rustc: Support --reproducible option for requesting reproducible output

compiler/rustc_session/src/config.rs

@@ -1754,6 +1754,21 @@ fn parse_jobs_all(
         },
     };
 
+    // If reproducible compilation is enabled we lower frontend parallelism to 1,
+    // hopefully we'll be able to do better in the future.
+    let reproducible = match parse_reproducible(matches.opt_str("reproducible").as_deref()) {
+        Ok(reproducible) => reproducible,
+        Err(()) => {
+            let msg = "`--reproducible`: expected of of `none`, `binaries`, `diagnostics`, \
+                       or `binaries,diagnostics`";
+            early_dcx.early_fatal(msg);
+        }
+    };
+    let frontend = match frontend {
+        Some(n) if !reproducible || n.get() == 1 => Some(n), // respect `sync`
+        _ => None,
+    };
+
     Jobs { frontend, backend, linker }
 }
 
@@ -1788,6 +1803,16 @@ fn parse_jobs_one(
     (n > 1).then_some(NonZero::new(usize::from(n)).unwrap())
 }
 
+fn parse_reproducible(s: Option<&str>) -> Result<bool, ()> {
+    match s {
+        None | Some("none") => Ok(false),
+        Some("binaries" | "diagnostics" | "binaries,diagnostics" | "diagnostics,binaries") => {
+            Ok(true)
+        }
+        _ => Err(()),
+    }
+}
+
 pub fn build_configuration(sess: &Session, mut user_cfg: Cfg) -> Cfg {
     // First disallow some configuration given on the command line
     cfg::disallow_cfgs(sess, &user_cfg);
@@ -2128,6 +2153,14 @@ pub fn rustc_optgroups() -> Vec<RustcOptGroup> {
             "Limit on the number of parallel jobs used by linker",
             "<N>",
         ),
+        opt(
+            Unstable,
+            Opt,
+            "",
+            "reproducible",
+            "Try making some of the compiler outputs reproducible, possibly at cost of compilation speed",
+            "BOOL",
+        ),
     ];
     options.extend(verbose_only.into_iter().map(|mut opt| {
         opt.is_verbose_help_only = true;

src/doc/rustc/src/command-line-arguments.md

@@ -531,6 +531,35 @@ then we cannot limit it.
 
 Currently only LLD supports controlling parallelism.
 
+<a id="reproducible"></a>
+## `--reproducible`: request reproducible output
+
+By reproducible or deterministic compilation here we'll understand obtaining identical binaries
+or other compiler outputs when rustc is run multiple times on the same source code with the same
+options and environment.
+
+The reproducibility may be needed in a number of scenarios:
+- When compiler outputs are compared to some existing snapshots for testing
+  (e.g. rustc's own UI test suite).
+- When compiler outputs are cached between multiple compilations using tools like sccache.
+  In this case different outputs will break the caching even if they are functionally equivalent.
+- Other scenarios, including security-oriented.
+
+However, in many other scenarios the reproducibility is not an issue, for example:
+- Building and running the project to check whether the tests pass, on CI or locally.
+- Compiling the project with cargo check to see and fix the reported errors.
+
+This option accepts the following values - `none`, `binaries`, `diagnostics`,
+`binaries,diagnostics`, or `diagnostics,binaries`.
+
+The default is `binaries,diagnostics`, unless rustc frontend parallelism is enabled with
+`--jobs` or `--jobs-frontend`.
+
+Currently `--reproducible=value` for any value except `none` will just reduce the frontend
+parallelism to 1, but we hope to do something better in the future. For example keeping at least
+some parallelism in `--reproducible=binaries,diagnostics` cases, or enabling more optimizations for
+`--reproducible=none`, even in single-threaded mode.
+
 <a id="at-path"></a>
 ## `@path`: load command-line flags from a path
 

tests/ui/compile-flags/jobs/jobs-fail-conflict.h.stderr

@@ -0,0 +1,2 @@
+error: Option 'reproducible' given more than once
+

tests/ui/compile-flags/jobs/jobs-fail-conflict.rs

@@ -1,4 +1,4 @@
-//@ revisions: a b c d e f g
+//@ revisions: a b c d e f g h
 //@ ignore-parallel-frontend option conflicts
 //@ compile-flags: -Z unstable-options
 
