Re-do recursive const stability checks

Fundamentally, we have *three* disjoint categories of functions:
1. const-stable functions
2. private/unstable functions that are meant to be callable from const-stable functions
3. functions that can make use of unstable const features

This PR implements the following system:
- `#[rustc_const_stable]` puts functions in the first category. It may only be applied to `#[stable]` functions.
- `#[rustc_const_unstable]` by default puts functions in the third category. The new attribute `#[rustc_const_stable_indirect]` can be added to such a function to move it into the second category.
- `const fn` without a const stability marker are in the second category if they are still unstable. They automatically inherit the feature gate for regular calls, it can now also be used for const-calls.

Also, several holes in recursive const stability checking are being closed.
There's still one potential hole that is hard to avoid, which is when MIR
building automatically inserts calls to a particular function in stable
functions -- which happens in the panic machinery. Those need to *not* be
`rustc_const_unstable` (or manually get a `rustc_const_stable_indirect`) to be
sure they follow recursive const stability. But that's a fairly rare and special
case so IMO it's fine.

The net effect of this is that a `#[unstable]` or unmarked function can be
constified simply by marking it as `const fn`, and it will then be
const-callable from stable `const fn` and subject to recursive const stability
requirements. If it is publicly reachable (which implies it cannot be unmarked),
it will be const-unstable under the same feature gate. Only if the function ever
becomes `#[stable]` does it need a `#[rustc_const_unstable]` or
`#[rustc_const_stable]` marker to decide if this should also imply
const-stability.

Adding `#[rustc_const_unstable]` is only needed for (a) functions that need to
use unstable const lang features (including intrinsics), or (b) `#[stable]`
functions that are not yet intended to be const-stable. Adding
`#[rustc_const_stable]` is only needed for functions that are actually meant to
be directly callable from stable const code. `#[rustc_const_stable_indirect]` is
used to mark intrinsics as const-callable and for `#[rustc_const_unstable]`
functions that are actually called from other, exposed-on-stable `const fn`. No
other attributes are required.

1 files changed, 35 insertions, 19 deletions

diff --git a/tests/ui/consts/rustc-const-stability-require-const.stderr b/tests/ui/consts/rustc-const-stability-require-const.stderr
index 1027b9311b7..d9a7d37cbcd 100644
--- a/tests/ui/consts/rustc-const-stability-require-const.stderr
+++ b/tests/ui/consts/rustc-const-stability-require-const.stderr
@@ -1,8 +1,6 @@
-error: attributes `#[rustc_const_unstable]` and `#[rustc_const_stable]` require the function or method to be `const`
+error: attributes `#[rustc_const_unstable]`, `#[rustc_const_stable]` and `#[rustc_const_stable_indirect]` require the function or method to be `const`
   --> $DIR/rustc-const-stability-require-const.rs:7:1
    |
-LL | #[rustc_const_unstable(feature = "const_foo", issue = "none")]
-   | -------------------------------------------------------------- attribute specified here
 LL | pub fn foo() {}
    | ^^^^^^^^^^^^
    |
@@ -12,11 +10,9 @@ help: make the function or method const
 LL | pub fn foo() {}
    | ^^^^^^^^^^^^
 
-error: attributes `#[rustc_const_unstable]` and `#[rustc_const_stable]` require the function or method to be `const`
+error: attributes `#[rustc_const_unstable]`, `#[rustc_const_stable]` and `#[rustc_const_stable_indirect]` require the function or method to be `const`
   --> $DIR/rustc-const-stability-require-const.rs:12:1
    |
-LL | #[rustc_const_stable(feature = "const_bar", since = "1.0.0")]
-   | ------------------------------------------------------------- attribute specified here
 LL | pub fn bar() {}
    | ^^^^^^^^^^^^
    |
@@ -26,11 +22,9 @@ help: make the function or method const
 LL | pub fn bar() {}
    | ^^^^^^^^^^^^
 
