cmd/go, go/build: update docs to use //go:build syntax

Fixes #46124.

Change-Id: I6b8179032102a14befc37719f64ddace98397c97
Reviewed-on: https://go-review.googlesource.com/c/go/+/326931
Trust: Damien Neil <dneil@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/go/alldocs.go

@@ -1568,7 +1568,7 @@
 //
 // A build constraint, also known as a build tag, is a line comment that begins
 //
-// 	// +build
+// 	//go:build
 //
 // that lists the conditions under which a file should be included in the package.
 // Constraints may appear in any kind of source file (not just Go), but
@@ -1576,30 +1576,20 @@
 // only by blank lines and other line comments. These rules mean that in Go
 // files a build constraint must appear before the package clause.
 //
-// To distinguish build constraints from package documentation, a series of
-// build constraints must be followed by a blank line.
+// To distinguish build constraints from package documentation,
+// a build constraint should be followed by a blank line.
 //
-// A build constraint is evaluated as the OR of space-separated options.
-// Each option evaluates as the AND of its comma-separated terms.
-// Each term consists of letters, digits, underscores, and dots.
-// A term may be negated with a preceding !.
-// For example, the build constraint:
+// A build constraint is evaluated as an expression containing options
+// combined by ||, &&, and ! operators and parentheses. Operators have
+// the same meaning as in Go.
 //
-// 	// +build linux,386 darwin,!cgo
+// For example, the following build constraint constrains a file to
+// build when the "linux" and "386" constraints are satisfied, or when
+// "darwin" is satisfied and "cgo" is not:
 //
-// corresponds to the boolean formula:
+// 	//go:build (linux && 386) || (darwin && !cgo)
 //
-// 	(linux AND 386) OR (darwin AND (NOT cgo))
-//
-// A file may have multiple build constraints. The overall constraint is the AND
-// of the individual constraints. That is, the build constraints:
-//
-// 	// +build linux darwin
-// 	// +build amd64
-//
-// corresponds to the boolean formula:
-//
-// 	(linux OR darwin) AND amd64
+// It is an error for a file to have more than one //go:build line.
 //
 // During a particular build, the following words are satisfied:
 //
@@ -1637,24 +1627,28 @@
 //
 // To keep a file from being considered for the build:
 //
-// 	// +build ignore
+// 	//go:build ignore
 //
 // (any other unsatisfied word will work as well, but "ignore" is conventional.)
 //
 // To build a file only when using cgo, and only on Linux and OS X:
 //
-// 	// +build linux,cgo darwin,cgo
+// 	//go:build cgo && (linux || darwin)
 //
 // Such a file is usually paired with another file implementing the
 // default functionality for other systems, which in this case would
 // carry the constraint:
 //
-// 	// +build !linux,!darwin !cgo
+// 	//go:build !(cgo && (linux || darwin))
 //
 // Naming a file dns_windows.go will cause it to be included only when
 // building the package for Windows; similarly, math_386.s will be included
 // only when building the package for 32-bit x86.
 //
+// Go versions 1.16 and earlier used a different syntax for build constraints,
+// with a "// +build" prefix. The gofmt command will add an equivalent //go:build
+// constraint when encountering the older syntax.
+//
 //
 // Build modes
 //

src/cmd/go/internal/help/helpdoc.go

@@ -784,7 +784,7 @@ var HelpBuildConstraint = &base.Command{
 	Long: `
 A build constraint, also known as a build tag, is a line comment that begins
 
-	// +build
+	//go:build
 
 that lists the conditions under which a file should be included in the package.
 Constraints may appear in any kind of source file (not just Go), but
@@ -792,30 +792,20 @@ they must appear near the top of the file, preceded
 only by blank lines and other line comments. These rules mean that in Go
 files a build constraint must appear before the package clause.
 
-To distinguish build constraints from package documentation, a series of
-build constraints must be followed by a blank line.
+To distinguish build constraints from package documentation,
+a build constraint should be followed by a blank line.
 
-A build constraint is evaluated as the OR of space-separated options.
-Each option evaluates as the AND of its comma-separated terms.
-Each term consists of letters, digits, underscores, and dots.
-A term may be negated with a preceding !.
-For example, the build constraint:
+A build constraint is evaluated as an expression containing options
+combined by ||, &&, and ! operators and parentheses. Operators have
+the same meaning as in Go.
 
-	// +build linux,386 darwin,!cgo
+For example, the following build constraint constrains a file to
+build when the "linux" and "386" constraints are satisfied, or when
+"darwin" is satisfied and "cgo" is not:
 
-corresponds to the boolean formula:
+	//go:build (linux && 386) || (darwin && !cgo)
 
-	(linux AND 386) OR (darwin AND (NOT cgo))
-
-A file may have multiple build constraints. The overall constraint is the AND
-of the individual constraints. That is, the build constraints:
-
-	// +build linux darwin
-	// +build amd64
-
-corresponds to the boolean formula:
-
-	(linux OR darwin) AND amd64
+It is an error for a file to have more than one //go:build line.
 
 During a particular build, the following words are satisfied:
 
@@ -853,22 +843,26 @@ in addition to ios tags and files.
 
 To keep a file from being considered for the build:
 
-	// +build ignore
+	//go:build ignore
 
 (any other unsatisfied word will work as well, but "ignore" is conventional.)
 
 To build a file only when using cgo, and only on Linux and OS X:
 
-	// +build linux,cgo darwin,cgo
+	//go:build cgo && (linux || darwin)
 
 Such a file is usually paired with another file implementing the
 default functionality for other systems, which in this case would
 carry the constraint:
 
-	// +build !linux,!darwin !cgo
+	//go:build !(cgo && (linux || darwin))
 
 Naming a file dns_windows.go will cause it to be included only when
 building the package for Windows; similarly, math_386.s will be included
 only when building the package for 32-bit x86.
+
+Go versions 1.16 and earlier used a different syntax for build constraints,
+with a "// +build" prefix. The gofmt command will add an equivalent //go:build
+constraint when encountering the older syntax.
 `,
 }

src/go/build/doc.go

@@ -59,7 +59,7 @@
 //
 // A build constraint, also known as a build tag, is a line comment that begins
 //
-// 	// +build
+// 	//go:build
 //
 // that lists the conditions under which a file should be included in the
 // package. Build constraints may also be part of a file's name