Refactor contract builtin macro + error handling

Instead of parsing the different components of a function signature,
eagerly look for either the `where` keyword or the function body.

- Also address feedback to use `From` instead of `TryFrom` in cranelift
  contract and ubcheck codegen.

tests/ui/contracts/contract-annotation-limitations.rs

@@ -0,0 +1,27 @@
+//! Test for some of the existing limitations and the current error messages.
+//! Some of these limitations may be removed in the future.
+
+#![feature(rustc_contracts)]
+#![allow(dead_code)]
+
+/// Represent a 5-star system.
+struct Stars(u8);
+
+impl Stars {
+    fn is_valid(&self) -> bool {
+        self.0 <= 5
+    }
+}
+
+trait ParseStars {
+    #[core::contracts::ensures(|ret| ret.is_none_or(Stars::is_valid))]
+    //~^ ERROR contract annotations is only supported in functions with bodies
+    fn parse_string(input: String) -> Option<Stars>;
+
+    #[core::contracts::ensures(|ret| ret.is_none_or(Stars::is_valid))]
+    //~^ ERROR contract annotations is only supported in functions with bodies
+    fn parse<T>(input: T) -> Option<Stars> where T: for<'a> Into<&'a str>;
+}
+
+fn main() {
+}

tests/ui/contracts/contract-annotation-limitations.stderr

@@ -0,0 +1,14 @@
+error: contract annotations is only supported in functions with bodies
+  --> $DIR/contract-annotation-limitations.rs:17:5
+   |
+LL |     #[core::contracts::ensures(|ret| ret.is_none_or(Stars::is_valid))]
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: contract annotations is only supported in functions with bodies
+  --> $DIR/contract-annotation-limitations.rs:21:5
+   |
+LL |     #[core::contracts::ensures(|ret| ret.is_none_or(Stars::is_valid))]
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+

tests/ui/contracts/contract-attributes-generics.rs

@@ -0,0 +1,70 @@
+//! Test that contracts can be applied to generic functions.
+
+//@ revisions: unchk_pass chk_pass chk_fail_pre chk_fail_post chk_const_fail
+//
+//@ [unchk_pass] run-pass
+//@ [chk_pass] run-pass
+//
+//@ [chk_fail_pre] run-fail
+//@ [chk_fail_post] run-fail
+//@ [chk_const_fail] run-fail
+//
+//@ [unchk_pass] compile-flags: -Zcontract-checks=no
+//
+//@ [chk_pass] compile-flags: -Zcontract-checks=yes
+//@ [chk_fail_pre] compile-flags: -Zcontract-checks=yes
+//@ [chk_fail_post] compile-flags: -Zcontract-checks=yes
+//@ [chk_const_fail] compile-flags: -Zcontract-checks=yes
+
+#![feature(rustc_contracts)]
+
+use std::ops::Sub;
+
+/// Dummy fn contract that precondition fails for val < 0, and post-condition fail for val == 1
+#[core::contracts::requires(val > 0u8.into())]
+#[core::contracts::ensures(|ret| *ret > 0u8.into())]
+fn decrement<T>(val: T) -> T
+where T: PartialOrd + Sub<Output=T> + From<u8>
+{
+    val - 1u8.into()
+}
+
+/// Create a structure that takes a constant parameter.
+#[allow(dead_code)]
+struct Capped<const MAX: usize>(usize);
+
+/// Now declare a function to create stars which shouldn't exceed 5 stars.
+// Add redundant braces to ensure the built-in macro can handle this syntax.
+#[allow(unused_braces)]
+#[core::contracts::requires(num <= 5)]
+unsafe fn stars_unchecked(num: usize) -> Capped<{ 5 }> {
+    Capped(num)
+}
+
+
+fn main() {
+    check_decrement();
+    check_stars();
+}
+
+fn check_stars() {
+    // This should always pass.
+    let _ = unsafe { stars_unchecked(3) };
+
+    // This violates the contract.
+    #[cfg(any(unchk_pass, chk_const_fail))]
+    let _ = unsafe { stars_unchecked(10) };
+}
+
+fn check_decrement() {
+    // This should always pass
+    assert_eq!(decrement(10u8), 9u8);
+
+    // This should fail requires but pass with no contract check.
+    #[cfg(any(unchk_pass, chk_fail_pre))]
+    assert_eq!(decrement(-2i128), -3i128);
+
+    // This should fail ensures but pass with no contract check.
+    #[cfg(any(unchk_pass, chk_fail_post))]
+    assert_eq!(decrement(1i32), 0i32);
+}

