Remove default config from bootstrap

[Shourya742]

This PR removes the default config initialization from parse_inner, as it introduced many assumptions during config setup. Instead, each variable is now manually initialized to eliminate certain invariants in parse_inner and streamline the process.

The PR is still a WIP and has been opened for an initial review.

r? @Kobzol

[rust-log-analyzer]

The job tidy failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

   Compiling termcolor v1.4.1
error[E0609]: no field `download_rustc_commit` on type `std::option::Option<PathBuf>`
##[error]    --> src/bootstrap/src/core/config/config.rs:1118:49
     |
1118 |         if debug_assertions_requested && config.download_rustc_commit.is_some() {
     |                                                 ^^^^^^^^^^^^^^^^^^^^^ unknown field

For more information about this error, try `rustc --explain E0609`.
error: could not compile `bootstrap` (lib) due to 1 previous error
failed to run: /checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo build --manifest-path /checkout/src/bootstrap/Cargo.toml -Zroot-dir=/checkout --locked --features build-metrics
---
   Compiling bootstrap v0.0.0 (/checkout/src/bootstrap)
error[E0609]: no field `download_rustc_commit` on type `std::option::Option<PathBuf>`
##[error]    --> src/bootstrap/src/core/config/config.rs:1118:49
     |
1118 |         if debug_assertions_requested && config.download_rustc_commit.is_some() {
     |                                                 ^^^^^^^^^^^^^^^^^^^^^ unknown field

For more information about this error, try `rustc --explain E0609`.
error: could not compile `bootstrap` (lib) due to 1 previous error
failed to run: /checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo build --manifest-path /checkout/src/bootstrap/Cargo.toml -Zroot-dir=/checkout --locked --features build-metrics
---
   Compiling bootstrap v0.0.0 (/checkout/src/bootstrap)
error[E0609]: no field `download_rustc_commit` on type `std::option::Option<PathBuf>`
##[error]    --> src/bootstrap/src/core/config/config.rs:1118:49
     |
1118 |         if debug_assertions_requested && config.download_rustc_commit.is_some() {
     |                                                 ^^^^^^^^^^^^^^^^^^^^^ unknown field

For more information about this error, try `rustc --explain E0609`.
error: could not compile `bootstrap` (lib) due to 1 previous error
failed to run: /checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo build --manifest-path /checkout/src/bootstrap/Cargo.toml -Zroot-dir=/checkout --locked --features build-metrics
---
   Compiling bootstrap v0.0.0 (/checkout/src/bootstrap)
error[E0609]: no field `download_rustc_commit` on type `std::option::Option<PathBuf>`
##[error]    --> src/bootstrap/src/core/config/config.rs:1118:49
     |
1118 |         if debug_assertions_requested && config.download_rustc_commit.is_some() {
     |                                                 ^^^^^^^^^^^^^^^^^^^^^ unknown field

For more information about this error, try `rustc --explain E0609`.
error: could not compile `bootstrap` (lib) due to 1 previous error
failed to run: /checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo build --manifest-path /checkout/src/bootstrap/Cargo.toml -Zroot-dir=/checkout --locked --features build-metrics
---
   Compiling bootstrap v0.0.0 (/checkout/src/bootstrap)
error[E0609]: no field `download_rustc_commit` on type `std::option::Option<PathBuf>`
##[error]    --> src/bootstrap/src/core/config/config.rs:1118:49
     |
1118 |         if debug_assertions_requested && config.download_rustc_commit.is_some() {
     |                                                 ^^^^^^^^^^^^^^^^^^^^^ unknown field

For more information about this error, try `rustc --explain E0609`.
error: could not compile `bootstrap` (lib) due to 1 previous error
failed to run: /checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo build --manifest-path /checkout/src/bootstrap/Cargo.toml -Zroot-dir=/checkout --locked --features build-metrics

[Kobzol]

Heh, I had feedback for you, but I see that Clippy did it for me in one of the commits xD Let me know once it's ready and I'll check the changes locally, but in general looks great.

[Kobzol]

What does it mean for a variable to not be mut here? If the variable is initialized only once, I would just initialize it at the end of the function, in the Config { ... } struct literal.

[Kobzol]

Could you please group the variables that have an "interesting" (i.e. non-Default) default together? Although I assume that once you remove the let XXX vars and move them to the Config constructor, this will be less of an issue.

[Kobzol]

Let's just initialize the variables at places where they are first used (let config;). I don't think that the ultra long list of variable declarations at the start of the function win us anything.