resolve: Reintroduce feature gate for uniform paths in imports

Author: petrochenkov

src/doc/unstable-book/src/language-features/extern-in-paths.md

@@ -11,8 +11,8 @@ introducing `extern crate` items, using keyword `extern`.
 
 For example, `extern::my_crat::a::b` will resolve to path `a::b` in crate `my_crate`.
 
-`feature(extern_absolute_paths)` mode provides the same effect by resolving absolute paths like
-`::my_crate::a::b` to paths from extern crates by default.
+Absolute paths on 2018 edition (e.g. `::my_crate::a::b`) provide the same effect
+and resolve to extern crates (built-in or passed with `--extern`).
 
 ```rust,ignore
 #![feature(extern_in_paths)]

src/librustc_resolve/lib.rs

@@ -4670,6 +4670,34 @@ impl<'a, 'crateloader: 'a> Resolver<'a, 'crateloader> {
         }
     }
 
+    fn binding_description(&self, b: &NameBinding, ident: Ident, from_prelude: bool) -> String {
+        if b.span.is_dummy() {
+            let add_built_in = match b.def() {
+                // These already contain the "built-in" prefix or look bad with it.
+                Def::NonMacroAttr(..) | Def::PrimTy(..) | Def::ToolMod => false,
+                _ => true,
+            };
+            let (built_in, from) = if from_prelude {
+                ("", " from prelude")
+            } else if b.is_extern_crate() && !b.is_import() &&
+                        self.session.opts.externs.get(&ident.as_str()).is_some() {
+                ("", " passed with `--extern`")
+            } else if add_built_in {
+                (" built-in", "")
+            } else {
+                ("", "")
+            };
+
+            let article = if built_in.is_empty() { b.article() } else { "a" };
+            format!("{a}{built_in} {thing}{from}",
+                    a = article, thing = b.descr(), built_in = built_in, from = from)
+        } else {
+            let introduced = if b.is_import() { "imported" } else { "defined" };
+            format!("the {thing} {introduced} here",
+                    thing = b.descr(), introduced = introduced)
+        }
+    }
+
     fn report_ambiguity_error(&self, ambiguity_error: &AmbiguityError) {
         let AmbiguityError { kind, ident, b1, b2, misc1, misc2 } = *ambiguity_error;
         let (b1, b2, misc1, misc2, swapped) = if b2.span.is_dummy() && !b1.span.is_dummy() {
@@ -4685,31 +4713,7 @@ impl<'a, 'crateloader: 'a> Resolver<'a, 'crateloader> {
         err.span_label(ident.span, "ambiguous name");
 
         let mut could_refer_to = |b: &NameBinding, misc: AmbiguityErrorMisc, also: &str| {
-            let what = if b.span.is_dummy() {
-                let add_built_in = match b.def() {
-                    // These already contain the "built-in" prefix or look bad with it.
-                    Def::NonMacroAttr(..) | Def::PrimTy(..) | Def::ToolMod => false,
-                    _ => true,
-                };
-                let (built_in, from) = if misc == AmbiguityErrorMisc::FromPrelude {
-                    ("", " from prelude")
-                } else if b.is_extern_crate() && !b.is_import() &&
-                          self.session.opts.externs.get(&ident.as_str()).is_some() {
-                    ("", " passed with `--extern`")
-                } else if add_built_in {
-                    (" built-in", "")
-                } else {
-                    ("", "")
-                };
-
-                let article = if built_in.is_empty() { b.article() } else { "a" };
-                format!("{a}{built_in} {thing}{from}",
-                        a = article, thing = b.descr(), built_in = built_in, from = from)
-            } else {
-                let participle = if b.is_import() { "imported" } else { "defined" };
-                format!("the {thing} {introduced} here",
-                        thing = b.descr(), introduced = participle)
