Auto merge of #46616 - cramertj:impl-trait-elision, r=nikomatsakis

Implement impl Trait lifetime elision

Fixes #43396.

There's one weird ICE in the interaction with argument-position `impl Trait`. I'm still debugging it-- I've left a test for it commented out with a FIXME.

Also included a FIXME to ensure that `impl Trait` traits are caught under the lint in #45992.

r? @nikomatsakis

src/librustc/hir/lowering.rs

@@ -990,8 +990,9 @@ impl<'a> LoweringContext<'a> {
         struct ImplTraitLifetimeCollector<'r, 'a: 'r> {
             context: &'r mut LoweringContext<'a>,
             parent: DefIndex,
-            currently_bound_lifetimes: Vec<Name>,
-            already_defined_lifetimes: HashSet<Name>,
+            collect_elided_lifetimes: bool,
+            currently_bound_lifetimes: Vec<hir::LifetimeName>,
+            already_defined_lifetimes: HashSet<hir::LifetimeName>,
             output_lifetimes: Vec<hir::Lifetime>,
             output_lifetime_defs: Vec<hir::LifetimeDef>,
         }
@@ -1002,6 +1003,30 @@ impl<'a> LoweringContext<'a> {
                 hir::intravisit::NestedVisitorMap::None
             }
 
+            fn visit_path_parameters(&mut self, span: Span, parameters: &'v hir::PathParameters) {
+                // Don't collect elided lifetimes used inside of `Fn()` syntax.
+                if parameters.parenthesized {
+                    let old_collect_elided_lifetimes = self.collect_elided_lifetimes;
+                    self.collect_elided_lifetimes = false;
+                    hir::intravisit::walk_path_parameters(self, span, parameters);
+                    self.collect_elided_lifetimes = old_collect_elided_lifetimes;
+                } else {
+                    hir::intravisit::walk_path_parameters(self, span, parameters);
+                }
+            }
+
+            fn visit_ty(&mut self, t: &'v hir::Ty) {
+                // Don't collect elided lifetimes used inside of `fn()` syntax
+                if let &hir::Ty_::TyBareFn(_) = &t.node {
+                    let old_collect_elided_lifetimes = self.collect_elided_lifetimes;
+                    self.collect_elided_lifetimes = false;
+                    hir::intravisit::walk_ty(self, t);
+                    self.collect_elided_lifetimes = old_collect_elided_lifetimes;
+                } else {
+                    hir::intravisit::walk_ty(self, t);
+                }
+            }
+
             fn visit_poly_trait_ref(&mut self,
                                     polytr: &'v hir::PolyTraitRef,
                                     _: hir::TraitBoundModifier) {
@@ -1010,10 +1035,8 @@ impl<'a> LoweringContext<'a> {
                 // Record the introduction of 'a in `for<'a> ...`
                 for lt_def in &polytr.bound_lifetimes {
                     // Introduce lifetimes one at a time so that we can handle
-                    // cases like `fn foo<'d>() -> impl for<'a, 'b: 'a, 'c: 'b + 'd> ...`
-                    if let hir::LifetimeName::Name(name) = lt_def.lifetime.name {
-                        self.currently_bound_lifetimes.push(name);
-                    }
+                    // cases like `fn foo<'d>() -> impl for<'a, 'b: 'a, 'c: 'b + 'd>`
+                    self.currently_bound_lifetimes.push(lt_def.lifetime.name);
 
                     // Visit the lifetime bounds
                     for lt_bound in &lt_def.bounds {
@@ -1027,47 +1050,58 @@ impl<'a> LoweringContext<'a> {
             }
 
