rustc: Fix crate lint for single-item paths

This commit fixes recommending the `crate` prefix when migrating to 2018 for
paths that look like `use foo;` or `use {bar, baz}`

Closes #50660

Author: alexcrichton

src/librustc_resolve/lib.rs

@@ -3232,6 +3232,7 @@ impl<'a> Resolver<'a> {
                     -> PathResult<'a> {
         let mut module = None;
         let mut allow_super = true;
+        let mut second_binding = None;
 
         for (i, &ident) in path.iter().enumerate() {
             debug!("resolve_path ident {} {:?}", i, ident);
@@ -3334,13 +3335,22 @@ impl<'a> Resolver<'a> {
 
             match binding {
                 Ok(binding) => {
+                    if i == 1 {
+                        second_binding = Some(binding);
+                    }
                     let def = binding.def();
                     let maybe_assoc = opt_ns != Some(MacroNS) && PathSource::Type.is_expected(def);
                     if let Some(next_module) = binding.module() {
                         module = Some(next_module);
                     } else if def == Def::Err {
                         return PathResult::NonModule(err_path_resolution());
                     } else if opt_ns.is_some() && (is_last || maybe_assoc) {
+                        self.lint_if_path_starts_with_module(
+                            node_id,
+                            path,
+                            path_span,
+                            second_binding,
+                        );
                         return PathResult::NonModule(PathResolution::with_unresolved_segments(
                             def, path.len() - i - 1
                         ));
@@ -3349,33 +3359,6 @@ impl<'a> Resolver<'a> {
                                                   format!("Not a module `{}`", ident),
                                                   is_last);
                     }
-
-                    if let Some(id) = node_id {
-                        if i == 1 && self.session.features_untracked().crate_in_paths
-                                  && !self.session.rust_2018() {
-                            let prev_name = path[0].name;
-                            if prev_name == keywords::Extern.name() ||
-                               prev_name == keywords::CrateRoot.name() {
-                                let mut is_crate = false;
-                                if let NameBindingKind::Import { directive: d, .. } = binding.kind {
-                                    if let ImportDirectiveSubclass::ExternCrate(..) = d.subclass {
-                                        is_crate = true;
-                                    }
-                                }
-
-                                if !is_crate {
-                                    let diag = lint::builtin::BuiltinLintDiagnostics
-                                                   ::AbsPathWithModule(path_span);
-                                    self.session.buffer_lint_with_diagnostic(
-                                        lint::builtin::ABSOLUTE_PATH_STARTING_WITH_MODULE,
-                                        id, path_span,
-                                        "Absolute paths must start with `self`, `super`, \
-                                        `crate`, or an external crate name in the 2018 edition",
-                                        diag);
-                                }
-                            }
-                        }
-                    }
                 }
                 Err(Undetermined) => return PathResult::Indeterminate,
                 Err(Determined) => {
@@ -3408,9 +3391,67 @@ impl<'a> Resolver<'a> {
             }
         }
 
+        self.lint_if_path_starts_with_module(node_id, path, path_span, second_binding);
+
         PathResult::Module(module.unwrap_or(self.graph_root))
     }
 
+    fn lint_if_path_starts_with_module(&self,
+                                       id: Option<NodeId>,
+                                       path: &[Ident],
+                                       path_span: Span,
+                                       second_binding: Option<&NameBinding>) {
+        // In the 2018 edition this lint is a hard error, so nothing to do
+        if self.session.rust_2018() {
+            return
+        }
+        // In the 2015 edition there's no use in emitting lints unless the
+        // crate's already enabled the feature that we're going to suggest
+        if !self.session.features_untracked().crate_in_paths {
+            return
+        }
+        let id = match id {
+            Some(id) => id,
+            None => return,
+        };
+        let first_name = match path.get(0) {
+            Some(ident) => ident.name,
+            None => return,
+        };
+
+        // We're only interested in `use` paths which should start with
+        // `{{root}}` or `extern` currently.
+        if first_name != keywords::Extern.name() && first_name != keywords::CrateRoot.name() {
+            return
+        }
+
+        if let Some(part) = path.get(1) {
+            if part.name == keywords::Crate.name() {
+                return
+            }
+        }
+
+        // If the first element of our path was actually resolved to an
+        // `ExternCrate` (also used for `crate::...`) then no need to issue a
+        // warning, this looks all good!
+        if let Some(binding) = second_binding {
+            if let NameBindingKind::Import { directive: d, .. } = binding.kind {
+                if let ImportDirectiveSubclass::ExternCrate(..) = d.subclass {
+                    return
+                }
+            }
+        }
+
+        let diag = lint::builtin::BuiltinLintDiagnostics
+            ::AbsPathWithModule(path_span);
+        self.session.buffer_lint_with_diagnostic(
+            lint::builtin::ABSOLUTE_PATH_STARTING_WITH_MODULE,
+            id, path_span,
+            "Absolute paths must start with `self`, `super`, \
+            `crate`, or an external crate name in the 2018 edition",
+            diag);
+    }
+
     // Resolve a local definition, potentially adjusting for closures.
     fn adjust_local_def(&mut self,
                         ns: Namespace,

