, "{1:0$}^", indent, "")?;
+                print_error(w, indent, cur_inst.to_string(), err)?;
-                for _c in cur_inst.to_string().chars() {
-                    write!(w, "~")?;
-                }
-                writeln!(w, " verifier {}", err.to_string())?;
             }
             ir::entities::AnyEntity::Inst(_) => i += 1,
             _ => unreachable!(),
@@ -133,11 +129,7 @@ fn pretty_preamble_error(
                 printed_entity = true;
             }
 
-            write!(w, "{1:0$}^", indent, "")?;
+            print_error(w, indent, entity.to_string(), err)?;
-            for _c in entity.to_string().chars() {
-                write!(w, "~")?;
-            }
-            writeln!(w, " verifier {}", err.to_string())?;
         } else {
             i += 1
         }
@@ -152,6 +144,18 @@ fn pretty_preamble_error(
     Ok(())
 }
 
+/// Prints ;   ^~~~~~ verifier [ERROR BODY]
+fn print_error(w: &mut Write, indent: usize, s: String, err: VerifierError) -> fmt::Result {
+    let indent = if indent < 1 { 0 } else { indent - 1 };
+
+    write!(w, ";{1:0$}^", indent, "")?;
+    for _c in s.chars() {
+        write!(w, "~")?;
+    }
+    writeln!(w, " verifier {}", err.to_string())?;
+    Ok(())
+}
+
 /// Pretty-print a Cranelift error.
 pub fn pretty_error(func: &ir::Function, isa: Option<&TargetIsa>, err: CodegenError) -> String {
     if let CodegenError::Verifier(e) = err {