runtime: elide instantiated types in tracebacks

They tend to be things like ".shape.int" which are noisy, if not
otherwise confusing.

It would be nice to somehow print the real instantiations here, but that
requires keeping track of the dictionary argument so the instantiating
types could be found. One day, maybe, but not today.

Fixes #48578

Change-Id: I0968d24e110b6d47c9468c45372a6979575a8d29
Reviewed-on: https://go-review.googlesource.com/c/go/+/352118
Trust: Keith Randall <khr@golang.org>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>

src/cmd/link/internal/ld/pcln.go

@@ -14,6 +14,7 @@ import (
 	"internal/buildcfg"
 	"os"
 	"path/filepath"
+	"strings"
 )
 
 // pclntab holds the state needed for pclntab generation.
@@ -286,11 +287,35 @@ func walkFuncs(ctxt *Link, funcs []loader.Sym, f func(loader.Sym)) {
 func (state *pclntab) generateFuncnametab(ctxt *Link, funcs []loader.Sym) map[loader.Sym]uint32 {
 	nameOffsets := make(map[loader.Sym]uint32, state.nfunc)
 
+	// The name used by the runtime is the concatenation of the 3 returned strings.
+	// For regular functions, only one returned string is nonempty.
+	// For generic functions, we use three parts so that we can print everything
+	// within the outermost "[]" as "...".
+	nameParts := func(name string) (string, string, string) {
+		i := strings.IndexByte(name, '[')
+		if i < 0 {
+			return name, "", ""
+		}
+		// TODO: use LastIndexByte once the bootstrap compiler is >= Go 1.5.
+		j := len(name) - 1
+		for j > i && name[j] != ']' {
+			j--
+		}
+		if j <= i {
+			return name, "", ""
+		}
+		return name[:i], "[...]", name[j+1:]
+	}
+
 	// Write the null terminated strings.
 	writeFuncNameTab := func(ctxt *Link, s loader.Sym) {
 		symtab := ctxt.loader.MakeSymbolUpdater(s)
 		for s, off := range nameOffsets {
-			symtab.AddStringAt(int64(off), ctxt.loader.SymName(s))
+			a, b, c := nameParts(ctxt.loader.SymName(s))
+			o := int64(off)
+			o = symtab.AddStringAt(o, a)
+			o = symtab.AddStringAt(o, b)
+			_ = symtab.AddCStringAt(o, c)
 		}
 	}
 
@@ -298,7 +323,8 @@ func (state *pclntab) generateFuncnametab(ctxt *Link, funcs []loader.Sym) map[lo
 	var size int64
 	walkFuncs(ctxt, funcs, func(s loader.Sym) {
 		nameOffsets[s] = uint32(size)
-		size += int64(ctxt.loader.SymNameLen(s)) + 1 // NULL terminate
+		a, b, c := nameParts(ctxt.loader.SymName(s))
+		size += int64(len(a) + len(b) + len(c) + 1) // NULL terminate
 	})
 
 	state.funcnametab = state.addGeneratedSym(ctxt, "runtime.funcnametab", size, writeFuncNameTab)

src/cmd/link/internal/loader/loader.go

@@ -752,22 +752,6 @@ func (l *Loader) NReachableSym() int {
 	return l.attrReachable.Count()
 }
 
-// SymNameLen returns the length of the symbol name, trying hard not to load
-// the name.
-func (l *Loader) SymNameLen(i Sym) int {
-	// Not much we can do about external symbols.
-	if l.IsExternal(i) {
-		return len(l.SymName(i))
-	}
-	r, li := l.toLocal(i)
-	le := r.Sym(li).NameLen(r.Reader)
-	if !r.NeedNameExpansion() {
-		return le
-	}
-	// Just load the symbol name. We don't know how expanded it'll be.
-	return len(l.SymName(i))
-}
-
 // Returns the raw (unpatched) name of the i-th symbol.
 func (l *Loader) RawSymName(i Sym) string {
 	if l.IsExternal(i) {

src/cmd/link/internal/loader/symbolbuilder.go

@@ -308,6 +308,16 @@ func (sb *SymbolBuilder) SetAddr(arch *sys.Arch, off int64, tgt Sym) int64 {
 }
 
 func (sb *SymbolBuilder) AddStringAt(off int64, str string) int64 {
+	strLen := int64(len(str))
+	if off+strLen > int64(len(sb.data)) {
+		panic("attempt to write past end of buffer")
+	}
+	copy(sb.data[off:off+strLen], str)
+	return off + strLen
+}
+
+// AddCStringAt adds str plus a null terminating byte.
+func (sb *SymbolBuilder) AddCStringAt(off int64, str string) int64 {
 	strLen := int64(len(str))
 	if off+strLen+1 > int64(len(sb.data)) {
 		panic("attempt to write past end of buffer")