more precise span for erroneous consts during CTFE/Miri

Author: RalfJung

src/librustc_mir/interpret/eval_context.rs

@@ -83,9 +83,11 @@ pub struct Frame<'mir, 'tcx, Tag = (), Extra = ()> {
     ////////////////////////////////////////////////////////////////////////////////
     // Current position within the function
     ////////////////////////////////////////////////////////////////////////////////
-    /// If this is `None`, we are unwinding and this function doesn't need any clean-up.
-    /// Just continue the same as with `Resume`.
-    pub loc: Option<mir::Location>,
+    /// If this is `Err`, we are not currently executing any particular statement in
+    /// this frame (can happen e.g. during frame initialziation, and during unwinding on
+    /// frames without cleanup code).
+    /// We basically abuse `Result` as `Either`.
+    pub(super) loc: Result<mir::Location, Span>,
 }
 
 /// What we store about a frame in an interpreter backtrace.
@@ -189,11 +191,14 @@ impl<'mir, 'tcx, Tag> Frame<'mir, 'tcx, Tag> {
 impl<'mir, 'tcx, Tag, Extra> Frame<'mir, 'tcx, Tag, Extra> {
     /// Return the `SourceInfo` of the current instruction.
     pub fn current_source_info(&self) -> Option<&mir::SourceInfo> {
-        self.loc.map(|loc| self.body.source_info(loc))
+        self.loc.ok().map(|loc| self.body.source_info(loc))
     }
 
     pub fn current_span(&self) -> Span {
-        self.current_source_info().map(|si| si.span).unwrap_or(self.body.span)
+        match self.loc {
+            Ok(loc) => self.body.source_info(loc).span,
+            Err(span) => span,
+        }
     }
 }
 
@@ -640,7 +645,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         // first push a stack frame so we have access to the local substs
         let pre_frame = Frame {
             body,
-            loc: None, // `None` for errors generated before we start evaluating.
+            loc: Err(body.span), // Span used for errors caused during preamble.
             return_to_block,
             return_place,
             // empty local array, we fill it in below, after we are inside the stack frame and
@@ -654,9 +659,15 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
 
         // Make sure all the constants required by this frame evaluate successfully (post-monomorphization check).
         for const_ in &body.required_consts {
+            let span = const_.span;
             let const_ =
                 self.subst_from_current_frame_and_normalize_erasing_regions(const_.literal);
-            self.const_to_op(const_, None)?;
+            self.const_to_op(const_, None).map_err(|err| {
+                // If there was an error, set the span of the current frame so this constant.
+                // Avoiding doing this when evaluation succeeds.
+                self.frame_mut().loc = Err(span);
+                err
+            })?;
         }
 
         // Locals are initially uninitialized.
@@ -683,9 +694,8 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         }
         // done
         self.frame_mut().locals = locals;
-        self.frame_mut().loc = Some(mir::Location::START);
-
         M::after_stack_push(self)?;
+        self.frame_mut().loc = Ok(mir::Location::START);
         info!("ENTERING({}) {}", self.frame_idx(), self.frame().instance);
 
         Ok(())
@@ -694,7 +704,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
     /// Jump to the given block.
     #[inline]
     pub fn go_to_block(&mut self, target: mir::BasicBlock) {
-        self.frame_mut().loc = Some(mir::Location { block: target, statement_index: 0 });
+        self.frame_mut().loc = Ok(mir::Location { block: target, statement_index: 0 });
     }
 
     /// *Return* to the given `target` basic block.
@@ -716,7 +726,10 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
     /// If `target` is `None`, that indicates the function does not need cleanup during
     /// unwinding, and we will just keep propagating that upwards.
     pub fn unwind_to_block(&mut self, target: Option<mir::BasicBlock>) {
-        self.frame_mut().loc = target.map(|block| mir::Location { block, statement_index: 0 });
+        self.frame_mut().loc = match target {
+            Some(block) => Ok(mir::Location { block, statement_index: 0 }),
+            None => Err(self.frame_mut().body.span),
+        };
     }
 
     /// Pops the current frame from the stack, deallocating the
@@ -744,8 +757,8 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         assert_eq!(
             unwinding,
             match self.frame().loc {
-                None => true,
-                Some(loc) => self.body().basic_blocks()[loc.block].is_cleanup,
+                Ok(loc) => self.body().basic_blocks()[loc.block].is_cleanup,
+                Err(_) => true,
             }
         );
 

src/librustc_mir/interpret/step.rs

@@ -47,8 +47,8 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         }
 
         let loc = match self.frame().loc {
-            Some(loc) => loc,
-            None => {
+            Ok(loc) => loc,
+            Err(_) => {
                 // We are unwinding and this fn has no cleanup code.
                 // Just go on unwinding.
                 trace!("unwinding: skipping frame");
@@ -283,7 +283,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
 
         self.eval_terminator(terminator)?;
         if !self.stack().is_empty() {
-            if let Some(loc) = self.frame().loc {
+            if let Ok(loc) = self.frame().loc {
                 info!("// executing {:?}", loc.block);
             }
         }

src/test/ui/consts/const-err-multi.stderr

@@ -13,22 +13,28 @@ LL | #![deny(const_err)]
    |         ^^^^^^^^^
 
 error: any use of this value will cause an error
-  --> $DIR/const-err-multi.rs:5:1
+  --> $DIR/const-err-multi.rs:5:19
    |
 LL | pub const B: i8 = A;
-   | ^^^^^^^^^^^^^^^^^^^^ referenced constant has errors
+   | ------------------^-
+   |                   |
+   |                   referenced constant has errors
 
 error: any use of this value will cause an error
-  --> $DIR/const-err-multi.rs:7:1
+  --> $DIR/const-err-multi.rs:7:19
    |
 LL | pub const C: u8 = A as u8;
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^ referenced constant has errors
+   | ------------------^-------
+   |                   |
+   |                   referenced constant has errors
 
 error: any use of this value will cause an error
-  --> $DIR/const-err-multi.rs:9:1
+  --> $DIR/const-err-multi.rs:9:24
    |
 LL | pub const D: i8 = 50 - A;
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^ referenced constant has errors
+   | -----------------------^-
+   |                        |
+   |                        referenced constant has errors
 
 error: aborting due to 4 previous errors
 

src/test/ui/consts/const-eval/erroneous-const.rs

@@ -7,9 +7,9 @@ impl<T> PrintName<T> {
     //~^ WARN this operation will panic at runtime
 }
 
-const fn no_codegen<T>() { //~ERROR evaluation of constant value failed
+const fn no_codegen<T>() {
     if false {
-        let _ = PrintName::<T>::VOID;
+        let _ = PrintName::<T>::VOID; //~ERROR evaluation of constant value failed
     }
 }
 

src/test/ui/consts/const-eval/erroneous-const.stderr

@@ -25,14 +25,10 @@ LL | #![warn(const_err, unconditional_panic)]
    |         ^^^^^^^^^
 
 error[E0080]: evaluation of constant value failed
-  --> $DIR/erroneous-const.rs:10:1
-   |
-LL | / const fn no_codegen<T>() {
-LL | |     if false {
-LL | |         let _ = PrintName::<T>::VOID;
-LL | |     }
-LL | | }
-   | |_^ referenced constant has errors
+  --> $DIR/erroneous-const.rs:12:17
+   |
+LL |         let _ = PrintName::<T>::VOID;
+   |                 ^^^^^^^^^^^^^^^^^^^^ referenced constant has errors
 
 error[E0080]: could not evaluate static initializer
   --> $DIR/erroneous-const.rs:16:22