introduce hir debugging infra

This is to make debugging rust-analyzer easier.

The idea is that `dbg!(krate.debug(db))` will print the actual, fuzzy
crate name, instead of precise ID. Debug printing infra is a separate
thing, to make sure that the actual hir doesn't have access to global
information.

Do not use `.debug` for `log::` logging: debugging executes queries,
and might introduce unneded dependencies to the crate graph

crates/ra_hir/src/debug.rs

@@ -0,0 +1,64 @@
+use std::{cell::Cell, fmt};
+
+use ra_db::{CrateId, FileId};
+
+use crate::{db::HirDatabase, Crate, Module, Name};
+
+impl Crate {
+    pub fn debug(self, db: &impl HirDebugDatabase) -> impl fmt::Debug + '_ {
+        debug_fn(move |fmt| db.debug_crate(self, fmt))
+    }
+}
+
+impl Module {
+    pub fn debug(self, db: &impl HirDebugDatabase) -> impl fmt::Debug + '_ {
+        debug_fn(move |fmt| db.debug_module(self, fmt))
+    }
+}
+
+pub trait HirDebugHelper: HirDatabase {
+    fn crate_name(&self, _krate: CrateId) -> Option<String> {
+        None
+    }
+    fn file_path(&self, _file_id: FileId) -> Option<String> {
+        None
+    }
+}
+
+pub trait HirDebugDatabase {
+    fn debug_crate(&self, krate: Crate, fmt: &mut fmt::Formatter<'_>) -> fmt::Result;
+    fn debug_module(&self, module: Module, fmt: &mut fmt::Formatter<'_>) -> fmt::Result;
+}
+
+impl<DB: HirDebugHelper> HirDebugDatabase for DB {
+    fn debug_crate(&self, krate: Crate, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let mut builder = fmt.debug_tuple("Crate");
+        match self.crate_name(krate.crate_id) {
+            Some(name) => builder.field(&name),
+            None => builder.field(&krate.crate_id),
+        }
+        .finish()
+    }
+
+    fn debug_module(&self, module: Module, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let file_id = module.definition_source(self).file_id.original_file(self);
+        let path = self.file_path(file_id);
+        fmt.debug_struct("Module")
+            .field("name", &module.name(self).unwrap_or_else(Name::missing))
+            .field("path", &path.unwrap_or_else(|| "N/A".to_string()))
+            .finish()
+    }
+}
+
+fn debug_fn(f: impl FnOnce(&mut fmt::Formatter<'_>) -> fmt::Result) -> impl fmt::Debug {
+    struct DebugFn<F>(Cell<Option<F>>);
+
+    impl<F: FnOnce(&mut fmt::Formatter<'_>) -> fmt::Result> fmt::Debug for DebugFn<F> {
+        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+            let f = self.0.take().unwrap();
+            f(fmt)
+        }
+    }
+
+    DebugFn(Cell::new(Some(f)))
+}