put empty generic lists behind a pointer

This reduces the size of hir::Expr from 128 to 88 bytes (!) and shaves
200MB out of #36799.

src/librustc/hir/intravisit.rs

@@ -627,7 +627,9 @@ pub fn walk_path_segment<'v, V: Visitor<'v>>(visitor: &mut V,
                                              path_span: Span,
                                              segment: &'v PathSegment) {
     visitor.visit_name(path_span, segment.name);
-    visitor.visit_path_parameters(path_span, &segment.parameters);
+    if let Some(ref parameters) = segment.parameters {
+        visitor.visit_path_parameters(path_span, parameters);
+    }
 }
 
 pub fn walk_path_parameters<'v, V: Visitor<'v>>(visitor: &mut V,

src/librustc/hir/lowering.rs

@@ -690,10 +690,9 @@ impl<'a> LoweringContext<'a> {
             TyKind::ImplicitSelf => {
                 hir::TyPath(hir::QPath::Resolved(None, P(hir::Path {
                     def: self.expect_full_def(t.id),
-                    segments: hir_vec![hir::PathSegment {
-                        name: keywords::SelfType.name(),
-                        parameters: hir::PathParameters::none()
-                    }],
+                    segments: hir_vec![
+                        hir::PathSegment::from_name(keywords::SelfType.name())
+                    ],
                     span: t.span,
                 })))
             }
@@ -914,12 +913,8 @@ impl<'a> LoweringContext<'a> {
             segments: segments.map(|segment| {
                 self.lower_path_segment(p.span, segment, param_mode, 0,
                                         ParenthesizedGenericArgs::Err)
-            }).chain(name.map(|name| {
-                hir::PathSegment {
-                    name,
-                    parameters: hir::PathParameters::none()
-                }
-            })).collect(),
+            }).chain(name.map(|name| hir::PathSegment::from_name(name)))
+              .collect(),
             span: p.span,
         }
     }
@@ -940,7 +935,7 @@ impl<'a> LoweringContext<'a> {
                           expected_lifetimes: usize,
                           parenthesized_generic_args: ParenthesizedGenericArgs)
                           -> hir::PathSegment {
-        let mut parameters = if let Some(ref parameters) = segment.parameters {
+        let (mut parameters, infer_types) = if let Some(ref parameters) = segment.parameters {
             let msg = "parenthesized parameters may only be used with a trait";
             match **parameters {
                 PathParameters::AngleBracketed(ref data) => {
@@ -951,12 +946,12 @@ impl<'a> LoweringContext<'a> {
                     ParenthesizedGenericArgs::Warn => {
                         self.sess.buffer_lint(PARENTHESIZED_PARAMS_IN_TYPES_AND_MODULES,
                                               CRATE_NODE_ID, data.span, msg.into());
-                        hir::PathParameters::none()
+                        (hir::PathParameters::none(), true)
                     }
                     ParenthesizedGenericArgs::Err => {
                         struct_span_err!(self.sess, data.span, E0214, "{}", msg)
                             .span_label(data.span, "only traits may use parentheses").emit();
-                        hir::PathParameters::none()
+                        (hir::PathParameters::none(), true)
                     }
                 }
             }
@@ -970,40 +965,39 @@ impl<'a> LoweringContext<'a> {
             }).collect();
         }
 
-        hir::PathSegment {
-            name: self.lower_ident(segment.identifier),
+        hir::PathSegment::new(
+            self.lower_ident(segment.identifier),
             parameters,
-        }
+            infer_types
+        )
     }
 
     fn lower_angle_bracketed_parameter_data(&mut self,
                                             data: &AngleBracketedParameterData,
                                             param_mode: ParamMode)
-                                            -> hir::PathParameters {
+                                            -> (hir::PathParameters, bool) {
         let &AngleBracketedParameterData { ref lifetimes, ref types, ref bindings, .. } = data;
-        hir::PathParameters {
+        (hir::PathParameters {
             lifetimes: self.lower_lifetimes(lifetimes),
             types: types.iter().map(|ty| self.lower_ty(ty)).collect(),
-            infer_types: types.is_empty() && param_mode == ParamMode::Optional,
             bindings: bindings.iter().map(|b| self.lower_ty_binding(b)).collect(),
             parenthesized: false,
-        }
+        }, types.is_empty() && param_mode == ParamMode::Optional)
     }
 
