New lint no_effect_replace

Closes #1595

changelog: Add no_effect_replace lint.

clippy_lints/src/methods/no_effect_replace.rs

@@ -0,0 +1,47 @@
+use clippy_utils::diagnostics::span_lint;
+use clippy_utils::ty::is_type_diagnostic_item;
+use clippy_utils::SpanlessEq;
+use if_chain::if_chain;
+use rustc_ast::LitKind;
+use rustc_hir::ExprKind;
+use rustc_lint::LateContext;
+use rustc_span::sym;
+
+use super::NO_EFFECT_REPLACE;
+
+pub(super) fn check<'tcx>(
+    cx: &LateContext<'tcx>,
+    expr: &'tcx rustc_hir::Expr<'_>,
+    arg1: &'tcx rustc_hir::Expr<'_>,
+    arg2: &'tcx rustc_hir::Expr<'_>,
+) {
+    let ty = cx.typeck_results().expr_ty(expr).peel_refs();
+    if !(ty.is_str() || is_type_diagnostic_item(cx, ty, sym::String)) {
+        return;
+    }
+
+    if_chain! {
+        if let ExprKind::Lit(spanned) = &arg1.kind;
+        if let Some(param1) = lit_string_value(&spanned.node);
+
+        if let ExprKind::Lit(spanned) = &arg2.kind;
+        if let LitKind::Str(param2, _) = &spanned.node;
+        if param1 == param2.as_str();
+
+        then {
+            span_lint(cx, NO_EFFECT_REPLACE, expr.span, "replacing text with itself");
+        }
+    }
+
+    if SpanlessEq::new(cx).eq_expr(arg1, arg2) {
+        span_lint(cx, NO_EFFECT_REPLACE, expr.span, "replacing text with itself");
+    }
+}
+
+fn lit_string_value(node: &LitKind) -> Option<String> {
+    match node {
+        LitKind::Char(value) => Some(value.to_string()),
+        LitKind::Str(value, _) => Some(value.as_str().to_owned()),
+        _ => None,
+    }
+}