go/doc/go1.5.html

<!--{
	"Title": "Go 1.5 Release Notes",
	"Path":  "/doc/go1.5",
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<h2 id="introduction">Introduction to Go 1.5</h2>

<p>
The latest Go release, version 1.5,
is a significant release, including major architectural changes to the implementation.
Despite that, we expect almost all Go programs to continue to compile and run as before,
because the release still maintains the Go 1 <a href="/doc/go1compat.html">promise
of compatibility</a>.
</p>

<p>
The biggest developments in the implementation are:
</p>

<ul>

<li>
The compiler and runtime are now written entirely in Go (with a little assembler).
C is no longer involved in the implementation, and so the C compiler that was
once necessary for building the distribution is gone.
</li>

<li>
The garbage collector is now <a href="/s/go14gc">concurrent</a> and provides dramatically lower
pause times by running, when possible, in parallel with other goroutines.
</li>

<li>
By default, Go programs run with <code>GOMAXPROCS</code> set to the
number of cores available; in prior releases it defaulted to 1.
</li>

<li>
Support for <a href="http://golang.org/s/go14internal">internal packages</a>
is now provided for all repositories, not just the Go core.
</li>

<li>
The <code>go</code> command now provides <a href="/s/go15vendor">experimental
support</a> for "vendoring" external dependencies.
</li>

</ul>

<p>
These and a number of other changes to the implementation and tools
are discussed below.
</p>

<p>
The release also contains one small language change involving map literals.
</p>

<p>
Finally, the timing of the <a href="/s/releasesched">release</a>
strays from the usual six-month interval,
both to provide more time to prepare this major release and to shift the schedule thereafter to
time the release dates more conveniently.
</p>

<h2 id="language">Changes to the language</h2>

<h3 id="mapliterals">Map literals</h3>

<p>
Due to an oversight, the rule that allowed the element type to be elided from slice literals was not
applied to map keys.
This has been <a href="/cl/2591">corrected</a> in Go 1.5.
An example will make this clear: as of Go 1.5, this map literal,
</p>

<pre>
m := map[Point]string{
    Point{29.935523, 52.891566}:   "Persepolis",
    Point{-25.352594, 131.034361}: "Uluru",
    Point{37.422455, -122.084306}: "Googleplex",
}
</pre>

<p>
may be written as follows, without the <code>Point</code> type listed explicitly:
</p>

<pre>
m := map[Point]string{
    {29.935523, 52.891566}:   "Persepolis",
    {-25.352594, 131.034361}: "Uluru",
    {37.422455, -122.084306}: "Googleplex",
}
</pre>

<h2 id="implementation">The Implementation</h2>

<h3 id="c">No more C</h3>

<p>
The compiler and runtime are now implemented in Go and assembler, without C.
The only C source left in the tree is related to testing or to <code>cgo</code>.
There was a C compiler in the tree in 1.4 and earlier.
It was used to build the runtime; a custom compiler was necessary in part to
guarantee the C code would work with the stack management of goroutines.
Since the runtime is in Go now, there is no need for this C compiler and it is gone.
Details of the process to eliminate C are discussed <a href="/s/go13compiler">elsewhere</a>.
</p>

<p>
The conversion from C was done with the help of custom tools created for the job.
Most important, the compiler was actually moved by automatic translation of
the C code into Go.
It is in effect the same program in a different language.
It is not a new implementation
of the compiler so we expect the process will not have introduced new compiler
bugs.
An overview of this process is available in the slides for
<a href="https://talks.golang.org/2015/gogo.slide">this presentation</a>.
</p>

<h3 id="compiler">Compiler and tools</h3>

<p>
Independent of but encouraged by the move to Go, the names of the tools have changed.
The old names <code>6g</code>, <code>8g</code> and so on are gone; instead there
is just one binary, accessible as <code>go</code> <code>tool</code> <code>compile</code>,
that compiles Go source into binaries suitable for the architecture and operating system
specified by <code>$GOARCH</code> and <code>$GOOS</code>.
Similarly, there is now one linker (<code>go</code> <code>tool</code> <code>link</code>)
and one assembler (<code>go</code> <code>tool</code> <code>asm</code>).
The linker was translated automatically from the old C implementation,
but the assembler is a new native Go implementation discussed
in more detail below.
</p>

<p>
Similar to the drop of the names <code>6g</code>, <code>8g</code>, and so on,
the output of the compiler and assembler are now given a plain <code>.o</code> suffix
rather than <code>.8</code>, <code>.6</code>, etc.
</p>


