Auto merge of #38072 - nikomatsakis:bootstrap-incremental, r=acrichto

add preliminary support for incremental compilation to rustbuild.py

This implements the integration described in #37929. It requires the use of a local nightly as your bootstrap compiler. The setup is described in `src/bootstrap/README.md`.

This does NOT implement the "copy stage0 libs to stage1" optimization described in #37929, just because that seems orthogonal to me.

In local testing, I do not yet see any incremental re-use when building rustc. I'm not sure why that is, more investigation needed.

(For these reasons, this is not marked as fixing the relevant issue.)

r? @alexcrichton -- I included one random cleanup (`Step::noop()`) that turned out to not be especially relevant. Feel free to tell me you liked it better the old way.

1 files changed, 27 insertions, 0 deletions

diff --git a/src/bootstrap/bin/rustc.rs b/src/bootstrap/bin/rustc.rs
index c5684e69994..2fa4363e658 100644
--- a/src/bootstrap/bin/rustc.rs
+++ b/src/bootstrap/bin/rustc.rs
@@ -29,6 +29,9 @@ extern crate bootstrap;
 
 use std::env;
 use std::ffi::OsString;
+use std::io;
+use std::io::prelude::*;
+use std::str::FromStr;
 use std::path::PathBuf;
 use std::process::{Command, ExitStatus};
 
@@ -41,6 +44,11 @@ fn main() {
         .and_then(|w| w[1].to_str());
     let version = args.iter().find(|w| &**w == "-vV");
 
+    let verbose = match env::var("RUSTC_VERBOSE") {
+        Ok(s) => usize::from_str(&s).expect("RUSTC_VERBOSE should be an integer"),
+        Err(_) => 0,
+    };
+
     // Build scripts always use the snapshot compiler which is guaranteed to be
     // able to produce an executable, whereas intermediate compilers may not
     // have the standard library built yet and may not be able to produce an
@@ -95,6 +103,15 @@ fn main() {
             cmd.args(&s.split(" ").filter(|s| !s.is_empty()).collect::<Vec<_>>());
         }
 
+        // Pass down incremental directory, if any.
+        if let Ok(dir) = env::var("RUSTC_INCREMENTAL") {
+            cmd.arg(format!("-Zincremental={}", dir));
+
+            if verbose > 0 {
+                cmd.arg("-Zincremental-info");
+            }
+        }
+
         // If we're compiling specifically the `panic_abort` crate then we pass
         // the `-C panic=abort` option. Note that we do not do this for any
         // other crate intentionally as this is the only crate for now that we
@@ -176,9 +193,19 @@ fn main() {
             if let Some(rpath) = rpath {
                 cmd.arg("-C").arg(format!("link-args={}", rpath));
             }
+
+            if let Ok(s) = env::var("RUSTFLAGS") {
+                for flag in s.split_whitespace() {
+                    cmd.arg(flag);
+                }
+            }
         }
     }
 
+    if verbose > 1 {
+        writeln!(&mut io::stderr(), "rustc command: {:?}", cmd).unwrap();
+    }
+
     // Actually run the compiler!
     std::process::exit(match exec_cmd(&mut cmd) {
         Ok(s) => s.code().unwrap_or(0xfe),