rustc: Extract --crate-type parsing to its own function

Helpful for users of rustc as a library.

src/librustc/driver/config.rs

@@ -593,24 +593,10 @@ fn parse_cfgspecs(cfgspecs: Vec<String> ) -> ast::CrateConfig {
 }
 
 pub fn build_session_options(matches: &getopts::Matches) -> Options {
-    let mut crate_types: Vec<CrateType> = Vec::new();
+
     let unparsed_crate_types = matches.opt_strs("crate-type");
-    for unparsed_crate_type in unparsed_crate_types.iter() {
-        for part in unparsed_crate_type.as_slice().split(',') {
-            let new_part = match part {
-                "lib"       => default_lib_output(),
-                "rlib"      => CrateTypeRlib,
-                "staticlib" => CrateTypeStaticlib,
-                "dylib"     => CrateTypeDylib,
-                "bin"       => CrateTypeExecutable,
-                _ => {
-                    early_error(format!("unknown crate type: `{}`",
-                                        part).as_slice())
-                }
-            };
-            crate_types.push(new_part)
-        }
-    }
+    let crate_types = parse_crate_types_from_list(unparsed_crate_types)
+        .unwrap_or_else(|e| early_error(e.as_slice()));
 
     let parse_only = matches.opt_present("parse-only");
     let no_trans = matches.opt_present("no-trans");
@@ -804,6 +790,29 @@ pub fn build_session_options(matches: &getopts::Matches) -> Options {
     }
 }
 
+pub fn parse_crate_types_from_list(crate_types_list_list: Vec<String>) -> Result<Vec<CrateType>, String> {
+
+    let mut crate_types: Vec<CrateType> = Vec::new();
+    for unparsed_crate_type in crate_types_list_list.iter() {
+        for part in unparsed_crate_type.as_slice().split(',') {
+            let new_part = match part {
+                "lib"       => default_lib_output(),
+                "rlib"      => CrateTypeRlib,
+                "staticlib" => CrateTypeStaticlib,
+                "dylib"     => CrateTypeDylib,
+                "bin"       => CrateTypeExecutable,
+                _ => {
+                    return Err(format!("unknown crate type: `{}`",
+                                       part));
+                }
+            };
+            crate_types.push(new_part)
+        }
+    }
+
+    return Ok(crate_types);
+}
+
 impl fmt::Show for CrateType {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         match *self {