src/test/ui/edition-lint-paths.rs

@@ -18,6 +18,22 @@ pub mod foo {
     //~| WARN this was previously accepted
     use super::bar::Bar2;
     use crate::bar::Bar3;
+
+    use bar;
+    //~^ ERROR Absolute
+    //~| WARN this was previously accepted
+    use crate::{bar as something_else};
+
+    use {Bar as SomethingElse, main};
+    //~^ ERROR Absolute
+    //~| WARN this was previously accepted
+    //~| ERROR Absolute
+    //~| WARN this was previously accepted
+
+    use crate::{Bar as SomethingElse2, main as another_main};
+
+    pub fn test() {
+    }
 }
 
 
@@ -38,4 +54,5 @@ fn main() {
     let x = bar::Bar;
     let x = ::crate::bar::Bar;
     let x = self::bar::Bar;
+    foo::test();
 }

src/test/ui/edition-lint-paths.stderr

@@ -13,7 +13,34 @@ LL | #![deny(absolute_path_starting_with_module)]
    = note: for more information, see issue TBD
 
 error: Absolute paths must start with `self`, `super`, `crate`, or an external crate name in the 2018 edition
-  --> $DIR/edition-lint-paths.rs:24:5
+  --> $DIR/edition-lint-paths.rs:22:9
+   |
+LL |     use bar;
+   |         ^^^ help: use `crate`: `crate::bar`
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in the 2018 edition!
+   = note: for more information, see issue TBD
+
+error: Absolute paths must start with `self`, `super`, `crate`, or an external crate name in the 2018 edition
+  --> $DIR/edition-lint-paths.rs:27:10
+   |
+LL |     use {Bar as SomethingElse, main};
+   |          ^^^^^^^^^^^^^^^^^^^^ help: use `crate`: `crate::Bar as SomethingElse`
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in the 2018 edition!
+   = note: for more information, see issue TBD
+
+error: Absolute paths must start with `self`, `super`, `crate`, or an external crate name in the 2018 edition
+  --> $DIR/edition-lint-paths.rs:27:32
+   |
+LL |     use {Bar as SomethingElse, main};
+   |                                ^^^^ help: use `crate`: `crate::main`
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in the 2018 edition!
+   = note: for more information, see issue TBD
+
+error: Absolute paths must start with `self`, `super`, `crate`, or an external crate name in the 2018 edition
+  --> $DIR/edition-lint-paths.rs:40:5
    |
 LL | use bar::Bar;
    |     ^^^^^^^^ help: use `crate`: `crate::bar::Bar`
@@ -22,13 +49,13 @@ LL | use bar::Bar;
    = note: for more information, see issue TBD
 
 error: Absolute paths must start with `self`, `super`, `crate`, or an external crate name in the 2018 edition
-  --> $DIR/edition-lint-paths.rs:35:13
+  --> $DIR/edition-lint-paths.rs:51:13
    |
 LL |     let x = ::bar::Bar;
    |             ^^^^^^^^^^ help: use `crate`: `crate::bar::Bar`
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in the 2018 edition!
    = note: for more information, see issue TBD
 
-error: aborting due to 3 previous errors
+error: aborting due to 6 previous errors