Make explicit_deref_methods check for multiple deref calls

Fix suggestion for `explicit_deref_methods`. Sometimes `&**` is needed, sometimes nothing is needed.
Allow `explicit_deref_methods` to trigger in a few new contexts.
`explicit_deref_methods` will now consider ufcs calls

Author: Jarcho

clippy_lints/src/dereference.rs

@@ -1,11 +1,13 @@
-use crate::utils::{get_parent_expr, implements_trait, snippet, span_lint_and_sugg};
-use if_chain::if_chain;
-use rustc_ast::util::parser::{ExprPrecedence, PREC_POSTFIX, PREC_PREFIX};
+use crate::utils::{
+    get_node_span, get_parent_node, in_macro, is_allowed, peel_mid_ty_refs, snippet_with_context, span_lint_and_sugg,
+};
+use rustc_ast::util::parser::PREC_PREFIX;
 use rustc_errors::Applicability;
-use rustc_hir::{Expr, ExprKind};
+use rustc_hir::{BorrowKind, Destination, Expr, ExprKind, HirId, MatchSource, Mutability, Node, UnOp};
 use rustc_lint::{LateContext, LateLintPass};
-use rustc_session::{declare_lint_pass, declare_tool_lint};
-use rustc_span::source_map::Span;
+use rustc_middle::ty::{self, adjustment::Adjustment, Ty, TyCtxt, TyS, TypeckResults};
+use rustc_session::{declare_tool_lint, impl_lint_pass};
+use rustc_span::{symbol::sym, Span};
 
 declare_clippy_lint! {
     /// **What it does:** Checks for explicit `deref()` or `deref_mut()` method calls.
@@ -34,76 +36,313 @@ declare_clippy_lint! {
     "Explicit use of deref or deref_mut method while not in a method chain."
 }
 
-declare_lint_pass!(Dereferencing => [
-    EXPLICIT_DEREF_METHODS
+impl_lint_pass!(Dereferencing => [
+    EXPLICIT_DEREF_METHODS,
 ]);
 