<h3 id="gc">Garbage collector</h3>

<p>
TODO
</p>

<h3 id="runtime">Runtime</h3>

<p>
In Go 1.5, the order in which goroutines are scheduled has been changed.
The properties of the scheduler were never defined by the language,
but programs that depended on the scheduling order may be broken
by this change.
We have seen a few (erroneous) programs affected by this change.
If you have programs that implicitly depend on the scheduling
order, you will need to update them.
</p>

<p>
Another potentially breaking change is that the runtime now
sets the default number of threads to run simultaneously,
defined by <code>GOMAXPROCS</code>, to the number
of cores available on the CPU.
In prior releases it defaulted to 1.
Programs that do not expect to run with multiple cores may
break inadvertently.
They can be updated by removing the restriction or by setting
<code>GOMAXPROCS</code> explicitly. 
</p>

<h3 id="build">Build</h3>

<p>
Now that the Go compiler and runtime are implemented in Go, a Go compiler
must be available to compile the distribution from source.
Thus, to build the Go core, a working Go distribution must already be in place.
(Go programmers who do not work on the core are unaffected by this change.)
Any Go 1.4 or later distribution (including <code>gccgo</code>) will serve.
For details, see the <a href="/s/go15bootstrap">design document</a>.
</p>

<h2 id="ports">Ports</h2>

<p>
Due mostly to the industry's move away the 32-bit x86 architecture,
the set of binary downloads provided is reduced in 1.5.
A distribution for the OS X operating system is provided only for the
<code>amd64</code> architecture, not <code>386</code>.
Similarly, the ports for Snow Leopard (Apple OS X 10.6) still work but are no
longer released as a download or maintained since Apple no longer maintains that version
of the operating system.
Also, the <code>dragonfly/386</code> port is no longer supported at all
because DragonflyBSD itself no longer supports the 32-bit 386 architecture.
</p>

<p>
There are however several new ports available to be built from source.
These include <code>darwin/arm</code> and <code>darwin/arm64</code>.
The new port <code>linux/arm64</code> is mostly in place, but <code>cgo</code>
is only supported using external linking.
</p>

<p>
On FreeBSD, Go 1.5 requires FreeBSD 8-STABLE+ because of its new use of the <code>SYSCALL</code> instruction.
</p>

<p>
On NaCl, Go 1.5 requires SDK version pepper-39 or above because it now uses the
<code>get_random_bytes</code> system call.
</p>

<h2 id="tools">Tools</h2>

<pre>
build: external linking support for windows (https://golang.org/cl/7163, 7282, 7283, 7284, 7534, 7535)
cmd/cover: tool now lives in the standard repository (https://golang.org/cl/9560)
cmd/gc: constant arithmetic is based on math/big (https://golang.org/cl/7830, 7851, 7857, 8426, 7858, 7912, 8171)
cmd/go, go/build: add ${SRCDIR} variable expansion to cgo lines (https://golang.org/cl/1756)
cmd/go: add $DOLLAR to generate's variables (https://golang.org/cl/8091)
cmd/go: std wildcard now excludes commands in main repo (https://golang.org/cl/5550)
cmd/go: .swig/.swigcxx files now require SWIG 3.0.6 or later
cmd/go: add -run flag to go generate (https://golang.org/cl/9005)
cmd/go: add $GOLINE to generate's variables (https://golang.org/cl/9007)
cmd/go: add go doc (https://golang.org/cl/9227)
cmd/go: internal enforced even outside standard library (golang.org/s/go14internal; https://golang.org/cl/9156)
cmd/go, testing: add go test -count (https://golang.org/cl/10669)
cmd/go: add preliminary support for vendor directories (https://golang.org/cl/10923)
cmd/vet: better validation of struct tags (https://golang.org/cl/2685)
cmd/ld: no longer record build timestamp in Windows PE file header (https://golang.org/cl/3740)
cmd/go: add -toolexec build option
cmd/go: drop -ccflags build option
cmd/go: add -asmflags build option
cmd/go: add -buildmode build option
cmd/gc: add -dynlink option (for amd64 only)
cmd/ld: add -buildmode option
cmd/trace: new command to view traces (https://golang.org/cl/3601)

</pre>

<h3 id="assembler">Assembler</h3>

<p>
The assembler in Go 1.5 is a single new Go program that replaces
the suite of C-language assemblers (<code>6a</code>,
<code>8a</code>, etc.) in previous releases.
The values of the environment variables
<code>GOARCH</code> and <code>GOOS</code>
choose which architecture and operating system the generated
code will be for.
This is practical because the assembly language syntax has always
been idiosyncratic and nearly uniform across architectures;
what differs is just the list of instructions available and the
syntax of some addressing modes.
With the variation easily configured at startup, a single
assembler binary can cover all architectures.
(See the updated <a href="/doc/asm">assembler guide</a>
for more information about the language and some of
the changes listed below.)
</p>
 
