compiletest: remove pretty-expanded directive and infra

Foreword
========

Let us begin the journey to rediscover what the `//@ pretty-expanded`
directive does, brave traveller --

    "My good friend, [..] when I wrote that passage, God and I knew what
    it meant. It is possible that God knows it still; but as for me, I
    have totally forgotten."

                                 -- Johann Paul Friedrich Richter, 1826

We must retrace the steps of those before us, for history shall guide us
in the present and inform us of the future.

The Past
========

Originally there was some effort to introduce more test coverage for `-Z
unpretty=expanded` (in 2015 this was called `--pretty=expanded`). In
[Make it an error to not declare used features #23598][pr-23598], there
was a flip from `//@ no-pretty-expanded` (opt-out of `-Z
unpretty=expanded` test) to `//@ pretty-expanded` (opt-in to `-Z
unpretty=expanded` test). This was needed because back then the
dedicated `tests/pretty` ("pretty") test suite did not existed, and the
pretty tests were grouped together under `run-pass` tests (I believe
`ui` test suite didn't exist back then either). Unfortunately, in this
process the replacement `//@ pretty-expanded` directives contained a
`FIXME #23616` linking to [There are very few tests for `-Z unpretty`
expansion #23616][issue-23616]. But this was arguably backwards and
somewhat misleading, as noted in
<#23616 (comment)>:

    The attribute is off by default and things just work if you don't
    test it, people have not been adding the `pretty-expanded`
    annotation to new tests even if it would work.

Which basically renders this useless.

The Present
===========

As of Nov 2024, we have a dedicated `pretty` test suite, and some time
over the years the split between `run-pass` into `ui` and `pretty` test
suites caused all of the `//@ pretty-expanded` in `ui` tests to do
absolutely nothing -- the compiletest logic for `pretty-expanded` only
triggered in the *pretty* test suite, but none of the pretty tests use
it. Oops.

The Future
==========

Nobody remembers this, nobody uses this, it's misleading in ui tests.
Let's get rid of this directive altogether.

[pr-23598]: #23598
[issue-23616]: #23616

src/tools/compiletest/src/directive-list.rs

@@ -214,7 +214,6 @@ const KNOWN_DIRECTIVE_NAMES: &[&str] = &[
     "only-x86_64-unknown-linux-gnu",
     "pp-exact",
     "pretty-compare-only",
-    "pretty-expanded",
     "pretty-mode",
     "reference",
     "regex-error-pattern",

src/tools/compiletest/src/header.rs

@@ -124,8 +124,6 @@ pub struct TestProps {
     // a proc-macro and needs `#![crate_type = "proc-macro"]`. This ensures
     // that the aux file is compiled as a `proc-macro` and not as a `dylib`.
     pub no_prefer_dynamic: bool,
-    // Run -Zunpretty expanded when running pretty printing tests
-    pub pretty_expanded: bool,
     // Which pretty mode are we testing with, default to 'normal'
     pub pretty_mode: String,
     // Only compare pretty output and don't try compiling
@@ -218,7 +216,6 @@ mod directives {
     pub const DONT_CHECK_COMPILER_STDOUT: &'static str = "dont-check-compiler-stdout";
     pub const DONT_CHECK_COMPILER_STDERR: &'static str = "dont-check-compiler-stderr";
     pub const NO_PREFER_DYNAMIC: &'static str = "no-prefer-dynamic";
-    pub const PRETTY_EXPANDED: &'static str = "pretty-expanded";
     pub const PRETTY_MODE: &'static str = "pretty-mode";
     pub const PRETTY_COMPARE_ONLY: &'static str = "pretty-compare-only";
     pub const AUX_BIN: &'static str = "aux-bin";
@@ -278,7 +275,6 @@ impl TestProps {
             dont_check_compiler_stderr: false,
             compare_output_lines_by_subset: false,
             no_prefer_dynamic: false,
-            pretty_expanded: false,
             pretty_mode: "normal".to_string(),
             pretty_compare_only: false,
             forbid_output: vec![],
@@ -425,7 +421,6 @@ impl TestProps {
                         &mut self.dont_check_compiler_stderr,
                     );
                     config.set_name_directive(ln, NO_PREFER_DYNAMIC, &mut self.no_prefer_dynamic);
-                    config.set_name_directive(ln, PRETTY_EXPANDED, &mut self.pretty_expanded);
 
                     if let Some(m) = config.parse_name_value_directive(ln, PRETTY_MODE) {
                         self.pretty_mode = m;

src/tools/compiletest/src/runtest/pretty.rs

@@ -84,21 +84,5 @@ impl TestCx<'_> {
         if !proc_res.status.success() {
             self.fatal_proc_rec("pretty-printed source does not typecheck", &proc_res);
         }
-
-        if !self.props.pretty_expanded {
-            return;
-        }
-
-        // additionally, run `-Zunpretty=expanded` and try to build it.
-        let proc_res = self.print_source(ReadFrom::Path, "expanded");
-        if !proc_res.status.success() {
-            self.fatal_proc_rec("pretty-printing (expanded) failed", &proc_res);
-        }
-
-        let ProcRes { stdout: expanded_src, .. } = proc_res;
-        let proc_res = self.typecheck_source(expanded_src);
-        if !proc_res.status.success() {
-            self.fatal_proc_rec("pretty-printed source (expanded) does not typecheck", &proc_res);
-        }
     }
 }