In Rust 1.78+, too many errors get printed when building with neither "std" nor "alloc"

[dtolnay]

This bisects to a regression in nightly-2024-02-09, and specifically rust-lang/rust#120550. I noticed this while testing #1124.

For the following dependency:

[dependencies]
serde_json = { version = "1", default-features = false }

in Rust 1.77 or nightly-2024-02-08 and older, the error you get is:

error: expected item, found `"serde_json requires that either `std` (default) or `alloc` feature is enabled"`
 --> ~/.cargo/registry/src/index.crates.io-6f17d22bba15001f/serde_json-1.0.115/src/features_check/error.rs:1:1
  |
1 | "serde_json requires that either `std` (default) or `alloc` feature is enabled"
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected item
  |
  = note: for a full list of items that can appear in modules, see <https://doc.rust-lang.org/reference/items.html>

Whereas with Rust 1.78-beta, nightly-2024-02-09 or newer:

error: expected item, found `"serde_json requires that either `std` (default) or `alloc` feature is enabled"`
 --> ~/.cargo/registry/src/index.crates.io-6f17d22bba15001f/serde_json-1.0.115/src/features_check/error.rs:1:1
  |
1 | "serde_json requires that either `std` (default) or `alloc` feature is enabled"
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected item
  |
  = note: for a full list of items that can appear in modules, see <https://doc.rust-lang.org/reference/items.html>

error[E0004]: non-exhaustive patterns: `Value::String(_)` not covered
   --> ~/.cargo/registry/src/index.crates.io-6f17d22bba15001f/serde_json-1.0.115/src/value/de.rs:216:15
    |
216 |         match self {
    |               ^^^^ pattern `Value::String(_)` not covered
    |
note: `Value` defined here
   --> ~/.cargo/registry/src/index.crates.io-6f17d22bba15001f/serde_json-1.0.115/src/value/mod.rs:116:10
    |
116 | pub enum Value {
    |          ^^^^^
...
151 |     String(String),
    |     ------ not covered
    = note: the matched value is of type `Value`
help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
    |
223 ~             Value::Object(v) => visit_object(v, visitor),
224 ~             Value::String(_) => todo!(),
    |

error[E0004]: non-exhaustive patterns: `Cow::Owned(_)` not covered
    --> ~/.cargo/registry/src/index.crates.io-6f17d22bba15001f/serde_json-1.0.115/src/value/de.rs:1338:15
     |
1338 |         match self.value {
     |               ^^^^^^^^^^ pattern `Cow::Owned(_)` not covered
     |
note: `Cow<'_, str>` defined here
    --> /rustc/98aa3624be70462d6a25ed5544333e3df62f4c66/library/alloc/src/borrow.rs:180:1
    ::: /rustc/98aa3624be70462d6a25ed5544333e3df62f4c66/library/alloc/src/borrow.rs:190:5
     |
     = note: not covered
     = note: the matched value is of type `Cow<'_, str>`
help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
     |
1339 ~             Cow::Borrowed(string) => visitor.visit_borrowed_str(string),
1340 ~             Cow::Owned(_) => todo!(),
     |

error[E0004]: non-exhaustive patterns: `&Value::Object(_)` not covered
   --> ~/.cargo/registry/src/index.crates.io-6f17d22bba15001f/serde_json-1.0.115/src/value/ser.rs:17:15
    |
17  |         match self {
    |               ^^^^ pattern `&Value::Object(_)` not covered
    |
note: `Value` defined here
   --> /home/david/.cargo/registry/src/index.crates.io-6f17d22bba15001f/serde_json-1.0.115/src/value/mod.rs:116:10
    |
116 | pub enum Value {
    |          ^^^^^
...
175 |     Object(Map<String, Value>),
    |     ------ not covered
    = note: the matched value is of type `&Value`
help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
    |
22  ~             Value::Array(v) => v.serialize(serializer),
23  ~             &Value::Object(_) => todo!(),
    |

[dtolnay]

The current approach was put in place before serde_json had a build.rs. But now we have a build.rs anyway for a different reason, that might be a better place to do this check, if it can produce a good enough error.

With #1124:

error: serde_json requires that either `std` (default) or `alloc` feature is enabled
   --> serde-json/src/lib.rs:367:1
    |
367 | / compile_error! {
368 | |     "serde_json requires that either `std` (default) or `alloc` feature is enabled"
369 | | }
    | |_^

error: could not compile `serde_json` (lib) due to 1 previous error

Instead with this code in build.rs:

if cfg!(not(any(feature = "std", feature = "alloc"))) {
    eprintln!("serde_json requires that either `std` (default) or `alloc` feature is enabled");
    process::exit(1);
}

error: failed to run custom build command for `serde_json v1.0.115`

Caused by:
  process didn't exit successfully: `target/debug/build/serde_json-9e37cf325e9030a2/build-script-build` (exit status: 1)
  --- stderr
  serde_json requires that either `std` (default) or `alloc` feature is enabled

[dtolnay]

Some old workarounds that we could drop if build.rs does the check, meaning the library code would never even begin to build with an invalid combination of features: #643.