Add one more prelude layer for extern crate names passed with --extern

src/librustc_resolve/lib.rs

@@ -1407,6 +1407,7 @@ pub struct Resolver<'a> {
     graph_root: Module<'a>,
 
     prelude: Option<Module<'a>>,
+    extern_prelude: FxHashSet<Name>,
 
     /// n.b. This is used only for better diagnostics, not name resolution itself.
     has_self: FxHashSet<DefId>,
@@ -1715,6 +1716,7 @@ impl<'a> Resolver<'a> {
             // AST.
             graph_root,
             prelude: None,
+            extern_prelude: session.opts.externs.iter().map(|kv| Symbol::intern(kv.0)).collect(),
 
             has_self: FxHashSet(),
             field_names: FxHashMap(),
@@ -1970,13 +1972,32 @@ impl<'a> Resolver<'a> {
             }
         }
 
-        match self.prelude {
-            Some(prelude) if !module.no_implicit_prelude => {
-                self.resolve_ident_in_module_unadjusted(prelude, ident, ns, false, false, path_span)
-                    .ok().map(LexicalScopeBinding::Item)
+        if !module.no_implicit_prelude {
+            // `record_used` means that we don't try to load crates during speculative resolution
+            if record_used && ns == TypeNS && self.extern_prelude.contains(&ident.name) {
+                if !self.session.features_untracked().extern_prelude {
+                    feature_err(&self.session.parse_sess, "extern_prelude",
+                                ident.span, GateIssue::Language,
+                                "access to extern crates through prelude is experimental").emit();
                 }
-            _ => None,
+
+                let crate_id = self.crate_loader.process_path_extern(ident.name, ident.span);
+                let crate_root = self.get_module(DefId { krate: crate_id, index: CRATE_DEF_INDEX });
+                self.populate_module_if_necessary(crate_root);
+
+                let binding = (crate_root, ty::Visibility::Public,
+                               ident.span, Mark::root()).to_name_binding(self.arenas);
+                return Some(LexicalScopeBinding::Item(binding));
             }
+            if let Some(prelude) = self.prelude {
+                if let Ok(binding) = self.resolve_ident_in_module_unadjusted(prelude, ident, ns,
+                                                                        false, false, path_span) {
+                    return Some(LexicalScopeBinding::Item(binding));
+                }
+            }
+        }
+
+        None
     }
 
     fn hygienic_lexical_parent(&mut self, mut module: Module<'a>, span: &mut Span)
@@ -3587,8 +3608,9 @@ impl<'a> Resolver<'a> {
                         // We can see through blocks
                     } else {
                         // Items from the prelude
-                        if let Some(prelude) = self.prelude {
                         if !module.no_implicit_prelude {
+                            names.extend(self.extern_prelude.iter().cloned());
+                            if let Some(prelude) = self.prelude {
                                 add_module_candidates(prelude, &mut names);
                             }
                         }

src/libsyntax/feature_gate.rs

@@ -397,7 +397,7 @@ declare_features! (
     (active, generic_associated_types, "1.23.0", Some(44265), None),
 
     // Resolve absolute paths as paths from other crates
-    (active, extern_absolute_paths, "1.24.0", Some(44660), None),
+    (active, extern_absolute_paths, "1.24.0", Some(44660), Some(Edition::Edition2018)),
 
     // `foo.rs` as an alternative to `foo/mod.rs`
     (active, non_modrs_mods, "1.24.0", Some(44660), Some(Edition::Edition2018)),
@@ -466,6 +466,9 @@ declare_features! (
 
