Fix alloc_instead_of_core FP when alloc is an alias (#15581)

Closes #15579

changelog: `alloc_instead_of_core` fix FP when `alloc` is an alias

Author: Jarcho

clippy_lints/src/std_instead_of_core.rs

@@ -126,6 +126,8 @@ impl<'tcx> LateLintPass<'tcx> for StdReexports {
             && !is_from_proc_macro(cx, &first_segment.ident)
             && !matches!(def_kind, DefKind::Macro(_))
             && let Some(last_segment) = path.segments.last()
+            && let Res::Def(DefKind::Mod, crate_def_id) = first_segment.res
+            && crate_def_id.is_crate_root()
         {
             let (lint, used_mod, replace_with) = match first_segment.ident.name {
                 sym::std => match cx.tcx.crate_name(def_id.krate) {

tests/ui/std_instead_of_core.fixed

@@ -89,3 +89,10 @@ fn msrv_1_76(_: std::net::IpAddr) {}
 #[clippy::msrv = "1.77"]
 fn msrv_1_77(_: core::net::IpAddr) {}
 //~^ std_instead_of_core
+
+#[warn(clippy::alloc_instead_of_core)]
+fn issue15579() {
+    use std::alloc;
+
+    let layout = alloc::Layout::new::<u8>();
+}

tests/ui/std_instead_of_core.rs

@@ -89,3 +89,10 @@ fn msrv_1_76(_: std::net::IpAddr) {}
 #[clippy::msrv = "1.77"]
 fn msrv_1_77(_: std::net::IpAddr) {}
 //~^ std_instead_of_core
+
+#[warn(clippy::alloc_instead_of_core)]
+fn issue15579() {
+    use std::alloc;
+
+    let layout = alloc::Layout::new::<u8>();
+}