Rollup merge of rust-lang#52658 - Wallacoloo:topics/use-option-methods, r=cramertj

Prefer `Option::map`/etc over `match` wherever it improves clarity

This isn't intended to change behavior anywhere. A lot of times statements like `match x { None => None, Some(y) => [...] }` can be rewritten using `Option::map` or `Option::and_then` in a way that preserves or improves clarity, so that's what I've done here.

I think it's particularly valuable to keep things in `libcore` and `libstd` pretty/idiomatic since it's not uncommon to follow the `[src]` links when browsing the rust-lang.org docs for std/core. If there's any concern about pushing style-based changes though, I'll happily back out the non-std/core commits here.

Author: Mark-Simulacrum

src/libcore/str/mod.rs

@@ -696,13 +696,10 @@ impl<'a> Iterator for CharIndices<'a> {
 impl<'a> DoubleEndedIterator for CharIndices<'a> {
     #[inline]
     fn next_back(&mut self) -> Option<(usize, char)> {
-        match self.iter.next_back() {
-            None => None,
-            Some(ch) => {
-                let index = self.front_offset + self.iter.iter.len();
-                Some((index, ch))
-            }
-        }
+        self.iter.next_back().map(|ch| {
+            let index = self.front_offset + self.iter.iter.len();
+            (index, ch)
+        })
     }
 }
 

src/librustc/traits/on_unimplemented.rs

@@ -190,11 +190,10 @@ impl<'a, 'gcx, 'tcx> OnUnimplementedDirective {
         for command in self.subcommands.iter().chain(Some(self)).rev() {
             if let Some(ref condition) = command.condition {
                 if !attr::eval_condition(condition, &tcx.sess.parse_sess, &mut |c| {
-                    options.contains(&(c.name().as_str().to_string(),
-                                      match c.value_str().map(|s| s.as_str().to_string()) {
-                                          Some(s) => Some(s),
-                                          None => None
-                                      }))
+                    options.contains(&(
+                        c.name().as_str().to_string(),
+                        c.value_str().map(|s| s.as_str().to_string())
+                    ))
                 }) {
                     debug!("evaluate: skipping {:?} due to condition", command);
                     continue

src/librustc/ty/mod.rs

@@ -2697,15 +2697,12 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
             self.opt_associated_item(def_id)
         };
 
-        match item {
-            Some(trait_item) => {
-                match trait_item.container {
-                    TraitContainer(_) => None,
-                    ImplContainer(def_id) => Some(def_id),
-                }
+        item.and_then(|trait_item|
+            match trait_item.container {
+                TraitContainer(_) => None,
+                ImplContainer(def_id) => Some(def_id),
             }
-            None => None
-        }
+        )
     }
 
     /// Looks up the span of `impl_did` if the impl is local; otherwise returns `Err`

src/librustc_metadata/locator.rs

@@ -824,17 +824,14 @@ impl<'a> Context<'a> {
         if rlib.is_none() && rmeta.is_none() && dylib.is_none() {
             return None;
         }
-        match slot {
-            Some((_, metadata)) => {
-                Some(Library {
-                    dylib,
-                    rlib,
-                    rmeta,
-                    metadata,
-                })
+        slot.map(|(_, metadata)|
+            Library {
+                dylib,
+                rlib,
+                rmeta,
+                metadata,
             }
-            None => None,
-        }
+        )
     }
 }
 

src/libstd/net/parser.rs

@@ -53,10 +53,7 @@ impl<'a> Parser<'a> {
         F: FnOnce(&mut Parser) -> Option<T>,
     {
         self.read_atomically(move |p| {
-            match cb(p) {
-                Some(x) => if p.is_eof() {Some(x)} else {None},
-                None => None,
-            }
+            cb(p).filter(|_| p.is_eof())
         })
     }
 

src/libstd/path.rs

@@ -1065,10 +1065,7 @@ impl<'a> Iterator for Ancestors<'a> {
 
     fn next(&mut self) -> Option<Self::Item> {
         let next = self.next;
-        self.next = match next {
-            Some(path) => path.parent(),
-            None => None,
-        };
+        self.next = next.and_then(Path::parent);
         next
     }
 }

src/libstd/sys_common/backtrace.rs

@@ -156,16 +156,15 @@ pub fn log_enabled() -> Option<PrintFormat> {
         _ => return Some(PrintFormat::Full),
     }
 
-    let val = match env::var_os("RUST_BACKTRACE") {
-        Some(x) => if &x == "0" {
+    let val = env::var_os("RUST_BACKTRACE").and_then(|x|
+        if &x == "0" {
             None
         } else if &x == "full" {
             Some(PrintFormat::Full)
         } else {
             Some(PrintFormat::Short)
-        },
-        None => None,
-    };
+        }
+    );
     ENABLED.store(match val {
         Some(v) => v as isize,
         None => 1,

src/libsyntax_ext/deriving/generic/ty.rs

@@ -138,17 +138,13 @@ pub fn nil_ty<'r>() -> Ty<'r> {
 }
 
 fn mk_lifetime(cx: &ExtCtxt, span: Span, lt: &Option<&str>) -> Option<ast::Lifetime> {
-    match *lt {
-        Some(s) => Some(cx.lifetime(span, Ident::from_str(s))),
-        None => None,
-    }
+    lt.map(|s|
+        cx.lifetime(span, Ident::from_str(s))
+    )
 }
 
 fn mk_lifetimes(cx: &ExtCtxt, span: Span, lt: &Option<&str>) -> Vec<ast::Lifetime> {
-    match *lt {
-        Some(s) => vec![cx.lifetime(span, Ident::from_str(s))],
-        None => vec![],
-    }
+    mk_lifetime(cx, span, lt).into_iter().collect()
 }
 
 impl<'a> Ty<'a> {