Add some comments for magic numbers + Add tests

src/test/ui/proc-macro/doc-comment-preserved.rs

@@ -0,0 +1,24 @@
+// check-pass
+// aux-build:test-macros.rs
+
+// Anonymize unstable non-dummy spans while still showing dummy spans `0..0`.
+// normalize-stdout-test "bytes\([^0]\w*\.\.(\w+)\)" -> "bytes(LO..$1)"
+// normalize-stdout-test "bytes\((\w+)\.\.[^0]\w*\)" -> "bytes($1..HI)"
+
+#[macro_use]
+extern crate test_macros;
+
+print_bang! {
+
+/**
+*******
+* DOC *
+* DOC *
+* DOC *
+*******
+*/
+pub struct S;
+
+}
+
+fn main() {}

src/test/ui/proc-macro/doc-comment-preserved.stdout

@@ -0,0 +1,54 @@
+PRINT-BANG INPUT (DISPLAY): /**
+*******
+* DOC *
+* DOC *
+* DOC *
+*******
+*/
+ pub struct S ;
+PRINT-BANG RE-COLLECTED (DISPLAY): #[doc = "\n*******\n* DOC *\n* DOC *\n* DOC *\n*******\n"] pub struct S ;
+PRINT-BANG INPUT (DEBUG): TokenStream [
+    Punct {
+        ch: '#',
+        spacing: Alone,
+        span: #0 bytes(LO..HI),
+    },
+    Group {
+        delimiter: Bracket,
+        stream: TokenStream [
+            Ident {
+                ident: "doc",
+                span: #0 bytes(LO..HI),
+            },
+            Punct {
+                ch: '=',
+                spacing: Alone,
+                span: #0 bytes(LO..HI),
+            },
+            Literal {
+                kind: Str,
+                symbol: "\n*******\n* DOC *\n* DOC *\n* DOC *\n*******\n",
+                suffix: None,
+                span: #0 bytes(LO..HI),
+            },
+        ],
+        span: #0 bytes(LO..HI),
+    },
+    Ident {
+        ident: "pub",
+        span: #0 bytes(LO..HI),
+    },
+    Ident {
+        ident: "struct",
+        span: #0 bytes(LO..HI),
+    },
+    Ident {
+        ident: "S",
+        span: #0 bytes(LO..HI),
+    },
+    Punct {
+        ch: ';',
+        spacing: Alone,
+        span: #0 bytes(LO..HI),
+    },
+]