-            };
+            let what = self.binding_description(b, ident, misc == AmbiguityErrorMisc::FromPrelude);
             let note_msg = format!("`{ident}` could{also} refer to {what}",
                                    ident = ident, also = also, what = what);
 

src/librustc_resolve/macros.rs

@@ -890,7 +890,33 @@ impl<'a, 'cl> Resolver<'a, 'cl> {
         }
 
         // The first found solution was the only one, return it.
-        if let Some((binding, ..)) = innermost_result {
+        if let Some((binding, flags)) = innermost_result {
+            if is_import && !self.session.features_untracked().uniform_paths {
+                // We get to here only if there's no ambiguity, in ambiguous cases an error will
+                // be reported anyway, so there's no reason to report an additional feature error.
+                // The `binding` can actually be introduced by something other than `--extern`,
+                // but its `Def` should coincide with a crate passed with `--extern`
+                // (otherwise there would be ambiguity) and we can skip feature error in this case.
+                if ns != TypeNS || !use_prelude ||
+                   self.extern_prelude_get(ident, true, false).is_none() {
+                    let msg = "imports can only refer to extern crate names \
+                               passed with `--extern` on stable channel";
+                    let mut err = feature_err(&self.session.parse_sess, "uniform_paths",
+                                              ident.span, GateIssue::Language, msg);
+
+                    let what = self.binding_description(binding, ident,
+                                                        flags.contains(Flags::MISC_FROM_PRELUDE));
+                    let note_msg = format!("this import refers to {what}", what = what);
+                    if binding.span.is_dummy() {
+                        err.note(&note_msg);
+                    } else {
+                        err.span_note(binding.span, &note_msg);
+                        err.span_label(binding.span, "not an extern crate passed with `--extern`");
+                    }
+                    err.emit();
+                }
+            }
+
             return Ok(binding);
         }
 

src/test/ui/feature-gates/feature-gate-uniform-paths.rs

@@ -8,15 +8,22 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// edition:2018
+
 pub mod foo {
-    pub use bar::Bar;
-    //~^ ERROR unresolved import `bar`
+    pub use bar::Bar; //~ ERROR imports can only refer to extern crate names
 
     pub mod bar {
         pub struct Bar;
     }
 }
 
+use inline; //~ ERROR imports can only refer to extern crate names
+
+use Vec; //~ ERROR imports can only refer to extern crate names
+
+use vec; //~ ERROR imports can only refer to extern crate names
+
 fn main() {
     let _ = foo::Bar;
 }

src/test/ui/feature-gates/feature-gate-uniform-paths.stderr

@@ -1,9 +1,62 @@
-error[E0432]: unresolved import `bar`
-  --> $DIR/feature-gate-uniform-paths.rs:12:13
+error[E0658]: imports can only refer to extern crate names passed with `--extern` on stable channel (see issue #53130)
+  --> $DIR/feature-gate-uniform-paths.rs:14:13
    |
-LL |     pub use bar::Bar;
-   |             ^^^ Did you mean `self::bar`?
+LL |       pub use bar::Bar; //~ ERROR imports can only refer to extern crate names
+   |               ^^^
+LL | 
+LL | /     pub mod bar {
+LL | |         pub struct Bar;
+LL | |     }
+   | |_____- not an extern crate passed with `--extern`
+   |
+   = help: add #![feature(uniform_paths)] to the crate attributes to enable
+note: this import refers to the module defined here
+  --> $DIR/feature-gate-uniform-paths.rs:16:5
+   |
+LL | /     pub mod bar {
+LL | |         pub struct Bar;
+LL | |     }
+   | |_____^
+
