Make error messages great again (and fix clippy and add test)

compiler/rustc_typeck/src/check/mod.rs

@@ -366,7 +366,7 @@ fn typeck_with_fallback<'tcx>(
 
     let typeck_results = Inherited::build(tcx, def_id).enter(|inh| {
         let param_env = tcx.param_env(def_id);
-        let fcx = if let Some(hir::FnSig { header, decl, .. }) = fn_sig {
+        let mut fcx = if let Some(hir::FnSig { header, decl, .. }) = fn_sig {
             let fn_sig = if crate::collect::get_infer_ret_ty(&decl.output).is_some() {
                 let fcx = FnCtxt::new(&inh, param_env, body.value.hir_id);
                 <dyn AstConv<'_>>::ty_of_fn(&fcx, id, header.unsafety, header.abi, decl, None, None)
@@ -459,7 +459,11 @@ fn typeck_with_fallback<'tcx>(
 
         // Closure and generator analysis may run after fallback
         // because they don't constrain other type variables.
+        // Closure analysis only runs on closures. Therefore they only need to fulfill non-const predicates (as of now)
+        let prev_constness = fcx.param_env.constness();
+        fcx.param_env = fcx.param_env.without_const();
         fcx.closure_analyze(body);
+        fcx.param_env = fcx.param_env.with_constness(prev_constness);
         assert!(fcx.deferred_call_resolutions.borrow().is_empty());
         // Before the generator analysis, temporary scopes shall be marked to provide more
         // precise information on types to be captured.

src/test/ui/const-generics/issue-93647.rs

@@ -1,6 +1,6 @@
 struct X<const N: usize = {
     (||1usize)()
-    //~^ ERROR the trait bound
+    //~^ ERROR cannot call non-const closure
 }>;
 
 fn main() {}

src/test/ui/const-generics/issue-93647.stderr

@@ -1,17 +1,12 @@
-error[E0277]: the trait bound `[closure@$DIR/issue-93647.rs:2:6: 2:8]: Fn<()>` is not satisfied
-  --> $DIR/issue-93647.rs:2:5
-   |
-LL |     (||1usize)()
-   |     ^^^^^^^^^^^^ expected an `Fn<()>` closure, found `[closure@$DIR/issue-93647.rs:2:6: 2:8]`
-   |
-   = help: the trait `~const Fn<()>` is not implemented for closure `[closure@$DIR/issue-93647.rs:2:6: 2:8]`
-note: the trait `Fn<()>` is implemented for `[closure@$DIR/issue-93647.rs:2:6: 2:8]`, but that implementation is not `const`
+error[E0015]: cannot call non-const closure in constants
   --> $DIR/issue-93647.rs:2:5
    |
 LL |     (||1usize)()
    |     ^^^^^^^^^^^^
-   = note: wrap the `[closure@$DIR/issue-93647.rs:2:6: 2:8]` in a closure with no arguments: `|| { /* code */ }`
+   |
+   = note: closures need an RFC before allowed to be called in constants
+   = note: calls in constants are limited to constant functions, tuple structs and tuple variants
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0277`.
+For more information about this error, try `rustc --explain E0015`.

src/test/ui/consts/issue-28113.rs

@@ -2,7 +2,7 @@
 
 const X: u8 =
     || -> u8 { 5 }()
-    //~^ ERROR the trait bound
+    //~^ ERROR cannot call non-const closure
 ;
 
 fn main() {}

src/test/ui/consts/issue-28113.stderr

@@ -1,17 +1,12 @@
-error[E0277]: the trait bound `[closure@$DIR/issue-28113.rs:4:5: 4:13]: Fn<()>` is not satisfied
-  --> $DIR/issue-28113.rs:4:5
-   |
-LL |     || -> u8 { 5 }()
-   |     ^^^^^^^^^^^^^^^^ expected an `Fn<()>` closure, found `[closure@$DIR/issue-28113.rs:4:5: 4:13]`
-   |
-   = help: the trait `~const Fn<()>` is not implemented for closure `[closure@$DIR/issue-28113.rs:4:5: 4:13]`
-note: the trait `Fn<()>` is implemented for `[closure@$DIR/issue-28113.rs:4:5: 4:13]`, but that implementation is not `const`
+error[E0015]: cannot call non-const closure in constants
   --> $DIR/issue-28113.rs:4:5
    |
 LL |     || -> u8 { 5 }()
    |     ^^^^^^^^^^^^^^^^
-   = note: wrap the `[closure@$DIR/issue-28113.rs:4:5: 4:13]` in a closure with no arguments: `|| { /* code */ }`
+   |
+   = note: closures need an RFC before allowed to be called in constants
+   = note: calls in constants are limited to constant functions, tuple structs and tuple variants
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0277`.
+For more information about this error, try `rustc --explain E0015`.

