From 66afdd1f4292e7fda6ea89113c0c8343e3321d99 Mon Sep 17 00:00:00 2001
From: Cameron Steffen <cam.steffen94@gmail.com>
Date: Thu, 21 Jan 2021 19:21:12 -0600
Subject: [PATCH] Enhance collapsible_match for adjusted bindings

---
 clippy_lints/src/collapsible_match.rs | 22 +++++++++++++---
 tests/ui/collapsible_match2.rs        | 29 ++++++++++++++++++++
 tests/ui/collapsible_match2.stderr    | 38 ++++++++++++++++++++++++++-
 3 files changed, 85 insertions(+), 4 deletions(-)

diff --git a/clippy_lints/src/collapsible_match.rs b/clippy_lints/src/collapsible_match.rs
index c75aa2bde97..604ba102046 100644
--- a/clippy_lints/src/collapsible_match.rs
+++ b/clippy_lints/src/collapsible_match.rs
@@ -2,7 +2,7 @@
 use crate::utils::{span_lint_and_then, SpanlessEq};
 use if_chain::if_chain;
 use rustc_hir::def::{CtorKind, CtorOf, DefKind, Res};
-use rustc_hir::{Arm, Expr, ExprKind, Guard, HirId, Pat, PatKind, QPath, StmtKind};
+use rustc_hir::{Arm, Expr, ExprKind, Guard, HirId, Pat, PatKind, QPath, StmtKind, UnOp};
 use rustc_lint::{LateContext, LateLintPass};
 use rustc_middle::ty::{DefIdTree, TyCtxt};
 use rustc_session::{declare_lint_pass, declare_tool_lint};
@@ -72,8 +72,7 @@ fn check_arm(arm: &Arm<'_>, wild_outer_arm: &Arm<'_>, cx: &LateContext<'_>) {
         if arms_inner.iter().all(|arm| arm.guard.is_none());
         // match expression must be a local binding
         // match <local> { .. }
-        if let ExprKind::Path(QPath::Resolved(None, path)) = expr_in.kind;
-        if let Res::Local(binding_id) = path.res;
+        if let Some(binding_id) = addr_adjusted_binding(expr_in, cx);
         // one of the branches must be "wild-like"
         if let Some(wild_inner_arm_idx) = arms_inner.iter().rposition(|arm_inner| arm_is_wild_like(arm_inner, cx.tcx));
         let (wild_inner_arm, non_wild_inner_arm) =
@@ -175,3 +174,20 @@ fn is_none_ctor(res: Res, tcx: TyCtxt<'_>) -> bool {
     }
     false
 }
+
+/// Retrieves a binding ID with optional `&` and/or `*` operators removed. (e.g. `&**foo`)
+/// Returns `None` if a non-reference type is de-referenced.
+/// For example, if `Vec` is de-referenced to a slice, `None` is returned.
+fn addr_adjusted_binding(mut expr: &Expr<'_>, cx: &LateContext<'_>) -> Option<HirId> {
+    loop {
+        match expr.kind {
+            ExprKind::AddrOf(_, _, e) => expr = e,
+            ExprKind::Path(QPath::Resolved(None, path)) => match path.res {
+                Res::Local(binding_id) => break Some(binding_id),
+                _ => break None,
+            },
+            ExprKind::Unary(UnOp::UnDeref, e) if cx.typeck_results().expr_ty(e).is_ref() => expr = e,
+            _ => break None,
+        }
+    }
+}
diff --git a/tests/ui/collapsible_match2.rs b/tests/ui/collapsible_match2.rs
index d571ac4ab69..8372a212477 100644
--- a/tests/ui/collapsible_match2.rs
+++ b/tests/ui/collapsible_match2.rs
@@ -40,6 +40,35 @@ macro_rules! mac {
         // there is still a better way to write this.
         mac!(res_opt => Ok(val), val => Some(n), foo(n));
     }
+
+    // deref reference value
+    match Some(&[1]) {
+        Some(s) => match *s {
+            [n] => foo(n),
+            _ => (),
+        },
+        _ => (),
+    }
+
+    // ref pattern and deref
+    match Some(&[1]) {
+        Some(ref s) => match &*s {
+            [n] => foo(n),
+            _ => (),
+        },
+        _ => (),
+    }
+}
+
+fn no_lint() {
+    // deref inner value (cannot pattern match with Vec)
+    match Some(vec![1]) {
+        Some(s) => match *s {
+            [n] => foo(n),
+            _ => (),
+        },
+        _ => (),
+    }
 }
 
 fn make<T>() -> T {
diff --git a/tests/ui/collapsible_match2.stderr b/tests/ui/collapsible_match2.stderr
index 490d82d12cd..b2eb457d173 100644
--- a/tests/ui/collapsible_match2.stderr
+++ b/tests/ui/collapsible_match2.stderr
@@ -57,5 +57,41 @@ LL |         mac!(res_opt => Ok(val), val => Some(n), foo(n));
    |                            Replace this binding
    = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: aborting due to 3 previous errors
+error: Unnecessary nested match
+  --> $DIR/collapsible_match2.rs:46:20
+   |
+LL |           Some(s) => match *s {
+   |  ____________________^
+LL | |             [n] => foo(n),
+LL | |             _ => (),
+LL | |         },
+   | |_________^
+   |
+help: The outer pattern can be modified to include the inner pattern.
+  --> $DIR/collapsible_match2.rs:46:14
+   |
+LL |         Some(s) => match *s {
+   |              ^ Replace this binding
+LL |             [n] => foo(n),
+   |             ^^^ with this pattern
+
+error: Unnecessary nested match
+  --> $DIR/collapsible_match2.rs:55:24
+   |
+LL |           Some(ref s) => match &*s {
+   |  ________________________^
+LL | |             [n] => foo(n),
+LL | |             _ => (),
+LL | |         },
+   | |_________^
+   |
+help: The outer pattern can be modified to include the inner pattern.
+  --> $DIR/collapsible_match2.rs:55:14
+   |
+LL |         Some(ref s) => match &*s {
+   |              ^^^^^ Replace this binding
+LL |             [n] => foo(n),
+   |             ^^^ with this pattern
+
+error: aborting due to 5 previous errors