 <p>
The new assembler is very nearly compatible with the previous
one, but there are a few changes that may affect some
assembler source files.
</p>

<p>
First, the expression evaluation used for constants is a little
different.
It now uses unsigned 64-bit arithmetic and the precedence
of operators (<code>+</code>, <code>-</code>, <code><<</code>, etc.)
comes from Go, not C.
Since there are few assembly programs to start with, and few use
complex arithmetic expressions,
and of those even fewer will be affected by these changes, we expect
almost no programs will need to be updated.
</p>

<p>
Perhaps more important is that some discrepancies between the
architectures in how the PC and SP are handled have been
eliminated.
Sometimes these registers represented hardware
registers, and sometimes pseudo-registers.
As of Go 1.5, the names <code>PC</code> and <code>SP</code>
are always pseudo-registers.
To refer to the hardware register, use the alternate representation such
as <code>R13</code> for the stack pointer and
<code>R15</code> for the hardware program counter on x86.
(The names are different on other architectures.)
To help enforce this change, references to the
<code>SP</code> and <code>PC</code>
pseudo-registers now always require an identifier:
<code>f+4(SP)</code> not <code>4(SP)</code>;
it is a syntax error to omit the identifier.
Uses of <code>SP</code> (say) as a hardware register
tend to omit the name, and they will now be flagged by
the assembler.
</p>

<p>
One minor change is that some of the old assemblers
permitted the notation
</p>

<pre>
constant=value
</pre>

<p>
to define a named constant.
Since this is always possible to do with the traditional
C-like <code>#define</code> notation, which is still
supported (the assembler includes an implementation
of a simplified C preprocessor), the feature was removed.
</p>

<h2 id="performance">Performance</h2>

<pre>

cmd/gc: evaluate concrete == interface without allocating (https://golang.org/cl/2096)
cmd/gc: optimize memclr of slices and arrays (https://golang.org/cl/2520)
cmd/gc: transform closure calls to function calls (https://golang.org/cl/4050)
cmd/gc: transitive inlining (https://golang.org/cl/5952)
cmd/gc, runtime: speed up some cases of _, ok := i.(T) (https://golang.org/cl/7697)
cmd/gc: speed up large string switches (https://golang.org/cl/7698)
cmd/gc: inline x := y.(*T) and x, ok := y.(*T) (https://golang.org/cl/7862)
cmd/gc: allocate backing storage for non-escaping interfaces on stack (https://golang.org/cl/8201)
encoding/xml: avoid an allocation for tags without attributes (https://golang.org/cl/4160)
image: many optimizations
runtime: add ARM runtime.cmpstring and bytes.Compare (https://golang.org/cl/8010)
runtime: do not scan maps when k/v do not contain pointers (https://golang.org/cl/3288)
runtime: reduce thrashing of gs between ps (https://golang.org/cl/9872)
sort: number of Sort performance optimizations (https://golang.org/cl/2100, https://golang.org/cl/2614, ...)
strconv: optimize decimal to string conversion (https://golang.org/cl/2105)
strconv: optimize float to string conversion (https://golang.org/cl/5600)
sync: add active spinning to Mutex (https://golang.org/cl/5430)
math/big: faster assembly kernels for amd64 and 386 (https://golang.org/cl/2503, https://golang.org/cl/2560)
math/big: faster "pure Go" kernels for platforms w/o assembly kernels (https://golang.org/cl/2480)
regexp: port RE2's bitstate backtracker to the regexp package (https://golang.org/cl/2153)
</pre>

<h3 id="library">Core library</h3>

<h3 id="flag">Flag</h3>

<p>
The flag package's
<a href="/pkg/flag/#PrintDefaults"><code>PrintDefaults</code></a> 
function, and method on <a href="/pkg/flag/#FlagSet"><code>FlagSet</code></a>,
have been modified to create nicer usage messages.
The format has been changed to be more human-friendly and in the usage
messages a word quoted with `backquotes` is taken to be the name of the
flag's operand to display in the usage message.
For instance, a flag created with the invocation,
</p>

<pre>
cpuFlag = flag.Int("cpu", 1, "run `N` processes in parallel")
</pre>

<p>
will show the help message,
</p>

<pre>
-cpu N
    	run N processes in parallel (default 1)
</pre>

<p>
Also, the default is now listed only when it is not the zero value for the type.
</p>

