Rollup merge of rust-lang#54295 - ljedrz:cleanups_rustc_traits, r=nikomatsakis

A few cleanups and minor improvements to rustc/traits

It's a little bigger than usual, so bear with me ^^:

- introduce `TyCtxt::all_impls` and use it to avoid inefficiently allocating push loops
- modify `ArgKind::from_expected_ty` to take an `Option<Span>` argument to make it more versatile
- replace `ArgKind::Arg("_".to_owned(), "_".to_owned())` with `ArgKind::empty`
- move early `return`s earlier where possible
- if all branches of a `match` end with the same expression, move it after it
- change a hacky `match` expression to an `if else` chain
- move the `push` out from a push loop closure to reduce the number of allocations
- correct the vector size for `pretty_predicates` (under `specialize`)
- take advantage of the fact that `if else` is an expression
- prefer `cloned()` to `map(|&x| x)` and `map(|x| *x)`
- prefer `vec![x; y.len()]` to `y.map(|_| x).collect()`
- use `unwrap_or_else` instead of `match` where applicable
- use `if let` instead of `match` when only one branch matters
- prefer `to_owned` to `to_string` for string literals
- remove explicit `return`s
- remove superfluous braces
- whitespace fixes
- several other minor improvements

Author: kennytm

src/librustc/traits/auto_trait.rs

@@ -112,17 +112,18 @@ impl<'a, 'tcx> AutoTraitFinder<'a, 'tcx> {
                 orig_params,
                 trait_pred.to_poly_trait_predicate(),
             ));
+
             match result {
                 Ok(Some(Vtable::VtableImpl(_))) => {
                     debug!(
                         "find_auto_trait_generics(did={:?}, trait_did={:?}, generics={:?}): \
                          manual impl found, bailing out",
                         did, trait_did, generics
                     );
-                    return true;
+                    true
                 }
-                _ => return false,
-            };
+                _ => false
+            }
         });
 
         // If an explicit impl exists, it always takes priority over an auto impl
@@ -426,6 +427,7 @@ impl<'a, 'tcx> AutoTraitFinder<'a, 'tcx> {
                     if new_trait.def_id() == old_trait.def_id() {
                         let new_substs = new_trait.skip_binder().trait_ref.substs;
                         let old_substs = old_trait.skip_binder().trait_ref.substs;
+
                         if !new_substs.types().eq(old_substs.types()) {
                             // We can't compare lifetimes if the types are different,
                             // so skip checking old_pred
@@ -489,12 +491,12 @@ impl<'a, 'tcx> AutoTraitFinder<'a, 'tcx> {
 
     pub fn get_lifetime(&self, region: Region, names_map: &FxHashMap<String, String>) -> String {
         self.region_name(region)
-            .map(|name| {
-                names_map.get(&name).unwrap_or_else(|| {
+            .map(|name|
+                names_map.get(&name).unwrap_or_else(||
                     panic!("Missing lifetime with name {:?} for {:?}", name, region)
-                })
-            })
-            .unwrap_or(&"'static".to_string())
+                )
+            )
+            .unwrap_or(&"'static".to_owned())
             .clone()
     }
 

src/librustc/traits/codegen/mod.rs

@@ -39,7 +39,7 @@ pub fn codegen_fulfill_obligation<'a, 'tcx>(ty: TyCtxt<'a, 'tcx, 'tcx>,
     let trait_ref = ty.erase_regions(&trait_ref);
 
     debug!("codegen_fulfill_obligation(trait_ref={:?}, def_id={:?})",
-            (param_env, trait_ref), trait_ref.def_id());
+        (param_env, trait_ref), trait_ref.def_id());
 