     fn lower_parenthesized_parameter_data(&mut self,
                                           data: &ParenthesizedParameterData)
-                                          -> hir::PathParameters {
+                                          -> (hir::PathParameters, bool) {
         let &ParenthesizedParameterData { ref inputs, ref output, span } = data;
         let inputs = inputs.iter().map(|ty| self.lower_ty(ty)).collect();
         let mk_tup = |this: &mut Self, tys, span| {
             let LoweredNodeId { node_id, hir_id } = this.next_id();
             P(hir::Ty { node: hir::TyTup(tys), id: node_id, hir_id, span })
         };
 
-        hir::PathParameters {
+        (hir::PathParameters {
             lifetimes: hir::HirVec::new(),
             types: hir_vec![mk_tup(self, inputs, span)],
-            infer_types: false,
             bindings: hir_vec![hir::TypeBinding {
                 id: self.next_id().node_id,
                 name: Symbol::intern(FN_OUTPUT_NAME),
@@ -1012,7 +1006,7 @@ impl<'a> LoweringContext<'a> {
                 span: output.as_ref().map_or(span, |ty| ty.span),
             }],
             parenthesized: true,
-        }
+        }, false)
     }
 
     fn lower_local(&mut self, l: &Local) -> P<hir::Local> {

src/librustc/hir/mod.rs

@@ -236,17 +236,49 @@ pub struct PathSegment {
     /// this is more than just simple syntactic sugar; the use of
     /// parens affects the region binding rules, so we preserve the
     /// distinction.
-    pub parameters: PathParameters,
+    pub parameters: Option<P<PathParameters>>,
+
+    /// Whether to infer remaining type parameters, if any.
+    /// This only applies to expression and pattern paths, and
+    /// out of those only the segments with no type parameters
+    /// to begin with, e.g. `Vec::new` is `<Vec<..>>::new::<..>`.
+    pub infer_types: bool,
 }
 
 impl PathSegment {
     /// Convert an identifier to the corresponding segment.
     pub fn from_name(name: Name) -> PathSegment {
         PathSegment {
             name,
-            parameters: PathParameters::none()
+            infer_types: true,
+            parameters: None
+        }
+    }
+
+    pub fn new(name: Name, parameters: PathParameters, infer_types: bool) -> Self {
+        PathSegment {
+            name,
+            infer_types,
+            parameters: if parameters.is_empty() {
+                None
+            } else {
+                Some(P(parameters))
+            }
         }
     }
+
+    // FIXME: hack required because you can't create a static
+    // PathParameters, so you can't just return a &PathParameters.
+    pub fn with_parameters<F, R>(&self, f: F) -> R
+        where F: FnOnce(&PathParameters) -> R
+    {
+        let dummy = PathParameters::none();
+        f(if let Some(ref params) = self.parameters {
+            &params
+        } else {
+            &dummy
+        })
+    }
 }
 
 #[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug)]
@@ -255,11 +287,6 @@ pub struct PathParameters {
     pub lifetimes: HirVec<Lifetime>,
     /// The type parameters for this path segment, if present.
     pub types: HirVec<P<Ty>>,
-    /// Whether to infer remaining type parameters, if any.
-    /// This only applies to expression and pattern paths, and
-    /// out of those only the segments with no type parameters
-    /// to begin with, e.g. `Vec::new` is `<Vec<..>>::new::<..>`.
-    pub infer_types: bool,
     /// Bindings (equality constraints) on associated types, if present.
     /// E.g., `Foo<A=Bar>`.
     pub bindings: HirVec<TypeBinding>,
@@ -274,12 +301,16 @@ impl PathParameters {
         Self {
             lifetimes: HirVec::new(),
             types: HirVec::new(),
-            infer_types: true,
             bindings: HirVec::new(),
             parenthesized: false,
         }
     }
 