<h3 id="math_big">Floats in math/big</h3>

<p>
The <a href="/pkg/math/big/"><code>math/big</code></a> package
has a new, fundamental data type,
<a href="/pkg/math/big/#Float"><code>Float</code></a>,
which implements arbitrary-precision floating-point numbers.
A <code>Float</code> value is represented by a boolean sign,
a variable-length mantissa, and a 32-bit fixed-size signed exponent.
The precision of a <code>Float</code> (the mantissa size in bits)
can be specified explicitly or is otherwise determined by the first
operation that creates the value.
Once created, the size of a <code>Float</code>'s mantissa may be modified with the
<a href="/pkg/math/big/#Float.SetPrec"><code>SetPrec</code></a> method.
<code>Floats</code> support the concept of infinities, such as are created by
overflow, but values that would lead to the equivalent of IEEE 754 NaNs
trigger a panic.
<code>Float</code> operations support all IEEE-754 rounding modes.
When the precision is set to 24 (53) bits,
operations that stay within the range of normalized <code>float32</code>
(<code>float64</code>)
values produce the same results as the corresponding IEEE-754
arithmetic on those values.
</p>

<h3 id="go_types">Go types</h3>

<p>
The <a href="/pkg/go/types/"><code>go/types</code></a> package
up to now has been maintained in the <code>golang.org/x</code>
repository; as of Go 1.5 it has been relocated to the main repository.
The code at the old location is now deprecated.
There is also a modest API change in the package, discussed below.
</p>

<p>
Associated with this move, the
<a href="/pkg/go/constant/"><code>go/constant</code></a>
package also moved to the main repository;
it was <code>golang.org/x/tools/exact</code> before.
The <a href="/pkg/go/importer/"><code>go/importer</code></a> package
also moved to the main repository,
as well as some tools described above.
</p>

<h3 id="net">Net</h3>

<p>
The DNS resolver in the net package has almost always used <code>cgo</code> to access
the system interface.
A change in Go 1.5 means that on most Unix systems DNS resolution
will no longer require <code>cgo</code>, which simplifies execution
on those platforms.
Now, if the system's networking configuration permits, the native Go resolver
will suffice.
The important effect of this change is that each DNS resolution occupies a goroutine
rather than a thread,
so a program with multiple outstanding DNS requests will consume fewer operating
system resources.
</p>

<p>
The decision of how to run the resolver applies at run time, not build time.
The <code>netgo</code> build tag that has been used to enforce the use
of the Go resolver is no longer necessary, although it still works.
</p>

<p>
This change applies to Unix systems only.
Windows, Mac OS X, and Plan 9 systems behave as before.
</p>

<h3 id="reflect">Reflect</h3>

<p>
The <a href="/pkg/reflect/"><code>reflect</code></a> package
has two new functions: <a href="/pkg/reflect/#ArrayOf"><code>ArrayOf</code></a>
and <a href="/pkg/reflect/#FuncOf"><code>FuncOf</code></a>.
These functions, analogous to the extant
<a href="/pkg/reflect/#SliceOf"><code>SliceOf</code></a> function,
create new types at runtime to describe arrays and functions.
</p>

<h3 id="hardening">Hardening</h3>

<p>
Several dozen bugs were found in the standard library
through randomized testing with the
<a href="https://github.com/dvyukov/go-fuzz"><code>go-fuzz</code></a> tool.
Bugs were fixed in the
<a href="/pkg/archive/tar/"><code>archive/tar</code></a>,
<a href="/pkg/archive/zip/"><code>archive/zip</code></a>,
<a href="/pkg/compress/flate/"><code>compress/flate</code></a>,
<a href="/pkg/encoding/gob/"><code>encoding/gob</code></a>,
<a href="/pkg/fmt/"><code>fmt</code></a>,
<a href="/pkg/html/template/"><code>html/template</code></a>,
<a href="/pkg/image/gif/"><code>image/gif</code></a>,
<a href="/pkg/image/jpeg/"><code>image/jpeg</code></a>,
<a href="/pkg/image/png/"><code>image/png</code></a>, and
<a href="/pkg/text/template/"><code>text/template</code></a>,
packages.
The fixes harden the implementation against incorrect and malicious inputs.
</p>

<h3 id="minor_library_changes">Minor changes to the library</h3>

<ul>

<li>
The <a href="/pkg/archive/zip/"><code>archive/zip</code></a> package's
<a href="/pkg/archive/zip/#Writer"><code>Writer</code></a> type now has a
<a href="/pkg/archive/zip/#Writer.SetOffset"><code>SetOffset</code></a>
method to specify the location within the output stream at which to write the archive.
</li>

<li>
The <a href="/pkg/bufio/#Reader"><code>Reader</code></a> in the
<a href="/pkg/bufio/"><code>bufio</code></a> package now has a
<a href="/pkg/bufio/#Reader.Discard"><code>Discard</code></a>
method to discard data from the input.
</li>

