cmd/compile/internal/inline: ignore superfluous slicing

When slicing, ignore expressions which could be elided, as in slicing
starting at 0 or ending at len(v).

Fixes #75278

Change-Id: I9c18e29c3d4da9bef89bd25bb261d3cb60e66392
Reviewed-on: https://go-review.googlesource.com/c/go/+/701216
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Mark Freeman <markfreeman@google.com>

src/cmd/compile/internal/inline/inl.go

@@ -738,6 +738,17 @@ opSwitch:
 		if n.X.Op() == ir.OINDEX && isIndexingCoverageCounter(n.X) {
 			return false
 		}
+
+	case ir.OSLICE, ir.OSLICEARR, ir.OSLICESTR, ir.OSLICE3, ir.OSLICE3ARR:
+		n := n.(*ir.SliceExpr)
+
+		// Ignore superfluous slicing.
+		if n.Low != nil && n.Low.Op() == ir.OLITERAL && ir.Int64Val(n.Low) == 0 {
+			v.budget++
+		}
+		if n.High != nil && n.High.Op() == ir.OLEN && n.High.(*ir.UnaryExpr).X == n.X {
+			v.budget += 2
+		}
 	}
 
 	v.budget--

src/cmd/compile/internal/test/inl_test.go

@@ -234,6 +234,7 @@ func TestIntendedInlining(t *testing.T) {
 			"(*B).Loop",
 		},
 		"path": {
+			"Base",
 			"scanChunk",
 		},
 		"path/filepath": {

test/fixedbugs/issue75278.go

@@ -0,0 +1,25 @@
+// errorcheck -0 -m=2
+
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package p
+
+var a, b []int
+
+func NoIndices() { // ERROR "can inline NoIndices with cost 4 as:.*"
+	b = a[:]
+}
+
+func LowIndex() { // ERROR "can inline LowIndex with cost 4 as:.*"
+	b = a[0:]
+}
+
+func HighIndex() { // ERROR "can inline HighIndex with cost 4 as:.*"
+	b = a[:len(a)]
+}
+
+func BothIndices() { // ERROR "can inline BothIndices with cost 4 as:.*"
+	b = a[0:len(a)]
+}