fix: Reimplement crate_supports_no_std syntactic heuristic

crates/hir-def/src/attrs.rs

@@ -380,6 +380,29 @@ pub fn parse_extra_crate_attrs(db: &dyn SourceDatabase, krate: Crate) -> Option<
     Some(p.tree())
 }
 
+/// Whether the crate root declares `#![no_std]`, looking through `cfg_attr` gating.
+#[salsa::tracked(returns(copy))]
+pub(crate) fn crate_supports_no_std(db: &dyn SourceDatabase, krate: Crate) -> bool {
+    fn contains_no_std(meta: ast::Meta) -> bool {
+        match meta {
+            ast::Meta::PathMeta(meta) => meta.path().is1("no_std"),
+            ast::Meta::CfgAttrMeta(meta) => meta.metas().any(contains_no_std),
+            ast::Meta::UnsafeMeta(meta) => meta.meta().is_some_and(contains_no_std),
+            ast::Meta::CfgMeta(_) | ast::Meta::KeyValueMeta(_) | ast::Meta::TokenTreeMeta(_) => {
+                false
+            }
+        }
+    }
+
+    let root_file = krate.root_file_id(db).parse(db).tree();
+    parse_extra_crate_attrs(db, krate)
+        .into_iter()
+        .flat_map(|extra| extra.attrs())
+        .chain(root_file.attrs())
+        .filter_map(|attr| attr.meta())
+        .any(contains_no_std)
+}
+
 fn attrs_source(
     db: &dyn SourceDatabase,
     owner: AttrDefId,

crates/hir-def/src/find_path.rs

@@ -1485,7 +1485,7 @@ pub mod fmt {
             "#]],
         );
 
-        // Should also work (on a best-effort basis) if `no_std` is conditional.
+        // Should also work (on a best-effort basis) if `no_std` is conditionally enabled.
         check_found_path(
             r#"
 //- /main.rs crate:main deps:core,std
@@ -1501,6 +1501,37 @@ pub mod fmt {
 
 //- /zzz.rs crate:core
 
+pub mod fmt {
+    pub struct Error;
+}
+        "#,
+            "core::fmt::Error",
+            expect![[r#"
+                Plain  (imports ✔): core::fmt::Error
+                Plain  (imports ✖): core::fmt::Error
+                ByCrate(imports ✔): core::fmt::Error
+                ByCrate(imports ✖): core::fmt::Error
+                BySelf (imports ✔): core::fmt::Error
+                BySelf (imports ✖): core::fmt::Error
+            "#]],
+        );
+
+        // Should also work (on a best-effort basis) if `no_std` is conditionally disabled.
+        check_found_path(
+            r#"
+//- /main.rs crate:main deps:core,std cfg:test
+#![cfg_attr(not(test), no_std)]
+
+$0
+
+//- /std.rs crate:std deps:core
+
+pub mod fmt {
+    pub use core::fmt::Error;
+}
+
+//- /zzz.rs crate:core
+
 pub mod fmt {
     pub struct Error;
 }