correct suggestion for repeat_vec_with_capacity in a no_std environment

Author: lapla-cogito

clippy_lints/src/repeat_vec_with_capacity.rs

@@ -3,7 +3,7 @@ use clippy_utils::diagnostics::span_lint_and_then;
 use clippy_utils::higher::VecArgs;
 use clippy_utils::macros::matching_root_macro_call;
 use clippy_utils::source::snippet;
-use clippy_utils::{expr_or_init, fn_def_id};
+use clippy_utils::{expr_or_init, fn_def_id, std_or_core};
 use rustc_errors::Applicability;
 use rustc_hir::{Expr, ExprKind};
 use rustc_lint::{LateContext, LateLintPass};
@@ -93,14 +93,18 @@ fn check_repeat_fn(cx: &LateContext<'_>, expr: &Expr<'_>) {
         && let ExprKind::Call(_, [repeat_expr]) = expr.kind
         && fn_def_id(cx, repeat_expr).is_some_and(|did| cx.tcx.is_diagnostic_item(sym::vec_with_capacity, did))
         && !repeat_expr.span.from_expansion()
+        && let Some(exec_context) = std_or_core(cx)
     {
         emit_lint(
             cx,
             expr.span,
             "iter::repeat",
             "none of the yielded `Vec`s will have the requested capacity",
             "if you intended to create an iterator that yields `Vec`s with an initial capacity, try",
-            format!("std::iter::repeat_with(|| {})", snippet(cx, repeat_expr.span, "..")),
+            format!(
+                "{exec_context}::iter::repeat_with(|| {})",
+                snippet(cx, repeat_expr.span, "..")
+            ),
         );
     }
 }

tests/ui/repeat_vec_with_capacity_nostd.fixed

@@ -0,0 +1,10 @@
+#![warn(clippy::repeat_vec_with_capacity)]
+#![allow(clippy::manual_repeat_n)]
+#![no_std]
+use core::iter;
+extern crate alloc;
+use alloc::vec::Vec;
+
+fn nostd() {
+    let _: Vec<Vec<u8>> = core::iter::repeat_with(|| Vec::with_capacity(42)).take(123).collect();
+}

tests/ui/repeat_vec_with_capacity_nostd.rs

@@ -0,0 +1,10 @@
+#![warn(clippy::repeat_vec_with_capacity)]
+#![allow(clippy::manual_repeat_n)]
+#![no_std]
+use core::iter;
+extern crate alloc;
+use alloc::vec::Vec;
+
+fn nostd() {
+    let _: Vec<Vec<u8>> = iter::repeat(Vec::with_capacity(42)).take(123).collect();
+}

tests/ui/repeat_vec_with_capacity_nostd.stderr

@@ -0,0 +1,16 @@
+error: repeating `Vec::with_capacity` using `iter::repeat`, which does not retain capacity
+  --> tests/ui/repeat_vec_with_capacity_nostd.rs:9:27
+   |
+LL |     let _: Vec<Vec<u8>> = iter::repeat(Vec::with_capacity(42)).take(123).collect();
+   |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: none of the yielded `Vec`s will have the requested capacity
+   = note: `-D clippy::repeat-vec-with-capacity` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::repeat_vec_with_capacity)]`
+help: if you intended to create an iterator that yields `Vec`s with an initial capacity, try
+   |
+LL |     let _: Vec<Vec<u8>> = core::iter::repeat_with(|| Vec::with_capacity(42)).take(123).collect();
+   |                           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+error: aborting due to 1 previous error
+