<li>
Also in the <a href="/pkg/bytes/"><code>bytes</code></a> package,
the <a href="/pkg/bytes/#Buffer"><code>Buffer</code></a> type
now has a <a href="/pkg/bytes/#Buffer.Cap"><code>Cap</code></a> method
that reports the number of bytes allocated within the buffer.
Similarly, both the <a href="/pkg/bytes/"><code>bytes</code></a>
and <a href="/pkg/strings/"><code>strings</code></a> packages,
the <a href="/pkg/bytes/#Reader"><code>Reader</code></a>
type now has a <a href="/pkg/bytes/#Reader.Size"><code>Size</code></a>
method that reports the original length of the underlying slice or string.
</li>

<li>
Both the <a href="/pkg/bytes/"><code>bytes</code></a> and
<a href="/pkg/strings/"><code>strings</code></a> packages
also now have a <a href="/pkg/bytes/#LastIndexByte"><code>LastIndexByte</code></a>
function that locates the rightmost byte with that value in the argument.
</li>

<li>
The <a href="/pkg/crypto/"><code>crypto</code></a> package
has a new interface, <a href="/pkg/crypto/#Decrypter"><code>Decrypter</code></a>,
that abstracts the behavior of a private key used in asymmetric decryption.
</li>

<li>
In the <a href="/pkg/crypto/cipher/"><code>crypto/cipher</code></a> package,
the documentation for the <a href="/pkg/crypto/cipher/#Stream"><code>Stream</code></a>
interface has been clarified regarding the behavior when the source and destination are
different lengths.
If the destination is shorter than the source, the method will panic.
This is not a change in the implementation, only the documentation.
</li>

<li>
Also in the <a href="/pkg/crypto/cipher/"><code>crypto/cipher</code></a> package,
there is now support for nonce lengths other than 96 bytes in AES's Galois/Counter mode (GCM),
which some protocols require.
</li>

<li>
In the <a href="/pkg/crypto/elliptic/"><code>crypto/elliptic</code></a> package,
there is now a <code>Name</code> field in the
<a href="/pkg/crypto/elliptic/#CurveParams"><code>CurveParams</code></a> struct,
and the curves implemented in the package have been given names.
These names provide a safer way to select a curve, as opposed to
selecting its bit size, for cryptographic systems that are curve-dependent.
</li>

<li>
Also in the <a href="/pkg/crypto/elliptic/"><code>crypto/elliptic</code></a> package,
the <a href="/pkg/crypto/elliptic/#Unmarshal"><code>Unmarshal</code></a> function
now verifies that the point is actually on the curve.
(If it is not, the function returns nils).
This change guards against certain attacks.
</li>

<li>
The <a href="/pkg/crypto/tls/"><code>crypto/tls</code></a> package
now defaults to TLS 1.0.
The old default, SSLv3, is still available through <a href="/pkg/crypto/tls/#Config"><code>Config</code></a> if needed.
</li>

<li>
Also, the <a href="/pkg/crypto/tls/"><code>crypto/tls</code></a> package
now supports Signed Certificate Timestamps (SCTs) as specified in RFC 6962.
The server serves them if they are listed in the
<a href="/pkg/crypto/tls/#Certificate"><code>Certificate</code></a> struct,
and the client reqeusts them and exposes them, if present,
in its <a href="/pkg/crypto/tls/#ConnectionState"><code>ConnectionState</code></a> struct.
The <a href="/pkg/crytpo/tls/"><code>crytpo/tls</code></a> server implementation
will also now always call the
<code>GetCertificate</code> function in
the <a href="/pkg/crypto/tls/#Config"><code>Config</code></a> struct
to select a certificate for the connection when none is supplied.
</li>

<li>
Finally, the session ticket keys in the
<a href="/pkg/crypto/tls/"><code>crypto/tls</code></a> package
can now be rotated (changed periodically during an active connection).
This is done through the new 
<a href="/pkg/crypto/tls/#Config.SetSessionTicketKeys"><code>SetSessionTicketKeys</code></a>
method of the
<a href="/pkg/crypto/tls/#Config"><code>Config</code></a> type.
</li>

<li>
In the <a href="/pkg/crypto/x509/"><code>crypto/x509</code></a> package,
wildcards are now accepted only in the leftmost label as defined in
<a href="https://tools.ietf.org/html/rfc6125#section-6.4.3">the specification</a>.
</li>

<li>
Also in the <a href="/pkg/crypto/x509/"><code>crypto/x509</code></a> package,
the handling of unknown critical extensions has been changed.
They used to cause parse errors but now they are parsed and caused errors only
in <a href="/pkg/crypto/x509/#Certificate.Verify"><code>Verify</code></a>.
The new field <code>UnhandledCriticalExtensions</code> of
<a href="/pkg/crypto/x509/#Certificate"><code>Certificate</code></a> records these extensions.
</li>