src/test/ui/consts/issue-56164.rs

@@ -1,8 +1,11 @@
 const fn foo() { (||{})() }
-//~^ ERROR the trait bound
+//~^ ERROR cannot call non-const closure
+//~| ERROR erroneous constant
+//~| WARN this was previously accepted
 
 const fn bad(input: fn()) {
     input()
+    //~^ ERROR function pointer calls are not allowed
 }
 
 fn main() {

src/test/ui/consts/issue-56164.stderr

@@ -1,17 +1,39 @@
-error[E0277]: the trait bound `[closure@$DIR/issue-56164.rs:1:19: 1:21]: Fn<()>` is not satisfied
-  --> $DIR/issue-56164.rs:1:18
-   |
-LL | const fn foo() { (||{})() }
-   |                  ^^^^^^^^ expected an `Fn<()>` closure, found `[closure@$DIR/issue-56164.rs:1:19: 1:21]`
-   |
-   = help: the trait `~const Fn<()>` is not implemented for closure `[closure@$DIR/issue-56164.rs:1:19: 1:21]`
-note: the trait `Fn<()>` is implemented for `[closure@$DIR/issue-56164.rs:1:19: 1:21]`, but that implementation is not `const`
+error[E0015]: cannot call non-const closure in constant functions
   --> $DIR/issue-56164.rs:1:18
    |
 LL | const fn foo() { (||{})() }
    |                  ^^^^^^^^
-   = note: wrap the `[closure@$DIR/issue-56164.rs:1:19: 1:21]` in a closure with no arguments: `|| { /* code */ }`
+   |
+   = note: closures need an RFC before allowed to be called in constant functions
+   = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
 
-error: aborting due to previous error
+error: function pointer calls are not allowed in constant functions
+  --> $DIR/issue-56164.rs:7:5
+   |
+LL |     input()
+   |     ^^^^^^^
+
+error: erroneous constant used
+  --> $DIR/issue-56164.rs:1:18
+   |
+LL | const fn foo() { (||{})() }
+   |                  ^^^^^^ referenced constant has errors
+   |
+   = note: `#[deny(const_err)]` on by default
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0015`.
+Future incompatibility report: Future breakage diagnostic:
+error: erroneous constant used
+  --> $DIR/issue-56164.rs:1:18
+   |
+LL | const fn foo() { (||{})() }
+   |                  ^^^^^^ referenced constant has errors
+   |
+   = note: `#[deny(const_err)]` on by default
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>
 
-For more information about this error, try `rustc --explain E0277`.

src/test/ui/consts/issue-68542-closure-in-array-len.rs

@@ -3,7 +3,7 @@
 // in the length part of an array.
 
 struct Bug {
-    a: [(); (|| { 0 })()] //~ ERROR the trait bound
+    a: [(); (|| { 0 })()] //~ ERROR cannot call non-const closure
 }
 
 fn main() {}

src/test/ui/consts/issue-68542-closure-in-array-len.stderr

@@ -1,17 +1,12 @@
-error[E0277]: the trait bound `[closure@$DIR/issue-68542-closure-in-array-len.rs:6:14: 6:16]: Fn<()>` is not satisfied
-  --> $DIR/issue-68542-closure-in-array-len.rs:6:13
-   |
-LL |     a: [(); (|| { 0 })()]
-   |             ^^^^^^^^^^^^ expected an `Fn<()>` closure, found `[closure@$DIR/issue-68542-closure-in-array-len.rs:6:14: 6:16]`
-   |
-   = help: the trait `~const Fn<()>` is not implemented for closure `[closure@$DIR/issue-68542-closure-in-array-len.rs:6:14: 6:16]`
-note: the trait `Fn<()>` is implemented for `[closure@$DIR/issue-68542-closure-in-array-len.rs:6:14: 6:16]`, but that implementation is not `const`
+error[E0015]: cannot call non-const closure in constants
   --> $DIR/issue-68542-closure-in-array-len.rs:6:13
    |
 LL |     a: [(); (|| { 0 })()]
    |             ^^^^^^^^^^^^
-   = note: wrap the `[closure@$DIR/issue-68542-closure-in-array-len.rs:6:14: 6:16]` in a closure with no arguments: `|| { /* code */ }`
+   |
+   = note: closures need an RFC before allowed to be called in constants
+   = note: calls in constants are limited to constant functions, tuple structs and tuple variants
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0277`.
+For more information about this error, try `rustc --explain E0015`.

src/test/ui/rfc-2632-const-trait-impl/nested-closure.rs

@@ -0,0 +1,12 @@
+// check-pass
+
+#![feature(const_trait_impl, once_cell)]
+
+use std::sync::LazyLock;
+
+static EXTERN_FLAGS: LazyLock<String> = LazyLock::new(|| {
+    let x = || String::new();
+    x()
+});
+
+fn main() {}