+#[derive(Default)]
+pub struct Dereferencing {
+    state: Option<(State, StateData)>,
+
+    // While parsing a `deref` method call in ufcs form, the path to the function is itself an
+    // expression. This is to store the id of that expression so it can be skipped when
+    // `check_expr` is called for it.
+    skip_expr: Option<HirId>,
+}
+
+struct StateData {
+    /// Span of the top level expression
+    span: Span,
+    /// The required mutability
+    target_mut: Mutability,
+}
+
+enum State {
+    // Any number of deref method calls.
+    DerefMethod {
+        // The number of calls in a sequence which changed the referenced type
+        ty_changed_count: usize,
+        is_final_ufcs: bool,
+    },
+}
+
+// A reference operation considered by this lint pass
+enum RefOp {
+    Method,
+    Deref,
+    AddrOf,
+}
+
 impl<'tcx> LateLintPass<'tcx> for Dereferencing {
     fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
-        if_chain! {
-            if !expr.span.from_expansion();
-            if let ExprKind::MethodCall(ref method_name, _, ref args, _) = &expr.kind;
-            if args.len() == 1;
-
-            then {
-                if let Some(parent_expr) = get_parent_expr(cx, expr) {
-                    // Check if we have the whole call chain here
-                    if let ExprKind::MethodCall(..) = parent_expr.kind {
-                        return;
-                    }
-                    // Check for Expr that we don't want to be linted
-                    let precedence = parent_expr.precedence();
-                    match precedence {
-                        // Lint a Call is ok though
-                        ExprPrecedence::Call | ExprPrecedence::AddrOf => (),
-                        _ => {
-                            if precedence.order() >= PREC_PREFIX && precedence.order() <= PREC_POSTFIX {
-                                return;
-                            }
-                        }
+        // Skip path expressions from deref calls. e.g. `Deref::deref(e)`
+        if Some(expr.hir_id) == self.skip_expr.take() {
+            return;
+        }
+
+        // Stop processing sub expressions when a macro call is seen
+        if in_macro(expr.span) {
+            if let Some((state, data)) = self.state.take() {
+                report(cx, expr, state, data);
+            }
+            return;
+        }
+
+        let typeck = cx.typeck_results();
+        let (kind, sub_expr) = if let Some(x) = try_parse_ref_op(cx.tcx, typeck, expr) {
+            x
+        } else {
+            // The whole chain of reference operations has been seen
+            if let Some((state, data)) = self.state.take() {
+                report(cx, expr, state, data);
+            }
+            return;
+        };
+
+        match (self.state.take(), kind) {
+            (None, kind) => {
+                let parent = get_parent_node(cx.tcx, expr.hir_id);
+                // This is an odd case. The expression is a macro argument, but the top level
+                // address of expression is inserted by the compiler.
+                if matches!(kind, RefOp::AddrOf) && parent.and_then(get_node_span).map_or(false, in_macro) {
+                    return;
+                }
+
+                let expr_adjustments = find_adjustments(cx.tcx, typeck, expr);
+                let expr_ty = typeck.expr_ty(expr);
+                let target_mut =
+                    if let ty::Ref(_, _, mutability) = *expr_adjustments.last().map_or(expr_ty, |a| a.target).kind() {
+                        mutability
+                    } else {
+                        Mutability::Not
+                    };
+
+                match kind {
+                    RefOp::Method
+                        if !is_allowed(cx, EXPLICIT_DEREF_METHODS, expr.hir_id)
+                            && is_linted_explicit_deref_position(parent, expr.hir_id) =>
+                    {
+                        self.state = Some((
+                            State::DerefMethod {
+                                ty_changed_count: if deref_method_same_type(expr_ty, typeck.expr_ty(sub_expr)) {
+                                    0
+                                } else {
+                                    1
+                                },
+                                is_final_ufcs: matches!(expr.kind, ExprKind::Call(..)),
+                            },
+                            StateData {
+                                span: expr.span,
+                                target_mut,
+                            },
+                        ));
                     }
+                    _ => (),
                 }
-                let name = method_name.ident.as_str();
-                lint_deref(cx, &*name, &args[0], args[0].span, expr.span);
-            }
+            },
+            (Some((State::DerefMethod { ty_changed_count, .. }, data)), RefOp::Method) => {
+                self.state = Some((
+                    State::DerefMethod {
+                        ty_changed_count: if deref_method_same_type(typeck.expr_ty(expr), typeck.expr_ty(sub_expr)) {
+                            ty_changed_count
+                        } else {
+                            ty_changed_count + 1
+                        },
+                        is_final_ufcs: matches!(expr.kind, ExprKind::Call(..)),
+                    },
+                    data,
+                ));
+            },
+
+            (Some((state, data)), _) => report(cx, expr, state, data),
         }
     }
 }
 
