Rollup merge of rust-lang#68102 - lzutao:inline, r=alexcrichton

Inline some conversion methods around OsStr

Diff on the assembly of this snippet before and after this PR: https://www.diffchecker.com/NeGMjaJ2
```rust
use std::env;
use std::io;
use std::path::{Path, PathBuf};

pub fn cargo_home_with_cwd(cwd: &Path) -> io::Result<PathBuf> {
    match env::var_os("CARGO_HOME").filter(|h| !h.is_empty()) {
        Some(home) => {
            let home = PathBuf::from(home);
            if home.is_absolute() {
                Ok(home)
            } else {
                Ok(cwd.join(&home))
            }
        }
        _ => env::home_dir()
            .map(|p| p.join(".cargo"))
            .ok_or_else(|| io::Error::new(io::ErrorKind::Other, "could not find cargo home dir")),
    }
}
```

src/libstd/error.rs

@@ -250,6 +250,7 @@ impl From<String> for Box<dyn Error + Send + Sync> {
     /// assert!(
     ///     mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))
     /// ```
+    #[inline]
     fn from(err: String) -> Box<dyn Error + Send + Sync> {
         struct StringError(String);
 
@@ -317,6 +318,7 @@ impl<'a> From<&str> for Box<dyn Error + Send + Sync + 'a> {
     /// assert!(
     ///     mem::size_of::<Box<dyn Error + Send + Sync>>() == mem::size_of_val(&a_boxed_error))
     /// ```
+    #[inline]
     fn from(err: &str) -> Box<dyn Error + Send + Sync + 'a> {
         From::from(String::from(err))
     }

src/libstd/ffi/os_str.rs

@@ -615,6 +615,7 @@ impl OsStr {
     /// assert!(!os_str.is_empty());
     /// ```
     #[stable(feature = "osstring_simple_functions", since = "1.9.0")]
+    #[inline]
     pub fn is_empty(&self) -> bool {
         self.inner.inner.is_empty()
     }
@@ -965,6 +966,7 @@ impl AsRef<OsStr> for OsStr {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl AsRef<OsStr> for OsString {
+    #[inline]
     fn as_ref(&self) -> &OsStr {
         self
     }

src/libstd/path.rs

@@ -1475,6 +1475,7 @@ impl From<OsString> for PathBuf {
     /// Converts a `OsString` into a `PathBuf`
     ///
     /// This conversion does not allocate or copy memory.
+    #[inline]
     fn from(s: OsString) -> PathBuf {
         PathBuf { inner: s }
     }
@@ -1535,7 +1536,7 @@ impl fmt::Debug for PathBuf {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl ops::Deref for PathBuf {
     type Target = Path;
-
+    #[inline]
     fn deref(&self) -> &Path {
         Path::new(&self.inner)
     }
@@ -2655,6 +2656,7 @@ impl AsRef<Path> for OsString {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl AsRef<Path> for str {
+    #[inline]
     fn as_ref(&self) -> &Path {
         Path::new(self)
     }
@@ -2669,6 +2671,7 @@ impl AsRef<Path> for String {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl AsRef<Path> for PathBuf {
+    #[inline]
     fn as_ref(&self) -> &Path {
         self
     }

src/libstd/sys_common/os_str_bytes.rs

@@ -104,6 +104,7 @@ impl Buf {
         self.inner.shrink_to(min_capacity)
     }
 
+    #[inline]
     pub fn as_slice(&self) -> &Slice {
         unsafe { mem::transmute(&*self.inner) }
     }