bootstrap: Use cargo's build.warnings=deny rather than -Dwarnings

[jyn514]

This has two major advantages. First, it makes us less dependent on the rustc shim, which is nice but not super important. More importantly, it gives us much nicer caching properties.

Previously, x build --warnings warn && x build --warnings deny would rebuild all of bootstrap, and if there were any warnings emitted on the last build of the compiler, they would not fail the build, because cargo would cache the output rather than rerunning the shim.

After this change, bootstrap rebuilds instantly, and cargo knows that it should fail the build but without invalidating the cache.

An example of rebuilding bootstrap after a switch from warn->deny:

INFO: Downloading and building bootstrap before processing --help command.
      See src/bootstrap/README.md for help with common commands.
Building bootstrap
   Compiling bootstrap v0.0.0 (/Users/jyn/src/ferrocene3/src/bootstrap)
warning: unused variable: `x`
    --> src/bootstrap/src/core/builder/mod.rs:1792:13
     |
1792 |         let x: ();
     |             ^
     |
     = note: `#[warn(unused_variables)]` (part of `#[warn(unused)]`) on by default
help: if this is intentional, prefix it with an underscore
     |
1792 |         let _x: ();
     |             +
help: you might have meant to pattern match on the similarly named constant `_`
     |
1792 -         let x: ();
1792 +         let utils::render_tests::_: ();
     |

warning: `bootstrap` (lib) generated 1 warning (run `cargo fix --lib -p bootstrap` to apply 1 suggestion)
    Finished `dev` profile [unoptimized] target(s) in 5.14s
error: warnings are denied by `build.warnings` configuration
failed to run: /Users/jyn/src/ferrocene3/build/aarch64-apple-darwin/stage0/bin/cargo build --jobs=default --manifest-path /Users/jyn/src/ferrocene3/src/bootstrap/Cargo.toml -Zroot-dir=/Users/jyn/src/ferrocene3 -Zwarnings

building the compiler from scratch with deny: https://gist.github.com/jyn514/493ed26c2aa6f61bf47c21e75efa2175

and rebuilding the compiler after switching from deny->warn (note that it reuses the whole cache, there are no invalidations):

$ x c compiler
Building bootstrap
    Finished `dev` profile [unoptimized] target(s) in 0.15s
Checking stage1 compiler artifacts{rustc-main, rustc_abi, rustc_arena, rustc_ast, rustc_ast_ir, rustc_ast_lowering, rustc_ast_passes, rustc_ast_pretty, rustc_attr_parsing, rustc_baked_icu_data, rustc_borrowck, rustc_builtin_macros, rustc_codegen_llvm, rustc_codegen_ssa, rustc_const_eval, rustc_data_structures, rustc_driver, rustc_driver_impl, rustc_error_codes, rustc_error_messages, rustc_errors, rustc_expand, rustc_feature, rustc_fluent_macro, rustc_fs_util, rustc_graphviz, rustc_hashes, rustc_hir, rustc_hir_analysis, rustc_hir_id, rustc_hir_pretty, rustc_hir_typeck, rustc_incremental, rustc_index, rustc_index_macros, rustc_infer, rustc_interface, rustc_lexer, rustc_lint, rustc_lint_defs, rustc_llvm, rustc_log, rustc_macros, rustc_metadata, rustc_middle, rustc_mir_build, rustc_mir_dataflow, rustc_mir_transform, rustc_monomorphize, rustc_next_trait_solver, rustc_parse, rustc_parse_format, rustc_passes, rustc_pattern_analysis, rustc_privacy, rustc_proc_macro, rustc_public, rustc_public_bridge, rustc_query_impl, rustc_query_system, rustc_resolve, rustc_sanitizers, rustc_serialize, rustc_session, rustc_span, rustc_symbol_mangling, rustc_target, rustc_thread_pool, rustc_trait_selection, rustc_traits, rustc_transmute, rustc_ty_utils, rustc_type_ir, rustc_type_ir_macros, rustc_windows_rc} (stage0 -> stage1, aarch64-apple-darwin)
warning: function `foo` is never used
  --> compiler/rustc_hashes/src/lib.rs:16:4
   |
16 | fn foo() {}
   |    ^^^
   |
   = note: `#[warn(dead_code)]` (part of `#[warn(unused)]`) on by default

warning: `rustc_hashes` (lib) generated 1 warning
    Finished `release` profile [optimized + debuginfo] target(s) in 0.53s
Build completed successfully in 0:00:08

thanks to @epage for the help finding this!

r? bootstrap

[epage]

This has two major advantages.

Another advantage is you get clear signal what is only a warning (at least for me that is nice)

[Kobzol]

Ooh, this is so cool! Especially the bootstrap part, it was always so annoying that bootstrap was rebuilt when rust.deny-warnings was modified. Thank you!

r? @Kobzol

@bors r+ rollup

[bors]

📌 Commit e1e8913 has been approved by Kobzol

It is now in the queue for this repository.