Correctly suggest std or core path depending if this is a no_std crate

clippy_lints/src/default_instead_of_iter_empty.rs

@@ -1,6 +1,6 @@
 use clippy_utils::diagnostics::span_lint_and_sugg;
-use clippy_utils::last_path_segment;
 use clippy_utils::source::snippet_with_context;
+use clippy_utils::{is_no_std_crate, last_path_segment};
 use rustc_errors::Applicability;
 use rustc_hir::{def, Expr, ExprKind, GenericArg, QPath, TyKind};
 use rustc_lint::{LateContext, LateLintPass};
@@ -42,12 +42,17 @@ impl<'tcx> LateLintPass<'tcx> for DefaultIterEmpty {
             && ty.span.ctxt() == ctxt
         {
             let mut applicability = Applicability::MachineApplicable;
-            let sugg = make_sugg(cx, ty_path, ctxt, &mut applicability);
+            let path = if is_no_std_crate(cx) {
+                "core::iter::empty"
+            } else {
+                "std::iter::empty"
+            };
+            let sugg = make_sugg(cx, ty_path, ctxt, &mut applicability, path);
             span_lint_and_sugg(
                 cx,
                 DEFAULT_INSTEAD_OF_ITER_EMPTY,
                 expr.span,
-                "`std::iter::empty()` is the more idiomatic way",
+                &format!("`{path}()` is the more idiomatic way"),
                 "try",
                 sugg,
                 applicability,
@@ -61,6 +66,7 @@ fn make_sugg(
     ty_path: &rustc_hir::QPath<'_>,
     ctxt: SyntaxContext,
     applicability: &mut Applicability,
+    path: &str,
 ) -> String {
     if let Some(last) = last_path_segment(ty_path).args
         && let Some(iter_ty) = last.args.iter().find_map(|arg| match arg {
@@ -69,10 +75,10 @@ fn make_sugg(
         })
     {
         format!(
-            "std::iter::empty::<{}>()",
+            "{path}::<{}>()",
             snippet_with_context(cx, iter_ty.span, ctxt, "..", applicability).0
         )
     } else {
-        "std::iter::empty()".to_owned()
+        format!("{path}()")
     }
 }

clippy_lints/src/mem_replace.rs

@@ -3,7 +3,7 @@ use clippy_utils::diagnostics::{span_lint_and_help, span_lint_and_sugg, span_lin
 use clippy_utils::source::{snippet, snippet_with_applicability};
 use clippy_utils::sugg::Sugg;
 use clippy_utils::ty::is_non_aggregate_primitive_type;
-use clippy_utils::{is_default_equivalent, is_res_lang_ctor, path_res, peel_ref_operators};
+use clippy_utils::{is_default_equivalent, is_no_std_crate, is_res_lang_ctor, path_res, peel_ref_operators};
 use rustc_errors::Applicability;
 use rustc_hir::LangItem::OptionNone;
 use rustc_hir::{Expr, ExprKind};
@@ -123,6 +123,10 @@ fn check_replace_option_with_none(cx: &LateContext<'_>, dest: &Expr<'_>, expr_sp
     );
 }
 
