stage2: fix incremental compilation handling of parse errors

Before, incremental compilation would crash when trying to emit compile
errors for the update after introducing a parse error.

Parse errors are handled by not invalidating any existing semantic
analysis. However, only the parse error must be reported, with all the
other errors suppressed. Once the parse error is fixed, the new file can
be treated as an update to the previously-succeeded update.

src/Compilation.zig

@@ -1346,9 +1346,9 @@ pub fn update(self: *Compilation) !void {
             module.generation += 1;
 
             // TODO Detect which source files changed.
-            // Until then we simulate a full cache miss. Source files could have been loaded for any reason;
-            // to force a refresh we unload now.
-            module.root_scope.unload(module.gpa);
+            // Until then we simulate a full cache miss. Source files could have been loaded
+            // for any reason; to force a refresh we unload now.
+            module.unloadFile(module.root_scope);
             module.failed_root_src_file = null;
             module.analyzeContainer(&module.root_scope.root_container) catch |err| switch (err) {
                 error.AnalysisFail => {
@@ -1362,7 +1362,7 @@ pub fn update(self: *Compilation) !void {
 
             // TODO only analyze imports if they are still referenced
             for (module.import_table.items()) |entry| {
-                entry.value.unload(module.gpa);
+                module.unloadFile(entry.value);
                 module.analyzeContainer(&entry.value.root_container) catch |err| switch (err) {
                     error.AnalysisFail => {
                         assert(self.totalErrorCount() != 0);
@@ -1432,11 +1432,25 @@ pub fn totalErrorCount(self: *Compilation) usize {
     var total: usize = self.failed_c_objects.items().len;
 
     if (self.bin_file.options.module) |module| {
-        total += module.failed_decls.count() +
-            module.emit_h_failed_decls.count() +
-            module.failed_exports.items().len +
+        total += module.failed_exports.items().len +
             module.failed_files.items().len +
             @boolToInt(module.failed_root_src_file != null);
+        // Skip errors for Decls within files that failed parsing.
+        // When a parse error is introduced, we keep all the semantic analysis for
+        // the previous parse success, including compile errors, but we cannot
+        // emit them until the file succeeds parsing.
+        for (module.failed_decls.items()) |entry| {
+            if (entry.key.container.file_scope.status == .unloaded_parse_failure) {
+                continue;
+            }
+            total += 1;
+        }
+        for (module.emit_h_failed_decls.items()) |entry| {
+            if (entry.key.container.file_scope.status == .unloaded_parse_failure) {
+                continue;
+            }
+            total += 1;
+        }
     }
 
     // The "no entry point found" error only counts if there are no other errors.
@@ -1483,9 +1497,19 @@ pub fn getAllErrorsAlloc(self: *Compilation) !AllErrors {
             try AllErrors.add(module, &arena, &errors, entry.value.*);
         }
         for (module.failed_decls.items()) |entry| {
+            if (entry.key.container.file_scope.status == .unloaded_parse_failure) {
+                // Skip errors for Decls within files that had a parse failure.
+                // We'll try again once parsing succeeds.
+                continue;
+            }
             try AllErrors.add(module, &arena, &errors, entry.value.*);
         }
         for (module.emit_h_failed_decls.items()) |entry| {
+            if (entry.key.container.file_scope.status == .unloaded_parse_failure) {
+                // Skip errors for Decls within files that had a parse failure.
+                // We'll try again once parsing succeeds.
+                continue;
+            }
             try AllErrors.add(module, &arena, &errors, entry.value.*);
         }
         for (module.failed_exports.items()) |entry| {

src/Module.zig

@@ -65,8 +65,8 @@ emit_h_failed_decls: std.AutoArrayHashMapUnmanaged(*Decl, *ErrorMsg) = .{},
 /// Keep track of one `@compileLog` callsite per owner Decl.
 compile_log_decls: std.AutoArrayHashMapUnmanaged(*Decl, SrcLoc) = .{},
 /// Using a map here for consistency with the other fields here.
-/// The ErrorMsg memory is owned by the `Scope`, using Module's general purpose allocator.
-failed_files: std.AutoArrayHashMapUnmanaged(*Scope, *ErrorMsg) = .{},
+/// The ErrorMsg memory is owned by the `Scope.File`, using Module's general purpose allocator.
+failed_files: std.AutoArrayHashMapUnmanaged(*Scope.File, *ErrorMsg) = .{},
 /// Using a map here for consistency with the other fields here.
 /// The ErrorMsg memory is owned by the `Export`, using Module's general purpose allocator.
 failed_exports: std.AutoArrayHashMapUnmanaged(*Export, *ErrorMsg) = .{},
@@ -732,10 +732,12 @@ pub const Scope = struct {
 
         pub fn unload(file: *File, gpa: *Allocator) void {
             switch (file.status) {
-                .never_loaded,
                 .unloaded_parse_failure,
+                .never_loaded,
                 .unloaded_success,
-                => {},
+                => {
+                    file.status = .unloaded_success;
+                },
 
                 .loaded_success => {
                     file.tree.deinit(gpa);
@@ -3241,7 +3243,7 @@ pub fn getAstTree(mod: *Module, root_scope: *Scope.File) !*const ast.Tree {
                     .msg = msg.toOwnedSlice(),
                 };
 
-                mod.failed_files.putAssumeCapacityNoClobber(&root_scope.base, err_msg);
+                mod.failed_files.putAssumeCapacityNoClobber(root_scope, err_msg);
                 root_scope.status = .unloaded_parse_failure;
                 return error.AnalysisFail;
             }
@@ -4691,3 +4693,10 @@ pub fn parseStrLit(
         },
     }
 }
+
+pub fn unloadFile(mod: *Module, file_scope: *Scope.File) void {
+    if (file_scope.status == .unloaded_parse_failure) {
+        mod.failed_files.swapRemove(file_scope).?.value.destroy(mod.gpa);
+    }
+    file_scope.unload(mod.gpa);
+}

test/stage2/cbe.zig

@@ -600,6 +600,7 @@ pub fn addCases(ctx: *TestContext) !void {
         , "");
 
         // Specifying alignment is a parse error.
+        // This also tests going from a successful build to a parse error.
         case.addError(
             \\const E1 = enum {
             \\    a,
@@ -612,6 +613,24 @@ pub fn addCases(ctx: *TestContext) !void {
         , &.{
             ":3:7: error: expected ',', found 'align'",
         });
+
+        // Redundant non-exhaustive enum mark.
+        // This also tests going from a parse error to an AstGen error.
+        case.addError(
+            \\const E1 = enum {
+            \\    a,
+            \\    _,
+            \\    b,
+            \\    c,
+            \\    _,
+            \\};
+            \\export fn foo() void {
+            \\    const x = E1.a;
+            \\}
+        , &.{
+            ":6:5: error: redundant non-exhaustive enum mark",
+            ":3:5: note: other mark here",
+        });
     }
 
     ctx.c("empty start function", linux_x64,