Auto merge of #43604 - abonander:proc_macro_span_api, r=jseyfried

Improvements to `proc_macro::Span` API

Motivation: https://internals.rust-lang.org/t/better-panic-location-reporting-for-unwrap-and-friends/5042/12?u=logician

TODO:
- [x] Bikeshedding/complete API
- [x] Implement tests/verify return values

cc @jseyfried @nrc

src/test/run-pass-fulldeps/proc-macro/auxiliary/span-api-tests.rs

@@ -0,0 +1,45 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// force-host
+// no-prefer-dynamic
+
+#![crate_type = "proc-macro"]
+#![feature(proc_macro)]
+
+extern crate proc_macro;
+
+use proc_macro::*;
+
+// Re-emits the input tokens by parsing them from strings
+#[proc_macro]
+pub fn reemit(input: TokenStream) -> TokenStream {
+    input.to_string().parse().unwrap()
+}
+
+#[proc_macro]
+pub fn assert_fake_source_file(input: TokenStream) -> TokenStream {
+    for tk in input {
+        let source_file = tk.span.source_file();
+        assert!(!source_file.is_real(), "Source file is real: {:?}", source_file);
+    }
+
+    "".parse().unwrap()
+}
+
+#[proc_macro]
+pub fn assert_source_file(input: TokenStream) -> TokenStream {
+    for tk in input {
+        let source_file = tk.span.source_file();
+        assert!(source_file.is_real(), "Source file is not real: {:?}", source_file);
+    }
+
+    "".parse().unwrap()
+}

src/test/run-pass-fulldeps/proc-macro/auxiliary/span-test-macros.rs

@@ -0,0 +1,19 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#[macro_export]
+macro_rules! reemit_legacy {
+    ($($tok:tt)*) => ($($tok)*)
+}
+
+#[macro_export]
+macro_rules! say_hello_extern {
+    ($macname:ident) => ( $macname! { "Hello, world!" })
+}

src/test/run-pass-fulldeps/proc-macro/span-api-tests.rs

@@ -0,0 +1,43 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// aux-build:span-api-tests.rs
+// aux-build:span-test-macros.rs
+
+// ignore-pretty
+
+#![feature(proc_macro)]
+
+#[macro_use]
+extern crate span_test_macros;
+
+extern crate span_api_tests;
+
+use span_api_tests::{reemit, assert_fake_source_file, assert_source_file};
+
+macro_rules! say_hello {
+    ($macname:ident) => ( $macname! { "Hello, world!" })
+}
+
+assert_source_file! { "Hello, world!" }
+
+say_hello! { assert_source_file }
+
+reemit_legacy! {
+    assert_source_file! { "Hello, world!" }
+}
+
+say_hello_extern! { assert_fake_source_file }
+
+reemit! {
+    assert_source_file! { "Hello, world!" }
+}
+
+fn main() {}