<li>
The <a href="/pkg/database/sql/#DB"><code>DB</code></a> type of the
<a href="/pkg/database/sql/"><code>database/sql</code></a> package
now has a <a href="/pkg/database/sql/#DB.Stats"><code>Stats</code></a> method
to retrieve database statistics.
</li>

<li>
The <a href="/pkg/debug/dwarf/"><code>debug/dwarf</code></a>
package has extensive additions to better support DWARF version 4.
See for example the definition of the new type
<a href="/pkg/debug/dwarf/#Class"><code>Class</code></a>.
</li>

<li>
The <a href="/pkg/debug/elf/"><code>debug/elf</code></a>
package now has support for the 64-bit Power architecture.
</li>

<li>
The <a href="/pkg/encoding/base64/"><code>encoding/base64</code></a> package
now supports unpadded encodings through two new encoding variables,
<a href="/pkg/encoding/base64/#RawStdEncoding"><code>RawStdEncoding</code></a> and
<a href="/pkg/encoding/base64/#RawURLEncoding"><code>RawURLEncoding</code></a>.
</li>

<li>
The <a href="/pkg/encoding/json/"><code>encoding/json</code></a> package
now returns an <a href="/pkg/encoding/json/#UnmarshalTypeError"><code>UnmarshalTypeError</code></a>
if a JSON value is not appropriate for the target variable or component
to which it is being unmarshaled.
</li>

<li>
The <a href="/pkg/flag/"><code>flag</code></a> package
has a new function, <a href="/pkg/flag/#UnquoteUsage"><code>UnquoteUsage</code></a>,
to assist in the creation of usage messages using the new convention
described above.
</li>

<li>
Also in the <a href="/pkg/fmt/"><code>fmt</code></a> package,
a value of type <a href="/pkg/reflect/#Value"><code>Value</code></a> now
prints what it holds, rather than use the <code>reflect.Value</code>'s <code>Stringer</code>
method, which produces things like <code>&lt;int Value&gt;</code>.
</li>

<li>
The <a href="/pkg/ast/#EmptyStmt"><code>EmptyStmt</code></a> type
in the <a href="/pkg/go/ast/"><code>go/ast</code></a> package now
has a boolean <code>Implicit</code> field that records whether the
semicolon was implicitly added or was present in the source.
</li>

<li>
For forward compatibility the <a href="/pkg/go/build/"><code>go/build</code></a> package
reserves <code>GOARCH</code> values for  a number of architectures that Go might support one day.
This is not a promise that it will.
Also, the <a href="/pkg/go/build/#Package"><code>Package</code></a> struct
now has a <code>PkgTargetRoot</code> field that stores the
architecture-dependent root directory in which to install, if known.
</li>

<li>
The (newly migrated) <a href="/pkg/go/types/"><code>go/types</code></a>
package allows one to control the prefix attached to package-level names using
the new <a href="/pkg/go/types/#Qualifier"><code>Qualifier</code></a>
function type as an argument to several functions. This is an API change for
the package, but since it is new to the core, it is not breaking the Go 1 compatibility
rules since code that uses the package must explicitly ask for it at its new location.
TODO: There should be a gofix for this.
</li>

<li>
In the <a href="/pkg/image/"><code>image</code></a> package,
the <a href="/pkg/image/#Rectangle"><code>Rectangle</code></a> type
now implements the <a href="/pkg/image/#Image"><code>Image</code></a> interface,
mask image when drawing.
</li>

<li>
Also in the <a href="/pkg/image/"><code>image</code></a> package,
to assist in the handling of some JPEG images,
there is now support for 4:1:1 and 4:1:0 YCbCr subsampling and basic
CMYK support, represented by the new image.CMYK struct.
</li>

<li>
The <a href="/pkg/image/color/"><code>image/color</code></a> package
adds basic CMYK support, through the new
<a href="/pkg/image/color/#CMYK"><code>CMYK</code></a> struct,
the <a href="/pkg/image/color/#CMYKModel"><code>CMYKModel</code></a> color model, and the
<a href="/pkg/image/color/#CMYKToRGB"><code>CMYKToRGB</code></a> function, as
needed by some JPEG images.
</li>

<li>
The <a href="/pkg/image/gif/"><code>image/gif</code></a> package
includes a couple of generalizations.
A multiple-frame GIF file can now have an overall bounds different
from all the contained single frames' bounds.
Also, the <a href="/pkg/image/gif/#GIF"><code>GIF</code></a> struct
now has a <code>Disposal</code> field
that specifies the disposal method for each frame.
</li>

