Rollup merge of rust-lang#42497 - qnighy:just-use-try-in-three-places, r=eddyb

Replace some matches with try.

This patch just replaces `match`es with `?` in the compiler, which I came across when I'm reading the parser.

Author: frewsxcv

src/librustc_resolve/macros.rs

@@ -285,10 +285,7 @@ impl<'a> base::Resolver for Resolver<'a> {
                      -> Result<Option<Rc<SyntaxExtension>>, Determinacy> {
         let def = match invoc.kind {
             InvocationKind::Attr { attr: None, .. } => return Ok(None),
-            _ => match self.resolve_invoc_to_def(invoc, scope, force) {
-                Ok(def) => def,
-                Err(determinacy) => return Err(determinacy),
-            },
+            _ => self.resolve_invoc_to_def(invoc, scope, force)?,
         };
 
         self.macro_defs.insert(invoc.expansion_data.mark, def.def_id());

src/libsyntax/parse/parser.rs

@@ -1249,10 +1249,7 @@ impl<'a> Parser<'a> {
             let mac = respan(lo.to(self.prev_span), Mac_ { path: pth, tts: tts });
             (keywords::Invalid.ident(), ast::TraitItemKind::Macro(mac))
         } else {
-            let (constness, unsafety, abi) = match self.parse_fn_front_matter() {
-                Ok(cua) => cua,
-                Err(e) => return Err(e),
-            };
+            let (constness, unsafety, abi) = self.parse_fn_front_matter()?;
 
             let ident = self.parse_ident()?;
             let mut generics = self.parse_generics()?;

src/libsyntax/ptr.rs

@@ -211,10 +211,7 @@ impl<T: Encodable> Encodable for P<[T]> {
 
 impl<T: Decodable> Decodable for P<[T]> {
     fn decode<D: Decoder>(d: &mut D) -> Result<P<[T]>, D::Error> {
-        Ok(P::from_vec(match Decodable::decode(d) {
-            Ok(t) => t,
-            Err(e) => return Err(e)
-        }))
+        Ok(P::from_vec(Decodable::decode(d)?))
     }
 }