Auto merge of #2282 - mcarton:clippy, r=alexcrichton

For information, here is the [log before and after](https://gist.github.com/mcarton/684c030321c4c60d6bc9) with Clippy.
Remaining warnings from Clippy are mostly false positive or `str` to `string` conversion (too many of them for this PR).

```
warnings before:
     cmp_owned : 2
     cyclomatic_complexity : 1
     deprecated : 11
     explicit_iter_loop : 57
     identity_op : 3
     len_without_is_empty : 3
     len_zero : 20
     let_and_return : 3
     map_clone : 14
     needless_lifetimes : 4
     needless_return : 24
     ok_expect : 2
     option_map_unwrap_or : 10
     private_in_public : 28
     redundant_closure : 2
     should_implement_trait : 2
     single_match : 3
     str_to_string : 112
     string_to_string : 12
     type_complexity : 3
     unnecessary_mut_passed : 2
     unneeded_field_pattern : 3
     unused_lifetimes : 1
     unused_variables : 2
     while_let_loop : 2

warnings after:
     cmp_owned : 2
     cyclomatic_complexity : 1
     identity_op : 1
     let_and_return : 3
     map_clone : 1
     needless_return : 3
     ok_expect : 2
     option_map_unwrap_or : 1
     private_in_public : 28
     should_implement_trait : 2
     str_to_string : 80
     type_complexity : 3
     while_let_loop : 2
```

Author: bors

src/cargo/core/package.rs

@@ -129,6 +129,10 @@ impl PackageSet {
         PackageSet { packages: packages.to_vec() }
     }
 
+    pub fn is_empty(&self) -> bool {
+        self.packages.is_empty()
+    }
+
     pub fn len(&self) -> usize {
         self.packages.len()
     }

src/cargo/core/registry.rs

@@ -15,7 +15,7 @@ pub trait Registry {
 impl Registry for Vec<Summary> {
     fn query(&mut self, dep: &Dependency) -> CargoResult<Vec<Summary>> {
         Ok(self.iter().filter(|summary| dep.matches(*summary))
-               .map(|summary| summary.clone()).collect())
+               .cloned().collect())
     }
 }
 
@@ -295,7 +295,7 @@ impl<'cfg> Registry for PackageRegistry<'cfg> {
     fn query(&mut self, dep: &Dependency) -> CargoResult<Vec<Summary>> {
         let overrides = try!(self.query_overrides(dep));
 
-        let ret = if overrides.len() == 0 {
+        let ret = if overrides.is_empty() {
             // Ensure the requested source_id is loaded
             try!(self.ensure_loaded(dep.source_id(), Kind::Normal));
             let mut ret = Vec::new();

src/cargo/core/resolver/mod.rs

@@ -384,7 +384,7 @@ fn find_candidate(backtrack_stack: &mut Vec<BacktrackFrame>,
             return Some(candidate)
         }
     }
-    return None
+    None
 }
 
 #[allow(deprecated)] // connect => join in 1.3
@@ -453,7 +453,7 @@ fn activation_error(cx: &Context,
     candidates.sort_by(|a, b| {
         b.version().cmp(a.version())
     });
-    if candidates.len() > 0 {
+    if !candidates.is_empty() {
         msg.push_str("\nversions found: ");
         for (i, c) in candidates.iter().take(3).enumerate() {
             if i != 0 { msg.push_str(", "); }
@@ -469,7 +469,7 @@ fn activation_error(cx: &Context,
     // update`. In this case try to print a helpful error!
     if dep.source_id().is_path() &&
        dep.version_req().to_string().starts_with("=") &&
-       candidates.len() > 0 {
+       !candidates.is_empty() {
         msg.push_str("\nconsider running `cargo update` to update \
                       a path dependency's locked version");
 
@@ -607,7 +607,7 @@ impl Context {
                     (!use_default || prev.contains("default") ||
                      !has_default_feature)
             }
-            None => features.len() == 0 && (!use_default || !has_default_feature)
+            None => features.is_empty() && (!use_default || !has_default_feature)
         }
     }
 
@@ -686,18 +686,18 @@ impl Context {
         // they should have all been weeded out by the above iteration. Any
         // remaining features are bugs in that the package does not actually
         // have those features.
-        if feature_deps.len() > 0 {
+        if !feature_deps.is_empty() {
             let unknown = feature_deps.keys().map(|s| &s[..])
                                       .collect::<Vec<&str>>();
-            if unknown.len() > 0 {
+            if !unknown.is_empty() {
                 let features = unknown.connect(", ");
                 bail!("Package `{}` does not have these features: `{}`",
                       parent.package_id(), features)
             }
         }
 
