fix(mbe): desugar doc correctly for mbe

Fixes #16110.

The way rust desugars doc comments when expanding macros
is rendering it as raw strings delimited with hashes.
Rust-analyzer wasn't aware of this, so the desugared doc
comments wouldn't match correctly when on the LHS of macro
declarations.

This PR fixes this by porting the code used by rustc: 4cfdbd328b/compiler/rustc_ast/src/tokenstream.rs (L6837)

crates/hir-def/src/macro_expansion_tests/mbe.rs

@@ -1218,8 +1218,10 @@ m! {
 macro_rules! m {
     ($(#[$m:meta])+) => ( $(#[$m])+ fn bar() {} )
 }
-#[doc = " Single Line Doc 1"]
-#[doc = "\n        MultiLines Doc\n    "] fn bar() {}
+#[doc = r" Single Line Doc 1"]
+#[doc = "
+        MultiLines Doc
+    "] fn bar() {}
 "##]],
     );
 }
@@ -1260,8 +1262,10 @@ m! {
 macro_rules! m {
     ($(#[$ m:meta])+) => ( $(#[$m])+ fn bar() {} )
 }
-#[doc = " 錦瑟無端五十弦，一弦一柱思華年。"]
-#[doc = "\n        莊生曉夢迷蝴蝶，望帝春心託杜鵑。\n    "] fn bar() {}
+#[doc = r" 錦瑟無端五十弦，一弦一柱思華年。"]
+#[doc = r"
+        莊生曉夢迷蝴蝶，望帝春心託杜鵑。
+    "] fn bar() {}
 "##]],
     );
 }
@@ -1281,7 +1285,7 @@ m! {
 macro_rules! m {
     ($(#[$m:meta])+) => ( $(#[$m])+ fn bar() {} )
 }
-#[doc = " \\ \" \'"] fn bar() {}
+#[doc = r#" \ " '"#] fn bar() {}
 "##]],
     );
 }

crates/mbe/src/syntax_bridge.rs

@@ -406,9 +406,20 @@ fn doc_comment_text(comment: &ast::Comment) -> SmolStr {
         text = &text[0..text.len() - 2];
     }
 
-    // Quote the string
+    let mut num_of_hashes = 0;
+    let mut count = 0;
+    for ch in text.chars() {
+        count = match ch {
+            '"' => 1,
+            '#' if count > 0 => count + 1,
+            _ => 0,
+        };
+        num_of_hashes = num_of_hashes.max(count);
+    }
+
+    // Quote raw string with delimiters
     // Note that `tt::Literal` expect an escaped string
-    let text = format!("\"{}\"", text.escape_debug());
+    let text = format!("r{delim}\"{text}\"{delim}", delim = "#".repeat(num_of_hashes));
     text.into()
 }