             fn visit_lifetime(&mut self, lifetime: &'v hir::Lifetime) {
-                // Exclude '_, 'static, and elided lifetimes (there should be no elided lifetimes)
-                if let hir::LifetimeName::Name(lifetime_name) = lifetime.name {
-                    if !self.currently_bound_lifetimes.contains(&lifetime_name) &&
-                       !self.already_defined_lifetimes.contains(&lifetime_name)
-                    {
-                        self.already_defined_lifetimes.insert(lifetime_name);
-                        let name = hir::LifetimeName::Name(lifetime_name);
-
-                        self.output_lifetimes.push(hir::Lifetime {
-                            id: self.context.next_id().node_id,
-                            span: lifetime.span,
-                            name,
-                        });
+                let name = match lifetime.name {
+                    hir::LifetimeName::Implicit |
+                    hir::LifetimeName::Underscore =>
+                        if self.collect_elided_lifetimes {
+                            // Use `'_` for both implicit and underscore lifetimes in
+                            // `abstract type Foo<'_>: SomeTrait<'_>;`
+                            hir::LifetimeName::Underscore
+                        } else {
+                            return
+                        }
+                    name @ hir::LifetimeName::Name(_) => name,
+                    hir::LifetimeName::Static => return,
+                };
 
-                        let def_node_id = self.context.next_id().node_id;
-                        self.context.resolver.definitions().create_def_with_parent(
-                            self.parent,
-                            def_node_id,
-                            DefPathData::LifetimeDef(lifetime_name.as_str()),
-                            DefIndexAddressSpace::High,
-                            Mark::root()
-                        );
-                        let def_lifetime = hir::Lifetime {
-                            id: def_node_id,
-                            span: lifetime.span,
-                            name,
-                        };
-                        self.output_lifetime_defs.push(hir::LifetimeDef {
-                            lifetime: def_lifetime,
-                            bounds: Vec::new().into(),
-                            pure_wrt_drop: false,
-                            in_band: false,
-                        });
-                    }
+                if !self.currently_bound_lifetimes.contains(&name) &&
+                   !self.already_defined_lifetimes.contains(&name)
+                {
+                    self.already_defined_lifetimes.insert(name);
+
+                    self.output_lifetimes.push(hir::Lifetime {
+                        id: self.context.next_id().node_id,
+                        span: lifetime.span,
+                        name,
+                    });
+
+                    let def_node_id = self.context.next_id().node_id;
+                    self.context.resolver.definitions().create_def_with_parent(
+                        self.parent,
+                        def_node_id,
+                        DefPathData::LifetimeDef(name.name().as_str()),
+                        DefIndexAddressSpace::High,
+                        Mark::root()
+                    );
+                    let def_lifetime = hir::Lifetime {
+                        id: def_node_id,
+                        span: lifetime.span,
+                        name: name,
+                    };
+                    self.output_lifetime_defs.push(hir::LifetimeDef {
+                        lifetime: def_lifetime,
+                        bounds: Vec::new().into(),
+                        pure_wrt_drop: false,
+                        in_band: false,
+                    });
                 }
             }
         }
 
         let mut lifetime_collector = ImplTraitLifetimeCollector {
             context: self,
             parent: parent_index,
+            collect_elided_lifetimes: true,
             currently_bound_lifetimes: Vec::new(),
             already_defined_lifetimes: HashSet::new(),
             output_lifetimes: Vec::new(),

src/librustc/middle/resolve_lifetime.rs

@@ -600,24 +600,45 @@ impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
                 } = *exist_ty;
                 let mut index = self.next_early_index();
                 debug!("visit_ty: index = {}", index);
-                let lifetimes = generics
-                    .lifetimes
-                    .iter()
-                    .map(|lt_def| Region::early(&self.tcx.hir, &mut index, lt_def))
-                    .collect();
 
