Rollup merge of #52703 - ljedrz:vec_improvements, r=nikomatsakis

Improve a few vectors - calculate capacity or build from iterators

Collecting from iterators improves readability and tailoring vector capacities should be beneficial in terms of performance.

src/librustc/hir/map/hir_id_validator.rs

@@ -105,7 +105,7 @@ impl<'a, 'hir: 'a> HirIdValidator<'a, 'hir> {
               .collect();
 
             // Try to map those to something more useful
-            let mut missing_items = vec![];
+            let mut missing_items = Vec::with_capacity(missing.len());
 
             for local_id in missing {
                 let hir_id = HirId {

src/librustc/infer/outlives/obligations.rs

@@ -505,11 +505,7 @@ where
     }
 
     fn recursive_type_bound(&self, ty: Ty<'tcx>) -> VerifyBound<'tcx> {
-        let mut bounds = vec![];
-
-        for subty in ty.walk_shallow() {
-            bounds.push(self.type_bound(subty));
-        }
+        let mut bounds = ty.walk_shallow().map(|subty| self.type_bound(subty)).collect::<Vec<_>>();
 
         let mut regions = ty.regions();
         regions.retain(|r| !r.is_late_bound()); // ignore late-bound regions

src/librustc/lint/context.rs

@@ -219,7 +219,7 @@ impl LintStore {
             }
         }
 
-        let mut future_incompatible = vec![];
+        let mut future_incompatible = Vec::with_capacity(lints.len());
         for lint in lints {
             future_incompatible.push(lint.id);
             self.future_incompatible.insert(lint.id, lint);

src/librustc/traits/object_safety.rs

@@ -98,13 +98,10 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
     pub fn astconv_object_safety_violations(self, trait_def_id: DefId)
                                             -> Vec<ObjectSafetyViolation>
     {
-        let mut violations = vec![];
-
-        for def_id in traits::supertrait_def_ids(self, trait_def_id) {
-            if self.predicates_reference_self(def_id, true) {
-                violations.push(ObjectSafetyViolation::SupertraitSelf);
-            }
-        }
+        let violations = traits::supertrait_def_ids(self, trait_def_id)
+            .filter(|&def_id| self.predicates_reference_self(def_id, true))
+            .map(|_| ObjectSafetyViolation::SupertraitSelf)
+            .collect();
 
         debug!("astconv_object_safety_violations(trait_def_id={:?}) = {:?}",
                trait_def_id,

src/librustc_codegen_llvm/back/link.rs

@@ -40,6 +40,7 @@ use std::env;
 use std::fmt;
 use std::fs;
 use std::io;
+use std::iter;
 use std::path::{Path, PathBuf};
 use std::process::{Output, Stdio};
 use std::str;
@@ -885,9 +886,9 @@ fn exec_linker(sess: &Session, cmd: &mut Command, out_filename: &Path, tmpdir: &
     }
     let file = tmpdir.join("linker-arguments");
     let bytes = if sess.target.target.options.is_like_msvc {
-        let mut out = vec![];
+        let mut out = Vec::with_capacity((1 + args.len()) * 2);
         // start the stream with a UTF-16 BOM
-        for c in vec![0xFEFF].into_iter().chain(args.encode_utf16()) {
+        for c in iter::once(0xFEFF).chain(args.encode_utf16()) {
             // encode in little endian
             out.push(c as u8);
             out.push((c >> 8) as u8);

src/libsyntax_pos/lib.rs

@@ -632,15 +632,14 @@ impl MultiSpan {
     /// `SpanLabel` instances with empty labels.
     pub fn span_labels(&self) -> Vec<SpanLabel> {
         let is_primary = |span| self.primary_spans.contains(&span);
-        let mut span_labels = vec![];
 
-        for &(span, ref label) in &self.span_labels {
-            span_labels.push(SpanLabel {
+        let mut span_labels = self.span_labels.iter().map(|&(span, ref label)|
+            SpanLabel {
                 span,
                 is_primary: is_primary(span),
                 label: Some(label.clone())
-            });
-        }
+            }
+        ).collect::<Vec<_>>();
 
         for &span in &self.primary_spans {
             if !span_labels.iter().any(|sl| sl.span == span) {