Use the correct edition when syntax highlighting doctests

Previously it would unconditionally use edition 2015, which was
incorrect.

src/librustdoc/passes/check_code_block_syntax.rs

@@ -4,7 +4,7 @@ use rustc_middle::lint::LintDiagnosticBuilder;
 use rustc_parse::parse_stream_from_source_str;
 use rustc_session::parse::ParseSess;
 use rustc_span::source_map::{FilePathMapping, SourceMap};
-use rustc_span::{FileName, InnerSpan};
+use rustc_span::{hygiene::AstPass, ExpnData, ExpnKind, FileName, InnerSpan, DUMMY_SP};
 
 use crate::clean;
 use crate::core::DocContext;
@@ -36,12 +36,22 @@ impl<'a, 'tcx> SyntaxChecker<'a, 'tcx> {
         let source = dox[code_block.code].to_owned();
         let sess = ParseSess::with_span_handler(handler, sm);
 
+        let edition = code_block.lang_string.edition.unwrap_or(self.cx.tcx.sess.edition());
+        let expn_data = ExpnData::default(
+            ExpnKind::AstPass(AstPass::TestHarness),
+            DUMMY_SP,
+            edition,
+            None,
+            None,
+        );
+        let span = DUMMY_SP.fresh_expansion(expn_data, self.cx.tcx.create_stable_hashing_context());
+
         let is_empty = rustc_driver::catch_fatal_errors(|| {
             parse_stream_from_source_str(
                 FileName::Custom(String::from("doctest")),
                 source,
                 &sess,
-                None,
+                Some(span),
             )
             .is_empty()
         })

src/test/rustdoc-ui/doctest-edition.rs

@@ -0,0 +1,16 @@
+// edition:2021
+
+#![deny(rustdoc::invalid_rust_codeblocks)]
+//~^ NOTE lint level is defined here
+
+// By default, rustdoc should use the edition of the crate.
+//! ```
+//! foo'b'
+//! ```
+//~^^^ ERROR could not parse
+//~| NOTE prefix `foo` is unknown
+
+// Rustdoc should respect `edition2018` when highlighting syntax.
+//! ```edition2018
+//! foo'b'
+//! ```

src/test/rustdoc-ui/doctest-edition.stderr

@@ -0,0 +1,22 @@
+error: could not parse code block as Rust code
+  --> $DIR/doctest-edition.rs:7:5
+   |
+LL |   //! ```
+   |  _____^
+LL | | //! foo'b'
+LL | | //! ```
+   | |_______^
+   |
+note: the lint level is defined here
+  --> $DIR/doctest-edition.rs:3:9
+   |
+LL | #![deny(rustdoc::invalid_rust_codeblocks)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: error from rustc: prefix `foo` is unknown
+help: mark blocks that do not contain Rust code as text
+   |
+LL | //! ```text
+   |        ++++
+
+error: aborting due to previous error
+