+fn get_top_crate(cx: &LateContext<'_>) -> &'static str {
+    if is_no_std_crate(cx) { "core" } else { "std" }
+}
+
 fn check_replace_with_uninit(cx: &LateContext<'_>, src: &Expr<'_>, dest: &Expr<'_>, expr_span: Span) {
     if let Some(method_def_id) = cx.typeck_results().type_dependent_def_id(src.hir_id)
         // check if replacement is mem::MaybeUninit::uninit().assume_init()
@@ -136,7 +140,8 @@ fn check_replace_with_uninit(cx: &LateContext<'_>, src: &Expr<'_>, dest: &Expr<'
             "replacing with `mem::MaybeUninit::uninit().assume_init()`",
             "consider using",
             format!(
-                "std::ptr::read({})",
+                "{}::ptr::read({})",
+                get_top_crate(cx),
                 snippet_with_applicability(cx, dest.span, "", &mut applicability)
             ),
             applicability,
@@ -157,7 +162,8 @@ fn check_replace_with_uninit(cx: &LateContext<'_>, src: &Expr<'_>, dest: &Expr<'
                 "replacing with `mem::uninitialized()`",
                 "consider using",
                 format!(
-                    "std::ptr::read({})",
+                    "{}::ptr::read({})",
+                    get_top_crate(cx),
                     snippet_with_applicability(cx, dest.span, "", &mut applicability)
                 ),
                 applicability,
@@ -184,14 +190,17 @@ fn check_replace_with_default(cx: &LateContext<'_>, src: &Expr<'_>, dest: &Expr<
         return;
     }
     if is_default_equivalent(cx, src) && !in_external_macro(cx.tcx.sess, expr_span) {
+        let top_crate = get_top_crate(cx);
         span_lint_and_then(
             cx,
             MEM_REPLACE_WITH_DEFAULT,
             expr_span,
-            "replacing a value of type `T` with `T::default()` is better expressed using `std::mem::take`",
+            &format!(
+                "replacing a value of type `T` with `T::default()` is better expressed using `{top_crate}::mem::take`"
+            ),
             |diag| {
                 if !expr_span.from_expansion() {
-                    let suggestion = format!("std::mem::take({})", snippet(cx, dest.span, ""));
+                    let suggestion = format!("{top_crate}::mem::take({})", snippet(cx, dest.span, ""));
 
                     diag.span_suggestion(
                         expr_span,

clippy_lints/src/methods/iter_on_single_or_empty_collections.rs

@@ -58,10 +58,10 @@ pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, method_name: &str, re
         return;
     }
 
+    let top_crate = if is_no_std_crate(cx) { "core" } else { "std" };
     if let Some(i) = item {
         let sugg = format!(
-            "{}::iter::once({}{})",
-            if is_no_std_crate(cx) { "core" } else { "std" },
+            "{top_crate}::iter::once({}{})",
             iter_type.ref_prefix(),
             snippet(cx, i.span, "...")
         );
@@ -81,11 +81,7 @@ pub(super) fn check(cx: &LateContext<'_>, expr: &Expr<'_>, method_name: &str, re
             expr.span,
             &format!("`{method_name}` call on an empty collection"),
             "try",
-            if is_no_std_crate(cx) {
-                "core::iter::empty()".to_string()
-            } else {
-                "std::iter::empty()".to_string()
-            },
+            format!("{top_crate}::iter::empty()"),
             Applicability::MaybeIncorrect,
         );
     }

clippy_lints/src/methods/unnecessary_sort_by.rs

@@ -204,7 +204,7 @@ pub(super) fn check<'tcx>(
             cx,
             UNNECESSARY_SORT_BY,
             expr.span,
-            "use Vec::sort_by_key here instead",
+            "consider using `sort_by_key`",
             "try",
             format!(
                 "{}.sort{}_by_key(|{}| {})",
@@ -227,7 +227,7 @@ pub(super) fn check<'tcx>(
             cx,
             UNNECESSARY_SORT_BY,
             expr.span,
-            "use Vec::sort here instead",
+            "consider using `sort`",
             "try",
             format!(
                 "{}.sort{}()",

clippy_lints/src/operators/ptr_eq.rs

@@ -1,13 +1,12 @@
 use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::is_no_std_crate;
 use clippy_utils::source::snippet_opt;
 use rustc_errors::Applicability;
 use rustc_hir::{BinOpKind, Expr, ExprKind};
 use rustc_lint::LateContext;
 
 use super::PTR_EQ;
 