<li>
The <a href="/pkg/io/"><code>io</code></a> package
adds a <a href="/pkg/io/#CopyBuffer"><code>CopyBuffer</code></a> function
that is like <a href="/pkg/io/#Copy"><code>Copy</code></a> but
uses a caller-provided buffer, permitting control of allocation and buffer size.
</li>

<li>
The <a href="/pkg/log/"><code>log</code></a> package
has a new <a href="/pkg/log/#LUTC"><code>LUTC</code></a> flag
that causes time stamps to be printed in the UTC time zone.
It also adds a <a href="/pkg/log/#SetOutput"><code>SetOutput</code></a> function
to set the output destination for the standard logger
and a corresponding method for user-created loggers.
</li>

<li>
In Go 1.4, <a href="/pkg/math/#Max"><code>Max</code></a> was not detecting all possible NaN bit patterns.
This is fixed in Go 1.5, so programs that use <code>math.Max</code> on data including NaNs may behave differently,
but now correctly according to the IEEE754 definition of NaNs.
</li>

<li>
The <a href="/pkg/math/big/"><code>math/big</code></a> package
adds a new <a href="/pkg/math/big/#Jacobi"><code>Jacobi</code></a>
function for integers and a new method
<a href="/pkg/math/big/#Int.ModSqrt"><code>ModSqrt</code></a>
method for the <a href="/pkg/math/big/#Int"><code>Int</code></a> type.
</li>

<li>
The mime package
adds a new <a href="/pkg/mime/#WordDecoder"><code>WordDecoder</code></a> type
to decode MIME headers containing RFC 204-encoded words.
It also provides <a href="/pkg/mime/#BEncoding"><code>BEncoding</code></a> and
<a href="/pkg/mime/#QEncoding"><code>QEncoding</code></a>
as implementations of the encoding schemes of RFC 2045 and RFC 2047.
</li>

<li>
The <a href="/pkg/mime/"><code>mime</code></a> package also adds an
<a href="/pkg/mime/#ExtensionsByType"><code>ExtensionsByType</code></a>
function that returns the MIME extensions know to be associated with a given MIME type.
</li>

<li>
There is a new <a href="/pkg/mime/quotedprintable/"><code>mime/quotedprintable</code></a>
package that implements the quoted-printable encoding defined by RFC 2045.
</li>

<li>
Go 1.5's <a href="/pkg/net/"><code>net</code></a> package
adds RFC-6555-compliant dialing for sites with multiple TCP
addresses listed in DNS.
A new <code>DualStack</code> field
in <a href="/pkg/net/#Dialer"><code>Dialer</code></a> enables the feature. 
</li>

<li>
A number of inconsistencies in the types returned by errors in the
<a href="/pkg/net/"><code>net</code></a> package have been
tidied up.
Most now return an
<a href="/pkg/net/#OpError"><code>OpError</code></a> value
with more information than before.
Also, the <a href="/pkg/net/#OpError"><code>OpError</code></a>
type now includes a <code>Source</code> field that holds the local
network address.
</li>

<li>
The <a href="/pkg/net/http/"><code>net/http</code></a> package now
has support for setting trailers from a server <a href="/pkg/net/http/#Handler"><code>Handler</code></a>.
For details, see the documentation for
<a href="/pkg/net/http/#ResponseWriter"><code>ResponseWriter</code></a>.
</li>

<li>
Also in the <a href="/pkg/net/http/"><code>net/http</code></a> package,
there is code to ignore the zero <a href="/pkg/time/#Time"><code>Time</code></a> value
in the <a href="/pkg/net/#ServeContent"><code>ServeContent</code></a> function.
As of Go 1.5, it now also ignores a time value equal to the Unix epoch.
</li>

<li>
The <a href="/pkg/net/http/fcgi/"><code>net/http/fcgi</code></a> package
exports two new errors,
<a href="/pkg/net/http/fcgi/#ErrConnClosed"><code>ErrConnClosed</code></a> and
<a href="/pkg/net/http/fcgi/#ErrRequestAborted"><code>ErrRequestAborted</code></a>,
to report the corresponding error conditions.
</li>

<li>
The <a href="/pkg/net/http/cgi/"><code>net/http/cgi</code></a> package
had a bug that mishandled the values of the environment variables
<code>REMOTE_ADDR</code> ad <code>REMOTE_HOST</code>.
This has been fixed.
Also, starting with Go 1.5 the package sets the <code>REMOTE_PORT</code>
variable.
</li>

<li>
The <a href="/pkg/net/mail/"><code>net/mail</code></a> package
adds a <a href="/pkg/net/mail/#AddressParser"><code>AddressParser</code></a>
type that can parse mail addresses.
</li>

