[gcc/devel/rust/master] Fix lexing of empty comments continuing till next line

[Thomas Schwinge <tschwinge@gcc.gnu.org>]

https://gcc.gnu.org/g:11493cccd7e7fc5ea31e37581323f85eca58ca0c

commit 11493cccd7e7fc5ea31e37581323f85eca58ca0c
Author: Nirmal Patel <nirmal@nirmal.dev>
Date:   Sun Jun 12 19:23:30 2022 -0400

    Fix lexing of empty comments continuing till next line
    
    Empty comments (comments without any characters including spaces after
    //) had a bug during lexing. The lexer did not recheck the current
    character after skipping / characters. When there was no character after
    //, the lexer skipped the next newline character. This caused lexer to
    count the next line as a part of the comment to. This commit fixes this
    bug by rechecking current character after skipping two / characters.
    
    Signed-off-by: Nirmal Patel <nirmal@nirmal.dev>

Diff:
---
 gcc/rust/lex/rust-lex.cc                                 | 1 +
 gcc/testsuite/rust/compile/empty_comment_before_match.rs | 7 +++++++
 2 files changed, 8 insertions(+)

diff --git a/gcc/rust/lex/rust-lex.cc b/gcc/rust/lex/rust-lex.cc
index 023b67651b7..9e0595b02fb 100644
--- a/gcc/rust/lex/rust-lex.cc
+++ b/gcc/rust/lex/rust-lex.cc
@@ -489,6 +489,7 @@ Lexer::build_token ()
 	      // (but not an inner or outer doc comment)
 	      skip_input ();
 	      current_column += 2;
+	      current_char = peek_input ();
 
 	      // basically ignore until line finishes
 	      while (current_char != '\n' && current_char != EOF)
diff --git a/gcc/testsuite/rust/compile/empty_comment_before_match.rs b/gcc/testsuite/rust/compile/empty_comment_before_match.rs
new file mode 100644
index 00000000000..3d344d3e758
--- /dev/null
+++ b/gcc/testsuite/rust/compile/empty_comment_before_match.rs
@@ -0,0 +1,7 @@
+fn foo (x: i8) -> i32 { // { dg-warning "function is never used" }
+    //
+    match x {
+        1 => { return 1; }
+        _ => { return 0; }
+    }
+}