Rollup merge of #3851 - phansch:refactor_trait_stuff, r=flip1995

Refactor: Extract `trait_ref_of_method` function

This pattern was used in three places after #3844, so I think it's worth moving it into `utils/mod.rs` and documenting it.

Author: flip1995

clippy_lints/src/assign_ops.rs

@@ -1,4 +1,6 @@
-use crate::utils::{get_trait_def_id, implements_trait, snippet_opt, span_lint_and_then, SpanlessEq};
+use crate::utils::{
+    get_trait_def_id, implements_trait, snippet_opt, span_lint_and_then, trait_ref_of_method, SpanlessEq,
+};
 use crate::utils::{higher, sugg};
 use if_chain::if_chain;
 use rustc::hir;
@@ -140,13 +142,8 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for AssignOps {
                                         };
                                         // check that we are not inside an `impl AssignOp` of this exact operation
                                         let parent_fn = cx.tcx.hir().get_parent_item(e.hir_id);
-                                        let parent_impl = cx.tcx.hir().get_parent_item(parent_fn);
-                                        // the crate node is the only one that is not in the map
                                         if_chain! {
-                                            if parent_impl != hir::CRATE_HIR_ID;
-                                            if let hir::Node::Item(item) = cx.tcx.hir().get_by_hir_id(parent_impl);
-                                            if let hir::ItemKind::Impl(_, _, _, _, Some(trait_ref), _, _) =
-                                                &item.node;
+                                            if let Some(trait_ref) = trait_ref_of_method(cx, parent_fn);
                                             if trait_ref.path.def.def_id() == trait_id;
                                             then { return; }
                                         }

clippy_lints/src/missing_const_for_fn.rs

@@ -1,5 +1,4 @@
-use crate::utils::{is_entrypoint_fn, span_lint};
-use if_chain::if_chain;
+use crate::utils::{is_entrypoint_fn, span_lint, trait_ref_of_method};
 use rustc::hir;
 use rustc::hir::intravisit::FnKind;
 use rustc::hir::{Body, Constness, FnDecl, HirId};
@@ -96,7 +95,7 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for MissingConstForFn {
                 }
             },
             FnKind::Method(_, sig, ..) => {
-                if is_trait_method(cx, hir_id) || already_const(sig.header) {
+                if trait_ref_of_method(cx, hir_id).is_some() || already_const(sig.header) {
                     return;
                 }
             },
@@ -115,18 +114,6 @@ impl<'a, 'tcx> LateLintPass<'a, 'tcx> for MissingConstForFn {
     }
 }
 
-fn is_trait_method(cx: &LateContext<'_, '_>, hir_id: HirId) -> bool {
-    // Get the implemented trait for the current function
-    let parent_impl = cx.tcx.hir().get_parent_item(hir_id);
-    if_chain! {
-        if parent_impl != hir::CRATE_HIR_ID;
-        if let hir::Node::Item(item) = cx.tcx.hir().get_by_hir_id(parent_impl);
-        if let hir::ItemKind::Impl(_, _, _, _, Some(_trait_ref), _, _) = &item.node;
-        then { return true; }
-    }
-    false
-}
-
 // We don't have to lint on something that's already `const`
 fn already_const(header: hir::FnHeader) -> bool {
     header.constness == Constness::Const

clippy_lints/src/suspicious_trait_impl.rs

@@ -1,4 +1,4 @@
-use crate::utils::{get_trait_def_id, span_lint};
+use crate::utils::{get_trait_def_id, span_lint, trait_ref_of_method};
 use if_chain::if_chain;
 use rustc::hir;
 use rustc::hir::intravisit::{walk_expr, NestedVisitorMap, Visitor};
@@ -177,12 +177,9 @@ fn check_binop<'a>(
 
     // Get the actually implemented trait
     let parent_fn = cx.tcx.hir().get_parent_item(expr.hir_id);
-    let parent_impl = cx.tcx.hir().get_parent_item(parent_fn);
 
     if_chain! {
-        if parent_impl != hir::CRATE_HIR_ID;
-        if let hir::Node::Item(item) = cx.tcx.hir().get_by_hir_id(parent_impl);
-        if let hir::ItemKind::Impl(_, _, _, _, Some(ref trait_ref), _, _) = item.node;
+        if let Some(trait_ref) = trait_ref_of_method(cx, parent_fn);
         if let Some(idx) = trait_ids.iter().position(|&tid| tid == trait_ref.path.def.def_id());
         if binop != expected_ops[idx];
         then{

clippy_lints/src/utils/mod.rs

@@ -301,6 +301,33 @@ pub fn implements_trait<'a, 'tcx>(
         .enter(|infcx| infcx.predicate_must_hold_modulo_regions(&obligation))
 }
 
+/// Get the `hir::TraitRef` of the trait the given method is implemented for
+///
+/// Use this if you want to find the `TraitRef` of the `Add` trait in this example:
+///
+/// ```rust
+/// struct Point(isize, isize);
+///
+/// impl std::ops::Add for Point {
+///     type Output = Self;
+///
+///     fn add(self, other: Self) -> Self {
+///         Point(0, 0)
+///     }
+/// }
+/// ```
+pub fn trait_ref_of_method(cx: &LateContext<'_, '_>, hir_id: HirId) -> Option<TraitRef> {
+    // Get the implemented trait for the current function
+    let parent_impl = cx.tcx.hir().get_parent_item(hir_id);
+    if_chain! {
+        if parent_impl != hir::CRATE_HIR_ID;
+        if let hir::Node::Item(item) = cx.tcx.hir().get_by_hir_id(parent_impl);
+        if let hir::ItemKind::Impl(_, _, _, _, trait_ref, _, _) = &item.node;
+        then { return trait_ref.clone(); }
+    }
+    None
+}
+
 /// Check whether this type implements Drop.
 pub fn has_drop<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, ty: Ty<'tcx>) -> bool {
     match ty.ty_adt_def() {