Auto merge of #49789 - petrochenkov:prelext, r=nikomatsakis

Module experiments: Add one more prelude layer for extern crate names passed with `--extern`

Implements one item from https://internals.rust-lang.org/t/the-great-module-adventure-continues/6678/183

When some name is looked up in lexical scope (`name`, i.e. not module-relative scope `some_mod::name` or `::name`), it's searched roughly in the next order:
- local variables
- items in unnamed blocks
- items in the current module
- ✨ NEW! ✨ crate names passed with `--extern` ("extern prelude")
- standard library prelude (`Vec`, `drop`)
- language prelude (built-in types like `u8`, `str`, etc)

The last two layers contain a limited set of names controlled by us and not arbitrary user-defined names like upper layers. We want to be able to add new names into these two layers without breaking user code, so "extern prelude" names have higher priority than std prelude and built-in types.
This is a one-time breaking change, that's why it would be nice to run this through crater.
Practical impact is expected to be minimal though due to stylistic reasons (there are not many `Uppercase` crates) and due to the way how primitive types are resolved (#32131).

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();
+                }
+
+                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,
         }
+
+        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 {
+                        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

@@ -391,7 +391,7 @@ 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)),
@@ -460,6 +460,9 @@ 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
+