gccrs: add self_constructor_from_outer_item lint

Warn when the `Self` constructor is used from an item nested inside an
impl or trait, such as a function defined in a method body.

gcc/rust/ChangeLog:

	* checks/lints/unused/rust-unused-checker.cc (UnusedChecker::visit):
	New.
	* checks/lints/unused/rust-unused-checker.h (UnusedChecker::visit):
	New.

gcc/testsuite/ChangeLog:

	* rust/compile/self-constructor-from-outer-item_0.rs: New test.

Signed-off-by: Lucas Ly Ba <lucas.ly-ba@outlook.com>

Author: lucasly-ba

gcc/rust/checks/lints/unused/rust-unused-checker.cc

@@ -21,6 +21,7 @@
 #include "rust-hir-generic-param.h"
 #include "rust-hir-item.h"
 #include "rust-hir-pattern.h"
+#include "rust-hir-path.h"
 
 #include "options.h"
 #include "rust-keyword-values.h"
@@ -165,7 +166,48 @@ UnusedChecker::visit (HIR::Function &fct)
     rust_warning_at (fct.get_locus (), OPT_Wunused_variable,
 		     "function %qs should have a snake case name",
 		     fct.get_function_name ().as_string ().c_str ());
+
+  bool is_associated = in_associated_scope;
+  bool saved_associated = in_associated_scope;
+  bool saved_self_ctor = self_ctor_from_outer;
+  in_associated_scope = false;
+  self_ctor_from_outer = impl_trait_nesting > 0 && !is_associated;
   walk (fct);
+  in_associated_scope = saved_associated;
+  self_ctor_from_outer = saved_self_ctor;
+}
+
+void
+UnusedChecker::visit (HIR::ImplBlock &impl)
+{
+  impl_trait_nesting++;
+  bool saved_associated = in_associated_scope;
+  in_associated_scope = true;
+  walk (impl);
+  in_associated_scope = saved_associated;
+  impl_trait_nesting--;
+}
+
+void
+UnusedChecker::visit (HIR::Trait &trait)
+{
+  impl_trait_nesting++;
+  bool saved_associated = in_associated_scope;
+  in_associated_scope = true;
+  walk (trait);
+  in_associated_scope = saved_associated;
+  impl_trait_nesting--;
+}
+
+void
+UnusedChecker::visit (HIR::PathInExpression &path)
+{
+  if (self_ctor_from_outer && path.get_segments ().size () == 1
+      && path.get_segments ()[0].get_segment ().to_string ()
+	   == Values::Keywords::SELF_ALIAS)
+    rust_warning_at (path.get_locus (), OPT_Wunused_variable,
+		     "cannot reference %<Self%> constructor from outer item");
+  walk (path);
 }
 
 void

gcc/rust/checks/lints/unused/rust-unused-checker.h

@@ -50,7 +50,14 @@ class UnusedChecker : public HIR::DefaultHIRVisitor
   virtual void visit (HIR::LifetimeParam &lft) override;
   virtual void visit (HIR::StructPatternFieldIdentPat &field) override;
   virtual void visit (HIR::MatchExpr &expr) override;
+  virtual void visit (HIR::ImplBlock &impl) override;
+  virtual void visit (HIR::Trait &trait) override;
+  virtual void visit (HIR::PathInExpression &path) override;
   virtual void visit_loop_label (HIR::LoopLabel &label) override;
+
+  bool in_associated_scope = false;
+  int impl_trait_nesting = 0;
+  bool self_ctor_from_outer = false;
 };
 } // namespace Analysis
 } // namespace Rust

gcc/testsuite/rust/compile/self-constructor-from-outer-item_0.rs

@@ -0,0 +1,19 @@
+// { dg-additional-options "-frust-unused-check-2.0" }
+#![feature(no_core)]
+#![no_core]
+
+pub struct S0(usize);
+
+impl S0 {
+    pub fn foo() -> S0 {
+        fn bar() -> S0 {
+            Self(0)
+// { dg-warning "cannot reference .Self. constructor from outer item" "" { target *-*-* } .-1 }
+        }
+        bar()
+    }
+
+    pub fn direct() -> S0 {
+        Self(0)
+    }
+}