Allow using an undeclared '_ as an anonymous input or inference region.

src/librustc/middle/resolve_lifetime.rs

@@ -37,6 +37,7 @@ use util::nodemap::NodeMap;
 #[derive(Clone, Copy, PartialEq, Eq, Hash, RustcEncodable, RustcDecodable, Debug)]
 pub enum DefRegion {
     DefStaticRegion,
+    DefAnonRegion,
     DefEarlyBoundRegion(/* space */ subst::ParamSpace,
                         /* index */ u32,
                         /* lifetime decl */ ast::NodeId),
@@ -72,6 +73,9 @@ struct LifetimeContext<'a> {
     // I'm sorry.
     trait_ref_hack: bool,
 
+    // Pre-interned "'_", if #[feature(anon_lifetime)] is enabled.
+    anon_lifetime_name: Option<ast::Name>,
+
     // List of labels in the function/method currently under analysis.
     labels_in_fn: Vec<(ast::Ident, Span)>,
 }
@@ -95,12 +99,18 @@ static ROOT_SCOPE: ScopeChain<'static> = RootScope;
 
 pub fn krate(sess: &Session, krate: &ast::Crate, def_map: &DefMap) -> NamedRegionMap {
     let mut named_region_map = NodeMap();
+    let anon_lifetime_name = if sess.features.borrow().anon_lifetime {
+        Some(token::intern("'_"))
+    } else {
+        None
+    };
     visit::walk_crate(&mut LifetimeContext {
         sess: sess,
         named_region_map: &mut named_region_map,
         scope: &ROOT_SCOPE,
         def_map: def_map,
         trait_ref_hack: false,
+        anon_lifetime_name: anon_lifetime_name,
         labels_in_fn: vec![],
     }, krate);
     sess.abort_if_errors();
@@ -224,11 +234,28 @@ impl<'a, 'v> Visitor<'v> for LifetimeContext<'a> {
     }
 
     fn visit_lifetime_ref(&mut self, lifetime_ref: &ast::Lifetime) {
-        if lifetime_ref.name == special_idents::static_lifetime.name {
-            self.insert_lifetime(lifetime_ref, DefStaticRegion);
-            return;
+        if lifetime_ref.id == ast::DUMMY_NODE_ID {
+            self.sess.span_bug(lifetime_ref.span,
+                               "lifetime reference not renumbered, \
+                                probably a bug in syntax::fold");
         }
-        self.resolve_lifetime_ref(lifetime_ref);
+
+        let def = if lifetime_ref.name == special_idents::static_lifetime.name {
+            DefStaticRegion
+        } else if let Ok(def) = self.resolve_lifetime_ref(lifetime_ref) {
+            def
+        } else if Some(lifetime_ref.name) == self.anon_lifetime_name {
+            DefAnonRegion
+        } else {
+            self.unresolved_lifetime_ref(lifetime_ref);
+            return;
+        };
+
+        debug!("lifetime_ref={:?} id={:?} resolved to {:?}",
+                lifetime_to_string(lifetime_ref),
+                lifetime_ref.id,
+                def);
+        self.named_region_map.insert(lifetime_ref.id, def);
     }
 
     fn visit_generics(&mut self, generics: &ast::Generics) {
@@ -478,6 +505,7 @@ impl<'a> LifetimeContext<'a> {
             scope: &wrap_scope,
             def_map: self.def_map,
             trait_ref_hack: self.trait_ref_hack,
+            anon_lifetime_name: self.anon_lifetime_name,
             labels_in_fn: self.labels_in_fn.clone(),
         };
         debug!("entering scope {:?}", this.scope);
@@ -525,7 +553,8 @@ impl<'a> LifetimeContext<'a> {
         });
     }
 
-    fn resolve_lifetime_ref(&mut self, lifetime_ref: &ast::Lifetime) {
+    fn resolve_lifetime_ref(&mut self, lifetime_ref: &ast::Lifetime)
+                            -> Result<DefRegion, ()> {
         // Walk up the scope chain, tracking the number of fn scopes
         // that we pass through, until we find a lifetime with the
         // given name or we run out of scopes. If we encounter a code
@@ -548,9 +577,7 @@ impl<'a> LifetimeContext<'a> {
                     match search_lifetimes(lifetimes, lifetime_ref) {
                         Some((index, lifetime_def)) => {
                             let decl_id = lifetime_def.id;
-                            let def = DefEarlyBoundRegion(space, index, decl_id);
-                            self.insert_lifetime(lifetime_ref, def);
-                            return;
+                            return Ok(DefEarlyBoundRegion(space, index, decl_id));
                         }
                         None => {
                             scope = s;
@@ -563,9 +590,7 @@ impl<'a> LifetimeContext<'a> {
                         Some((_index, lifetime_def)) => {
                             let decl_id = lifetime_def.id;
                             let debruijn = ty::DebruijnIndex::new(late_depth + 1);
-                            let def = DefLateBoundRegion(debruijn, decl_id);
-                            self.insert_lifetime(lifetime_ref, def);
-                            return;
+                            return Ok(DefLateBoundRegion(debruijn, decl_id));
                         }
 