-                let next_early_index = index + generics.ty_params.len() as u32;
-                let scope = Scope::Binder {
-                    lifetimes,
-                    next_early_index,
-                    s: self.scope,
-                };
-                self.with(scope, |_old_scope, this| {
-                    this.visit_generics(generics);
-                    for bound in bounds {
-                        this.visit_ty_param_bound(bound);
+                let mut elision = None;
+                let mut lifetimes = FxHashMap();
+                for lt_def in &generics.lifetimes {
+                    let (lt_name, region) = Region::early(&self.tcx.hir, &mut index, &lt_def);
+                    if let hir::LifetimeName::Underscore = lt_name {
+                        // Pick the elided lifetime "definition" if one exists and use it to make an
+                        // elision scope.
+                        elision = Some(region);
+                    } else {
+                        lifetimes.insert(lt_name, region);
                     }
-                });
+                }
+
+                let next_early_index = index + generics.ty_params.len() as u32;
+
+                if let Some(elision_region) = elision {
+                    let scope = Scope::Elision {
+                        elide: Elide::Exact(elision_region),
+                        s: self.scope
+                    };
+                    self.with(scope, |_old_scope, this| {
+                        let scope = Scope::Binder { lifetimes, next_early_index, s: this.scope };
+                        this.with(scope, |_old_scope, this| {
+                            this.visit_generics(generics);
+                            for bound in bounds {
+                                this.visit_ty_param_bound(bound);
+                            }
+                        });
+                    });
+                } else {
+                    let scope = Scope::Binder { lifetimes, next_early_index, s: self.scope };
+                    self.with(scope, |_old_scope, this| {
+                        this.visit_generics(generics);
+                        for bound in bounds {
+                            this.visit_ty_param_bound(bound);
+                        }
+                    });
+                }
             }
             _ => intravisit::walk_ty(self, ty),
         }

src/test/run-pass/impl-trait/lifetimes.rs

@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(conservative_impl_trait)]
+#![feature(conservative_impl_trait, underscore_lifetimes, universal_impl_trait)]
 #![allow(warnings)]
 
 use std::fmt::Debug;
@@ -32,12 +32,36 @@ fn no_params_or_lifetimes_is_static() -> impl Debug + 'static {
 fn static_input_type_is_static<T: Debug + 'static>(x: T) -> impl Debug + 'static { x }
 
 fn type_outlives_reference_lifetime<'a, T: Debug>(x: &'a T) -> impl Debug + 'a { x }
+fn type_outlives_reference_lifetime_elided<T: Debug>(x: &T) -> impl Debug + '_ { x }
 
 trait SingleRegionTrait<'a> {}
 impl<'a> SingleRegionTrait<'a> for u32 {}
+impl<'a> SingleRegionTrait<'a> for &'a u32 {}
+struct SingleRegionStruct<'a>(&'a u32);
 
 fn simple_type_hrtb<'b>() -> impl for<'a> SingleRegionTrait<'a> { 5 }
+// FIXME(cramertj) add test after #45992 lands to ensure lint is triggered
+fn elision_single_region_trait(x: &u32) -> impl SingleRegionTrait { x }
+fn elision_single_region_struct(x: SingleRegionStruct) -> impl Into<SingleRegionStruct> { x }
+
 fn closure_hrtb() -> impl for<'a> Fn(&'a u32) { |_| () }
+fn closure_hr_elided() -> impl Fn(&u32) { |_| () }
+fn closure_hr_elided_return() -> impl Fn(&u32) -> &u32 { |x| x }
+fn closure_pass_through_elided_return(x: impl Fn(&u32) -> &u32) -> impl Fn(&u32) -> &u32 { x }
+fn closure_pass_through_reference_elided(x: &impl Fn(&u32) -> &u32) -> &impl Fn(&u32) -> &u32 { x }
+
+fn pass_through_elision(x: &u32) -> impl Into<&u32> { x }
+fn pass_through_elision_with_fn_ptr(x: &fn(&u32) -> &u32) -> impl Into<&fn(&u32) -> &u32> { x }
+
+fn pass_through_elision_with_fn_path<T: Fn(&u32) -> &u32>(
+    x: &T
+) -> impl Into<&impl Fn(&u32) -> &u32> { x }
+
+// FIXME(cramertj) Currently ICEing, part of issue #46685:
+// fn foo(x: &impl Debug) -> impl Into<&impl Debug> { x }
+// Works:
+fn foo_no_outer_impl(x: &impl Debug) -> &impl Debug { x }
+fn foo_explicit_arg<T: Debug>(x: &T) -> impl Into<&impl Debug> { x }
 
 fn mixed_lifetimes<'a>() -> impl for<'b: 'a> Fn(&'b u32) { |_| () }
 fn mixed_as_static() -> impl Fn(&'static u32) { mixed_lifetimes() }