bootstrap: use the same version number for rustc and cargo

Historically the stable tarballs were named after the version number of
the specific tool, instead of the version number of Rust. For example,
both of the following tarballs were part of the same release:

    rustc-1.48.0-x86_64-unknown-linux-gnu.tar.xz
    cargo-0.49.0-x86_64-unknown-linux-gnu.tar.xz

PR #77336 changed the dist code to instead use Rust's version number for
all the tarballs, regardless of the tool they contain:

    rustc-1.48.0-x86_64-unknown-linux-gnu.tar.xz
    cargo-1.48.0-x86_64-unknown-linux-gnu.tar.xz

Because of that there is no need anymore to have a separate `cargo`
field in src/stage0.txt, as the Cargo version will always be the same as
the rustc version. This PR removes the field, simplifying the code and
the maintenance work required while producing releases.

src/bootstrap/bootstrap.py

@@ -360,7 +360,6 @@ def output(filepath):
 class RustBuild(object):
     """Provide all the methods required to build Rust"""
     def __init__(self):
-        self.cargo_channel = ''
         self.date = ''
         self._download_url = ''
         self.rustc_channel = ''
@@ -387,7 +386,6 @@ class RustBuild(object):
         will move all the content to the right place.
         """
         rustc_channel = self.rustc_channel
-        cargo_channel = self.cargo_channel
         rustfmt_channel = self.rustfmt_channel
 
         if self.rustc().startswith(self.bin_root()) and \
@@ -400,12 +398,15 @@ class RustBuild(object):
                 rustc_channel, self.build, tarball_suffix)
             pattern = "rust-std-{}".format(self.build)
             self._download_stage0_helper(filename, pattern, tarball_suffix)
-
             filename = "rustc-{}-{}{}".format(rustc_channel, self.build,
                                               tarball_suffix)
             self._download_stage0_helper(filename, "rustc", tarball_suffix)
+            filename = "cargo-{}-{}{}".format(rustc_channel, self.build,
+                                              tarball_suffix)
+            self._download_stage0_helper(filename, "cargo", tarball_suffix)
             self.fix_bin_or_dylib("{}/bin/rustc".format(self.bin_root()))
             self.fix_bin_or_dylib("{}/bin/rustdoc".format(self.bin_root()))
+            self.fix_bin_or_dylib("{}/bin/cargo".format(self.bin_root()))
             lib_dir = "{}/lib".format(self.bin_root())
             for lib in os.listdir(lib_dir):
                 if lib.endswith(".so"):
@@ -413,17 +414,6 @@ class RustBuild(object):
             with output(self.rustc_stamp()) as rust_stamp:
                 rust_stamp.write(self.date)
 
-        if self.cargo().startswith(self.bin_root()) and \
-                (not os.path.exists(self.cargo()) or
-                 self.program_out_of_date(self.cargo_stamp())):
-            tarball_suffix = '.tar.xz' if support_xz() else '.tar.gz'
-            filename = "cargo-{}-{}{}".format(cargo_channel, self.build,
-                                              tarball_suffix)
-            self._download_stage0_helper(filename, "cargo", tarball_suffix)
-            self.fix_bin_or_dylib("{}/bin/cargo".format(self.bin_root()))
-            with output(self.cargo_stamp()) as cargo_stamp:
-                cargo_stamp.write(self.date)
-
         if self.rustfmt() and self.rustfmt().startswith(self.bin_root()) and (
             not os.path.exists(self.rustfmt())
             or self.program_out_of_date(self.rustfmt_stamp(), self.rustfmt_channel)
@@ -601,16 +591,6 @@ class RustBuild(object):
         """
         return os.path.join(self.bin_root(), '.rustc-stamp')
 
-    def cargo_stamp(self):
-        """Return the path for .cargo-stamp
-
-        >>> rb = RustBuild()
-        >>> rb.build_dir = "build"
-        >>> rb.cargo_stamp() == os.path.join("build", "stage0", ".cargo-stamp")
-        True
-        """
-        return os.path.join(self.bin_root(), '.cargo-stamp')
-
     def rustfmt_stamp(self):
         """Return the path for .rustfmt-stamp
 
@@ -1056,7 +1036,6 @@ def bootstrap(help_triggered):
     data = stage0_data(build.rust_root)
     build.date = data['date']
     build.rustc_channel = data['rustc']
-    build.cargo_channel = data['cargo']
 
     if "rustfmt" in data:
         build.rustfmt_channel = data['rustfmt']

src/stage0.txt

@@ -1,6 +1,5 @@
 # This file describes the stage0 compiler that's used to then bootstrap the Rust
-# compiler itself. For the rustbuild build system, this also describes the
-# relevant Cargo revision that we're using.
+# compiler itself.
 #
 # Currently Rust always bootstraps from the previous stable release, and in our
 # train model this means that the master branch bootstraps from beta, beta
@@ -8,13 +7,13 @@
 # release.
 #
 # If you're looking at this file on the master branch, you'll likely see that
-# rustc and cargo are configured to `beta`, whereas if you're looking at a
-# source tarball for a stable release you'll likely see `1.x.0` for rustc and
-# `0.(x+1).0` for Cargo where they were released on `date`.
+# rustc is configured to `beta`, whereas if you're looking at a source tarball
+# for a stable release you'll likely see `1.x.0` for rustc, with the previous
+# stable release's version number. `date` is the date where the release we're
+# bootstrapping off was released.
 
 date: 2020-10-16
 rustc: beta
-cargo: beta
 
 # We use a nightly rustfmt to format the source because it solves some
 # bootstrapping issues with use of new syntax in this repo. If you're looking at