diagnostics: Be clear about "crate root" and ::foo paths in resolve diagnostics

compiler/rustc_resolve/src/late/diagnostics.rs

@@ -16,6 +16,7 @@ use rustc_hir::def::{self, CtorKind, CtorOf, DefKind};
 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 @@ impl<'a: 'ast, 'ast> LateResolutionVisitor<'a, '_, 'ast> {
         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 @@ impl<'a: 'ast, 'ast> LateResolutionVisitor<'a, '_, 'ast> {
             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];

compiler/rustc_resolve/src/lib.rs

@@ -2433,8 +2433,10 @@ impl<'a> Resolver<'a> {
                                     Applicability::MaybeIncorrect,
                                 )),
                             )
-                        } else {
+                        } else if self.session.edition() == Edition::Edition2015 {
                             (format!("maybe a missing crate `{}`?", ident), None)
+                        } else {
+                            (format!("could not find `{}` in the crate root", ident), None)
                         }
                     } else if i == 0 {
                         if ident
@@ -2450,10 +2452,16 @@ 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 +1,21 @@
+// 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 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

@@ -0,0 +1,28 @@
+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:7:30
+   |
+LL |     fn crate_inner(_: crate::nonexistant::Foo) {
+   |                              ^^^^^^^^^^^ maybe a missing crate `nonexistant`?
+
+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
+
+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

@@ -0,0 +1,21 @@
+// 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: could not find `nonexistant` in the crate root
+    }
+
+    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

@@ -0,0 +1,28 @@
+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: could not find `nonexistant` in the crate root
+  --> $DIR/editions-crate-root-2018.rs:7:30
+   |
+LL |     fn crate_inner(_: crate::nonexistant::Foo) {
+   |                              ^^^^^^^^^^^ could not find `nonexistant` in the crate root
+
+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
+
+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/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/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
 

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