Auto merge of #55780 - ogoffart:span_source_text, r=petrochenkov

Introduce proc_macro::Span::source_text

A function to extract the actual source behind a Span.

Background: I would like to use `syn` in a `build.rs` script to parse the rust code, and extract part of the source code. However, `syn` only gives access to proc_macro2::Span, and i would like to get the source code behind that.
I opened an issue on proc_macro2 bug tracker for this feature dtolnay/proc-macro2#110  and @alexcrichton said the feature should first go upstream in proc_macro.  So there it is!

Since most of the Span API is unstable anyway, this is guarded by the same `proc_macro_span` feature as everything else.

src/libproc_macro/bridge/mod.rs

@@ -155,6 +155,7 @@ macro_rules! with_api {
                 fn end($self: $S::Span) -> LineColumn;
                 fn join($self: $S::Span, other: $S::Span) -> Option<$S::Span>;
                 fn resolved_at($self: $S::Span, at: $S::Span) -> $S::Span;
+                fn source_text($self: $S::Span) -> Option<String>;
             },
         }
     };

src/libproc_macro/lib.rs

@@ -333,6 +333,18 @@ impl Span {
         self.0 == other.0
     }
 
+    /// Returns the source text behind a span. This preserves the original source
+    /// code, including spaces and comments. It only returns a result if the span
+    /// corresponds to real source code.
+    ///
+    /// Note: The observable result of a macro should only rely on the tokens and
+    /// not on this source text. The result of this function is a best effort to
+    /// be used for diagnostics only.
+    #[unstable(feature = "proc_macro_span", issue = "54725")]
+    pub fn source_text(&self) -> Option<String> {
+        self.0.source_text()
+    }
+
     diagnostic_method!(error, Level::Error);
     diagnostic_method!(warning, Level::Warning);
     diagnostic_method!(note, Level::Note);

src/libsyntax_ext/proc_macro_server.rs

@@ -740,4 +740,7 @@ impl server::Span for Rustc<'_> {
     fn resolved_at(&mut self, span: Self::Span, at: Self::Span) -> Self::Span {
         span.with_ctxt(at.ctxt())
     }
+    fn source_text(&mut self,  span: Self::Span) -> Option<String> {
+        self.sess.source_map().span_to_snippet(span).ok()
+    }
 }

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

@@ -33,3 +33,14 @@ pub fn assert_source_file(input: TokenStream) -> TokenStream {
 
     "".parse().unwrap()
 }
+
+#[proc_macro]
+pub fn macro_stringify(input: TokenStream) -> TokenStream {
+    let mut tokens = input.into_iter();
+    let first_span = tokens.next().expect("first token").span();
+    let last_span = tokens.last().map(|x| x.span()).unwrap_or(first_span);
+    let span = first_span.join(last_span).expect("joined span");
+    let src = span.source_text().expect("source_text");
+    TokenTree::Literal(Literal::string(&src)).into()
+}
+

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

@@ -3,12 +3,14 @@
 
 // ignore-pretty
 
+#![feature(proc_macro_hygiene)]
+
 #[macro_use]
 extern crate span_test_macros;
 
 extern crate span_api_tests;
 
-use span_api_tests::{reemit, assert_fake_source_file, assert_source_file};
+use span_api_tests::{reemit, assert_fake_source_file, assert_source_file, macro_stringify};
 
 macro_rules! say_hello {
     ($macname:ident) => ( $macname! { "Hello, world!" })
@@ -28,4 +30,32 @@ reemit! {
     assert_source_file! { "Hello, world!" }
 }
 
-fn main() {}
+fn main() {
+    let s = macro_stringify!(Hello, world!);
+    assert_eq!(s, "Hello, world!");
+    assert_eq!(macro_stringify!(Hello, world!), "Hello, world!");
+    assert_eq!(reemit_legacy!(macro_stringify!(Hello, world!)), "Hello, world!");
+    reemit_legacy!(assert_eq!(macro_stringify!(Hello, world!), "Hello, world!"));
+    // reemit change the span to be that of the call site
+    assert_eq!(
+        reemit!(macro_stringify!(Hello, world!)),
+        "reemit!(macro_stringify!(Hello, world!))"
+    );
+    let r = "reemit!(assert_eq!(macro_stringify!(Hello, world!), r));";
+    reemit!(assert_eq!(macro_stringify!(Hello, world!), r));
+
+    assert_eq!(macro_stringify!(
+        Hello,
+        world!
+    ), "Hello,\n        world!");
+
+    assert_eq!(macro_stringify!(Hello, /*world */ !), "Hello, /*world */ !");
+        assert_eq!(macro_stringify!(
+        Hello,
+        // comment
+        world!
+    ), "Hello,\n        // comment\n        world!");
+
+    assert_eq!(say_hello! { macro_stringify }, "\"Hello, world!\"");
+    assert_eq!(say_hello_extern! { macro_stringify }, "\"Hello, world!\"");
+}