+error[E0658]: imports can only refer to extern crate names passed with `--extern` on stable channel (see issue #53130)
+  --> $DIR/feature-gate-uniform-paths.rs:21:5
+   |
+LL | use inline; //~ ERROR imports can only refer to extern crate names
+   |     ^^^^^^ not an extern crate passed with `--extern`
+   |
+   = help: add #![feature(uniform_paths)] to the crate attributes to enable
+note: this import refers to the built-in attribute imported here
+  --> $DIR/feature-gate-uniform-paths.rs:21:5
+   |
+LL | use inline; //~ ERROR imports can only refer to extern crate names
+   |     ^^^^^^
+
+error[E0658]: imports can only refer to extern crate names passed with `--extern` on stable channel (see issue #53130)
+  --> $DIR/feature-gate-uniform-paths.rs:23:5
+   |
+LL | use Vec; //~ ERROR imports can only refer to extern crate names
+   |     ^^^ not an extern crate passed with `--extern`
+   |
+   = help: add #![feature(uniform_paths)] to the crate attributes to enable
+note: this import refers to the struct imported here
+  --> $DIR/feature-gate-uniform-paths.rs:23:5
+   |
+LL | use Vec; //~ ERROR imports can only refer to extern crate names
+   |     ^^^
+
+error[E0658]: imports can only refer to extern crate names passed with `--extern` on stable channel (see issue #53130)
+  --> $DIR/feature-gate-uniform-paths.rs:25:5
+   |
+LL | use vec; //~ ERROR imports can only refer to extern crate names
+   |     ^^^ not an extern crate passed with `--extern`
+   |
+   = help: add #![feature(uniform_paths)] to the crate attributes to enable
+note: this import refers to the macro imported here
+  --> $DIR/feature-gate-uniform-paths.rs:25:5
+   |
+LL | use vec; //~ ERROR imports can only refer to extern crate names
+   |     ^^^
 
-error: aborting due to previous error
+error: aborting due to 4 previous errors
 
-For more information about this error, try `rustc --explain E0432`.
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/rfc-2126-extern-absolute-paths/not-whitelisted.rs

@@ -10,6 +10,8 @@
 
 // edition:2018
 
+#![feature(uniform_paths)]
+
 // Tests that arbitrary crates (other than `core`, `std` and `meta`)
 // aren't allowed without `--extern`, even if they're in the sysroot.
 use alloc; //~ ERROR unresolved import `alloc`

src/test/ui/rfc-2126-extern-absolute-paths/not-whitelisted.stderr

@@ -1,11 +1,11 @@
 error: cannot import a built-in macro
-  --> $DIR/not-whitelisted.rs:16:5
+  --> $DIR/not-whitelisted.rs:18:5
    |
 LL | use test; //~ ERROR cannot import a built-in macro
    |     ^^^^
 
 error[E0432]: unresolved import `alloc`
-  --> $DIR/not-whitelisted.rs:15:5
+  --> $DIR/not-whitelisted.rs:17:5
    |
 LL | use alloc; //~ ERROR unresolved import `alloc`
    |     ^^^^^ no `alloc` external crate

src/test/ui/rust-2018/local-path-suggestions-2018.rs

@@ -12,6 +12,8 @@
 // compile-flags:--extern baz
 // edition:2018
 
+#![feature(uniform_paths)]
+
 mod foo {
     pub type Bar = u32;
 }

src/test/ui/rust-2018/local-path-suggestions-2018.stderr

@@ -1,13 +1,13 @@
 error[E0432]: unresolved import `foo`
-  --> $DIR/local-path-suggestions-2018.rs:20:9
+  --> $DIR/local-path-suggestions-2018.rs:22:9
    |
 LL |     use foo::Bar;
    |         ^^^ Did you mean `crate::foo`?
    |
    = note: `use` statements changed in Rust 2018; read more at <https://doc.rust-lang.org/edition-guide/rust-2018/module-system/path-clarity.html>
 
 error[E0432]: unresolved import `foobar`
-  --> $DIR/local-path-suggestions-2018.rs:29:5
+  --> $DIR/local-path-suggestions-2018.rs:31:5
    |
 LL | use foobar::Baz;
    |     ^^^^^^ Did you mean `baz::foobar`?

src/test/ui/rust-2018/uniform-paths-forward-compat/issue-54253.rs

@@ -10,7 +10,7 @@
 
 // edition:2018
 
-// Dummy import to introduce `uniform_paths` canaries.
+// Dummy import that previously introduced uniform path canaries.
 use std;
 
 // fn version() -> &'static str {""}