Auto merge of #48056 - ExpHP:macro-commas, r=dtolnay

Comprehensively support trailing commas in std/core macros

I carefully organized the changes into four commits:

* Test cases
* Fixes for `macro_rules!` macros
* Fixes for builtin macros
* Docs for builtins

**I can easily scale this back to just the first two commits for now if such is desired.**

### Breaking (?) changes

* This fixes #48042, which is a breaking change that I hope people can agree is just a bugfix for an extremely dark corner case.

* To fix five of the builtins, this changes `syntax::ext::base::get_single_str_from_tts` to accept a trailing comma, and revises the documentation so that this aspect is not surprising. **I made this change under the (hopefully correct) understanding that `libsyntax` is private rustc implementation detail.** After reviewing all call sites (which were, you guessed it, *precisely those five macros*), I believe the revised semantics are closer to the intended spirit of the function.

### Changes which may require concensus

Up until now, it could be argued that some or all the following macros did not conceptually take a comma-separated list, because they only took one argument:

  * **`cfg(unix,)`** (most notable since cfg! is unique in taking a meta tag)
  * **`include{,_bytes,_str}("file.rs",)`**  (in item form this might be written as "`include!{"file.rs",}`" which is even slightly more odd)
  * **`compile_error("message",);`**
  * **`option_env!("PATH",)`**
  * **`try!(Ok(()),)`**

So I think these particular changes may require some sort of consensus.  **All of the fixes for builtins are included this list, so if we want to defer these decisions to later then I can scale this PR back to just the first two commits.**

### Other notes/general requests for comment

* Do we have a big checklist somewhere of "things to do when adding macros?" My hope is for `run-pass/macro-comma-support.rs` to remain comprehensive.
* Originally I wanted the tests to also comprehensively forbid double trailing commas.  However, this didn't work out too well: [see this gist and the giant FIXME in it](https://gist.github.com/ExpHP/6fc40e82f3d73267c4e590a9a94966f1#file-compile-fail_macro-comma-support-rs-L33-L50)
* I did not touch `select!`. It appears to me to be a complete mess, and its trailing comma mishaps are only the tip of the iceberg.
* There are [some compile-fail test cases](https://github.com/ExpHP/rust/blob/5fa97c35da2f0ee/src/test/compile-fail/macro-comma-behavior.rs#L49-L52) that didn't seem to work (rustc emits errors, but compile-fail doesn't acknowledge them), so they are disabled. Any clues? (Possibly related: These happen to be precisely the set of errors which are tagged by rustc as "this error originates in a macro outside of the current crate".)

---

Fixes #48042
Closes #46241

src/libstd/macros.rs

@@ -68,6 +68,9 @@ macro_rules! panic {
     ($msg:expr) => ({
         $crate::rt::begin_panic($msg, &(file!(), line!(), __rust_unstable_column!()))
     });
+    ($msg:expr,) => ({
+        panic!($msg)
+    });
     ($fmt:expr, $($arg:tt)+) => ({
         $crate::rt::begin_panic_fmt(&format_args!($fmt, $($arg)+),
                                     &(file!(), line!(), __rust_unstable_column!()))
@@ -312,7 +315,10 @@ pub mod builtin {
     /// ```
     #[stable(feature = "compile_error_macro", since = "1.20.0")]
     #[macro_export]
-    macro_rules! compile_error { ($msg:expr) => ({ /* compiler built-in */ }) }
+    macro_rules! compile_error {
+        ($msg:expr) => ({ /* compiler built-in */ });
+        ($msg:expr,) => ({ /* compiler built-in */ });
+    }
 
     /// The core macro for formatted string creation & output.
     ///
@@ -400,7 +406,10 @@ pub mod builtin {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[macro_export]
-    macro_rules! option_env { ($name:expr) => ({ /* compiler built-in */ }) }
+    macro_rules! option_env {
+        ($name:expr) => ({ /* compiler built-in */ });
+        ($name:expr,) => ({ /* compiler built-in */ });
+    }
 
     /// Concatenate identifiers into one identifier.
     ///
@@ -580,7 +589,10 @@ pub mod builtin {
     /// Compiling 'main.rs' and running the resulting binary will print "adiós".
     #[stable(feature = "rust1", since = "1.0.0")]
     #[macro_export]
-    macro_rules! include_str { ($file:expr) => ({ /* compiler built-in */ }) }
+    macro_rules! include_str {
+        ($file:expr) => ({ /* compiler built-in */ });
+        ($file:expr,) => ({ /* compiler built-in */ });
+    }
 
     /// Includes a file as a reference to a byte array.
     ///
@@ -614,7 +626,10 @@ pub mod builtin {
     /// Compiling 'main.rs' and running the resulting binary will print "adiós".
     #[stable(feature = "rust1", since = "1.0.0")]
     #[macro_export]
-    macro_rules! include_bytes { ($file:expr) => ({ /* compiler built-in */ }) }
+    macro_rules! include_bytes {
+        ($file:expr) => ({ /* compiler built-in */ });
+        ($file:expr,) => ({ /* compiler built-in */ });
+    }
 
     /// Expands to a string that represents the current module path.
     ///
@@ -700,7 +715,10 @@ pub mod builtin {
     /// "🙈🙊🙉🙈🙊🙉".
     #[stable(feature = "rust1", since = "1.0.0")]
     #[macro_export]
-    macro_rules! include { ($file:expr) => ({ /* compiler built-in */ }) }
+    macro_rules! include {
+        ($file:expr) => ({ /* compiler built-in */ });
+        ($file:expr,) => ({ /* compiler built-in */ });
+    }
 }
 
 /// A macro for defining #[cfg] if-else statements.