diagnostics: Differentiate between edition meanings of ::foo in resolve diagnostics (for bare `::foo`)

compiler/rustc_resolve/src/lib.rs

@@ -2450,10 +2450,16 @@ impl<'a> Resolver<'a> {
                         }
                     } else {
                         let parent = path[i - 1].ident.name;
-                        let parent = if parent == kw::PathRoot {
-                            "crate root".to_owned()
-                        } else {
-                            format!("`{}`", parent)
+                        let parent = match parent {
+                            // ::foo is mounted at the crate root for 2015, and is the extern
+                            // prelude for 2018+
+                            kw::PathRoot if self.session.edition() > Edition::Edition2015 => {
+                                "the list of imported crates".to_owned()
+                            }
+                            kw::PathRoot | kw::Crate => "the crate root".to_owned(),
+                            _ => {
+                                format!("`{}`", parent)
+                            }
                         };
 
                         let mut msg = format!("could not find `{}` in {}", ident, parent);

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

@@ -0,0 +1,14 @@
+// edition:2015
+
+mod inner {
+    fn global_inner(_: ::nonexistant::Foo) {
+        //~^ ERROR failed to resolve: maybe a missing crate `nonexistant`?
+    }
+    fn crate_inner(_: crate::nonexistant::Foo) {
+        //~^ ERROR failed to resolve: maybe a missing crate `nonexistant`?
+    }
+}
+
+fn main() {
+
+}

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

@@ -0,0 +1,15 @@
+error[E0433]: failed to resolve: maybe a missing crate `nonexistant`?
+  --> $DIR/editions-crate-root-2015.rs:4:26
+   |
+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
+   |
+LL |     fn crate_inner(_: crate::nonexistant::Foo) {
+   |                              ^^^^^^^^^^^ maybe a missing crate `nonexistant`?
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0433`.

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

@@ -0,0 +1,14 @@
+// edition:2018
+
+mod inner {
+    fn global_inner(_: ::nonexistant::Foo) {
+        //~^ ERROR failed to resolve: could not find `nonexistant` in the list of imported crates
+    }
+    fn crate_inner(_: crate::nonexistant::Foo) {
+        //~^ ERROR failed to resolve: maybe a missing crate `nonexistant`?
+    }
+}
+
+fn main() {
+
+}

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

@@ -0,0 +1,15 @@
+error[E0433]: failed to resolve: could not find `nonexistant` in the list of imported crates
+  --> $DIR/editions-crate-root-2018.rs:4:26
+   |
+LL |     fn global_inner(_: ::nonexistant::Foo) {
+   |                          ^^^^^^^^^^^ could not find `nonexistant` in the list of imported crates
+
+error[E0433]: failed to resolve: maybe a missing crate `nonexistant`?
+  --> $DIR/editions-crate-root-2018.rs:7:30
+   |
+LL |     fn crate_inner(_: crate::nonexistant::Foo) {
+   |                              ^^^^^^^^^^^ maybe a missing crate `nonexistant`?
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0433`.

src/test/ui/editions/edition-imports-virtual-2015-gated.stderr

@@ -2,7 +2,7 @@ error[E0432]: unresolved import `E`
   --> $DIR/edition-imports-virtual-2015-gated.rs:8:5
    |
 LL |     gen_gated!();
-   |     ^^^^^^^^^^^^^ could not find `E` in crate root
+   |     ^^^^^^^^^^^^^ could not find `E` in the list of imported crates
    |
    = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
 

src/test/ui/rfc-2126-extern-absolute-paths/non-existent-2.rs

@@ -2,5 +2,5 @@
 
 fn main() {
     let s = ::xcrate::S;
-    //~^ ERROR failed to resolve: could not find `xcrate` in crate root
+    //~^ ERROR failed to resolve: could not find `xcrate` in the list of imported crates
 }

src/test/ui/rfc-2126-extern-absolute-paths/non-existent-2.stderr

@@ -1,8 +1,8 @@
-error[E0433]: failed to resolve: could not find `xcrate` in crate root
+error[E0433]: failed to resolve: could not find `xcrate` in the list of imported crates
   --> $DIR/non-existent-2.rs:4:15
    |
 LL |     let s = ::xcrate::S;
-   |               ^^^^^^ could not find `xcrate` in crate root
+   |               ^^^^^^ could not find `xcrate` in the list of imported crates
 
 error: aborting due to previous error