-fn lint_deref(cx: &LateContext<'_>, method_name: &str, call_expr: &Expr<'_>, var_span: Span, expr_span: Span) {
-    match method_name {
-        "deref" => {
-            let impls_deref_trait = cx.tcx.lang_items().deref_trait().map_or(false, |id| {
-                implements_trait(cx, cx.typeck_results().expr_ty(&call_expr), id, &[])
-            });
-            if impls_deref_trait {
-                span_lint_and_sugg(
-                    cx,
-                    EXPLICIT_DEREF_METHODS,
-                    expr_span,
-                    "explicit deref method call",
-                    "try this",
-                    format!("&*{}", &snippet(cx, var_span, "..")),
-                    Applicability::MachineApplicable,
-                );
-            }
+fn try_parse_ref_op(
+    tcx: TyCtxt<'tcx>,
+    typeck: &'tcx TypeckResults<'_>,
+    expr: &'tcx Expr<'_>,
+) -> Option<(RefOp, &'tcx Expr<'tcx>)> {
+    let (def_id, arg) = match expr.kind {
+        ExprKind::MethodCall(_, _, [arg], _) => (typeck.type_dependent_def_id(expr.hir_id)?, arg),
+        ExprKind::Call(
+            Expr {
+                kind: ExprKind::Path(path),
+                hir_id,
+                ..
+            },
+            [arg],
+        ) => (typeck.qpath_res(path, *hir_id).opt_def_id()?, arg),
+        ExprKind::Unary(UnOp::Deref, sub_expr) if !typeck.expr_ty(sub_expr).is_unsafe_ptr() => {
+            return Some((RefOp::Deref, sub_expr));
         },
-        "deref_mut" => {
-            let impls_deref_mut_trait = cx.tcx.lang_items().deref_mut_trait().map_or(false, |id| {
-                implements_trait(cx, cx.typeck_results().expr_ty(&call_expr), id, &[])
-            });
-            if impls_deref_mut_trait {
-                span_lint_and_sugg(
-                    cx,
-                    EXPLICIT_DEREF_METHODS,
-                    expr_span,
-                    "explicit deref_mut method call",
-                    "try this",
-                    format!("&mut *{}", &snippet(cx, var_span, "..")),
-                    Applicability::MachineApplicable,
-                );
-            }
+        ExprKind::AddrOf(BorrowKind::Ref, _, sub_expr) => return Some((RefOp::AddrOf, sub_expr)),
+        _ => return None,
+    };
+    (tcx.is_diagnostic_item(sym::deref_method, def_id)
+        || tcx.trait_of_item(def_id)? == tcx.lang_items().deref_mut_trait()?)
+    .then(|| (RefOp::Method, arg))
+}
+
+// Checks whether the type for a deref call actually changed the type, not just the mutability of
+// the reference.
+fn deref_method_same_type(result_ty: Ty<'tcx>, arg_ty: Ty<'tcx>) -> bool {
+    match (result_ty.kind(), arg_ty.kind()) {
+        (ty::Ref(_, result_ty, _), ty::Ref(_, arg_ty, _)) => TyS::same_type(result_ty, arg_ty),
+
+        // The result type for a deref method is always a reference
+        // Not matching the previous pattern means the argument type is not a reference
+        // This means that the type did change
+        _ => false,
+    }
+}
+
+// Adjustments are sometimes made in the parent block rather than the expression itself.
+fn find_adjustments(
+    tcx: TyCtxt<'tcx>,
+    typeck: &'tcx TypeckResults<'_>,
+    expr: &'tcx Expr<'_>,
+) -> &'tcx [Adjustment<'tcx>] {
+    let map = tcx.hir();
+    let mut iter = map.parent_iter(expr.hir_id);
+    let mut prev = expr;
+
+    loop {
+        match typeck.expr_adjustments(prev) {
+            [] => (),
+            a => break a,
+        };
+
+        match iter.next().map(|(_, x)| x) {
+            Some(Node::Block(_)) => {
+                if let Some((_, Node::Expr(e))) = iter.next() {
+                    prev = e;
+                } else {
+                    // This shouldn't happen. Blocks are always contained in an expression.
+                    break &[];
+                }
+            },
+            Some(Node::Expr(&Expr {
+                kind: ExprKind::Break(Destination { target_id: Ok(id), .. }, _),
+                ..
+            })) => {
+                if let Some(Node::Expr(e)) = map.find(id) {
+                    prev = e;
+                    iter = map.parent_iter(id);
+                    continue;
+                }