-error: attributes `#[rustc_const_unstable]` and `#[rustc_const_stable]` require the function or method to be `const`
+error: attributes `#[rustc_const_unstable]`, `#[rustc_const_stable]` and `#[rustc_const_stable_indirect]` require the function or method to be `const`
   --> $DIR/rustc-const-stability-require-const.rs:21:5
    |
-LL |     #[rustc_const_unstable(feature = "const_salad", issue = "none")]
-   |     ---------------------------------------------------------------- attribute specified here
 LL |     pub fn salad(&self) -> &'static str { "mmmmmm" }
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
@@ -40,11 +34,9 @@ help: make the function or method const
 LL |     pub fn salad(&self) -> &'static str { "mmmmmm" }
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: attributes `#[rustc_const_unstable]` and `#[rustc_const_stable]` require the function or method to be `const`
+error: attributes `#[rustc_const_unstable]`, `#[rustc_const_stable]` and `#[rustc_const_stable_indirect]` require the function or method to be `const`
   --> $DIR/rustc-const-stability-require-const.rs:26:5
    |
-LL |     #[rustc_const_unstable(feature = "const_roasted", issue = "none")]
-   |     ------------------------------------------------------------------ attribute specified here
 LL |     pub fn roasted(&self) -> &'static str { "mmmmmmmmmm" }
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
@@ -54,11 +46,9 @@ help: make the function or method const
 LL |     pub fn roasted(&self) -> &'static str { "mmmmmmmmmm" }
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: attributes `#[rustc_const_unstable]` and `#[rustc_const_stable]` require the function or method to be `const`
+error: attributes `#[rustc_const_unstable]`, `#[rustc_const_stable]` and `#[rustc_const_stable_indirect]` require the function or method to be `const`
   --> $DIR/rustc-const-stability-require-const.rs:32:1
    |
-LL | #[rustc_const_stable(feature = "const_bar", since = "1.0.0")]
-   | ------------------------------------------------------------- attribute specified here
 LL | pub extern "C" fn bar_c() {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^
    |
@@ -68,11 +58,9 @@ help: make the function or method const
 LL | pub extern "C" fn bar_c() {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: attributes `#[rustc_const_unstable]` and `#[rustc_const_stable]` require the function or method to be `const`
+error: attributes `#[rustc_const_unstable]`, `#[rustc_const_stable]` and `#[rustc_const_stable_indirect]` require the function or method to be `const`
   --> $DIR/rustc-const-stability-require-const.rs:37:1
    |
-LL | #[rustc_const_unstable(feature = "const_foo", issue = "none")]
-   | -------------------------------------------------------------- attribute specified here
 LL | pub extern "C" fn foo_c() {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^
    |
@@ -82,5 +70,33 @@ help: make the function or method const
 LL | pub extern "C" fn foo_c() {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 6 previous errors
+error: attribute `#[rustc_const_stable]` can only be applied to functions that are declared `#[stable]`
+  --> $DIR/rustc-const-stability-require-const.rs:52:1
+   |
+LL | #[rustc_const_stable(feature = "barfoo_const", since = "1.0.0")]
+   | ---------------------------------------------------------------- attribute specified here
+LL | const fn barfoo_unmarked() {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: attribute `#[rustc_const_stable]` can only be applied to functions that are declared `#[stable]`
+  --> $DIR/rustc-const-stability-require-const.rs:57:1
+   |
+LL | #[rustc_const_stable(feature = "barfoo_const", since = "1.0.0")]
+   | ---------------------------------------------------------------- attribute specified here
+LL | pub const fn barfoo_unstable() {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: attributes `#[rustc_const_unstable]`, `#[rustc_const_stable]` and `#[rustc_const_stable_indirect]` require the function or method to be `const`
+  --> $DIR/rustc-const-stability-require-const.rs:63:1
+   |
+LL | pub fn not_a_const_fn() {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: make the function or method const
+  --> $DIR/rustc-const-stability-require-const.rs:63:1
+   |
+LL | pub fn not_a_const_fn() {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 9 previous errors