     // #[doc(alias = "...")]
     (active, doc_alias, "1.27.0", Some(50146), None),
+
+    // Access to crate names passed via `--extern` through prelude
+    (active, extern_prelude, "1.27.0", Some(44660), Some(Edition::Edition2018)),
 );
 
 declare_features! (

src/test/run-make-fulldeps/extern-prelude/Makefile

@@ -0,0 +1,12 @@
+-include ../tools.mk
+
+all:
+	$(RUSTC) ep-lib.rs
+	$(RUSTC) ep-vec.rs
+
+	$(RUSTC) basic.rs --extern ep_lib=$(TMPDIR)/libep_lib.rlib
+	$(RUSTC) shadow-mod.rs --extern ep_lib=$(TMPDIR)/libep_lib.rlib
+	$(RUSTC) shadow-prelude.rs --extern Vec=$(TMPDIR)/libep_vec.rlib
+	$(RUSTC) feature-gate.rs --extern ep_lib=$(TMPDIR)/libep_lib.rlib 2>&1 | $(CGREP) "access to extern crates through prelude is experimental"
+	$(RUSTC) relative-only.rs --extern ep_lib=$(TMPDIR)/libep_lib.rlib 2>&1 | $(CGREP) "unresolved import"
+	$(RUSTC) relative-only.rs --extern ep_lib=$(TMPDIR)/libep_lib.rlib 2>&1 | $(CGREP) "failed to resolve"

src/test/run-make-fulldeps/extern-prelude/basic.rs

@@ -0,0 +1,16 @@
+// Copyright 2018 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.
+
+#![feature(extern_prelude)]
+
+fn main() {
+    let s = ep_lib::S; // It works
+    s.external();
+}

src/test/run-make-fulldeps/extern-prelude/ep-lib.rs

@@ -0,0 +1,17 @@
+// Copyright 2017 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.
+
+#![crate_type = "rlib"]
+
+pub struct S;
+
+impl S {
+    pub fn external(&self) {}
+}

src/test/run-make-fulldeps/extern-prelude/ep-vec.rs

@@ -0,0 +1,13 @@
+// Copyright 2017 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.
+
+#![crate_type = "rlib"]
+
+pub fn new(arg1: f32, arg2: ()) {}

src/test/run-make-fulldeps/extern-prelude/feature-gate.rs

@@ -0,0 +1,13 @@
+// Copyright 2018 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 s = ep_lib::S; // Feature error
+}

src/test/run-make-fulldeps/extern-prelude/relative-only.rs

@@ -0,0 +1,19 @@
+// Copyright 2018 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.
+
+// Extern prelude names are not available by absolute paths
+
+#![feature(extern_prelude)]
+
+use ep_lib::S;
+
+fn main() {
+    let s = ::ep_lib::S;
+}

src/test/run-make-fulldeps/extern-prelude/shadow-mod.rs

@@ -0,0 +1,24 @@
+// Copyright 2018 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.
+
+// Local module shadows `ep_lib` from extern prelude
+
+mod ep_lib {
+    pub struct S;
+
+    impl S {
+        pub fn internal(&self) {}
+    }
+}
+
+fn main() {
+    let s = ep_lib::S;
+    s.internal(); // OK
+}

src/test/run-make-fulldeps/extern-prelude/shadow-prelude.rs

@@ -0,0 +1,17 @@
+// Copyright 2018 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.
+
+// Extern prelude shadows standard library prelude
+
+#![feature(extern_prelude)]
+
+fn main() {
+    let x = Vec::new(0f32, ()); // OK
+}

src/test/run-pass/extern-prelude-no-speculative.rs

@@ -0,0 +1,21 @@
+// Copyright 2014 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.
+
+// compile-flags: --extern LooksLikeExternCrate=/path/to/nowhere
+
+mod m {
+    pub struct LooksLikeExternCrate;
+}
+
+fn main() {
+    // OK, speculative resolution for `unused_qualifications` doesn't try
+    // to resolve this as an extern crate and load that crate
+    let s = m::LooksLikeExternCrate {};
+}

src/test/ui/feature-gate-extern_prelude.rs

@@ -0,0 +1,11 @@
+// Copyright 2018 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.
+
+can-only-test-this-in-run-make-fulldeps //~ ERROR expected one of `!` or `::`, found `-`

src/test/ui/feature-gate-extern_prelude.stderr

@@ -0,0 +1,8 @@
+error: expected one of `!` or `::`, found `-`
+  --> $DIR/feature-gate-extern_prelude.rs:11:4
+   |
+LL | can-only-test-this-in-run-make-fulldeps //~ ERROR expected one of `!` or `::`, found `-`
+   |    ^ expected one of `!` or `::` here
+
+error: aborting due to previous error
+