New lint no_effect_replace

Closes #1595

changelog: Add no_effect_replace lint.

clippy_lints/src/lib.register_all.rs

@@ -190,6 +190,7 @@ store.register_group(true, "clippy::all", Some("clippy_all"), vec![
     LintId::of(methods::NEEDLESS_OPTION_TAKE),
     LintId::of(methods::NEEDLESS_SPLITN),
     LintId::of(methods::NEW_RET_NO_SELF),
+    LintId::of(methods::NO_EFFECT_REPLACE),
     LintId::of(methods::OK_EXPECT),
     LintId::of(methods::OPTION_AS_REF_DEREF),
     LintId::of(methods::OPTION_FILTER_MAP),

clippy_lints/src/lib.register_lints.rs

@@ -331,6 +331,7 @@ store.register_lints(&[
     methods::NEEDLESS_OPTION_TAKE,
     methods::NEEDLESS_SPLITN,
     methods::NEW_RET_NO_SELF,
+    methods::NO_EFFECT_REPLACE,
     methods::OK_EXPECT,
     methods::OPTION_AS_REF_DEREF,
     methods::OPTION_FILTER_MAP,

clippy_lints/src/lib.register_suspicious.rs

@@ -23,6 +23,7 @@ store.register_group(true, "clippy::suspicious", Some("clippy_suspicious"), vec!
     LintId::of(loops::EMPTY_LOOP),
     LintId::of(loops::FOR_LOOPS_OVER_FALLIBLES),
     LintId::of(loops::MUT_RANGE_BOUND),
+    LintId::of(methods::NO_EFFECT_REPLACE),
     LintId::of(methods::SUSPICIOUS_MAP),
     LintId::of(mut_key::MUTABLE_KEY_TYPE),
     LintId::of(octal_escapes::OCTAL_ESCAPES),

clippy_lints/src/methods/mod.rs

@@ -44,6 +44,7 @@ mod map_identity;
 mod map_unwrap_or;
 mod needless_option_as_deref;
 mod needless_option_take;
+mod no_effect_replace;
 mod ok_expect;
 mod option_as_ref_deref;
 mod option_map_or_none;
@@ -2197,6 +2198,24 @@ declare_clippy_lint! {
     "using `.as_ref().take()` on a temporary value"
 }
 
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for `replace` statements which have no effect.
+    ///
+    /// ### Why is this bad?
+    /// It's either a mistake or confusing.
+    ///
+    /// ### Example
+    /// ```rust
+    /// "1234".replace("12", "12");
+    /// "1234".replacen("12", "12", 1);
+    /// ```
+    #[clippy::version = "1.62.0"]
+    pub NO_EFFECT_REPLACE,
+    suspicious,
+    "replace with no effect"
+}
+
 pub struct Methods {
     avoid_breaking_exported_api: bool,
     msrv: Option<RustcVersion>,
@@ -2296,6 +2315,7 @@ impl_lint_pass!(Methods => [
     NEEDLESS_OPTION_AS_DEREF,
     IS_DIGIT_ASCII_RADIX,
     NEEDLESS_OPTION_TAKE,
+    NO_EFFECT_REPLACE,
 ]);
 
 /// Extracts a method call name, args, and `Span` of the method name.
@@ -2707,6 +2727,9 @@ impl Methods {
                         unnecessary_lazy_eval::check(cx, expr, recv, u_arg, "unwrap_or");
                     },
                 },
+                ("replace" | "replacen", [arg1, arg2] | [arg1, arg2, _]) => {
+                    no_effect_replace::check(cx, expr, arg1, arg2);
+                },
                 _ => {},
             }
         }

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,
+    }
+}