Rollup merge of #100507 - cameron1024:suggest-lazy, r=compiler-errors

suggest `once_cell::Lazy` for non-const statics

Addresses #100410

Some questions:
 - removing the `if` seems to include too many cases (e.g. calls to non-const functions inside a `const fn`), but this code excludes the following case:
```rust
const FOO: Foo = non_const_fn();
```
Should we suggest `once_cell` in this case as well?
 - The original issue mentions suggesting `AtomicI32` instead of `Mutex<i32>`, should this PR address that as well?

Author: matthiaskrgr

compiler/rustc_const_eval/src/transform/check_consts/ops.rs

@@ -1,6 +1,7 @@
 //! Concrete error types for all operations which may be invalid in a certain const context.
 
 use hir::def_id::LocalDefId;
+use hir::ConstContext;
 use rustc_errors::{
     error_code, struct_span_err, Applicability, DiagnosticBuilder, ErrorGuaranteed,
 };
@@ -331,6 +332,10 @@ impl<'tcx> NonConstOp<'tcx> for FnCallNonConst<'tcx> {
             ccx.const_kind(),
         ));
 
+        if let ConstContext::Static(_) = ccx.const_kind() {
+            err.note("consider wrapping this expression in `Lazy::new(|| ...)` from the `once_cell` crate: https://crates.io/crates/once_cell");
+        }
+
         err
     }
 }

src/test/ui/borrowck/issue-64453.stderr

@@ -14,6 +14,7 @@ LL | static settings_dir: String = format!("");
    |                               ^^^^^^^^^^^
    |
    = note: calls in statics are limited to constant functions, tuple structs and tuple variants
+   = note: consider wrapping this expression in `Lazy::new(|| ...)` from the `once_cell` crate: https://crates.io/crates/once_cell
    = note: this error originates in the macro `format` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error[E0507]: cannot move out of static item `settings_dir`

src/test/ui/check-static-values-constraints.stderr

@@ -22,6 +22,7 @@ LL |     field2: SafeEnum::Variant4("str".to_string())
    |                                      ^^^^^^^^^^^
    |
    = note: calls in statics are limited to constant functions, tuple structs and tuple variants
+   = note: consider wrapping this expression in `Lazy::new(|| ...)` from the `once_cell` crate: https://crates.io/crates/once_cell
 
 error[E0010]: allocations are not allowed in statics
   --> $DIR/check-static-values-constraints.rs:94:5

src/test/ui/consts/issue-32829-2.stderr

@@ -13,6 +13,7 @@ LL |         invalid();
    |         ^^^^^^^^^
    |
    = note: calls in statics are limited to constant functions, tuple structs and tuple variants
+   = note: consider wrapping this expression in `Lazy::new(|| ...)` from the `once_cell` crate: https://crates.io/crates/once_cell
 
 error[E0015]: cannot call non-const fn `invalid` in statics
   --> $DIR/issue-32829-2.rs:54:9
@@ -21,6 +22,7 @@ LL |         invalid();
    |         ^^^^^^^^^
    |
    = note: calls in statics are limited to constant functions, tuple structs and tuple variants
+   = note: consider wrapping this expression in `Lazy::new(|| ...)` from the `once_cell` crate: https://crates.io/crates/once_cell
 
 error: aborting due to 3 previous errors
 

src/test/ui/consts/mir_check_nonconst.stderr

@@ -5,6 +5,7 @@ LL | static foo: Foo = bar();
    |                   ^^^^^
    |
    = note: calls in statics are limited to constant functions, tuple structs and tuple variants
+   = note: consider wrapping this expression in `Lazy::new(|| ...)` from the `once_cell` crate: https://crates.io/crates/once_cell
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-16538.mir.stderr

@@ -5,6 +5,7 @@ LL | static foo: &Y::X = &*Y::foo(Y::x as *const Y::X);
    |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: calls in statics are limited to constant functions, tuple structs and tuple variants
+   = note: consider wrapping this expression in `Lazy::new(|| ...)` from the `once_cell` crate: https://crates.io/crates/once_cell
 
 error[E0133]: use of extern static is unsafe and requires unsafe function or block
   --> $DIR/issue-16538.rs:14:30

src/test/ui/issues/issue-16538.thir.stderr

@@ -21,6 +21,7 @@ LL | static foo: &Y::X = &*Y::foo(Y::x as *const Y::X);
    |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: calls in statics are limited to constant functions, tuple structs and tuple variants
+   = note: consider wrapping this expression in `Lazy::new(|| ...)` from the `once_cell` crate: https://crates.io/crates/once_cell
 
 error: aborting due to 3 previous errors
 

src/test/ui/issues/issue-25901.stderr

@@ -16,6 +16,7 @@ note: impl defined here, but it is not `const`
 LL | impl Deref for A {
    | ^^^^^^^^^^^^^^^^
    = note: calls in statics are limited to constant functions, tuple structs and tuple variants
+   = note: consider wrapping this expression in `Lazy::new(|| ...)` from the `once_cell` crate: https://crates.io/crates/once_cell
 
 error: aborting due to previous error
 

src/test/ui/static/static-vec-repeat-not-constant.stderr

@@ -5,6 +5,7 @@ LL | static a: [isize; 2] = [foo(); 2];
    |                         ^^^^^
    |
    = note: calls in statics are limited to constant functions, tuple structs and tuple variants
+   = note: consider wrapping this expression in `Lazy::new(|| ...)` from the `once_cell` crate: https://crates.io/crates/once_cell
 
 error: aborting due to previous error