Auto merge of #42938 - est31:col_number, r=eddyb

Output column number info when panicking

Outputs the column number when panicking. Useful when you e.g. have code like `foo[i] = bar[k] + bar[l]` and you get a panic with index out of bounds, or when you have an expression like `a = b + c + d + e` and the addition overflows. Now you know which operation to blame!

The format is `file:line:column`, just like for compiler errors. Example output with the patch:

```
thread 'main' panicked at 'index out of bounds: the len is 5 but the index is 8', src/main.rs:3:8
```

As some of the API between the compiler and the library landscape gets broken, this is a bit hackier than I'd originally wanted it to be.

* `panic` and `panic_bounds_check` lang items got an additional column param, on stage0 I still have to use the previous version. After a SNAP this should be resolved.
* For `#[derive(RustcDeserialze)]`, stage0 requires a fixed signature for `std::rt::begin_panic`, so we can't change it right away. What we need to do instead is to keep the signature, and add a `begin_panic_new` function that we use in later stages instead. After a SNAP we can change the `begin_panic` function and rely on it instead of `begin_panic_new`, and one SNAP later we can remove `begin_panic_new`.
* Fortunately I didn't have to break anything about the panic hook API, I could easily extend it.

Note that debuginfo remains unchanged, so RUST_BACKTRACE output won't contain any column info. See issue #42921 for discussion on including the column in debuginfo.

1 files changed, 6 insertions, 5 deletions

diff --git a/src/libsyntax/ext/build.rs b/src/libsyntax/ext/build.rs
index a4580ea3939..412a3493208 100644
--- a/src/libsyntax/ext/build.rs
+++ b/src/libsyntax/ext/build.rs
@@ -11,7 +11,7 @@
 use abi::Abi;
 use ast::{self, Ident, Generics, Expr, BlockCheckMode, UnOp, PatKind};
 use attr;
-use syntax_pos::{Span, DUMMY_SP};
+use syntax_pos::{Pos, Span, DUMMY_SP};
 use codemap::{dummy_spanned, respan, Spanned};
 use ext::base::ExtCtxt;
 use ptr::P;
@@ -768,14 +768,15 @@ impl<'a> AstBuilder for ExtCtxt<'a> {
         let loc = self.codemap().lookup_char_pos(span.lo);
         let expr_file = self.expr_str(span, Symbol::intern(&loc.file.name));
         let expr_line = self.expr_u32(span, loc.line as u32);
-        let expr_file_line_tuple = self.expr_tuple(span, vec![expr_file, expr_line]);
-        let expr_file_line_ptr = self.expr_addr_of(span, expr_file_line_tuple);
+        let expr_col = self.expr_u32(span, loc.col.to_usize() as u32 + 1);
+        let expr_loc_tuple = self.expr_tuple(span, vec![expr_file, expr_line, expr_col]);
+        let expr_loc_ptr = self.expr_addr_of(span, expr_loc_tuple);
         self.expr_call_global(
             span,
-            self.std_path(&["rt", "begin_panic"]),
+            self.std_path(&["rt", "begin_panic_new"]),
             vec![
                 self.expr_str(span, msg),
-                expr_file_line_ptr])
+                expr_loc_ptr])
     }
 
     fn expr_unreachable(&self, span: Span) -> P<ast::Expr> {