Auto merge of #53073 - Mark-Simulacrum:data-structures, r=pnkfelix

Cleanup to librustc::session and related code

No functional changes, just some cleanup.

This also creates the `rustc_fs_util` crate, but I can remove that change if desired. It felt a little odd to force crates to depend on librustc for some fs utilities; and also seemed good to generally keep the size of librustc lower (for compile times); fs_util will compile in parallel with essentially the first crate since it has no dependencies beyond std.

Author: bors

src/Cargo.lock

@@ -1887,6 +1887,7 @@ dependencies = [
  "rustc_apfloat 0.0.0",
  "rustc_data_structures 0.0.0",
  "rustc_errors 0.0.0",
+ "rustc_fs_util 0.0.0",
  "rustc_target 0.0.0",
  "scoped-tls 0.1.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "serialize 0.0.0",
@@ -2185,6 +2186,10 @@ dependencies = [
  "unicode-width 0.1.5 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
+[[package]]
+name = "rustc_fs_util"
+version = "0.0.0"
+
 [[package]]
 name = "rustc_incremental"
 version = "0.0.0"
@@ -2194,6 +2199,7 @@ dependencies = [
  "rand 0.4.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "rustc 0.0.0",
  "rustc_data_structures 0.0.0",
+ "rustc_fs_util 0.0.0",
  "serialize 0.0.0",
  "syntax 0.0.0",
  "syntax_pos 0.0.0",

src/librustc/Cargo.toml

@@ -32,6 +32,7 @@ backtrace = "0.3.3"
 parking_lot = "0.5.5"
 byteorder = { version = "1.1", features = ["i128"]}
 chalk-engine = { version = "0.6.0", default-features=false }
+rustc_fs_util = { path = "../librustc_fs_util" }
 
 # Note that these dependencies are a lie, they're just here to get linkage to
 # work.