tests/ui/contracts/disallow-contract-annotation-on-non-fn.rs

@@ -0,0 +1,52 @@
+//! Checks for compilation errors related to adding contracts to non-function items.
+
+#![feature(rustc_contracts)]
+#![allow(dead_code)]
+
+#[core::contracts::requires(true)]
+//~^ ERROR contract annotations can only be used on functions
+struct Dummy(usize);
+
+#[core::contracts::ensures(|v| v == 100)]
+//~^ ERROR contract annotations can only be used on functions
+const MAX_VAL: usize = 100;
+
+// FIXME: Improve the error message here. The macro thinks this is a function.
+#[core::contracts::ensures(|v| v == 100)]
+//~^ ERROR contract annotations is only supported in functions with bodies
+type NewDummy = fn(usize) -> Dummy;
+
+#[core::contracts::ensures(|v| v == 100)]
+//~^ ERROR contract annotations is only supported in functions with bodies
+const NEW_DUMMY_FN : fn(usize) -> Dummy = || { Dummy(0) };
+
+#[core::contracts::requires(true)]
+//~^ ERROR contract annotations can only be used on functions
+impl Dummy {
+
+    // This should work
+    #[core::contracts::ensures(|ret| ret.0 == v)]
+    fn new(v: usize) -> Dummy {
+        Dummy(v)
+    }
+}
+
+#[core::contracts::ensures(|dummy| dummy.0 > 0)]
+//~^ ERROR contract annotations can only be used on functions
+impl From<usize> for Dummy {
+    // This should work.
+    #[core::contracts::ensures(|ret| ret.0 == v)]
+    fn from(value: usize) -> Self {
+        Dummy::new(value)
+    }
+}
+
+/// You should not be able to annotate a trait either.
+#[core::contracts::requires(true)]
+//~^ ERROR contract annotations can only be used on functions
+pub trait DummyBuilder {
+    fn build() -> Dummy;
+}
+
+fn main() {
+}

tests/ui/contracts/disallow-contract-annotation-on-non-fn.stderr

@@ -0,0 +1,44 @@
+error: contract annotations can only be used on functions
+  --> $DIR/disallow-contract-annotation-on-non-fn.rs:6:1
+   |
+LL | #[core::contracts::requires(true)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: contract annotations can only be used on functions
+  --> $DIR/disallow-contract-annotation-on-non-fn.rs:10:1
+   |
+LL | #[core::contracts::ensures(|v| v == 100)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: contract annotations is only supported in functions with bodies
+  --> $DIR/disallow-contract-annotation-on-non-fn.rs:15:1
+   |
+LL | #[core::contracts::ensures(|v| v == 100)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: contract annotations is only supported in functions with bodies
+  --> $DIR/disallow-contract-annotation-on-non-fn.rs:19:1
+   |
+LL | #[core::contracts::ensures(|v| v == 100)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: contract annotations can only be used on functions
+  --> $DIR/disallow-contract-annotation-on-non-fn.rs:23:1
+   |
+LL | #[core::contracts::requires(true)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: contract annotations can only be used on functions
+  --> $DIR/disallow-contract-annotation-on-non-fn.rs:34:1
+   |
+LL | #[core::contracts::ensures(|dummy| dummy.0 > 0)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: contract annotations can only be used on functions
+  --> $DIR/disallow-contract-annotation-on-non-fn.rs:45:1
+   |
+LL | #[core::contracts::requires(true)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 7 previous errors
+