-static LINT_MSG: &str = "use `std::ptr::eq` when comparing raw pointers";
-
 pub(super) fn check<'tcx>(
     cx: &LateContext<'tcx>,
     expr: &'tcx Expr<'_>,
@@ -26,13 +25,14 @@ pub(super) fn check<'tcx>(
             && let Some(left_snip) = snippet_opt(cx, left_var.span)
             && let Some(right_snip) = snippet_opt(cx, right_var.span)
         {
+            let top_crate = if is_no_std_crate(cx) { "core" } else { "std" };
             span_lint_and_sugg(
                 cx,
                 PTR_EQ,
                 expr.span,
-                LINT_MSG,
+                &format!("use `{top_crate}::ptr::eq` when comparing raw pointers"),
                 "try",
-                format!("std::ptr::eq({left_snip}, {right_snip})"),
+                format!("{top_crate}::ptr::eq({left_snip}, {right_snip})"),
                 Applicability::MachineApplicable,
             );
         }

clippy_lints/src/transmute/transmute_int_to_char.rs

@@ -1,6 +1,6 @@
 use super::TRANSMUTE_INT_TO_CHAR;
 use clippy_utils::diagnostics::span_lint_and_then;
-use clippy_utils::sugg;
+use clippy_utils::{is_no_std_crate, sugg};
 use rustc_ast as ast;
 use rustc_errors::Applicability;
 use rustc_hir::Expr;
@@ -34,7 +34,10 @@ pub(super) fn check<'tcx>(
                     diag.span_suggestion(
                         e.span,
                         "consider using",
-                        format!("std::char::from_u32({arg}).unwrap()"),
+                        format!(
+                            "{}::char::from_u32({arg}).unwrap()",
+                            if is_no_std_crate(cx) { "core" } else { "std" }
+                        ),
                         Applicability::Unspecified,
                     );
                 },

clippy_lints/src/transmute/transmute_ref_to_ref.rs

@@ -1,7 +1,7 @@
 use super::{TRANSMUTE_BYTES_TO_STR, TRANSMUTE_PTR_TO_PTR};
 use clippy_utils::diagnostics::{span_lint_and_sugg, span_lint_and_then};
 use clippy_utils::source::snippet;
-use clippy_utils::sugg;
+use clippy_utils::{is_no_std_crate, sugg};
 use rustc_errors::Applicability;
 use rustc_hir::{Expr, Mutability};
 use rustc_lint::LateContext;
@@ -29,16 +29,18 @@ pub(super) fn check<'tcx>(
 
             let snippet = snippet(cx, arg.span, "..");
 
+            let top_crate = if is_no_std_crate(cx) { "core" } else { "std" };
+
             span_lint_and_sugg(
                 cx,
                 TRANSMUTE_BYTES_TO_STR,
                 e.span,
                 &format!("transmute from a `{from_ty}` to a `{to_ty}`"),
                 "consider using",
                 if const_context {
-                    format!("std::str::from_utf8_unchecked{postfix}({snippet})")
+                    format!("{top_crate}::str::from_utf8_unchecked{postfix}({snippet})")
                 } else {
-                    format!("std::str::from_utf8{postfix}({snippet}).unwrap()")
+                    format!("{top_crate}::str::from_utf8{postfix}({snippet}).unwrap()")
                 },
                 Applicability::MaybeIncorrect,
             );

tests/ui/default_instead_of_iter_empty_no_std.fixed

@@ -0,0 +1,28 @@
+#![warn(clippy::default_instead_of_iter_empty)]
+#![allow(dead_code)]
+#![feature(lang_items)]
+#![no_std]
+
+use core::panic::PanicInfo;
+
+#[lang = "eh_personality"]
+extern "C" fn eh_personality() {}
+
+#[panic_handler]
+fn panic(info: &PanicInfo) -> ! {
+    loop {}
+}
+
+#[derive(Default)]
+struct Iter {
+    iter: core::iter::Empty<usize>,
+}
+
+fn main() {
+    // Do lint.
+    let _ = core::iter::empty::<usize>();
+    let _foo: core::iter::Empty<usize> = core::iter::empty();
+
+    // Do not lint.
+    let _ = Iter::default();
+}