     // Do the initial selection for the obligation. This yields the
     // shallow result we are looking for -- that is, what specific impl.
@@ -48,8 +48,8 @@ pub fn codegen_fulfill_obligation<'a, 'tcx>(ty: TyCtxt<'a, 'tcx, 'tcx>,
 
         let obligation_cause = ObligationCause::dummy();
         let obligation = Obligation::new(obligation_cause,
-                                            param_env,
-                                            trait_ref.to_poly_trait_predicate());
+                                         param_env,
+                                         trait_ref.to_poly_trait_predicate());
 
         let selection = match selcx.select(&obligation) {
             Ok(Some(selection)) => selection,
@@ -61,12 +61,11 @@ pub fn codegen_fulfill_obligation<'a, 'tcx>(ty: TyCtxt<'a, 'tcx, 'tcx>,
                 // overflow bug, since I believe this is the only case
                 // where ambiguity can result.
                 bug!("Encountered ambiguity selecting `{:?}` during codegen, \
-                        presuming due to overflow",
-                        trait_ref)
+                      presuming due to overflow",
+                      trait_ref)
             }
             Err(e) => {
-                bug!("Encountered error `{:?}` selecting `{:?}` during codegen",
-                            e, trait_ref)
+                bug!("Encountered error `{:?}` selecting `{:?}` during codegen", e, trait_ref)
             }
         };
 
@@ -163,22 +162,16 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
         // In principle, we only need to do this so long as `result`
         // contains unbound type parameters. It could be a slight
         // optimization to stop iterating early.
-        match fulfill_cx.select_all_or_error(self) {
-            Ok(()) => { }
-            Err(errors) => {
-                span_bug!(span, "Encountered errors `{:?}` resolving bounds after type-checking",
-                          errors);
-            }
+        if let Err(errors) = fulfill_cx.select_all_or_error(self) {
+            span_bug!(span, "Encountered errors `{:?}` resolving bounds after type-checking",
+                      errors);
         }
 
         let result = self.resolve_type_vars_if_possible(result);
         let result = self.tcx.erase_regions(&result);
 
-        match self.tcx.lift_to_global(&result) {
-            Some(result) => result,
-            None => {
-                span_bug!(span, "Uninferred types/regions in `{:?}`", result);
-            }
-        }
+        self.tcx.lift_to_global(&result).unwrap_or_else(||
+            span_bug!(span, "Uninferred types/regions in `{:?}`", result)
+        )
     }
 }

src/librustc/traits/coherence.rs

@@ -115,9 +115,7 @@ fn overlap<'cx, 'gcx, 'tcx>(selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
                             b_def_id: DefId)
                             -> Option<OverlapResult<'tcx>>
 {
-    debug!("overlap(a_def_id={:?}, b_def_id={:?})",
-           a_def_id,
-           b_def_id);
+    debug!("overlap(a_def_id={:?}, b_def_id={:?})", a_def_id, b_def_id);
 
     // For the purposes of this check, we don't bring any skolemized
     // types into scope; instead, we replace the generic types with
@@ -133,10 +131,9 @@ fn overlap<'cx, 'gcx, 'tcx>(selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
 
     // Do `a` and `b` unify? If not, no overlap.
     let obligations = match selcx.infcx().at(&ObligationCause::dummy(), param_env)
-                                         .eq_impl_headers(&a_impl_header, &b_impl_header) {
-        Ok(InferOk { obligations, value: () }) => {
-            obligations
-        }
+                                         .eq_impl_headers(&a_impl_header, &b_impl_header)
+    {
+        Ok(InferOk { obligations, value: () }) => obligations,
         Err(_) => return None
     };
 
@@ -164,7 +161,7 @@ fn overlap<'cx, 'gcx, 'tcx>(selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
         return None
     }
 
-    let impl_header =  selcx.infcx().resolve_type_vars_if_possible(&a_impl_header);
+    let impl_header = selcx.infcx().resolve_type_vars_if_possible(&a_impl_header);
     let intercrate_ambiguity_causes = selcx.take_intercrate_ambiguity_causes();
     debug!("overlap: intercrate_ambiguity_causes={:#?}", intercrate_ambiguity_causes);
     Some(OverlapResult { impl_header, intercrate_ambiguity_causes })
@@ -471,14 +468,12 @@ fn ty_is_local_constructor(ty: Ty, in_crate: InCrate) -> bool {
         ty::Foreign(did) => def_id_is_local(did, in_crate),
 
         ty::Dynamic(ref tt, ..) => {
-            tt.principal().map_or(false, |p| {
+            tt.principal().map_or(false, |p|
                 def_id_is_local(p.def_id(), in_crate)
-            })
+            )
         }
 
-        ty::Error => {
-            true
-        }
+        ty::Error => true,
 
         ty::Closure(..) |
         ty::Generator(..) |