Merge commit '7c21f91b15' into clippyup

clippy_lints/src/unnecessary_owned_empty_strings.rs

@@ -0,0 +1,81 @@
+use clippy_utils::{diagnostics::span_lint_and_sugg, ty::is_type_diagnostic_item};
+use clippy_utils::{match_def_path, paths};
+use if_chain::if_chain;
+use rustc_ast::ast::LitKind;
+use rustc_errors::Applicability;
+use rustc_hir::{BorrowKind, Expr, ExprKind, Mutability};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_middle::ty;
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+use rustc_span::sym;
+
+declare_clippy_lint! {
+    /// ### What it does
+    ///
+    /// Detects cases of owned empty strings being passed as an argument to a function expecting `&str`
+    ///
+    /// ### Why is this bad?
+    ///
+    /// This results in longer and less readable code
+    ///
+    /// ### Example
+    /// ```rust
+    /// vec!["1", "2", "3"].join(&String::new());
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// vec!["1", "2", "3"].join("");
+    /// ```
+    #[clippy::version = "1.62.0"]
+    pub UNNECESSARY_OWNED_EMPTY_STRINGS,
+    style,
+    "detects cases of references to owned empty strings being passed as an argument to a function expecting `&str`"
+}
+declare_lint_pass!(UnnecessaryOwnedEmptyStrings => [UNNECESSARY_OWNED_EMPTY_STRINGS]);
+
+impl<'tcx> LateLintPass<'tcx> for UnnecessaryOwnedEmptyStrings {
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'tcx>) {
+        if_chain! {
+            if let ExprKind::AddrOf(BorrowKind::Ref, Mutability::Not, inner_expr) = expr.kind;
+            if let ExprKind::Call(fun, args) = inner_expr.kind;
+            if let ExprKind::Path(ref qpath) = fun.kind;
+            if let Some(fun_def_id) = cx.qpath_res(qpath, fun.hir_id).opt_def_id();
+            if let ty::Ref(_, inner_str, _) = cx.typeck_results().expr_ty_adjusted(expr).kind();
+            if inner_str.is_str();
+            then {
+                if match_def_path(cx, fun_def_id, &paths::STRING_NEW) {
+                     span_lint_and_sugg(
+                            cx,
+                            UNNECESSARY_OWNED_EMPTY_STRINGS,
+                            expr.span,
+                            "usage of `&String::new()` for a function expecting a `&str` argument",
+                            "try",
+                            "\"\"".to_owned(),
+                            Applicability::MachineApplicable,
+                        );
+                } else {
+                    if_chain! {
+                        if match_def_path(cx, fun_def_id, &paths::FROM_FROM);
+                        if let [.., last_arg] = args;
+                        if let ExprKind::Lit(spanned) = &last_arg.kind;
+                        if let LitKind::Str(symbol, _) = spanned.node;
+                        if symbol.is_empty();
+                        let inner_expr_type = cx.typeck_results().expr_ty(inner_expr);
+                        if is_type_diagnostic_item(cx, inner_expr_type, sym::String);
+                        then {
+                            span_lint_and_sugg(
+                                cx,
+                                UNNECESSARY_OWNED_EMPTY_STRINGS,
+                                expr.span,
+                                "usage of `&String::from(\"\")` for a function expecting a `&str` argument",
+                                "try",
+                                "\"\"".to_owned(),
+                                Applicability::MachineApplicable,
+                            );
+                        }
+                    }
+                }
+            }
+        }
+    }
+}