         // Record what list of features is active for this package.
-        if used_features.len() > 0 {
+        if !used_features.is_empty() {
             let pkgid = parent.package_id();
             self.resolve.features.entry(pkgid.clone())
                 .or_insert(HashSet::new())

src/cargo/core/shell.rs

@@ -81,7 +81,7 @@ impl MultiShell {
         where F: FnMut(&mut MultiShell) -> io::Result<()>
     {
         match self.verbosity {
-            Verbose => return callback(self),
+            Verbose => callback(self),
             _ => Ok(())
         }
     }
@@ -91,7 +91,7 @@ impl MultiShell {
     {
         match self.verbosity {
             Verbose => Ok(()),
-            _ => return callback(self)
+            _ => callback(self)
         }
     }
 

src/cargo/core/source.rs

@@ -419,6 +419,10 @@ impl<'src> SourceMap<'src> {
         self.map.insert(id.clone(), source);
     }
 
+    pub fn is_empty(&self) -> bool {
+        self.map.is_empty()
+    }
+
     pub fn len(&self) -> usize {
         self.map.len()
     }

src/cargo/lib.rs

@@ -241,7 +241,7 @@ pub fn version() -> String {
     })
 }
 
-fn flags_from_args<'a, T>(usage: &str, args: &[String],
+fn flags_from_args<T>(usage: &str, args: &[String],
                           options_first: bool) -> CliResult<T>
     where T: Decodable
 {

src/cargo/ops/cargo_clean.rs

@@ -22,7 +22,7 @@ pub fn clean(manifest_path: &Path, opts: &CleanOptions) -> CargoResult<()> {
 
     // If we have a spec, then we need to delete some packages, otherwise, just
     // remove the whole target directory and be done with it!
-    if opts.spec.len() == 0 {
+    if opts.spec.is_empty() {
         return rm_rf(&target_dir);
     }
 

src/cargo/ops/cargo_compile.rs

@@ -176,7 +176,7 @@ pub fn compile_pkg<'a>(root_package: &Package,
         vec![root_package.package_id()]
     };
 
-    if spec.len() > 0 && invalid_spec.len() > 0 {
+    if !spec.is_empty() && !invalid_spec.is_empty() {
         bail!("could not find package matching spec `{}`",
               invalid_spec.connect(", "))
     }
@@ -257,7 +257,7 @@ pub fn compile_pkg<'a>(root_package: &Package,
 
     ret.to_doc_test = to_builds.iter().map(|&p| p.clone()).collect();
 
-    return Ok(ret);
+    Ok(ret)
 }
 
 impl<'a> CompileFilter<'a> {
@@ -311,7 +311,7 @@ fn generate_targets<'a>(pkg: &'a Package,
         CompileMode::Build => build,
         CompileMode::Doc { .. } => &profiles.doc,
     };
-    return match *filter {
+    match *filter {
         CompileFilter::Everything => {
             match mode {
                 CompileMode::Bench => {
@@ -379,7 +379,7 @@ fn generate_targets<'a>(pkg: &'a Package,
             }
             Ok(targets)
         }
-    };
+    }
 }
 
 /// Read the `paths` configuration variable to discover all path overrides that

src/cargo/ops/cargo_doc.rs

@@ -18,7 +18,7 @@ pub fn doc(manifest_path: &Path,
 
     let mut lib_names = HashSet::new();
     let mut bin_names = HashSet::new();
-    if options.compile_opts.spec.len() == 0 {
+    if options.compile_opts.spec.is_empty() {
         for target in package.targets().iter().filter(|t| t.documented()) {
             if target.is_lib() {
                 assert!(lib_names.insert(target.crate_name()));
@@ -42,7 +42,7 @@ pub fn doc(manifest_path: &Path,
             bail!("Passing multiple packages and `open` is not supported")
         } else if options.compile_opts.spec.len() == 1 {
             try!(PackageIdSpec::parse(&options.compile_opts.spec[0]))
-                                             .name().replace("-", "_").to_string()
+                                             .name().replace("-", "_")
         } else {
             match lib_names.iter().chain(bin_names.iter()).nth(0) {
                 Some(s) => s.to_string(),

src/cargo/ops/cargo_generate_lockfile.rs

@@ -43,7 +43,7 @@ pub fn update_lockfile(manifest_path: &Path,
     let mut registry = PackageRegistry::new(opts.config);
     let mut to_avoid = HashSet::new();
 
-    if opts.to_update.len() == 0 {
+    if opts.to_update.is_empty() {
         to_avoid.extend(previous_resolve.iter());
     } else {
         let mut sources = Vec::new();