feat: add `const_is_empty` lint

2 files changed, 74 insertions, 1 deletions

diff --git a/clippy_lints/src/methods/is_empty.rs b/clippy_lints/src/methods/is_empty.rs
new file mode 100644
index 00000000000..4713c99d33b
--- /dev/null
+++ b/clippy_lints/src/methods/is_empty.rs
@@ -0,0 +1,40 @@
+use clippy_utils::diagnostics::span_lint_and_note;
+use clippy_utils::expr_or_init;
+use rustc_ast::LitKind;
+use rustc_hir::{Expr, ExprKind};
+use rustc_lint::{LateContext, LintContext};
+use rustc_middle::lint::in_external_macro;
+use rustc_span::source_map::Spanned;
+
+use super::CONST_IS_EMPTY;
+
+pub(super) fn check(cx: &LateContext<'_>, expr: &'_ Expr<'_>, receiver: &Expr<'_>) {
+    if in_external_macro(cx.sess(), expr.span) || !receiver.span.eq_ctxt(expr.span) {
+        return;
+    }
+    let init_expr = expr_or_init(cx, receiver);
+    if let Some(init_is_empty) = is_empty(init_expr)
+        && init_expr.span.eq_ctxt(receiver.span)
+    {
+        span_lint_and_note(
+            cx,
+            CONST_IS_EMPTY,
+            expr.span,
+            &format!("this expression always evaluates to {init_is_empty:?}"),
+            Some(init_expr.span),
+            "because its initialization value is constant",
+        );
+    }
+}
+
+fn is_empty(expr: &'_ rustc_hir::Expr<'_>) -> Option<bool> {
+    if let ExprKind::Lit(Spanned { node, .. }) = expr.kind {
+        match node {
+            LitKind::Str(sym, _) => Some(sym.is_empty()),
+            LitKind::ByteStr(value, _) | LitKind::CStr(value, _) => Some(value.is_empty()),
+            _ => None,
+        }
+    } else {
+        None
+    }
+}
diff --git a/clippy_lints/src/methods/mod.rs b/clippy_lints/src/methods/mod.rs
index 820f4c85890..b6894971acc 100644
--- a/clippy_lints/src/methods/mod.rs
+++ b/clippy_lints/src/methods/mod.rs
@@ -36,6 +36,7 @@ mod inefficient_to_string;
 mod inspect_for_each;
 mod into_iter_on_ref;
 mod is_digit_ascii_radix;
+mod is_empty;
 mod iter_cloned_collect;
 mod iter_count;
 mod iter_filter;
@@ -4041,6 +4042,31 @@ declare_clippy_lint! {
     "calling `.get().is_some()` or `.get().is_none()` instead of `.contains()` or `.contains_key()`"
 }
 
+declare_clippy_lint! {
+    /// ### What it does
+    /// It identifies calls to `.is_empty()` on constant values.
+    ///
+    /// ### Why is this bad?
+    /// String literals and constant values are known at compile time. Checking if they
+    /// are empty will always return the same value. This might not be the intention of
+    /// the expression.
+    ///
+    /// ### Example
+    /// ```no_run
+    /// let value = "";
+    /// if value.is_empty() {
+    ///     println!("the string is empty");
+    /// }
+    /// ```
+    /// Use instead:
+    /// ```no_run
+    /// println!("the string is empty");
+    /// ```
+    #[clippy::version = "1.78.0"]
+    pub CONST_IS_EMPTY,
+    suspicious,
+    "is_empty() called on strings known at compile time"
+}
 pub struct Methods {
     avoid_breaking_exported_api: bool,
     msrv: Msrv,
@@ -4089,6 +4115,7 @@ impl_lint_pass!(Methods => [
     CLONE_ON_COPY,
     CLONE_ON_REF_PTR,
     COLLAPSIBLE_STR_REPLACE,
+    CONST_IS_EMPTY,
     ITER_OVEREAGER_CLONED,
     CLONED_INSTEAD_OF_COPIED,
     FLAT_MAP_OPTION,
@@ -4442,7 +4469,7 @@ impl Methods {
                 ("as_deref" | "as_deref_mut", []) => {
                     needless_option_as_deref::check(cx, expr, recv, name);
                 },
-                ("as_bytes" | "is_empty", []) => {
+                ("as_bytes", []) => {
                     if let Some(("as_str", recv, [], as_str_span, _)) = method_call(recv) {
                         redundant_as_str::check(cx, expr, recv, as_str_span, span);
                     }
@@ -4616,6 +4643,12 @@ impl Methods {
                 ("hash", [arg]) => {
                     unit_hash::check(cx, expr, recv, arg);
                 },
+                ("is_empty", []) => {
+                    if let Some(("as_str", recv, [], as_str_span, _)) = method_call(recv) {
+                        redundant_as_str::check(cx, expr, recv, as_str_span, span);
+                    }
+                    is_empty::check(cx, expr, recv);
+                },
                 ("is_file", []) => filetype_is_file::check(cx, expr, recv),
                 ("is_digit", [radix]) => is_digit_ascii_radix::check(cx, expr, recv, radix, &self.msrv),
                 ("is_none", []) => check_is_some_is_none(cx, expr, recv, call_span, false),