[dev.typeparams] cmd/compile: add base.Assertf{,At} functions

We have almost 200 uses of the "assert" helper functions in noder and
typecheck. Clearly the tiny bit of extra convenience of writing a
one-line assertion rather than an if+panic is helpful, so we might as
well add functions for this to base itself so that it's easier to
write more informative error messages.

Change-Id: I06e2db2f0455af063937b25a53ca42f9413cf496
Reviewed-on: https://go-review.googlesource.com/c/go/+/336050
Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/base/print.go

@@ -233,6 +233,27 @@ func FatalfAt(pos src.XPos, format string, args ...interface{}) {
 	ErrorExit()
 }
 
+// Assert reports "assertion failed" with Fatalf, unless b is true.
+func Assert(b bool) {
+	if !b {
+		Fatalf("assertion failed")
+	}
+}
+
+// Assertf reports a fatal error with Fatalf, unless b is true.
+func Assertf(b bool, format string, args ...interface{}) {
+	if !b {
+		Fatalf(format, args...)
+	}
+}
+
+// AssertfAt reports a fatal error with FatalfAt, unless b is true.
+func AssertfAt(b bool, pos src.XPos, format string, args ...interface{}) {
+	if !b {
+		FatalfAt(pos, format, args...)
+	}
+}
+
 // hcrash crashes the compiler when -h is set, to find out where a message is generated.
 func hcrash() {
 	if Flag.LowerH != 0 {

src/cmd/compile/internal/noder/stencil.go

@@ -23,9 +23,7 @@ import (
 )
 
 func assert(p bool) {
-	if !p {
-		panic("assertion failed")
-	}
+	base.Assert(p)
 }
 
 // Temporary - for outputting information on derived types, dictionaries, sub-dictionaries.

src/cmd/compile/internal/typecheck/subr.go

@@ -981,9 +981,7 @@ func MakeDictName(gf *types.Sym, targs []*types.Type, hasBrackets bool) *types.S
 }
 
 func assert(p bool) {
-	if !p {
-		panic("assertion failed")
-	}
+	base.Assert(p)
 }
 
 // General type substituter, for replacing typeparams with type args.