Auto merge of rust-lang#18136 - valadaptive:no-mangle-lints, r=Veykril

Don't lint names of #[no_mangle] extern fns

[Rust doesn't run the `non_snake_case_name` lint on `extern fn`s with the `#[no_mangle]` attribute](rust-lang#44966).

The conditions are:
- The function must be `extern` and have a `#[no_mangle]` attribute.
- The function's ABI must not be explicitly set to "Rust".

This PR replicates that logic here.

src/tools/rust-analyzer/crates/hir-ty/src/diagnostics/decl_check.rs

@@ -24,6 +24,7 @@ use hir_expand::{
     name::{AsName, Name},
     HirFileId, HirFileIdExt,
 };
+use intern::sym;
 use stdx::{always, never};
 use syntax::{
     ast::{self, HasName},
@@ -197,12 +198,20 @@ impl<'a> DeclValidator<'a> {
         // Skipped if function is an associated item of a trait implementation.
         if !self.is_trait_impl_container(container) {
             let data = self.db.function_data(func);
-            self.create_incorrect_case_diagnostic_for_item_name(
-                func,
-                &data.name,
-                CaseType::LowerSnakeCase,
-                IdentType::Function,
-            );
+
+            // Don't run the lint on extern "[not Rust]" fn items with the
+            // #[no_mangle] attribute.
+            let no_mangle = data.attrs.by_key(&sym::no_mangle).exists();
+            if no_mangle && data.abi.as_ref().is_some_and(|abi| *abi != sym::Rust) {
+                cov_mark::hit!(extern_func_no_mangle_ignored);
+            } else {
+                self.create_incorrect_case_diagnostic_for_item_name(
+                    func,
+                    &data.name,
+                    CaseType::LowerSnakeCase,
+                    IdentType::Function,
+                );
+            }
         } else {
             cov_mark::hit!(trait_impl_assoc_func_name_incorrect_case_ignored);
         }

src/tools/rust-analyzer/crates/ide-diagnostics/src/handlers/incorrect_case.rs

@@ -418,6 +418,64 @@ fn f((_O): u8) {}
         )
     }
 
+    #[test]
+    fn ignores_no_mangle_items() {
+        cov_mark::check!(extern_func_no_mangle_ignored);
+        check_diagnostics(
+            r#"
+#[no_mangle]
+extern "C" fn NonSnakeCaseName(some_var: u8) -> u8;
+            "#,
+        );
+    }
+
+    #[test]
+    fn ignores_no_mangle_items_with_no_abi() {
+        cov_mark::check!(extern_func_no_mangle_ignored);
+        check_diagnostics(
+            r#"
+#[no_mangle]
+extern fn NonSnakeCaseName(some_var: u8) -> u8;
+            "#,
+        );
+    }
+
+    #[test]
+    fn no_mangle_items_with_rust_abi() {
+        check_diagnostics(
+            r#"
+#[no_mangle]
+extern "Rust" fn NonSnakeCaseName(some_var: u8) -> u8;
+              // ^^^^^^^^^^^^^^^^ 💡 warn: Function `NonSnakeCaseName` should have snake_case name, e.g. `non_snake_case_name`
+            "#,
+        );
+    }
+
+    #[test]
+    fn no_mangle_items_non_extern() {
+        check_diagnostics(
+            r#"
+#[no_mangle]
+fn NonSnakeCaseName(some_var: u8) -> u8;
+// ^^^^^^^^^^^^^^^^ 💡 warn: Function `NonSnakeCaseName` should have snake_case name, e.g. `non_snake_case_name`
+            "#,
+        );
+    }
+
+    #[test]
+    fn extern_fn_name() {
+        check_diagnostics(
+            r#"
+extern "C" fn NonSnakeCaseName(some_var: u8) -> u8;
+           // ^^^^^^^^^^^^^^^^ 💡 warn: Function `NonSnakeCaseName` should have snake_case name, e.g. `non_snake_case_name`
+extern "Rust" fn NonSnakeCaseName(some_var: u8) -> u8;
+              // ^^^^^^^^^^^^^^^^ 💡 warn: Function `NonSnakeCaseName` should have snake_case name, e.g. `non_snake_case_name`
+extern fn NonSnakeCaseName(some_var: u8) -> u8;
+       // ^^^^^^^^^^^^^^^^ 💡 warn: Function `NonSnakeCaseName` should have snake_case name, e.g. `non_snake_case_name`
+            "#,
+        );
+    }
+
     #[test]
     fn ignores_extern_items() {
         cov_mark::check!(extern_func_incorrect_case_ignored);