updated code according to review's feedback

Author: FrancescoV1985

CONTRIBUTING.md

@@ -34,9 +34,7 @@ The **sysroot** is the directory that stores the compiled standard
 library (`core`, `alloc`, `std`, `test`, …) and compiler built-ins.
 Rustup ships these libraries **pre-compiled with LLVM**.
 
-Because **rustc_codegen_gcc** replaces LLVM with the GCC backend, the shipped
-LLVM artefacts are **incompatible**.
-Therefore the standard library must be **rebuilt with GCC** before ordinary Rust crates can be compiled.
+**rustc_codegen_gcc** replaces LLVM with the GCC backend.
 
 The freshly compiled sysroot ends up in
 `build/build_sysroot/...`.
@@ -47,22 +45,22 @@ A rebuild of sysroot is needed when
 * the user switches toolchains / updates submodules.
 
 Both backend and sysroot can be built using different [profiles](https://doc.rust-lang.org/cargo/reference/profiles.html#default-profiles).
-That is exactly what the `--sysroot`, `--release-sysroot` and `--release` flag supported by the frontend script `y.sh` take care of.
+That is exactly what the `--sysroot`, `--release-sysroot` and `--release` flag supported by the build system script `y.sh` take care of.
 
 
 #### Typical flag combinations
 
 | Command                                   | Backend Profile               | Sysroot Profile                  | Usage Scenario                                              |
 |--------------------------------------------|-------------------------------|----------------------------------|------------------------------------------------------------|
-| `./y.sh build`                            |  dev (optimized + debuginfo)   |  X                                |  Optimize backend with debug capabilities                    |
-| `./y.sh build --release`                  |  release (optimized)           |  X                                |  Optimize backend without rebuilding sysroot                 |
-| `./y.sh build --release --release-sysroot`|  release (optimized)           |  X                                |  Same as --release                                           |
-| `./y.sh build --release --sysroot`        |  release (optimized)           |  dev (unoptimized + debuginfo)    |  Optimize backend for release without full sysroot rebuild   |
-| `./y.sh build --sysroot`                  |  dev (optimized + debuginfo)   |  dev (unoptimized + debuginfo)    |  Full debug capabilities with optimized backend              |
-| `./y.sh build --release-sysroot`          |  dev (optimized + debuginfo)   |  X                                |  Build only optimized backend with debug capabilities                    |
-| `./y.sh build --release-sysroot --sysroot`|  dev (optimized + debuginfo)   |  release (optimized)              |  Build optimized backend and sysroot for debugging and release, respectively                        |
+| `./y.sh build`                            |  dev (optimized + debuginfo)   |  X                                |  Build backend in dev. mode with optimized dependencies without rebuilding sysroot                |
+| `./y.sh build --release`                  |  release (optimized)           |  X                                |  Build backend in release mode with optimized dependencies without rebuilding sysroot                 |
+| `./y.sh build --release --sysroot`        |  release (optimized)           |  dev (unoptimized + debuginfo)    |  Build backend in release mode with optimized dependencies and sysroot in dev. mode (unoptimized)              |
+| `./y.sh build --sysroot`                  |  dev (optimized + debuginfo)   |  dev (unoptimized + debuginfo)    |  Build backend in dev. mode with optimized dependencies and sysroot in dev. mode (unoptimized)              |
+| `./y.sh build --release-sysroot --sysroot`|  dev (optimized + debuginfo)   |  release (optimized)              |  Build backend in dev. mode and sysroot in release mode, both with optimized dependencies             |
 
 
+Note: `--release-sysroot` should not be used without `--sysroot`.
+
 
 ### Common Development Tasks
 

build_system/src/build.rs

@@ -44,7 +44,17 @@ impl BuildArg {
     fn usage() {
         println!(
             r#"
-`build` command help:"#
+`build` command help:
+    --release              : Build backend in release mode with optimized dependencies without rebuilding sysroot
+    --sysroot              : When used on its own, build backend in dev. mode with optimized dependencies
+                             and sysroot in dev. mode (unoptimized)
+                             When used together with --release, build backend in release mode with optimized dependencies
+                             When used together with --release-sysroot,
+                             build the sysroot in release mode with optimized dependencies instead of in dev. mode
+    --release-sysroot      : When combined with --sysroot, additionally
+                             build the sysroot in release mode with optimized dependencies.
+                             When combined with --release, it has no effect.
+                             It should not be used on itw own."#
         );
         ConfigInfo::show_usage();
         println!("    --help                 : Show this help");

build_system/src/config.rs

@@ -477,17 +477,6 @@ impl ConfigInfo {
     --target-triple [arg]  : Set the target triple to [arg]
     --target [arg]         : Set the target to [arg]
     --out-dir              : Location where the files will be generated
-    --release              : Build and optimize the backend in release mode
-    --sysroot              : When used on its own, build both
-                             sysroot and backend in dev. mode. Only the backend is optimized.
-                             When used together with --release, build and optimize the backend
-                             in release mode instead of in dev. mode.
-                             When used together with --release-sysroot,
-                             build and optimize the sysroot in release mode instead of in dev. mode
-    --release-sysroot      : When used on its own, build and optimize only the backend in dev. mode.
-                             When combined with --release, it has no effect.
-                             When combined with --sysroot, additionally
-                             build and optimize the sysroot in release mode
     --sysroot-panic-abort  : Build the sysroot without unwinding support
     --config-file          : Location of the config file to be used
     --gcc-path             : Location of the GCC root folder