internal: start new macro test suite

I don't like our macro tests -- they are brittle and don't inspire
confidence. I think the reason for that is that we try to unit-test
them, but that is at odds with reality, where macro expansion
fundamentally depends on name resolution.

crates/hir_def/src/macro_expansion_tests.rs

@@ -0,0 +1,87 @@
+//! This module contains tests for macro expansion. Effectively, it covers `tt`,
+//! `mbe`, `proc_macro_api` and `hir_expand` crates. This might seem like a
+//! wrong architecture at the first glance, but is intentional.
+//!
+//! Physically, macro expansion process is intertwined with name resolution. You
+//! can not expand *just* the syntax. So, to be able to write integration tests
+//! of the "expand this code please" form, we have to do it after name
+//! resolution. That is, in this crate. We *could* fake some dependencies and
+//! write unit-tests (in fact, we used to do that), but that makes tests brittle
+//! and harder to understand.
+
+use std::ops::Range;
+
+use base_db::{fixture::WithFixture, SourceDatabase};
+use expect_test::{expect, Expect};
+use hir_expand::{db::AstDatabase, InFile, MacroFile};
+use stdx::format_to;
+use syntax::{ast, AstNode};
+
+use crate::{
+    db::DefDatabase, nameres::ModuleSource, resolver::HasResolver, test_db::TestDB, AsMacroCall,
+};
+
+fn check(ra_fixture: &str, expect: Expect) {
+    let db = TestDB::with_files(ra_fixture);
+    let krate = db.crate_graph().iter().next().unwrap();
+    let def_map = db.crate_def_map(krate);
+    let local_id = def_map.root();
+    let module = def_map.module_id(local_id);
+    let resolver = module.resolver(&db);
+    let source = def_map[local_id].definition_source(&db);
+    let source_file = match source.value {
+        ModuleSource::SourceFile(it) => it,
+        ModuleSource::Module(_) | ModuleSource::BlockExpr(_) => panic!(),
+    };
+
+    let mut expansions = Vec::new();
+    for macro_call in source_file.syntax().descendants().filter_map(ast::MacroCall::cast) {
+        let macro_call = InFile::new(source.file_id, &macro_call);
+        let macro_call_id = macro_call
+            .as_call_id_with_errors(
+                &db,
+                krate,
+                |path| resolver.resolve_path_as_macro(&db, &path),
+                &mut |err| panic!("{}", err),
+            )
+            .unwrap()
+            .unwrap();
+        let macro_file = MacroFile { macro_call_id };
+        let expansion_result = db.parse_macro_expansion(macro_file);
+        expansions.push((macro_call.value.clone(), expansion_result));
+    }
+
+    let mut expanded_text = source_file.to_string();
+    for (call, exp) in expansions.into_iter().rev() {
+        let mut expn_text = String::new();
+        if let Some(err) = exp.err {
+            format_to!(expn_text, "/* error: {} */", err);
+        }
+        if let Some((parse, _token_map)) = exp.value {
+            format_to!(expn_text, "{}", parse.syntax_node());
+        }
+        let range = call.syntax().text_range();
+        let range: Range<usize> = range.into();
+        expanded_text.replace_range(range, &expn_text)
+    }
+
+    expect.assert_eq(&expanded_text);
+}
+
+#[test]
+fn test_expand_rule() {
+    check(
+        r#"
+macro_rules! m {
+    ($($i:ident);*) => ($i)
+}
+m!{a}
+"#,
+        expect![[r#"
+macro_rules! m {
+    ($($i:ident);*) => ($i)
+}
+/* error: expected simple binding, found nested binding `i` */
+"#]],
+    );
+}