From 2c7e3b548d1b1e57dfc3eec8bf82d9589cf25fe8 Mon Sep 17 00:00:00 2001
From: Wesley Wiser <wwiser@gmail.com>
Date: Sun, 16 Feb 2020 20:32:25 -0500
Subject: [PATCH] WIP

---
 src/librustc/mir/interpret/error.rs | 34 ++++++++++++++++++-----------
 src/librustc_session/session.rs     | 16 ++++++++++++++
 2 files changed, 37 insertions(+), 13 deletions(-)

diff --git a/src/librustc/mir/interpret/error.rs b/src/librustc/mir/interpret/error.rs
index df3971a5ac3f1..4d668d22333e7 100644
--- a/src/librustc/mir/interpret/error.rs
+++ b/src/librustc/mir/interpret/error.rs
@@ -4,17 +4,20 @@ use crate::hir::map::definitions::DefPathData;
 use crate::mir;
 use crate::ty::layout::{Align, LayoutError, Size};
 use crate::ty::query::TyCtxtAt;
+use crate::ty::tls;
 use crate::ty::{self, layout, Ty};
 
 use backtrace::Backtrace;
 use hir::GeneratorKind;
+use rustc_data_structures::sync::Lock;
 use rustc_errors::{struct_span_err, DiagnosticBuilder};
 use rustc_hir as hir;
 use rustc_macros::HashStable;
+use rustc_session::CtfeBacktrace;
 use rustc_span::symbol::Symbol;
 use rustc_span::{Pos, Span};
 use rustc_target::spec::abi::Abi;
-use std::{any::Any, env, fmt};
+use std::{any::Any, fmt};
 
 #[derive(Debug, Copy, Clone, PartialEq, Eq, HashStable, RustcEncodable, RustcDecodable)]
 pub enum ErrorHandled {
@@ -241,21 +244,26 @@ impl From<ErrorHandled> for InterpErrorInfo<'tcx> {
 
 impl<'tcx> From<InterpError<'tcx>> for InterpErrorInfo<'tcx> {
     fn from(kind: InterpError<'tcx>) -> Self {
-        let backtrace = match env::var("RUSTC_CTFE_BACKTRACE") {
-            // Matching `RUST_BACKTRACE` -- we treat "0" the same as "not present".
-            Ok(ref val) if val != "0" => {
-                let mut backtrace = Backtrace::new_unresolved();
+        let capture_backtrace = tls::with_context_opt(|ctxt| {
+            if let Some(ctxt) = ctxt {
+                let l = Lock::borrow(&ctxt.tcx.sess.ctfe_backtrace);
+                *l
+            } else {
+                CtfeBacktrace::Disabled
+            }
+        });
 
-                if val == "immediate" {
-                    // Print it now.
-                    print_backtrace(&mut backtrace);
-                    None
-                } else {
-                    Some(Box::new(backtrace))
-                }
+        let backtrace = match capture_backtrace {
+            CtfeBacktrace::Disabled => None,
+            CtfeBacktrace::Capture => Some(Box::new(Backtrace::new_unresolved())),
+            CtfeBacktrace::Immediate => {
+                // Print it now.
+                let mut backtrace = Backtrace::new_unresolved();
+                print_backtrace(&mut backtrace);
+                None
             }
-            _ => None,
         };
+
         InterpErrorInfo { kind, backtrace }
     }
 }
diff --git a/src/librustc_session/session.rs b/src/librustc_session/session.rs
index 6f003043aa95c..69bea01a56cb5 100644
--- a/src/librustc_session/session.rs
+++ b/src/librustc_session/session.rs
@@ -49,6 +49,13 @@ pub struct OptimizationFuel {
     out_of_fuel: bool,
 }
 
+#[derive(Clone, Copy)]
+pub enum CtfeBacktrace {
+    Disabled,
+    Capture,
+    Immediate,
+}
+
 /// Represents the data associated with a compilation
 /// session for a single crate.
 pub struct Session {
@@ -136,6 +143,8 @@ pub struct Session {
     /// Path for libraries that will take preference over libraries shipped by Rust.
     /// Used by windows-gnu targets to priortize system mingw-w64 libraries.
     pub system_library_path: OneThread<RefCell<Option<Option<PathBuf>>>>,
+
+    pub ctfe_backtrace: Lock<CtfeBacktrace>,
 }
 
 pub struct PerfStats {
@@ -1036,6 +1045,12 @@ fn build_session_(
         sopts.debugging_opts.time_passes,
     );
 
+    let ctfe_backtrace = Lock::new(match env::var("RUSTC_CTFE_BACKTRACE") {
+        Ok(ref val) if val == "immediate" => CtfeBacktrace::Immediate,
+        Ok(ref val) if val != "0" => CtfeBacktrace::Capture,
+        _ => CtfeBacktrace::Disabled,
+    });
+
     let sess = Session {
         target: target_cfg,
         host,
@@ -1073,6 +1088,7 @@ fn build_session_(
         trait_methods_not_found: Lock::new(Default::default()),
         confused_type_with_std_module: Lock::new(Default::default()),
         system_library_path: OneThread::new(RefCell::new(Default::default())),
+        ctfe_backtrace,
     };
 
     validate_commandline_args_with_session_available(&sess);