go/types, types2: quote user-supplied names in error messages

Use `' quotes (as in `foo') to differentiate from Go quotes.
Quoting prevents confusion when user-supplied names alter
the meaning of the error message.

For instance, report

        duplicate method `wanted'

rather than

        duplicate method wanted

Exceptions:
- don't quote _:
        `_' is ugly and not necessary
- don't quote after a ":":
        undefined name: foo
- don't quote if the name is used correctly in a statement:
        goto L jumps over variable declaration

Quoting is done with a helper function and can be centrally adjusted
and fine-tuned as needed.

Adjusted some test cases to explicitly include the quoted names.

Fixes #65790.

Change-Id: Icce667215f303ab8685d3e5cb00d540a2fd372ca
Reviewed-on: https://go-review.googlesource.com/c/go/+/571396
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Emmanuel Odeke <emmanuel@orijtech.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Robert Griesemer <gri@google.com>

4 files changed, 6 insertions, 6 deletions

diff --git a/test/fixedbugs/issue11361.go b/test/fixedbugs/issue11361.go
index 1ef8735a52..84e71c73e6 100644
--- a/test/fixedbugs/issue11361.go
+++ b/test/fixedbugs/issue11361.go
@@ -8,4 +8,4 @@ package a
 
 import "fmt"  // GC_ERROR "imported and not used"
 
-const n = fmt // ERROR "fmt without selector|unexpected reference to package|use of package fmt not in selector"
+const n = fmt // ERROR "fmt without selector|unexpected reference to package|use of package `fmt' not in selector"
diff --git a/test/fixedbugs/issue21317.go b/test/fixedbugs/issue21317.go
index cebfc0f52d..d525f12a58 100644
--- a/test/fixedbugs/issue21317.go
+++ b/test/fixedbugs/issue21317.go
@@ -45,8 +45,8 @@ func main() {
 		log.Fatalf("expected cmd/compile to fail")
 	}
 	wantErrs := []string{
-		"7:9: n declared and not used",
-		"7:12: err declared and not used",
+		"7:9: `n' declared and not used",
+		"7:12: `err' declared and not used",
 	}
 	outStr := string(out)
 	for _, want := range wantErrs {
diff --git a/test/fixedbugs/issue28268.go b/test/fixedbugs/issue28268.go
index 263b2d1b42..b04e42555f 100644
--- a/test/fixedbugs/issue28268.go
+++ b/test/fixedbugs/issue28268.go
@@ -16,8 +16,8 @@ type T struct {
 
 type E struct{}
 
-func (T) b()  {} // ERROR "field and method named b|redeclares struct field name|field and method with the same name b"
-func (*T) E() {} // ERROR "field and method named E|redeclares struct field name|field and method with the same name E"
+func (T) b()  {} // ERROR "field and method named b|redeclares struct field name|field and method with the same name `b'"
+func (*T) E() {} // ERROR "field and method named E|redeclares struct field name|field and method with the same name `E'"
 
 func _() {
 	var x T
diff --git a/test/fixedbugs/issue34329.go b/test/fixedbugs/issue34329.go
index 790686e3dd..585770d87a 100644
--- a/test/fixedbugs/issue34329.go
+++ b/test/fixedbugs/issue34329.go
@@ -10,5 +10,5 @@ type I interface { M() }
 
 type _ interface {
 	I
-	I // ERROR "duplicate method M"
+	I // ERROR "duplicate method `M'"
 }