I ran into a few odd errors when compiling a Solana program:

$ npm run build

> hello-world@1.0.0 build
> cargo build-bpf --manifest-path=./src/program/Cargo.toml --bpf-out-dir=./dist/program

BPF SDK: /Users/matt/.local/share/solana/install/releases/1.8.6/solana-release/bin/sdk/bpf
cargo-build-bpf child: rustup toolchain list -v
cargo-build-bpf child: cargo +bpf build --target bpfel-unknown-unknown --release
   Compiling program v0.0.1 (/Users/matt/code/solana/hello-world/src/program)
error[E0433]: failed to resolve: maybe a missing crate `solana_program`?
 --> src/lib.rs:3:5
  |
3 | use solana_program::{
  |     ^^^^^^^^^^^^^^ maybe a missing crate `solana_program`?

error[E0432]: unresolved import `borsh`
 --> src/lib.rs:1:5
  |
1 | use borsh::{BorshDeserialize, BorshSerialize};
  |     ^^^^^ maybe a missing crate `borsh`?

error[E0432]: unresolved import `solana_program`
 --> src/lib.rs:3:5
  |
3 | use solana_program::{
  |     ^^^^^^^^^^^^^^ maybe a missing crate `solana_program`?

Took a while to figure out, but in the end the reason was that I was missing an edition value in Cargo.toml.

I had:

[package]
name = "program"
version = "0.0.1"

But needed:

[package]
name = "program"
version = "0.0.1"
edition = "2021"

Note that the name and package can be whatever you want here.

The edition is automatically set when you use cargo new, but at some point I had deleted it when cleaning up the manifest. Without it set, Rust assumes the edition is 2015 for backwards compatibility, but this causes problems during the Solana program compilation.