fix:: [std_instead_of_core] should check extern crate prelude

If `alloc` or `core` aren't present in the prelude, then downgrade suggestions to helps to avoid suggesting broken fixes.

Author: bushrat011899

clippy_lints/src/std_instead_of_core.rs

@@ -3,10 +3,12 @@ use clippy_utils::diagnostics::{span_lint_and_help, span_lint_and_sugg};
 use clippy_utils::is_from_proc_macro;
 use clippy_utils::msrvs::Msrv;
 use clippy_utils::paths::{PathNS, lookup_path};
+use rustc_data_structures::fx::FxHashSet;
 use rustc_errors::{Applicability, MultiSpan};
+use rustc_hir::attrs::AttributeKind;
 use rustc_hir::def::{DefKind, Namespace, Res};
 use rustc_hir::def_id::DefId;
-use rustc_hir::{Block, Body, HirId, Path, PathSegment, StabilityLevel, StableSince};
+use rustc_hir::{Attribute, Block, Body, HirId, Item, ItemKind, Node, Path, PathSegment, StabilityLevel, StableSince};
 use rustc_lint::{LateContext, LateLintPass, LintContext};
 use rustc_session::impl_lint_pass;
 use rustc_span::symbol::kw;
@@ -255,7 +257,18 @@ fn emit_lints(cx: &LateContext<'_>, lint_points: Option<(Span, Vec<LintPoint>)>)
         }
     }
 
+    let extern_prelude = extern_prelude(cx);
+
     for (lint, used_mod, replace_with) in suggestions {
+        // If the target crate isn't in the extern crate prelude,
+        // the suggestion is likely to fail.
+        // It's possible it could still work (e.g., extern crate in a local scope),
+        // but we'll be cautious and downgrade from a suggestion to a help.
+        if !extern_prelude.contains(replace_with) {
+            helps.push((lint, used_mod, replace_with, krate_span.into()));
+            continue;
+        }
+
         span_lint_and_sugg(
             cx,
             lint,
@@ -324,3 +337,43 @@ fn is_stable(cx: &LateContext<'_>, mut def_id: DefId, msrv: Msrv) -> bool {
         }
     }
 }
+
+fn extern_prelude(cx: &LateContext<'_>) -> FxHashSet<Symbol> {
+    let mut extern_prelude = cx
+        .tcx
+        .hir_root_module()
+        .item_ids
+        .iter()
+        .filter_map(|item| match cx.tcx.hir_node(item.hir_id()) {
+            Node::Item(Item {
+                kind: ItemKind::ExternCrate(original_name, ident),
+                ..
+            }) => Some(original_name.unwrap_or(ident.name)),
+            _ => None,
+        })
+        .collect::<FxHashSet<_>>();
+
+    let implicit_core = !cx.tcx.hir_krate_attrs().iter().any(|attr| {
+        matches!(
+            attr,
+            Attribute::Parsed(AttributeKind::NoCore | AttributeKind::NoImplicitPrelude)
+        )
+    });
+
+    if implicit_core {
+        extern_prelude.insert(sym::core);
+    }
+
+    let implicit_std = !cx.tcx.hir_krate_attrs().iter().any(|attr| {
+        matches!(
+            attr,
+            Attribute::Parsed(AttributeKind::NoStd | AttributeKind::NoImplicitPrelude)
+        )
+    });
+
+    if implicit_std {
+        extern_prelude.insert(sym::std);
+    }
+
+    extern_prelude
+}

tests/ui/std_instead_of_core_no_implicit_prelude.fixed

@@ -0,0 +1,16 @@
+#![warn(clippy::std_instead_of_core)]
+#![warn(clippy::std_instead_of_alloc)]
+#![allow(unused_imports)]
+#![no_implicit_prelude]
+
+extern crate alloc;
+extern crate core;
+extern crate std;
+
+fn pr17252() {
+    use core::result::Result;
+    //~^ std_instead_of_core
+
+    use alloc::vec::Vec;
+    //~^ std_instead_of_alloc
+}

tests/ui/std_instead_of_core_no_implicit_prelude.rs

@@ -0,0 +1,16 @@
+#![warn(clippy::std_instead_of_core)]
+#![warn(clippy::std_instead_of_alloc)]
+#![allow(unused_imports)]
+#![no_implicit_prelude]
+
+extern crate alloc;
+extern crate core;
+extern crate std;
+
+fn pr17252() {
+    use std::result::Result;
+    //~^ std_instead_of_core
+
+    use std::vec::Vec;
+    //~^ std_instead_of_alloc
+}

tests/ui/std_instead_of_core_no_implicit_prelude.stderr

@@ -0,0 +1,20 @@
+error: used import from `std` instead of `core`
+  --> tests/ui/std_instead_of_core_no_implicit_prelude.rs:11:9
+   |
+LL |     use std::result::Result;
+   |         ^^^ help: consider importing the item from `core`: `core`
+   |
+   = note: `-D clippy::std-instead-of-core` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::std_instead_of_core)]`
+
+error: used import from `std` instead of `alloc`
+  --> tests/ui/std_instead_of_core_no_implicit_prelude.rs:14:9
+   |
+LL |     use std::vec::Vec;
+   |         ^^^ help: consider importing the item from `alloc`: `alloc`
+   |
+   = note: `-D clippy::std-instead-of-alloc` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::std_instead_of_alloc)]`
+
+error: aborting due to 2 previous errors
+

tests/ui/std_instead_of_core_no_implicit_prelude_unfixable.rs

@@ -0,0 +1,27 @@
+#![warn(clippy::std_instead_of_core)]
+#![warn(clippy::std_instead_of_alloc)]
+#![allow(unused_imports)]
+#![no_implicit_prelude]
+
+extern crate std;
+
+fn pr17252_no_core() {
+    // This wont have a suggestion since `core` isn't in scope
+    use std::result::Result;
+    //~^ std_instead_of_core
+}
+
+fn pr17252_no_alloc() {
+    // This wont have a suggestion since `alloc` isn't in scope
+    use std::vec::Vec;
+    //~^ std_instead_of_alloc
+}
+
+fn pr17252_local_core() {
+    extern crate core;
+
+    // FIXME: This *should* have a suggestion, but we're not currently
+    // checking local scope, only the extern crate prelude.
+    use std::result::Result;
+    //~^ std_instead_of_core
+}

tests/ui/std_instead_of_core_no_implicit_prelude_unfixable.stderr

@@ -0,0 +1,30 @@
+error: used import from `std` instead of `core`
+  --> tests/ui/std_instead_of_core_no_implicit_prelude_unfixable.rs:10:9
+   |
+LL |     use std::result::Result;
+   |         ^^^
+   |
+   = help: consider importing the item from `core`
+   = note: `-D clippy::std-instead-of-core` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::std_instead_of_core)]`
+
+error: used import from `std` instead of `alloc`
+  --> tests/ui/std_instead_of_core_no_implicit_prelude_unfixable.rs:16:9
+   |
+LL |     use std::vec::Vec;
+   |         ^^^
+   |
+   = help: consider importing the item from `alloc`
+   = note: `-D clippy::std-instead-of-alloc` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::std_instead_of_alloc)]`
+
+error: used import from `std` instead of `core`
+  --> tests/ui/std_instead_of_core_no_implicit_prelude_unfixable.rs:25:9
+   |
+LL |     use std::result::Result;
+   |         ^^^
+   |
+   = help: consider importing the item from `core`
+
+error: aborting due to 3 previous errors
+