cmd/cover: go test -c -cover runtime fails

[dvyukov]

$ go test -c -cover runtime
# runtime
/var/folders/00/0zkq0000h01000cxqpysvccm003y_w/T/go-build973521540/runtime/_test/_obj_test/cgocall.go:152: args escapes to heap, not allowed in runtime.
/var/folders/00/0zkq0000h01000cxqpysvccm003y_w/T/go-build973521540/runtime/_test/_obj_test/chan.go:95: t escapes to heap, not allowed in runtime.
/var/folders/00/0zkq0000h01000cxqpysvccm003y_w/T/go-build973521540/runtime/_test/_obj_test/chan.go:312: c escapes to heap, not allowed in runtime.
/var/folders/00/0zkq0000h01000cxqpysvccm003y_w/T/go-build973521540/runtime/_test/_obj_test/chan.go:635: t escapes to heap, not allowed in runtime.
/var/folders/00/0zkq0000h01000cxqpysvccm003y_w/T/go-build973521540/runtime/_test/_obj_test/chan.go:653: t escapes to heap, not allowed in runtime.
/var/folders/00/0zkq0000h01000cxqpysvccm003y_w/T/go-build973521540/runtime/_test/_obj_test/env_posix.go:40: arg escapes to heap, not allowed in runtime.
/var/folders/00/0zkq0000h01000cxqpysvccm003y_w/T/go-build973521540/runtime/_test/_obj_test/env_posix.go:51: arg escapes to heap, not allowed in runtime.
/var/folders/00/0zkq0000h01000cxqpysvccm003y_w/T/go-build973521540/runtime/_test/_obj_test/extern.go:10: rpc escapes to heap, not allowed in runtime.
/var/folders/00/0zkq0000h01000cxqpysvccm003y_w/T/go-build973521540/runtime/_test/_obj_test/hashmap.go:218: t escapes to heap, not allowed in runtime.
/var/folders/00/0zkq0000h01000cxqpysvccm003y_w/T/go-build973521540/runtime/_test/_obj_test/hashmap.go:287: t escapes to heap, not allowed in runtime.
/var/folders/00/0zkq0000h01000cxqpysvccm003y_w/T/go-build973521540/runtime/_test/_obj_test/hashmap.go:287: too many errors

The problem is that cover strips all comments, including important //go:noescape/nosplit/linkname comments that affect behavior. We have such comments in runtime, bytes, crypto/md5, syscall and maybe other packages.

cover must not strip these comments.

FWIW, here is a patch that works for me:

diff --git a/cmd/cover/cover.go b/cmd/cover/cover.go
index 3aaf246..2bce5c6 100644
--- a/cmd/cover/cover.go
+++ b/cmd/cover/cover.go
@@ -19,6 +19,7 @@ import (
    "path/filepath"
    "sort"
    "strconv"
+   "strings"
 )

 const usageMessage = "" +
@@ -324,11 +325,26 @@ func annotate(name string) {
    if err != nil {
        log.Fatalf("cover: %s: %s", name, err)
    }
-   parsedFile, err := parser.ParseFile(fset, name, content, 0)
+   parsedFile, err := parser.ParseFile(fset, name, content, parser.ParseComments)
    if err != nil {
        log.Fatalf("cover: %s: %s", name, err)
    }

+   cgs := parsedFile.Comments
+   parsedFile.Comments = nil
+   for _, cg := range cgs {
+       cs := cg.List
+       cg.List = nil
+       for _, c := range cs {
+           if strings.HasPrefix(c.Text, "//go:") {
+               cg.List = append(cg.List, c)
+           }
+       }
+       if cg.List != nil {
+           parsedFile.Comments = append(parsedFile.Comments, cg)
+       }
+   }
+

[robpike]

Fixed by https://go.googlesource.com/tools/+/0c09ff325ac41535a3d5fb6d539c32aca981bada