                         None => {
@@ -577,13 +602,13 @@ impl<'a> LifetimeContext<'a> {
             }
         }
 
-        self.unresolved_lifetime_ref(lifetime_ref);
+        Err(())
     }
 
     fn resolve_free_lifetime_ref(&mut self,
                                  scope_data: region::DestructionScopeData,
                                  lifetime_ref: &ast::Lifetime,
-                                 scope: Scope) {
+                                 scope: Scope) -> Result<DefRegion, ()> {
         debug!("resolve_free_lifetime_ref \
                 scope_data: {:?} lifetime_ref: {:?} scope: {:?}",
                scope_data, lifetime_ref, scope);
@@ -621,13 +646,10 @@ impl<'a> LifetimeContext<'a> {
 
         match search_result {
             Some((_depth, lifetime)) => {
-                let def = DefFreeRegion(scope_data, lifetime.id);
-                self.insert_lifetime(lifetime_ref, def);
+                Ok(DefFreeRegion(scope_data, lifetime.id))
             }
 
-            None => {
-                self.unresolved_lifetime_ref(lifetime_ref);
-            }
+            None => Err(())
         }
 
     }
@@ -667,7 +689,7 @@ impl<'a> LifetimeContext<'a> {
             self.check_lifetime_def_for_shadowing(old_scope, &lifetime_i.lifetime);
 
             for bound in &lifetime_i.bounds {
-                self.resolve_lifetime_ref(bound);
+                self.visit_lifetime_ref(bound);
             }
         }
     }
@@ -713,22 +735,6 @@ impl<'a> LifetimeContext<'a> {
             }
         }
     }
-
-    fn insert_lifetime(&mut self,
-                       lifetime_ref: &ast::Lifetime,
-                       def: DefRegion) {
-        if lifetime_ref.id == ast::DUMMY_NODE_ID {
-            self.sess.span_bug(lifetime_ref.span,
-                               "lifetime reference not renumbered, \
-                               probably a bug in syntax::fold");
-        }
-
-        debug!("lifetime_ref={:?} id={:?} resolved to {:?}",
-                lifetime_to_string(lifetime_ref),
-                lifetime_ref.id,
-                def);
-        self.named_region_map.insert(lifetime_ref.id, def);
-    }
 }
 
 fn search_lifetimes<'a>(lifetimes: &'a Vec<ast::LifetimeDef>,

src/librustc_typeck/astconv.rs

@@ -145,7 +145,9 @@ pub trait AstConv<'tcx> {
                     -> Ty<'tcx>;
 }
 
-pub fn ast_region_to_region(tcx: &ty::ctxt, lifetime: &ast::Lifetime)
+pub fn ast_region_to_region(tcx: &ty::ctxt,
+                            rscope: &RegionScope,
+                            lifetime: &ast::Lifetime)
                             -> ty::Region {
     let r = match tcx.named_region_map.get(&lifetime.id) {
         None => {
@@ -157,6 +159,10 @@ pub fn ast_region_to_region(tcx: &ty::ctxt, lifetime: &ast::Lifetime)
             ty::ReStatic
         }
 
+        Some(&rl::DefAnonRegion) => {
+            opt_ast_region_to_region(tcx, rscope, lifetime.span, &None)
+        }
+
         Some(&rl::DefLateBoundRegion(debruijn, id)) => {
             ty::ReLateBound(debruijn, ty::BrNamed(ast_util::local_def(id), lifetime.name))
         }
@@ -239,24 +245,23 @@ fn report_elision_failure(
     }
 }
 
