Auto merge of #3516 - RalfJung:pathbuf, r=RalfJung

add smoke tests for basic PathBuf interactions

I wrote these while debugging [this](https://github.com/rust-lang/miri-test-libstd/actions/runs/8849912635/job/24302962983); it turns out the issue is [more complicated](rust-lang/rust#124409) but these tests still seemed worth keeping.

Author: bors

tests/pass/path.rs

@@ -0,0 +1,60 @@
+//@compile-flags: -Zmiri-disable-isolation
+use std::path::{absolute, Path, PathBuf};
+
+#[path = "../utils/mod.rs"]
+mod utils;
+
+#[track_caller]
+fn assert_absolute_eq(in_: &str, out: &str) {
+    assert_eq!(absolute(in_).unwrap().as_os_str(), Path::new(out).as_os_str());
+}
+
+fn test_absolute() {
+    if cfg!(unix) {
+        assert_absolute_eq("/a/b/c", "/a/b/c");
+        assert_absolute_eq("/a/b/c", "/a/b/c");
+        assert_absolute_eq("/a//b/c", "/a/b/c");
+        assert_absolute_eq("//a/b/c", "//a/b/c");
+        assert_absolute_eq("///a/b/c", "/a/b/c");
+        assert_absolute_eq("/a/b/c/", "/a/b/c/");
+        assert_absolute_eq("/a/./b/../c/.././..", "/a/b/../c/../..");
+    } else if cfg!(windows) {
+        // Test that all these are unchanged
+        assert_absolute_eq(r"C:\path\to\file", r"C:\path\to\file");
+        assert_absolute_eq(r"C:\path\to\file\", r"C:\path\to\file\");
+        assert_absolute_eq(r"\\server\share\to\file", r"\\server\share\to\file");
+        assert_absolute_eq(r"\\server.\share.\to\file", r"\\server.\share.\to\file");
+        assert_absolute_eq(r"\\.\PIPE\name", r"\\.\PIPE\name");
+        assert_absolute_eq(r"\\.\C:\path\to\COM1", r"\\.\C:\path\to\COM1");
+        assert_absolute_eq(r"\\?\C:\path\to\file", r"\\?\C:\path\to\file");
+        assert_absolute_eq(r"\\?\UNC\server\share\to\file", r"\\?\UNC\server\share\to\file");
+        assert_absolute_eq(r"\\?\PIPE\name", r"\\?\PIPE\name");
+        // Verbatim paths are always unchanged, no matter what.
+        assert_absolute_eq(r"\\?\path.\to/file..", r"\\?\path.\to/file..");
+
+        assert_absolute_eq(r"C:\path..\to.\file.", r"C:\path..\to\file");
+        assert_absolute_eq(r"COM1", r"\\.\COM1");
+    } else {
+        panic!("unsupported OS");
+    }
+}
+
+fn buf_smoke(mut p: PathBuf) {
+    for _c in p.components() {}
+
+    p.push("hello");
+    for _c in p.components() {}
+
+    if cfg!(windows) {
+        p.push(r"C:\mydir");
+    } else {
+        p.push(r"/mydir");
+    }
+    for _c in p.components() {}
+}
+
+fn main() {
+    buf_smoke(PathBuf::new());
+    buf_smoke(utils::tmp());
+    test_absolute();
+}