Fix limits and combinations with other params (#424)

* Fix limits and combinations with other params
* Read header before decoding (effectively unrolls reads)
* Fix duplicated error checks
* Limited fields now reuse destiantion registers if possible.
* Add (generated) tests.

Removes output changes from 1.5.0 (tested against larger codebase)

* We already handled nil, no need for special case.

* Simplify more.

Author: klauspost

_generated/def.go

@@ -31,6 +31,8 @@ type Fixed struct {
 	B bool
 }
 
+type StandaloneBytes []byte
+
 type AliasedType = Fixed
 type AliasedType2 = *Fixed
 type AliasedType3 = uint64

_generated/limits.go

@@ -26,10 +26,38 @@ type LimitTestData struct {
 // Test field limits vs file limits precedence
 // File limits: arrays:100 maps:50
 type FieldOverrideTestData struct {
-	TightSlice  []int          `msg:"tight_slice,limit=10"`   // Field limit (10) < file limit (100)
-	LooseSlice  []string       `msg:"loose_slice,limit=200"`  // Field limit (200) > file limit (100)
-	TightMap    map[string]int `msg:"tight_map,limit=5"`      // Field limit (5) < file limit (50)
-	LooseMap    map[int]string `msg:"loose_map,limit=80"`     // Field limit (80) > file limit (50)
-	DefaultSlice []byte        `msg:"default_slice"`          // No field limit, uses file limit (100)
-	DefaultMap   map[string]string `msg:"default_map"`        // No field limit, uses file limit (50)
+	TightSlice   []int             `msg:"tight_slice,limit=10"`  // Field limit (10) < file limit (100)
+	LooseSlice   []string          `msg:"loose_slice,limit=200"` // Field limit (200) > file limit (100)
+	TightMap     map[string]int    `msg:"tight_map,limit=5"`     // Field limit (5) < file limit (50)
+	LooseMap     map[int]string    `msg:"loose_map,limit=80"`    // Field limit (80) > file limit (50)
+	DefaultSlice []byte            `msg:"default_slice"`         // No field limit, uses file limit (100)
+	DefaultMap   map[string]string `msg:"default_map"`           // No field limit, uses file limit (50)
 }
+
+// Test allownil functionality with limits
+// File limits: arrays:100 maps:50
+type AllowNilTestData struct {
+	NilSlice        []byte                 `msg:"nil_slice,allownil"`                           // Can be nil, uses file limit (100)
+	NilTightSlice   []int                  `msg:"nil_tight_slice,allownil,limit=5"`             // Can be nil, field limit (5) < file limit (100)
+	NilLooseSlice   []string               `msg:"nil_loose_slice,allownil,limit=200"`           // Can be nil, field limit (200) > file limit (100)
+	NilZCSlice      []byte                 `msg:"nil_zc_slice,allownil,zerocopy"`               // Can be nil, zerocopy, uses file limit (100)
+	NilZCTightSlice []byte                 `msg:"nil_zc_tight_slice,allownil,zerocopy,limit=5"` // Can be nil, zerocopy, field limit (5)
+	NilMap          map[string]int         `msg:"nil_map,allownil"`                             // Can be nil, uses file limit (50)
+	NilTightMap     map[string]int         `msg:"nil_tight_map,allownil,limit=3"`               // Can be nil, field limit (3) < file limit (50)
+	NilLooseMap     map[int]string         `msg:"nil_loose_map,allownil,limit=75"`              // Can be nil, field limit (75) > file limit (50)
+	RegularSlice    []byte                 `msg:"regular_slice"`                                // Cannot be nil, uses file limit (100)
+	RegularMap      map[string]int         `msg:"regular_map"`                                  // Cannot be nil, uses file limit (50)
+	ObjSlice        []LimitedData          `msg:"obj_slice"`                                    // Cannot be nil, uses file limit (100)
+	ObjMap          map[string]LimitedData `msg:"obj_map"`                                      // Cannot be nil, uses file limit (50)
+}
+
+type ObjSlices []LimitedData
+type ObjMaps map[string]LimitedData
+
+// Test allownil with zero-sized allocations
+type AllowNilZeroTestData struct {
+	NilBytes     []byte `msg:"nil_bytes,allownil"` // Can be nil, should allocate zero-sized slice
+	RegularBytes []byte `msg:"regular_bytes"`      // Cannot be nil
+}
+
+type StandaloneBytesLimited []byte