-pub fn opt_ast_region_to_region<'tcx>(
-    this: &AstConv<'tcx>,
-    rscope: &RegionScope,
-    default_span: Span,
-    opt_lifetime: &Option<ast::Lifetime>) -> ty::Region
-{
+pub fn opt_ast_region_to_region(tcx: &ty::ctxt,
+                                rscope: &RegionScope,
+                                default_span: Span,
+                                opt_lifetime: &Option<ast::Lifetime>)
+                                -> ty::Region {
     let r = match *opt_lifetime {
         Some(ref lifetime) => {
-            ast_region_to_region(this.tcx(), lifetime)
+            ast_region_to_region(tcx, rscope, lifetime)
         }
 
         None => match rscope.anon_regions(default_span, 1) {
             Ok(rs) => rs[0],
             Err(params) => {
-                span_err!(this.tcx().sess, default_span, E0106,
+                span_err!(tcx.sess, default_span, E0106,
                           "missing lifetime specifier");
                 if let Some(params) = params {
-                    report_elision_failure(this.tcx(), default_span, params);
+                    report_elision_failure(tcx, default_span, params);
                 }
                 ty::ReStatic
             }
@@ -486,7 +491,7 @@ fn convert_angle_bracketed_parameters<'tcx>(this: &AstConv<'tcx>,
 {
     let regions: Vec<_> =
         data.lifetimes.iter()
-                      .map(|l| ast_region_to_region(this.tcx(), l))
+                      .map(|l| ast_region_to_region(this.tcx(), rscope, l))
                       .collect();
 
     let region_substs =
@@ -1544,7 +1549,7 @@ pub fn ast_ty_to_ty<'tcx>(this: &AstConv<'tcx>,
             })
         }
         ast::TyRptr(ref region, ref mt) => {
-            let r = opt_ast_region_to_region(this, rscope, ast_ty.span, region);
+            let r = opt_ast_region_to_region(tcx, rscope, ast_ty.span, region);
             debug!("TyRef r={:?}", r);
             let rscope1 =
                 &ObjectLifetimeDefaultRscope::new(
@@ -1813,7 +1818,7 @@ fn determine_explicit_self_category<'a, 'tcx>(this: &AstConv<'tcx>,
         ast::SelfValue(_) => ty::ByValueExplicitSelfCategory,
         ast::SelfRegion(ref lifetime, mutability, _) => {
             let region =
-                opt_ast_region_to_region(this,
+                opt_ast_region_to_region(this.tcx(),
                                          rscope,
                                          self_info.explicit_self.span,
                                          lifetime);
@@ -2061,7 +2066,7 @@ fn compute_object_lifetime_bound<'tcx>(
     if !explicit_region_bounds.is_empty() {
         // Explicitly specified region bound. Use that.
         let r = explicit_region_bounds[0];
-        return ast_region_to_region(tcx, r);
+        return ast_region_to_region(tcx, &ExplicitRscope, r);
     }
 
     if let Err(ErrorReported) = this.ensure_super_predicates(span,principal_trait_ref.def_id()) {

src/librustc_typeck/check/mod.rs

@@ -4598,7 +4598,7 @@ pub fn instantiate_path<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
             let region_count = region_defs.len(space);
             assert_eq!(substs.regions().len(space), 0);
             for (i, lifetime) in data.lifetimes.iter().enumerate() {
-                let r = ast_region_to_region(fcx.tcx(), lifetime);
+                let r = ast_region_to_region(fcx.tcx(), fcx, lifetime);
                 if i < region_count {
                     substs.mut_regions().push(space, r);
                 } else if i == region_count {

src/librustc_typeck/collect.rs

@@ -1763,7 +1763,7 @@ fn ty_generic_predicates<'a,'tcx>(ccx: &CrateCtxt<'a,'tcx>,
                 name: param.lifetime.name
             });
         for bound in &param.bounds {
-            let bound_region = ast_region_to_region(ccx.tcx, bound);
+            let bound_region = ast_region_to_region(tcx, &ExplicitRscope, bound);
             let outlives = ty::Binder(ty::OutlivesPredicate(region, bound_region));
             result.predicates.push(space, outlives.to_predicate());
         }
@@ -1797,7 +1797,7 @@ fn ty_generic_predicates<'a,'tcx>(ccx: &CrateCtxt<'a,'tcx>,
                         }
 
                         &ast::TyParamBound::RegionTyParamBound(ref lifetime) => {
-                            let region = ast_region_to_region(tcx, lifetime);
+                            let region = ast_region_to_region(tcx, &ExplicitRscope, lifetime);
                             let pred = ty::Binder(ty::OutlivesPredicate(ty, region));
                             result.predicates.push(space, ty::Predicate::TypeOutlives(pred))
                         }
@@ -1806,9 +1806,9 @@ fn ty_generic_predicates<'a,'tcx>(ccx: &CrateCtxt<'a,'tcx>,
             }
 
