feat: add const_is_empty lint

Author: samueltardieu

CHANGELOG.md

@@ -5109,6 +5109,7 @@ Released 2018-09-13
 [`collection_is_never_read`]: https://rust-lang.github.io/rust-clippy/master/index.html#collection_is_never_read
 [`comparison_chain`]: https://rust-lang.github.io/rust-clippy/master/index.html#comparison_chain
 [`comparison_to_empty`]: https://rust-lang.github.io/rust-clippy/master/index.html#comparison_to_empty
+[`const_is_empty`]: https://rust-lang.github.io/rust-clippy/master/index.html#const_is_empty
 [`const_static_lifetime`]: https://rust-lang.github.io/rust-clippy/master/index.html#const_static_lifetime
 [`copy_iterator`]: https://rust-lang.github.io/rust-clippy/master/index.html#copy_iterator
 [`crate_in_macro_def`]: https://rust-lang.github.io/rust-clippy/master/index.html#crate_in_macro_def

clippy_lints/src/declared_lints.rs

@@ -349,6 +349,7 @@ pub(crate) static LINTS: &[&crate::LintInfo] = &[
     crate::methods::CLONE_ON_COPY_INFO,
     crate::methods::CLONE_ON_REF_PTR_INFO,
     crate::methods::COLLAPSIBLE_STR_REPLACE_INFO,
+    crate::methods::CONST_IS_EMPTY_INFO,
     crate::methods::DRAIN_COLLECT_INFO,
     crate::methods::ERR_EXPECT_INFO,
     crate::methods::EXPECT_FUN_CALL_INFO,

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
+    }
+}

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;
@@ -4011,6 +4012,30 @@ declare_clippy_lint! {
     r#"creating a `CStr` through functions when `c""` literals can be used"#
 }
 
+declare_clippy_lint! {
+    /// ### What it does
+    /// It identifies calls to `.is_empty()` on a value initialized from a literal string.
+    ///
+    /// ### Why is this bad?
+    /// Literal strings are known at compile time. Their emptyness can be determined statically.
+    ///
+    /// ### 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,
@@ -4059,6 +4084,7 @@ 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,
@@ -4411,7 +4437,7 @@ 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);
                     }
@@ -4585,6 +4611,12 @@ 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, false),

tests/ui/const_is_empty.rs

@@ -0,0 +1,14 @@
+#![warn(clippy::const_is_empty)]
+
+fn main() {
+    let value = "foobar";
+    let _ = value.is_empty();
+    //~^ ERROR: this expression always evaluates to false
+    let x = value;
+    let _ = x.is_empty();
+    //~^ ERROR: this expression always evaluates to false
+    let _ = "".is_empty();
+    //~^ ERROR: this expression always evaluates to true
+    let _ = b"".is_empty();
+    //~^ ERROR: this expression always evaluates to true
+}

tests/ui/const_is_empty.stderr

@@ -0,0 +1,52 @@
+error: this expression always evaluates to false
+  --> tests/ui/const_is_empty.rs:5:13
+   |
+LL |     let _ = value.is_empty();
+   |             ^^^^^^^^^^^^^^^^
+   |
+note: because its initialization value is constant
+  --> tests/ui/const_is_empty.rs:4:17
+   |
+LL |     let value = "foobar";
+   |                 ^^^^^^^^
+   = note: `-D clippy::const-is-empty` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::const_is_empty)]`
+
+error: this expression always evaluates to false
+  --> tests/ui/const_is_empty.rs:8:13
+   |
+LL |     let _ = x.is_empty();
+   |             ^^^^^^^^^^^^
+   |
+note: because its initialization value is constant
+  --> tests/ui/const_is_empty.rs:4:17
+   |
+LL |     let value = "foobar";
+   |                 ^^^^^^^^
+
+error: this expression always evaluates to true
+  --> tests/ui/const_is_empty.rs:10:13
+   |
+LL |     let _ = "".is_empty();
+   |             ^^^^^^^^^^^^^
+   |
+note: because its initialization value is constant
+  --> tests/ui/const_is_empty.rs:10:13
+   |
+LL |     let _ = "".is_empty();
+   |             ^^
+
+error: this expression always evaluates to true
+  --> tests/ui/const_is_empty.rs:12:13
+   |
+LL |     let _ = b"".is_empty();
+   |             ^^^^^^^^^^^^^^
+   |
+note: because its initialization value is constant
+  --> tests/ui/const_is_empty.rs:12:13
+   |
+LL |     let _ = b"".is_empty();
+   |             ^^^
+
+error: aborting due to 4 previous errors
+