When the required discriminator value exceeds LLVM's limits, drop the debug info for the function instead of panicking.

The maximum discriminator value LLVM can currently encode is 2^12. If macro use
results in more than 2^12 calls to the same function attributed to the same
callsite, and those calls are MIR-inlined, we will require more than the maximum
discriminator value to completely represent the debug information. Once we reach
that point drop the debug info instead.

compiler/rustc_codegen_llvm/src/debuginfo/create_scope_map.rs

@@ -85,15 +85,23 @@ fn make_mir_scope<'ll, 'tcx>(
             discriminators,
             parent,
         );
-        debug_context.scopes[parent]
+        if let Some(parent_scope) = debug_context.scopes[parent] {
+            parent_scope
+        } else {
+            // If the parent scope could not be represented then no children
+            // can be either.
+            debug_context.scopes[scope] = None;
+            instantiated.insert(scope);
+            return;
+        }
     } else {
         // The root is the function itself.
         let file = cx.sess().source_map().lookup_source_file(mir.span.lo());
-        debug_context.scopes[scope] = DebugScope {
+        debug_context.scopes[scope] = Some(DebugScope {
             file_start_pos: file.start_pos,
             file_end_pos: file.end_position(),
-            ..debug_context.scopes[scope]
-        };
+            ..debug_context.scopes[scope].unwrap()
+        });
         instantiated.insert(scope);
         return;
     };
@@ -104,7 +112,7 @@ fn make_mir_scope<'ll, 'tcx>(
     {
         // Do not create a DIScope if there are no variables defined in this
         // MIR `SourceScope`, and it's not `inlined`, to avoid debuginfo bloat.
-        debug_context.scopes[scope] = parent_scope;
+        debug_context.scopes[scope] = Some(parent_scope);
         instantiated.insert(scope);
         return;
     }
@@ -137,13 +145,20 @@ fn make_mir_scope<'ll, 'tcx>(
         },
     };
 
-    let inlined_at = scope_data.inlined.map(|(_, callsite_span)| {
+    let mut debug_scope = Some(DebugScope {
+        dbg_scope,
+        inlined_at: parent_scope.inlined_at,
+        file_start_pos: loc.file.start_pos,
+        file_end_pos: loc.file.end_position(),
+    });
+
+    if let Some((_, callsite_span)) = scope_data.inlined {
         let callsite_span = hygiene::walk_chain_collapsed(callsite_span, mir.span);
         let callsite_scope = parent_scope.adjust_dbg_scope_for_span(cx, callsite_span);
         let loc = cx.dbg_loc(callsite_scope, parent_scope.inlined_at, callsite_span);
 
         // NB: In order to produce proper debug info for variables (particularly
-        // arguments) in multiply-inline functions, LLVM expects to see a single
+        // arguments) in multiply-inlined functions, LLVM expects to see a single
         // DILocalVariable with multiple different DILocations in the IR. While
         // the source information for each DILocation would be identical, their
         // inlinedAt attributes will be unique to the particular callsite.
@@ -151,7 +166,7 @@ fn make_mir_scope<'ll, 'tcx>(
         // We generate DILocations here based on the callsite's location in the
         // source code. A single location in the source code usually can't
         // produce multiple distinct calls so this mostly works, until
-        // proc-macros get involved. A proc-macro can generate multiple calls
+        // macros get involved. A macro can generate multiple calls
         // at the same span, which breaks the assumption that we're going to
         // produce a unique DILocation for every scope we process here. We
         // have to explicitly add discriminators if we see inlines into the
@@ -160,24 +175,29 @@ fn make_mir_scope<'ll, 'tcx>(
         // Note further that we can't key this hashtable on the span itself,
         // because these spans could have distinct SyntaxContexts. We have
         // to key on exactly what we're giving to LLVM.
-        match discriminators.entry(callsite_span.lo()) {
+        let inlined_at = match discriminators.entry(callsite_span.lo()) {
             Entry::Occupied(mut o) => {
                 *o.get_mut() += 1;
                 unsafe { llvm::LLVMRustDILocationCloneWithBaseDiscriminator(loc, *o.get()) }
-                    .expect("Failed to encode discriminator in DILocation")
             }
             Entry::Vacant(v) => {
                 v.insert(0);
-                loc
+                Some(loc)
+            }
+        };
+        match inlined_at {
+            Some(inlined_at) => {
+                debug_scope.as_mut().unwrap().inlined_at = Some(inlined_at);
+            }
+            None => {
+                // LLVM has a maximum discriminator that it can encode (currently
+                // it uses 12 bits for 4096 possible values). If we exceed that
+                // there is little we can do but drop the debug info.
+                debug_scope = None;
             }
         }
-    });
+    }
 
-    debug_context.scopes[scope] = DebugScope {
-        dbg_scope,
-        inlined_at: inlined_at.or(parent_scope.inlined_at),
-        file_start_pos: loc.file.start_pos,
-        file_end_pos: loc.file.end_position(),
-    };
+    debug_context.scopes[scope] = debug_scope;
     instantiated.insert(scope);
 }

compiler/rustc_codegen_llvm/src/debuginfo/mod.rs

@@ -294,12 +294,12 @@ impl<'ll, 'tcx> DebugInfoCodegenMethods<'tcx> for CodegenCx<'ll, 'tcx> {
         }
 
         // Initialize fn debug context (including scopes).
-        let empty_scope = DebugScope {
+        let empty_scope = Some(DebugScope {
             dbg_scope: self.dbg_scope_fn(instance, fn_abi, Some(llfn)),
             inlined_at: None,
             file_start_pos: BytePos(0),
             file_end_pos: BytePos(0),
-        };
+        });
         let mut fn_debug_context = FunctionDebugContext {
             scopes: IndexVec::from_elem(empty_scope, &mir.source_scopes),
             inlined_function_scopes: Default::default(),