+                // This shouldn't happen. The destination should definitely exist at this point.
+                break &[];
+            },
+            _ => break &[],
+        }
+    }
+}
+
+// Checks whether the parent node is a suitable context for switching from a deref method to the
+// deref operator.
+fn is_linted_explicit_deref_position(parent: Option<Node<'_>>, child_id: HirId) -> bool {
+    let parent = match parent {
+        Some(Node::Expr(e)) => e,
+        _ => return true,
+    };
+    match parent.kind {
+        // Leave deref calls in the middle of a method chain.
+        // e.g. x.deref().foo()
+        ExprKind::MethodCall(_, _, [self_arg, ..], _) if self_arg.hir_id == child_id => false,
+
+        // Leave deref calls resulting in a called function
+        // e.g. (x.deref())()
+        ExprKind::Call(func_expr, _) if func_expr.hir_id == child_id => false,
+
+        // Makes an ugly suggestion
+        // e.g. *x.deref() => *&*x
+        ExprKind::Unary(UnOp::Deref, _)
+        // Postfix expressions would require parens
+        | ExprKind::Match(_, _, MatchSource::TryDesugar | MatchSource::AwaitDesugar)
+        | ExprKind::Field(..)
+        | ExprKind::Index(..)
+        | ExprKind::Err => false,
+
+        ExprKind::Box(..)
+        | ExprKind::ConstBlock(..)
+        | ExprKind::Array(_)
+        | ExprKind::Call(..)
+        | ExprKind::MethodCall(..)
+        | ExprKind::Tup(..)
+        | ExprKind::Binary(..)
+        | ExprKind::Unary(..)
+        | ExprKind::Lit(..)
+        | ExprKind::Cast(..)
+        | ExprKind::Type(..)
+        | ExprKind::DropTemps(..)
+        | ExprKind::If(..)
+        | ExprKind::Loop(..)
+        | ExprKind::Match(..)
+        | ExprKind::Closure(..)
+        | ExprKind::Block(..)
+        | ExprKind::Assign(..)
+        | ExprKind::AssignOp(..)
+        | ExprKind::Path(..)
+        | ExprKind::AddrOf(..)
+        | ExprKind::Break(..)
+        | ExprKind::Continue(..)
+        | ExprKind::Ret(..)
+        | ExprKind::InlineAsm(..)
+        | ExprKind::LlvmInlineAsm(..)
+        | ExprKind::Struct(..)
+        | ExprKind::Repeat(..)
+        | ExprKind::Yield(..) => true,
+    }
+}
+
+#[allow(clippy::needless_pass_by_value)]
+fn report(cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>, state: State, data: StateData) {
+    match state {
+        State::DerefMethod {
+            ty_changed_count,
+            is_final_ufcs,
+        } => {
+            let mut app = Applicability::MachineApplicable;
+            let (expr_str, expr_is_macro_call) = snippet_with_context(cx, expr.span, data.span.ctxt(), "..", &mut app);
+            let ty = cx.typeck_results().expr_ty(expr);
+            let (_, ref_count) = peel_mid_ty_refs(ty);
+            let deref_str = if ty_changed_count >= ref_count && ref_count != 0 {
+                // a deref call changing &T -> &U requires two deref operators the first time
+                // this occurs. One to remove the reference, a second to call the deref impl.
+                "*".repeat(ty_changed_count + 1)
+            } else {
+                "*".repeat(ty_changed_count)
+            };
+            let addr_of_str = if ty_changed_count < ref_count {
+                // Check if a reborrow from &mut T -> &T is required.
+                if data.target_mut == Mutability::Not && matches!(ty.kind(), ty::Ref(_, _, Mutability::Mut)) {
+                    "&*"
+                } else {
+                    ""
+                }
+            } else if data.target_mut == Mutability::Mut {
+                "&mut "
+            } else {
+                "&"
+            };
+
+            let expr_str = if !expr_is_macro_call && is_final_ufcs && expr.precedence().order() < PREC_PREFIX {
+                format!("({})", expr_str)
+            } else {
+                expr_str.into_owned()
+            };
+
+            span_lint_and_sugg(
+                cx,
+                EXPLICIT_DEREF_METHODS,
+                data.span,
+                "explicit `deref` method call",
+                "try this",
+                format!("{}{}{}", addr_of_str, deref_str, expr_str),
+                app,
+            );
         },
-        _ => (),
     }
 }