@@ -21,4 +21,7 @@
 //@[g] compile-flags: --jobs 1 --jobs 2
 //[g]~? RAW Option 'jobs' given more than once
 
+//@[h] compile-flags: --reproducible true --reproducible false
+//[h]~? RAW Option 'reproducible' given more than once
+
 fn main() {}

tests/ui/compile-flags/jobs/jobs-fail-gate.g.stderr

@@ -0,0 +1,2 @@
+error: the `-Z unstable-options` flag must also be passed to enable the flag `reproducible`
+

tests/ui/compile-flags/jobs/jobs-fail-gate.rs

@@ -1,4 +1,4 @@
-//@ revisions: a b c d e f
+//@ revisions: a b c d e f g
 
 //@[a] compile-flags: -j 2
 //@[b] compile-flags: --jobs 2
@@ -14,4 +14,7 @@
 //@[f] compile-flags: --jobs-linker 2
 //[f]~? RAW the `-Z unstable-options` flag must also be passed to enable the flag `jobs-linker`
 
+//@[g] compile-flags: --reproducible=yes
+//[g]~? RAW the `-Z unstable-options` flag must also be passed to enable the flag `reproducible`
+
 fn main() {}

tests/ui/compile-flags/jobs/jobs-fail-range.m.stderr

@@ -0,0 +1,5 @@
+error: Argument to option 'reproducible' missing
+       Usage:
+           --reproducible BOOL Try making some of the compiler outputs reproducible,
+                               possibly at cost of compilation speed
+

tests/ui/compile-flags/jobs/jobs-fail-range.n.stderr

@@ -0,0 +1,2 @@
+error: `--reproducible`: expected of of `none`, `binaries`, `diagnostics`, or `binaries,diagnostics`
+

tests/ui/compile-flags/jobs/jobs-fail-range.rs

@@ -1,4 +1,4 @@
-//@ revisions: a b c d e f g h i j k l
+//@ revisions: a b c d e f g h i j k l m n
 //@ compile-flags: -Z unstable-options
 
 //@[a] compile-flags: -j 256
@@ -23,9 +23,13 @@
 //@[j] compile-flags: --jobs
 //@[k] compile-flags: -Zthreads
 //@[l] compile-flags: -Zno-parallel-backend
+//@[m] compile-flags: --reproducible
+//@[n] compile-flags: --reproducible nonsense
 //[i]~? RAW Argument to option 'j' missing
 //[j]~? RAW Argument to option 'jobs' missing
 //[k]~? ERROR unstable option `threads` requires a string
 //[l]~? ERROR `-Zno-parallel-backend` is removed, use `--jobs-backend=1` instead
+//[m]~? RAW Argument to option 'reproducible' missing
+//[n]~? ERROR `--reproducible`: expected of of `none`, `binaries`, `diagnostics`, or `binaries,diagnostics`
 
 fn main() {}

tests/ui/compile-flags/jobs/jobs-pass.rs

@@ -8,6 +8,7 @@
 //@[c] compile-flags: -j 255
 //@[d] compile-flags: --jobs 16 --jobs-frontend 8 --jobs-backend 4 --jobs-linker 2
 //@[e] compile-flags: --jobs 16 -Zthreads=8 --jobs-backend 4
-//@[g] compile-flags: -Zthreads=1 -Zthreads=2
+//@[f] compile-flags: -Zthreads=1 -Zthreads=2
+//@[g] compile-flags: --jobs 16 --reproducible=binaries,diagnostics
 
 fn main() {}