fix(macros-core): update unstable proc_macro APIs for recent nightly (#4227)

The unstable proc_macro tracking APIs were restructured in
rust-lang/rust#149400:

- modules `tracked_env` and `tracked_path` were merged into a
  single module `tracked`
- `proc_macro::tracked_env::var` -> `proc_macro::tracked::env_var`
- `proc_macro::tracked_path::path` -> `proc_macro::tracked::path`
- features `track_path` -> `proc_macro_tracked_path` (+
  `proc_macro_tracked_env`)

Building with --cfg=sqlx_macros_unstable on recent nightly fails
because of the renames and because `extern crate proc_macro;` was
only declared in `migrate.rs`, leaving `lib.rs` and
`query/cache.rs` without the crate in scope.

Move the gated `extern crate proc_macro;` to the crate root, drop
the duplicate from migrate.rs, and update the API call sites and
the clippy.toml reason field.

Fixes #4150

Author: barry3406

sqlx-macros-core/clippy.toml

@@ -1,3 +1,3 @@
 [[disallowed-methods]]
 path = "std::env::var"
-reason = "use `crate::env()` instead, which optionally calls `proc_macro::tracked_env::var()`"
+reason = "use `crate::env()` instead, which optionally calls `proc_macro::tracked::env_var()`"

sqlx-macros-core/src/lib.rs

@@ -16,9 +16,12 @@
 
 #![cfg_attr(
     any(sqlx_macros_unstable, procmacro2_semver_exempt),
-    feature(track_path)
+    feature(proc_macro_tracked_path, proc_macro_tracked_env)
 )]
 
+#[cfg(any(sqlx_macros_unstable, procmacro2_semver_exempt))]
+extern crate proc_macro;
+
 use cfg_if::cfg_if;
 use std::path::PathBuf;
 
@@ -96,7 +99,7 @@ pub fn env_opt(var: &str) -> Result<Option<String>> {
     use std::env::VarError;
 
     #[cfg(any(sqlx_macros_unstable, procmacro2_semver_exempt))]
-    let res: Result<String, VarError> = proc_macro::tracked_env::var(var);
+    let res: Result<String, VarError> = proc_macro::tracked::env_var(var);
 
     #[cfg(not(any(sqlx_macros_unstable, procmacro2_semver_exempt)))]
     let res: Result<String, VarError> = std::env::var(var);

sqlx-macros-core/src/migrate.rs

@@ -1,6 +1,3 @@
-#[cfg(any(sqlx_macros_unstable, procmacro2_semver_exempt))]
-extern crate proc_macro;
-
 use std::path::{Path, PathBuf};
 
 use proc_macro2::{Span, TokenStream};
@@ -132,7 +129,7 @@ pub fn expand_with_path(config: &Config, path: &Path) -> crate::Result<TokenStre
             )
         })?;
 
-        proc_macro::tracked_path::path(path);
+        proc_macro::tracked::path(path);
     }
 
     Ok(quote! {

sqlx-macros-core/src/query/cache.rs

@@ -71,7 +71,7 @@ impl MtimeCacheBuilder {
 
         #[cfg(any(sqlx_macros_unstable, procmacro2_semver_exempt))]
         {
-            proc_macro::tracked_path::path(&path);
+            proc_macro::tracked::path(&path);
         }
 
         self.file_mtimes.push((path, mtime));