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.

3 files changed, 42 insertions, 19 deletions

diff --git a/src/libcore/lib.rs b/src/libcore/lib.rs
index b6ab1ecaf4e..97aaaffe8bc 100644
--- a/src/libcore/lib.rs
+++ b/src/libcore/lib.rs
@@ -36,12 +36,12 @@
 //!   These functions are often provided by the system libc, but can also be
 //!   provided by the [rlibc crate](https://crates.io/crates/rlibc).
 //!
-//! * `rust_begin_panic` - This function takes three arguments, a
-//!   `fmt::Arguments`, a `&'static str`, and a `u32`. These three arguments
+//! * `rust_begin_panic` - This function takes four arguments, a
+//!   `fmt::Arguments`, a `&'static str`, and two `u32`'s. These four arguments
 //!   dictate the panic message, the file at which panic was invoked, and the
-//!   line. It is up to consumers of this core library to define this panic
-//!   function; it is only required to never return. This requires a `lang`
-//!   attribute named `panic_fmt`.
+//!   line and column inside the file. It is up to consumers of this core
+//!   library to define this panic function; it is only required to never
+//!   return. This requires a `lang` attribute named `panic_fmt`.
 //!
 //! * `rust_eh_personality` - is used by the failure mechanisms of the
 //!    compiler. This is often mapped to GCC's personality function, but crates
diff --git a/src/libcore/macros.rs b/src/libcore/macros.rs
index 5bb6a772aeb..70820dce5f9 100644
--- a/src/libcore/macros.rs
+++ b/src/libcore/macros.rs
@@ -17,16 +17,18 @@ macro_rules! panic {
         panic!("explicit panic")
     );
     ($msg:expr) => ({
-        static _MSG_FILE_LINE: (&'static str, &'static str, u32) = ($msg, file!(), line!());
-        $crate::panicking::panic(&_MSG_FILE_LINE)
+        static _MSG_FILE_LINE_COL: (&'static str, &'static str, u32, u32) =
+            ($msg, file!(), line!(), column!());
+        $crate::panicking::panic(&_MSG_FILE_LINE_COL)
     });
     ($fmt:expr, $($arg:tt)*) => ({
         // The leading _'s are to avoid dead code warnings if this is
         // used inside a dead function. Just `#[allow(dead_code)]` is
         // insufficient, since the user may have
         // `#[forbid(dead_code)]` and which cannot be overridden.
-        static _FILE_LINE: (&'static str, u32) = (file!(), line!());
-        $crate::panicking::panic_fmt(format_args!($fmt, $($arg)*), &_FILE_LINE)
+        static _MSG_FILE_LINE_COL: (&'static str, u32, u32) =
+            (file!(), line!(), column!());
+        $crate::panicking::panic_fmt(format_args!($fmt, $($arg)*), &_MSG_FILE_LINE_COL)
     });
 }
 
diff --git a/src/libcore/panicking.rs b/src/libcore/panicking.rs
index 60b7669f3b2..f5a7e78d0fa 100644
--- a/src/libcore/panicking.rs
+++ b/src/libcore/panicking.rs
@@ -17,7 +17,7 @@
 //!
 //! ```
 //! # use std::fmt;
-//! fn panic_impl(fmt: fmt::Arguments, file_line: &(&'static str, u32)) -> !
+//! fn panic_impl(fmt: fmt::Arguments, file_line_col: &(&'static str, u32, u32)) -> !
 //! # { loop {} }
 //! ```
 //!
@@ -39,34 +39,55 @@
 use fmt;
 
 #[cold] #[inline(never)] // this is the slow path, always
-#[lang = "panic"]
-pub fn panic(expr_file_line: &(&'static str, &'static str, u32)) -> ! {
+#[cfg_attr(not(stage0), lang = "panic")]
+pub fn panic(expr_file_line_col: &(&'static str, &'static str, u32, u32)) -> ! {
     // Use Arguments::new_v1 instead of format_args!("{}", expr) to potentially
     // reduce size overhead. The format_args! macro uses str's Display trait to
     // write expr, which calls Formatter::pad, which must accommodate string
     // truncation and padding (even though none is used here). Using
     // Arguments::new_v1 may allow the compiler to omit Formatter::pad from the
     // output binary, saving up to a few kilobytes.
+    let (expr, file, line, col) = *expr_file_line_col;
+    panic_fmt(fmt::Arguments::new_v1(&[expr], &[]), &(file, line, col))
+}
+
+// FIXME: remove when SNAP
+#[cold] #[inline(never)]
+#[cfg(stage0)]
+#[lang = "panic"]
+pub fn panic_old(expr_file_line: &(&'static str, &'static str, u32)) -> ! {
     let (expr, file, line) = *expr_file_line;
-    panic_fmt(fmt::Arguments::new_v1(&[expr], &[]), &(file, line))
+    let expr_file_line_col = (expr, file, line, 0);
+    panic(&expr_file_line_col)
+}
+
+#[cold] #[inline(never)]
+#[cfg_attr(not(stage0), lang = "panic_bounds_check")]
+fn panic_bounds_check(file_line_col: &(&'static str, u32, u32),
+                     index: usize, len: usize) -> ! {
+    panic_fmt(format_args!("index out of bounds: the len is {} but the index is {}",
+                           len, index), file_line_col)
 }
 
+// FIXME: remove when SNAP
 #[cold] #[inline(never)]
+#[cfg(stage0)]
 #[lang = "panic_bounds_check"]
-fn panic_bounds_check(file_line: &(&'static str, u32),
+fn panic_bounds_check_old(file_line: &(&'static str, u32),
                      index: usize, len: usize) -> ! {
+    let (file, line) = *file_line;
     panic_fmt(format_args!("index out of bounds: the len is {} but the index is {}",
-                           len, index), file_line)
+                           len, index), &(file, line, 0))
 }
 
 #[cold] #[inline(never)]
-pub fn panic_fmt(fmt: fmt::Arguments, file_line: &(&'static str, u32)) -> ! {
+pub fn panic_fmt(fmt: fmt::Arguments, file_line_col: &(&'static str, u32, u32)) -> ! {
     #[allow(improper_ctypes)]
     extern {
         #[lang = "panic_fmt"]
         #[unwind]
-        fn panic_impl(fmt: fmt::Arguments, file: &'static str, line: u32) -> !;
+        fn panic_impl(fmt: fmt::Arguments, file: &'static str, line: u32, col: u32) -> !;
     }
-    let (file, line) = *file_line;
-    unsafe { panic_impl(fmt, file, line) }
+    let (file, line, col) = *file_line_col;
+    unsafe { panic_impl(fmt, file, line, col) }
 }