+    pub fn is_empty(&self) -> bool {
+        self.lifetimes.is_empty() && self.types.is_empty() &&
+            self.bindings.is_empty() && !self.parenthesized
+    }
+
     pub fn inputs(&self) -> &[P<Ty>] {
         if self.parenthesized {
             if let Some(ref ty) = self.types.get(0) {

src/librustc/hir/print.rs

@@ -1213,11 +1213,17 @@ impl<'a> State<'a> {
         self.print_expr_maybe_paren(&args[0], parser::PREC_POSTFIX)?;
         self.s.word(".")?;
         self.print_name(segment.name)?;
-        if !segment.parameters.lifetimes.is_empty() ||
-                !segment.parameters.types.is_empty() ||
-                !segment.parameters.bindings.is_empty() {
-            self.print_path_parameters(&segment.parameters, true)?;
-        }
+
+        segment.with_parameters(|parameters| {
+            if !parameters.lifetimes.is_empty() ||
+                !parameters.types.is_empty() ||
+                !parameters.bindings.is_empty()
+            {
+                self.print_path_parameters(&parameters, segment.infer_types, true)
+            } else {
+                Ok(())
+            }
+        })?;
         self.print_call_post(base_args)
     }
 
@@ -1564,8 +1570,12 @@ impl<'a> State<'a> {
             }
             if segment.name != keywords::CrateRoot.name() &&
                segment.name != keywords::DollarCrate.name() {
-                self.print_name(segment.name)?;
-                self.print_path_parameters(&segment.parameters, colons_before_params)?;
+               self.print_name(segment.name)?;
+               segment.with_parameters(|parameters| {
+                   self.print_path_parameters(parameters,
+                                              segment.infer_types,
+                                              colons_before_params)
+               })?;
             }
         }
 
@@ -1593,29 +1603,42 @@ impl<'a> State<'a> {
                     if segment.name != keywords::CrateRoot.name() &&
                        segment.name != keywords::DollarCrate.name() {
                         self.print_name(segment.name)?;
-                        self.print_path_parameters(&segment.parameters, colons_before_params)?;
+                        segment.with_parameters(|parameters| {
+                            self.print_path_parameters(parameters,
+                                                       segment.infer_types,
+                                                       colons_before_params)
+                        })?;
                     }
                 }
 
                 self.s.word(">")?;
                 self.s.word("::")?;
                 let item_segment = path.segments.last().unwrap();
                 self.print_name(item_segment.name)?;
-                self.print_path_parameters(&item_segment.parameters, colons_before_params)
+                item_segment.with_parameters(|parameters| {
+                    self.print_path_parameters(parameters,
+                                               item_segment.infer_types,
+                                               colons_before_params)
+                })
             }
             hir::QPath::TypeRelative(ref qself, ref item_segment) => {
                 self.s.word("<")?;
                 self.print_type(qself)?;
                 self.s.word(">")?;
                 self.s.word("::")?;
                 self.print_name(item_segment.name)?;
-                self.print_path_parameters(&item_segment.parameters, colons_before_params)
+                item_segment.with_parameters(|parameters| {
+                    self.print_path_parameters(parameters,
+                                               item_segment.infer_types,
+                                               colons_before_params)
+                })
             }
         }
     }
 
     fn print_path_parameters(&mut self,
                              parameters: &hir::PathParameters,
+                             infer_types: bool,
                              colons_before_params: bool)
                              -> io::Result<()> {
         if parameters.parenthesized {
@@ -1652,7 +1675,7 @@ impl<'a> State<'a> {
 
             // FIXME(eddyb) This would leak into error messages, e.g.:
             // "non-exhaustive patterns: `Some::<..>(_)` not covered".
-            if parameters.infer_types && false {
+            if infer_types && false {
                 start_or_comma(self)?;
                 self.s.word("..")?;
             }

src/librustc/ich/impls_hir.rs

@@ -150,13 +150,13 @@ impl_stable_hash_for!(struct hir::Path {
 
 impl_stable_hash_for!(struct hir::PathSegment {
     name,
+    infer_types,
     parameters
 });
 
 impl_stable_hash_for!(struct hir::PathParameters {
     lifetimes,
     types,
-    infer_types,
     bindings,
     parenthesized
 });

src/librustc/middle/resolve_lifetime.rs

@@ -445,7 +445,9 @@ impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
     fn visit_path(&mut self, path: &'tcx hir::Path, _: ast::NodeId) {
         for (i, segment) in path.segments.iter().enumerate() {
             let depth = path.segments.len() - i - 1;
-            self.visit_segment_parameters(path.def, depth, &segment.parameters);
+            if let Some(ref parameters) = segment.parameters {
+                self.visit_segment_parameters(path.def, depth, parameters);
+            }
         }
     }