diagnostics: Differentiate between edition meanings of ::foo in resolve diagnostics for ::foo::Bar

compiler/rustc_resolve/src/late/diagnostics.rs

@@ -16,6 +16,7 @@
 use rustc_hir::def_id::{DefId, CRATE_DEF_INDEX, LOCAL_CRATE};
 use rustc_hir::PrimTy;
 use rustc_session::parse::feature_err;
+use rustc_span::edition::Edition;
 use rustc_span::hygiene::MacroKind;
 use rustc_span::lev_distance::find_best_match_for_name;
 use rustc_span::symbol::{kw, sym, Ident, Symbol};
@@ -133,7 +134,7 @@ pub(crate) fn smart_resolve_report_errors(
         let is_enum_variant = &|res| matches!(res, Res::Def(DefKind::Variant, _));
 
         // Make the base error.
-        let expected = source.descr_expected();
+        let mut expected = source.descr_expected();
         let path_str = Segment::names_to_string(path);
         let item_str = path.last().unwrap().ident;
         let (base_msg, fallback_label, base_span, could_be_expr) = if let Some(res) = res {
@@ -166,6 +167,15 @@ pub(crate) fn smart_resolve_report_errors(
             let (mod_prefix, mod_str) = if path.len() == 1 {
                 (String::new(), "this scope".to_string())
             } else if path.len() == 2 && path[0].ident.name == kw::PathRoot {
+                if self.r.session.edition() > Edition::Edition2015 {
+                    // In edition 2018 onwards, the `::foo` syntax may only pull from the extern prelude
+                    // which overrides all other expectations of item type
+                    expected = "crate";
+                    (String::new(), "the list of imported crates".to_string())
+                } else {
+                    (String::new(), "the crate root".to_string())
+                }
+            } else if path.len() == 2 && path[0].ident.name == kw::Crate {
                 (String::new(), "the crate root".to_string())
             } else {
                 let mod_path = &path[..path.len() - 1];

src/test/ui/editions-crate-root-2015.rs

@@ -7,6 +7,13 @@ fn global_inner(_: ::nonexistant::Foo) {
     fn crate_inner(_: crate::nonexistant::Foo) {
         //~^ ERROR failed to resolve: maybe a missing crate `nonexistant`?
     }
+
+    fn bare_global(_: ::nonexistant) {
+        //~^ ERROR cannot find type `nonexistant` in the crate root
+    }
+    fn bare_crate(_: crate::nonexistant) {
+        //~^ ERROR cannot find type `nonexistant` in the crate root
+    }
 }
 
 fn main() {

src/test/ui/editions-crate-root-2015.stderr

@@ -5,11 +5,24 @@ LL |     fn global_inner(_: ::nonexistant::Foo) {
    |                          ^^^^^^^^^^^ maybe a missing crate `nonexistant`?
 
 error[E0433]: failed to resolve: maybe a missing crate `nonexistant`?
-  --> $DIR/editions-crate-root-2015.rs:8:30
+  --> $DIR/editions-crate-root-2015.rs:7:30
    |
 LL |     fn crate_inner(_: crate::nonexistant::Foo) {
    |                              ^^^^^^^^^^^ maybe a missing crate `nonexistant`?
 
-error: aborting due to 2 previous errors
+error[E0412]: cannot find type `nonexistant` in the crate root
+  --> $DIR/editions-crate-root-2015.rs:11:25
+   |
+LL |     fn bare_global(_: ::nonexistant) {
+   |                         ^^^^^^^^^^^ not found in the crate root
 
-For more information about this error, try `rustc --explain E0433`.
+error[E0412]: cannot find type `nonexistant` in the crate root
+  --> $DIR/editions-crate-root-2015.rs:14:29
+   |
+LL |     fn bare_crate(_: crate::nonexistant) {
+   |                             ^^^^^^^^^^^ not found in the crate root
+
+error: aborting due to 4 previous errors
+
+Some errors have detailed explanations: E0412, E0433.
+For more information about an error, try `rustc --explain E0412`.

src/test/ui/editions-crate-root-2018.rs

@@ -7,6 +7,13 @@ fn global_inner(_: ::nonexistant::Foo) {
     fn crate_inner(_: crate::nonexistant::Foo) {
         //~^ ERROR failed to resolve: maybe a missing crate `nonexistant`?
     }
+
+    fn bare_global(_: ::nonexistant) {
+        //~^ ERROR cannot find crate `nonexistant` in the list of imported crates
+    }
+    fn bare_crate(_: crate::nonexistant) {
+        //~^ ERROR cannot find type `nonexistant` in the crate root
+    }
 }
 
 fn main() {

src/test/ui/editions-crate-root-2018.stderr

@@ -10,6 +10,19 @@ error[E0433]: failed to resolve: maybe a missing crate `nonexistant`?
 LL |     fn crate_inner(_: crate::nonexistant::Foo) {
    |                              ^^^^^^^^^^^ maybe a missing crate `nonexistant`?
 
-error: aborting due to 2 previous errors
+error[E0412]: cannot find crate `nonexistant` in the list of imported crates
+  --> $DIR/editions-crate-root-2018.rs:11:25
+   |
+LL |     fn bare_global(_: ::nonexistant) {
+   |                         ^^^^^^^^^^^ not found in the list of imported crates
 
-For more information about this error, try `rustc --explain E0433`.
+error[E0412]: cannot find type `nonexistant` in the crate root
+  --> $DIR/editions-crate-root-2018.rs:14:29
+   |
+LL |     fn bare_crate(_: crate::nonexistant) {
+   |                             ^^^^^^^^^^^ not found in the crate root
+
+error: aborting due to 4 previous errors
+
+Some errors have detailed explanations: E0412, E0433.
+For more information about an error, try `rustc --explain E0412`.

src/test/ui/resolve/raw-ident-in-path.rs

@@ -1,5 +1,5 @@
 // Regression test for issue #63882.
 
-type A = crate::r#break; //~ ERROR cannot find type `r#break` in module `crate`
+type A = crate::r#break; //~ ERROR cannot find type `r#break` in the crate root
 
 fn main() {}

src/test/ui/resolve/raw-ident-in-path.stderr

@@ -1,8 +1,8 @@
-error[E0412]: cannot find type `r#break` in module `crate`
+error[E0412]: cannot find type `r#break` in the crate root
   --> $DIR/raw-ident-in-path.rs:3:17
    |
 LL | type A = crate::r#break;
-   |                 ^^^^^^^ not found in `crate`
+   |                 ^^^^^^^ not found in the crate root
 
 error: aborting due to previous error