cmd/go/internal/mvs: indicate the actual version when printing a mismatched ModuleError

Previously, we suppressed the module version annotation if the last
error in the stack was a *module.ModuleError, regardless of its path.
However, if the error is for a replacement module, that produces a
confusing error message: the error is attributed to the last module in
the error path, but actually originates in the replacement (which is
not otherwise indicated).

Now, we print both the original and the replacement modules when they
differ, which may add some unfortunate redundancy in the output but at
least doesn't drop the very relevant information about replacements.

Fixes #35039

Change-Id: I631a7398033602b1bd5656150a4fad4945a87ade
Reviewed-on: https://go-review.googlesource.com/c/go/+/247765
Reviewed-by: Jay Conrod <[email protected]>

Author: Bryan C. Mills

src/cmd/go/internal/mvs/errors.go

@@ -78,16 +78,21 @@ func (e *BuildListError) Error() string {
 		b.WriteString(e.Err.Error())
 	} else {
 		for _, elem := range stack[:len(stack)-1] {
-			fmt.Fprintf(b, "%s@%s %s\n\t", elem.m.Path, elem.m.Version, elem.nextReason)
+			fmt.Fprintf(b, "%s %s\n\t", elem.m, elem.nextReason)
 		}
 		// Ensure that the final module path and version are included as part of the
 		// error message.
 		m := stack[len(stack)-1].m
-		if _, ok := e.Err.(*module.ModuleError); ok {
-			// TODO(bcmills): Also ensure that the module path and version match.
-			// (Otherwise, we may be reporting an error from a replacement without
-			// indicating the replacement path.)
-			fmt.Fprintf(b, "%v", e.Err)
+		if mErr, ok := e.Err.(*module.ModuleError); ok {
+			actual := module.Version{Path: mErr.Path, Version: mErr.Version}
+			if v, ok := mErr.Err.(*module.InvalidVersionError); ok {
+				actual.Version = v.Version
+			}
+			if actual == m {
+				fmt.Fprintf(b, "%v", e.Err)
+			} else {
+				fmt.Fprintf(b, "%s (replaced by %s): %v", m, actual, mErr.Err)
+			}
 		} else {
 			fmt.Fprintf(b, "%v", module.VersionError(m, e.Err))
 		}

src/cmd/go/testdata/script/mod_load_replace_mismatch.txt

@@ -18,6 +18,6 @@ package use
 import _ "rsc.io/quote"
 
 -- want --
-go: example.com/[email protected]: parsing go.mod:
+go: rsc.io/[email protected] (replaced by example.com/[email protected]): parsing go.mod:
 	module declares its path as: rsc.io/Quote
 	        but was required as: rsc.io/quote

src/cmd/go/testdata/script/mod_replace_gopkgin.txt

@@ -34,7 +34,7 @@ go list -m gopkg.in/src-d/go-git.v4
 # A mismatched gopkg.in path should not be able to replace a different major version.
 cd ../3-to-gomod-4
 ! go list -m gopkg.in/src-d/go-git.v3
-stderr '^go: gopkg\.in/src-d/go-git\.v3@v3.0.0-20190801152248-0d1a009cbb60: invalid version: go\.mod has non-\.\.\.\.v3 module path "gopkg\.in/src-d/go-git\.v4" at revision 0d1a009cbb60$'
+stderr '^go: gopkg\.in/src-d/go-git\.v3@v3\.2\.0 \(replaced by gopkg\.in/src-d/go-git\.v3@v3\.0\.0-20190801152248-0d1a009cbb60\): version "v3\.0\.0-20190801152248-0d1a009cbb60" invalid: go\.mod has non-\.\.\.\.v3 module path "gopkg\.in/src-d/go-git\.v4" at revision 0d1a009cbb60$'
 
 -- 4-to-4/go.mod --
 module golang.org/issue/34254