Working basic dereference clip

clippy_lints/src/dereference.rs

@@ -0,0 +1,74 @@
+use crate::rustc::hir::{Expr, ExprKind, QPath};
+use crate::rustc::lint::{LateContext, LateLintPass, LintArray, LintPass};
+use crate::rustc::{declare_tool_lint, lint_array};
+use crate::utils::{in_macro, span_lint_and_sugg};
+use if_chain::if_chain;
+
+/// **What it does:** Checks for explicit deref() or deref_mut() method calls.
+///
+/// **Why is this bad?** Derefencing by &*x or &mut *x is clearer and more concise,
+/// when not part of a method chain.
+///
+/// **Example:**
+/// ```rust
+/// let b = a.deref();
+/// let c = a.deref_mut();
+///
+/// // excludes
+/// let e = d.unwrap().deref();
+/// ```
+declare_clippy_lint! {
+    pub EXPLICIT_DEREF_METHOD,
+    pedantic,
+    "Explicit use of deref or deref_mut method while not in a method chain."
+}
+
+pub struct Pass;
+
+impl LintPass for Pass {
+    fn get_lints(&self) -> LintArray {
+        lint_array!(EXPLICIT_DEREF_METHOD)
+    }
+}
+
+impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Pass {
+    fn check_expr(&mut self, cx: &LateContext<'_, '_>, expr: &Expr) {
+        if in_macro(expr.span) {
+            return;
+        }
+
+        if_chain! {
+            // if this is a method call
+            if let ExprKind::MethodCall(ref method_name, _, ref args) = &expr.node;
+            // on a Path (i.e. a variable/name, not another method)
+            if let ExprKind::Path(QPath::Resolved(None, path)) = &args[0].node;
+            then {
+                let name = method_name.ident.as_str();
+                // alter help slightly to account for _mut
+                match &*name {
+                    "deref" => {
+                        span_lint_and_sugg(
+                            cx,
+                            EXPLICIT_DEREF_METHOD,
+                            expr.span,
+                            "explicit deref method call",
+                            "try this",
+                            format!("&*{}", path),
+                        );
+                    },
+                    "deref_mut" => {
+                        span_lint_and_sugg(
+                            cx,
+                            EXPLICIT_DEREF_METHOD,
+                            expr.span,
+                            "explicit deref_mut method call",
+                            "try this",
+                            format!("&mut *{}", path),
+                        );
+                    },
+                    _ => ()
+                };
+            }
+        }
+    }
+}