tests/ui/default_instead_of_iter_empty_no_std.rs

@@ -0,0 +1,28 @@
+#![warn(clippy::default_instead_of_iter_empty)]
+#![allow(dead_code)]
+#![feature(lang_items)]
+#![no_std]
+
+use core::panic::PanicInfo;
+
+#[lang = "eh_personality"]
+extern "C" fn eh_personality() {}
+
+#[panic_handler]
+fn panic(info: &PanicInfo) -> ! {
+    loop {}
+}
+
+#[derive(Default)]
+struct Iter {
+    iter: core::iter::Empty<usize>,
+}
+
+fn main() {
+    // Do lint.
+    let _ = core::iter::Empty::<usize>::default();
+    let _foo: core::iter::Empty<usize> = core::iter::Empty::default();
+
+    // Do not lint.
+    let _ = Iter::default();
+}

tests/ui/default_instead_of_iter_empty_no_std.stderr

@@ -0,0 +1,17 @@
+error: `core::iter::empty()` is the more idiomatic way
+  --> $DIR/default_instead_of_iter_empty_no_std.rs:23:13
+   |
+LL |     let _ = core::iter::Empty::<usize>::default();
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `core::iter::empty::<usize>()`
+   |
+   = note: `-D clippy::default-instead-of-iter-empty` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::default_instead_of_iter_empty)]`
+
+error: `core::iter::empty()` is the more idiomatic way
+  --> $DIR/default_instead_of_iter_empty_no_std.rs:24:42
+   |
+LL |     let _foo: core::iter::Empty<usize> = core::iter::Empty::default();
+   |                                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `core::iter::empty()`
+
+error: aborting due to 2 previous errors
+

tests/ui/mem_replace_no_std.fixed

@@ -0,0 +1,82 @@
+#![allow(unused)]
+#![warn(
+    clippy::all,
+    clippy::style,
+    clippy::mem_replace_option_with_none,
+    clippy::mem_replace_with_default
+)]
+#![feature(lang_items)]
+#![no_std]
+
+use core::mem;
+use core::panic::PanicInfo;
+
+#[lang = "eh_personality"]
+extern "C" fn eh_personality() {}
+
+#[panic_handler]
+fn panic(info: &PanicInfo) -> ! {
+    loop {}
+}
+
+fn replace_option_with_none() {
+    let mut an_option = Some(1);
+    let _ = an_option.take();
+    let an_option = &mut Some(1);
+    let _ = an_option.take();
+}
+
+fn replace_with_default() {
+    let mut refstr = "hello";
+    let _ = core::mem::take(&mut refstr);
+
+    let mut slice: &[i32] = &[1, 2, 3];
+    let _ = core::mem::take(&mut slice);
+}
+
+// lint is disabled for primitives because in this case `take`
+// has no clear benefit over `replace` and sometimes is harder to read
+fn dont_lint_primitive() {
+    let mut pbool = true;
+    let _ = mem::replace(&mut pbool, false);
+
+    let mut pint = 5;
+    let _ = mem::replace(&mut pint, 0);
+}
+
+fn main() {
+    replace_option_with_none();
+    replace_with_default();
+    dont_lint_primitive();
+}
+
+fn issue9824() {
+    struct Foo<'a>(Option<&'a str>);
+    impl<'a> core::ops::Deref for Foo<'a> {
+        type Target = Option<&'a str>;
+
+        fn deref(&self) -> &Self::Target {
+            &self.0
+        }
+    }
+    impl<'a> core::ops::DerefMut for Foo<'a> {
+        fn deref_mut(&mut self) -> &mut Self::Target {
+            &mut self.0
+        }
+    }
+
+    struct Bar {
+        opt: Option<u8>,
+        val: u8,
+    }
+
+    let mut f = Foo(Some("foo"));
+    let mut b = Bar { opt: Some(1), val: 12 };
+
+    // replace option with none
+    let _ = f.0.take();
+    let _ = (*f).take();
+    let _ = b.opt.take();
+    // replace with default
+    let _ = mem::replace(&mut b.val, u8::default());
+}