             &ast::WherePredicate::RegionPredicate(ref region_pred) => {
-                let r1 = ast_region_to_region(tcx, &region_pred.lifetime);
+                let r1 = ast_region_to_region(tcx, &ExplicitRscope, &region_pred.lifetime);
                 for bound in &region_pred.bounds {
-                    let r2 = ast_region_to_region(tcx, bound);
+                    let r2 = ast_region_to_region(tcx, &ExplicitRscope, bound);
                     let pred = ty::Binder(ty::OutlivesPredicate(r1, r2));
                     result.predicates.push(space, ty::Predicate::RegionOutlives(pred))
                 }
@@ -1838,7 +1838,7 @@ fn ty_generics<'a,'tcx>(ccx: &CrateCtxt<'a,'tcx>,
     let early_lifetimes = early_bound_lifetimes_from_generics(space, ast_generics);
     for (i, l) in early_lifetimes.iter().enumerate() {
         let bounds = l.bounds.iter()
-                             .map(|l| ast_region_to_region(tcx, l))
+                             .map(|l| ast_region_to_region(tcx, &ExplicitRscope, l))
                              .collect();
         let def = ty::RegionParameterDef { name: l.lifetime.name,
                                            space: space,
@@ -1954,7 +1954,7 @@ fn compute_object_lifetime_default<'a,'tcx>(ccx: &CrateCtxt<'a,'tcx>,
                       ast::TraitTyParamBound(..) =>
                           None,
                       ast::RegionTyParamBound(ref lifetime) =>
-                          Some(astconv::ast_region_to_region(ccx.tcx, lifetime)),
+                          Some(ast_region_to_region(ccx.tcx, &ExplicitRscope, lifetime)),
                   }
               })
               .collect()
@@ -2037,7 +2037,7 @@ fn predicates_from_bound<'tcx>(astconv: &AstConv<'tcx>,
                        .collect()
         }
         ast::RegionTyParamBound(ref lifetime) => {
-            let region = ast_region_to_region(astconv.tcx(), lifetime);
+            let region = ast_region_to_region(astconv.tcx(), &ExplicitRscope, lifetime);
             let pred = ty::Binder(ty::OutlivesPredicate(param_ty, region));
             vec![ty::Predicate::TypeOutlives(pred)]
         }
@@ -2085,7 +2085,7 @@ fn conv_param_bounds<'a,'tcx>(astconv: &AstConv<'tcx>,
 
     let region_bounds: Vec<ty::Region> =
         region_bounds.into_iter()
-                     .map(|r| ast_region_to_region(tcx, r))
+                     .map(|r| ast_region_to_region(tcx, &ExplicitRscope, r))
                      .collect();
 
     astconv::Bounds {

src/libsyntax/feature_gate.rs

@@ -155,6 +155,9 @@ const KNOWN_FEATURES: &'static [(&'static str, &'static str, Status)] = &[
 
     // Allows the definition of `const fn` functions.
     ("const_fn", "1.2.0", Active),
+
+    // Allows the usage of `'_` for function arguments and inference.
+    ("anon_lifetime", "1.2.0", Active),
 ];
 // (changing above list without updating src/doc/reference.md makes @cmr sad)
 
@@ -329,6 +332,7 @@ pub struct Features {
     /// #![feature] attrs for non-language (library) features
     pub declared_lib_features: Vec<(InternedString, Span)>,
     pub const_fn: bool,
+    pub anon_lifetime: bool,
 }
 
 impl Features {
@@ -350,6 +354,7 @@ impl Features {
             declared_stable_lang_features: Vec::new(),
             declared_lib_features: Vec::new(),
             const_fn: false,
+            anon_lifetime: false,
         }
     }
 }
@@ -789,6 +794,7 @@ fn check_crate_inner<F>(cm: &CodeMap, span_handler: &SpanHandler,
         declared_stable_lang_features: accepted_features,
         declared_lib_features: unknown_features,
         const_fn: cx.has_feature("const_fn"),
+        anon_lifetime: cx.has_feature("anon_lifetime"),
     }
 }
 

src/test/compile-fail/feature-gate-anon-lifetime.rs

@@ -0,0 +1,13 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+fn main() {
+    let _: &'_ () = &(); //~ ERROR use of undeclared lifetime name `'_`
+}