prefer `if let` to match with `None => {}` arm in some places

This is a spiritual succesor to #34268/8531d581, in which we replaced a
number of matches of None to the unit value with `if let` conditionals
where it was judged that this made for clearer/simpler code (as would be
recommended by Manishearth/rust-clippy's `single_match` lint). The same
rationale applies to matches of None to the empty block.

3 files changed, 17 insertions, 29 deletions

diff --git a/src/libsyntax_ext/deriving/generic/mod.rs b/src/libsyntax_ext/deriving/generic/mod.rs
index e01f4ed1f9b..f33898109cc 100644
--- a/src/libsyntax_ext/deriving/generic/mod.rs
+++ b/src/libsyntax_ext/deriving/generic/mod.rs
@@ -360,13 +360,10 @@ fn find_type_parameters(ty: &ast::Ty, ty_param_names: &[ast::Name], span: Span,
         fn visit_ty(&mut self, ty: &ast::Ty) {
             match ty.node {
                 ast::TyKind::Path(_, ref path) if !path.global => {
-                    match path.segments.first() {
-                        Some(segment) => {
-                            if self.ty_param_names.contains(&segment.identifier.name) {
-                                self.types.push(P(ty.clone()));
-                            }
+                    if let Some(segment) = path.segments.first() {
+                        if self.ty_param_names.contains(&segment.identifier.name) {
+                            self.types.push(P(ty.clone()));
                         }
-                        None => {}
                     }
                 }
                 _ => {}
diff --git a/src/libsyntax_ext/env.rs b/src/libsyntax_ext/env.rs
index 546f8eaa692..c6c4b6135c6 100644
--- a/src/libsyntax_ext/env.rs
+++ b/src/libsyntax_ext/env.rs
@@ -88,12 +88,9 @@ pub fn expand_env<'cx>(cx: &'cx mut ExtCtxt, sp: Span, tts: &[tokenstream::Token
         }
     };
 
-    match exprs.next() {
-        None => {}
-        Some(_) => {
-            cx.span_err(sp, "env! takes 1 or 2 arguments");
-            return DummyResult::expr(sp);
-        }
+    if let Some(_) = exprs.next() {
+        cx.span_err(sp, "env! takes 1 or 2 arguments");
+        return DummyResult::expr(sp);
     }
 
     let e = match env::var(&var[..]) {
diff --git a/src/libsyntax_ext/format.rs b/src/libsyntax_ext/format.rs
index f311f16f11b..dc572e652c6 100644
--- a/src/libsyntax_ext/format.rs
+++ b/src/libsyntax_ext/format.rs
@@ -126,16 +126,13 @@ fn parse_args(ecx: &mut ExtCtxt, sp: Span, tts: &[tokenstream::TokenTree])
 
             panictry!(p.expect(&token::Eq));
             let e = panictry!(p.parse_expr());
-            match names.get(name) {
-                None => {}
-                Some(prev) => {
-                    ecx.struct_span_err(e.span,
-                                        &format!("duplicate argument named `{}`",
-                                                 name))
-                       .span_note(prev.span, "previously here")
-                       .emit();
-                    continue
-                }
+            if let Some(prev) = names.get(name) {
+                ecx.struct_span_err(e.span,
+                                    &format!("duplicate argument named `{}`",
+                                             name))
+                    .span_note(prev.span, "previously here")
+                    .emit();
+                continue;
             }
             order.push(name.to_string());
             names.insert(name.to_string(), e);
@@ -665,13 +662,10 @@ pub fn expand_preparsed_format_args(ecx: &mut ExtCtxt, sp: Span,
             Some(piece) => {
                 if !parser.errors.is_empty() { break }
                 cx.verify_piece(&piece);
-                match cx.trans_piece(&piece) {
-                    Some(piece) => {
-                        let s = cx.trans_literal_string();
-                        cx.str_pieces.push(s);
-                        cx.pieces.push(piece);
-                    }
-                    None => {}
+                if let Some(piece) = cx.trans_piece(&piece) {
+                    let s = cx.trans_literal_string();
+                    cx.str_pieces.push(s);
+                    cx.pieces.push(piece);
                 }
             }
             None => break