<li>
The <a href="/pkg/net/smtp/"><code>net/smtp</code></a> package
now has a <a href="/pkg/net/smtp/#Client.TLSConnectionState"><code>TLSConnectionState</code></a>
accessor to the <a href="/pkg/net/smtp/#Client"><code>Client</code></a>
type that returns the client's TLS state.
</li>

<li>
The <a href="/pkg/os/"><code>os</code></a> package
has a new <a href="/pkg/os/#LookupEnv"><code>LookupEnv</code></a> function
that is similar to <a href="/pkg/os/#Getenv"><code>Getenv</code></a>
but can distinguish between an empty environment variable and a missing one.
</li>

<li>
The <a href="/pkg/os/signal/"><code>os/signal</code></a> package
adds new <a href="/pkg/os/signal/#Ignore"><code>Ignore</code></a> and
<a href="/pkg/os/signal/#Reset"><code>Reset</code></a> functions.
</li>

<li>
The <a href="/pkg/runtime/"><code>runtime</code></a>,
<a href="/pkg/runtime/pprof/"><code>runtime/pprof</code></a>,
and <a href="/pkg/net/http/pprof/"><code>net/http/pprof</code></a> packages
each have new functions to support the tracing facilities described above:
<a href="/pkg/runtime/#ReadTrace"><code>ReadTrace</code></a>,
<a href="/pkg/runtime/#StartTrace"><code>StartTrace</code></a>,
<a href="/pkg/runtime/#StopTrace"><code>StopTrace</code></a>,
<a href="/pkg/runtime/pprof/#StartTrace"><code>StartTrace</code></a>,
<a href="/pkg/runtime/pprof/#StopTrace"><code>StopTrace</code></a>, and
<a href="/pkg/net/http/pprof/#Trace"><code>Trace</code></a>.
See the respective documentation for details.
</li>

<li>
The <a href="/pkg/runtime/pprof/"><code>runtime/pprof</code></a> package
by default now includes overall memory statistics in all memory profiles.
</li>

<li>
The <a href="/pkg/strings/"><code>strings</code></a> package
has a new <a href="/pkg/strings/#Compare"><code>Compare</code></a> function.
This is present to provide symmetry with the <a href="/pkg/bytes/"><code>bytes</code></a> package
but is otherwise unnecessary as strings support comparison natively.
</li>

<li>
The <a href="/pkg/sync/#WaitGroup"><code>WaitGroup</code></a> function in
package <a href="/pkg/sync/"><code>sync</code></a>
now diagnoses code that races a call to <a href="/pkg/sync/#WaitGroup.Add"><code>Add</code></a>
against a return from <a href="/pkg/sync/#WaitGroup.Wait"><code>Wait</code></a>.
If it detects this condition, <code>WaitGroup</code> panics.
</li>

<li>
In the <a href="/pkg/syscall/"><code>syscall</code></a> package,
the Linux <code>SysProcAttr</code> struct now has a
<code>GidMappingsEnableSetgroups</code> field, made necessary
by security changes in Linux 3.19.
On all Unix systems, the struct also has new <code>Foreground</code> and <code>Pgid</code> fields
to provide more control when exec'ing.
On Darwin, there is now a <code>Syscall9</code> function
to support calls with too many arguments.
</li>

<li>
The <a href="/pkg/testing/quick/"><code>testing/quick</code></a> will now
generate <code>nil</code> values for pointer types,
making it possible to use with recursive data structures.
Also, the package now supports generation of array types.
</li>

<li>
In the <a href="/pkg/text/template/"><code>text/template</code></a> and
<a href="/pkg/html/template/"><code>html/template</code></a> packages,
integer constants too large to be represented as a Go integer now trigger a
parse error. Before, they were silently converted to floating point, losing
precision.
</li>

<li>
Also in the <a href="/pkg/text/template/"><code>text/template</code></a> and
<a href="/pkg/html/template/"><code>html/template</code></a> packages,
a new <a href="/pkg/text/template/#Option"><code>Option</code></a> type
allows customization of the behavior of the template during execution.
The sole implemented option allows control over how a missing key is
handled when indexing a map.
The default, which can now be overridden, is as before: to continue with an invalid value.
</li>

<li>
The <a href="/pkg/time/"><code>time</code></a> package's
<code>Time</code> type has a new method
<a href="/pkg/time/#Time.AppendFormat"><code>AppendFormat</code></a>, 
which can be used to avoid allocation when printing a time value.
</li>

<li>
The <a href="/pkg/unicode/"><code>unicode</code></a> package and associated
support throughout the system has been upgraded from version 7.0 to
<a href="http://www.unicode.org/versions/Unicode8.0.0/">Unicode 8.0</a>.
</li>

</ul>