Rollup merge of #83669 - kwj2104:issue-81508-fix, r=varkor

Issue 81508 fix

Fix #81508

**Problem**: When variable name is used incorrectly as path, error and warning point to undeclared/unused name, when in fact the name is used, just incorrectly (should be used as a variable, not part of a path).

**Summary for fix**: When path resolution errs, diagnostics checks for variables in ```ValueNS``` that have the same name (e.g., variable rather than path named Foo), and adds additional suggestion that user may actually intend to use the variable name rather than a path.

The fix does not suppress or otherwise change the *warning* that results. I did not find a straightforward way in the code to modify this, but would love to make changes here as well with any guidance.

compiler/rustc_resolve/src/late.rs

@@ -1031,7 +1031,6 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
             }
 
             ItemKind::Static(ref ty, _, ref expr) | ItemKind::Const(_, ref ty, ref expr) => {
-                debug!("resolve_item ItemKind::Const");
                 self.with_item_rib(HasGenericParams::No, |this| {
                     this.visit_ty(ty);
                     if let Some(expr) = expr {
@@ -1597,6 +1596,7 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
                         .try_resolve_as_non_binding(pat_src, pat, bmode, ident, has_sub)
                         .unwrap_or_else(|| self.fresh_binding(ident, pat.id, pat_src, bindings));
                     self.r.record_partial_res(pat.id, PartialRes::new(res));
+                    self.r.record_pat_span(pat.id, pat.span);
                 }
                 PatKind::TupleStruct(ref path, ref sub_patterns) => {
                     self.smart_resolve_path(

compiler/rustc_resolve/src/lib.rs

@@ -891,6 +891,10 @@ pub struct Resolver<'a> {
     /// "self-confirming" import resolutions during import validation.
     unusable_binding: Option<&'a NameBinding<'a>>,
 
+    // Spans for local variables found during pattern resolution.
+    // Used for suggestions during error reporting.
+    pat_span_map: NodeMap<Span>,
+
     /// Resolutions for nodes that have a single resolution.
     partial_res_map: NodeMap<PartialRes>,
     /// Resolutions for import nodes, which have multiple resolutions in different namespaces.
@@ -1270,6 +1274,7 @@ impl<'a> Resolver<'a> {
             last_import_segment: false,
             unusable_binding: None,
 
+            pat_span_map: Default::default(),
             partial_res_map: Default::default(),
             import_res_map: Default::default(),
             label_res_map: Default::default(),
@@ -1917,7 +1922,6 @@ impl<'a> Resolver<'a> {
                 return Some(LexicalScopeBinding::Item(binding));
             }
         }
-
         self.early_resolve_ident_in_lexical_scope(
             orig_ident,
             ScopeSet::Late(ns, module, record_used_id),
@@ -2394,7 +2398,59 @@ impl<'a> Resolver<'a> {
                             .next()
                             .map_or(false, |c| c.is_ascii_uppercase())
                         {
-                            (format!("use of undeclared type `{}`", ident), None)
+                            // Check whether the name refers to an item in the value namespace.
+                            let suggestion = if ribs.is_some() {
+                                let match_span = match self.resolve_ident_in_lexical_scope(
+                                    ident,
+                                    ValueNS,
+                                    parent_scope,
+                                    None,
+                                    path_span,
+                                    &ribs.unwrap()[ValueNS],
+                                ) {
+                                    // Name matches a local variable. For example:
+                                    // ```
+                                    // fn f() {
+                                    //     let Foo: &str = "";
+                                    //     println!("{}", Foo::Bar); // Name refers to local
+                                    //                               // variable `Foo`.
+                                    // }
+                                    // ```
+                                    Some(LexicalScopeBinding::Res(Res::Local(id))) => {
+                                        Some(*self.pat_span_map.get(&id).unwrap())
+                                    }
+
+                                    // Name matches item from a local name binding
+                                    // created by `use` declaration. For example:
+                                    // ```
+                                    // pub Foo: &str = "";
+                                    //
+                                    // mod submod {
+                                    //     use super::Foo;
+                                    //     println!("{}", Foo::Bar); // Name refers to local
+                                    //                               // binding `Foo`.
+                                    // }
+                                    // ```
+                                    Some(LexicalScopeBinding::Item(name_binding)) => {
+                                        Some(name_binding.span)
+                                    }
+                                    _ => None,
+                                };
+
+                                if let Some(span) = match_span {
+                                    Some((
+                                        vec![(span, String::from(""))],
+                                        format!("`{}` is defined here, but is not a type", ident),
+                                        Applicability::MaybeIncorrect,
+                                    ))
+                                } else {
+                                    None
+                                }
+                            } else {
+                                None
+                            };
+
+                            (format!("use of undeclared type `{}`", ident), suggestion)
                         } else {
                             (format!("use of undeclared crate or module `{}`", ident), None)
                         }
@@ -2805,6 +2861,11 @@ impl<'a> Resolver<'a> {
         }
     }
 
+    fn record_pat_span(&mut self, node: NodeId, span: Span) {
+        debug!("(recording pat) recording {:?} for {:?}", node, span);
+        self.pat_span_map.insert(node, span);
+    }
+
     fn is_accessible_from(&self, vis: ty::Visibility, module: Module<'a>) -> bool {
         vis.is_accessible_from(module.nearest_parent_mod, self)
     }

src/test/ui/resolve/issue-81508.rs

@@ -0,0 +1,22 @@
+// Confusing diagnostic when using variable as a type:
+//
+// Previous warnings indicate Foo is not used, when in fact it is
+// used improperly as a variable or constant. New warning points
+// out user may be trying to use variable as a type. Test demonstrates
+// cases for both local variable and const.
+
+fn main() {
+    let Baz: &str = "";
+
+    println!("{}", Baz::Bar); //~ ERROR: failed to resolve: use of undeclared type `Baz`
+}
+
+#[allow(non_upper_case_globals)]
+pub const Foo: &str = "";
+
+mod submod {
+    use super::Foo;
+    fn function() {
+        println!("{}", Foo::Bar); //~ ERROR: failed to resolve: use of undeclared type `Foo`
+    }
+}

src/test/ui/resolve/issue-81508.stderr

@@ -0,0 +1,21 @@
+error[E0433]: failed to resolve: use of undeclared type `Baz`
+  --> $DIR/issue-81508.rs:11:20
+   |
+LL |     let Baz: &str = "";
+   |         --- help: `Baz` is defined here, but is not a type
+LL | 
+LL |     println!("{}", Baz::Bar);
+   |                    ^^^ use of undeclared type `Baz`
+
+error[E0433]: failed to resolve: use of undeclared type `Foo`
+  --> $DIR/issue-81508.rs:20:24
+   |
+LL |     use super::Foo;
+   |         ---------- help: `Foo` is defined here, but is not a type
+LL |     fn function() {
+LL |         println!("{}", Foo::Bar);
+   |                        ^^^ use of undeclared type `Foo`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0433`.