Make Get<T> const friendly + Clean the runtime files a bit. (#6132)

* Make Get const friendly

* Better doc

* Grumble

* Better doc

* Clean runtime files more

substrate/frame/democracy/src/tests.rs

@@ -110,7 +110,7 @@ impl frame_system::Trait for Test {
 	type OnKilledAccount = ();
 }
 parameter_types! {
-	pub const MaximumSchedulerWeight: Weight = Perbill::from_percent(80) * MaximumBlockWeight::get();
+	pub MaximumSchedulerWeight: Weight = Perbill::from_percent(80) * MaximumBlockWeight::get();
 }
 impl pallet_scheduler::Trait for Test {
 	type Event = Event;

substrate/frame/grandpa/src/mock.rs

@@ -232,7 +232,7 @@ impl staking::Trait for Test {
 }
 
 parameter_types! {
-	pub const OffencesWeightSoftLimit: Weight = Perbill::from_percent(60) * MaximumBlockWeight::get();
+	pub OffencesWeightSoftLimit: Weight = Perbill::from_percent(60) * MaximumBlockWeight::get();
 }
 
 impl offences::Trait for Test {

substrate/frame/offences/benchmarking/src/mock.rs

@@ -187,7 +187,7 @@ impl pallet_im_online::Trait for Test {
 }
 
 parameter_types! {
-	pub const OffencesWeightSoftLimit: Weight = Perbill::from_percent(60) * MaximumBlockWeight::get();
+	pub OffencesWeightSoftLimit: Weight = Perbill::from_percent(60) * MaximumBlockWeight::get();
 }
 
 impl pallet_offences::Trait for Test {

substrate/frame/offences/src/mock.rs

@@ -122,7 +122,7 @@ impl frame_system::Trait for Runtime {
 }
 
 parameter_types! {
-	pub const OffencesWeightSoftLimit: Weight = Perbill::from_percent(60) * MaximumBlockWeight::get();
+	pub OffencesWeightSoftLimit: Weight = Perbill::from_percent(60) * MaximumBlockWeight::get();
 }
 
 impl Trait for Runtime {

substrate/frame/scheduler/src/lib.rs

@@ -515,7 +515,7 @@ mod tests {
 		type Event = ();
 	}
 	parameter_types! {
-		pub const MaximumSchedulerWeight: Weight = Perbill::from_percent(80) * MaximumBlockWeight::get();
+		pub MaximumSchedulerWeight: Weight = Perbill::from_percent(80) * MaximumBlockWeight::get();
 	}
 	impl Trait for Test {
 		type Event = ();

substrate/frame/support/src/lib.rs

@@ -84,27 +84,62 @@ pub use sp_runtime::{self, ConsensusEngineId, print, traits::Printable};
 #[derive(Debug)]
 pub enum Never {}
 
-/// Macro for easily creating a new implementation of the `Get` trait. Use similarly to
-/// how you would declare a `const`:
+/// Macro for easily creating a new implementation of the `Get` trait. If `const` token is used, the
+/// rhs of the expression must be `const`-only, and get is implemented as `const`:
 ///
-/// ```no_compile
+/// ```
+/// # use frame_support::traits::Get;
+/// # use frame_support::parameter_types;
+/// // This function cannot be used in a const context.
+/// fn non_const_expression() -> u64 { 99 }
+///
+/// const FIXED_VALUE: u64 = 10;
 /// parameter_types! {
-///   pub const Argument: u64 = 42;
+///   pub const Argument: u64 = 42 + FIXED_VALUE;
+///   pub OtherArgument: u64 = non_const_expression();
 /// }
+///
 /// trait Config {
 ///   type Parameter: Get<u64>;
+///   type OtherParameter: Get<u64>;
 /// }
+///
 /// struct Runtime;
 /// impl Config for Runtime {
 ///   type Parameter = Argument;
+///   type OtherParameter = OtherArgument;
 /// }
 /// ```
+///
+/// Invalid example:
+///
+/// ```compile_fail
+/// # use frame_support::traits::Get;
+/// # use frame_support::parameter_types;
+/// // This function cannot be used in a const context.
+/// fn non_const_expression() -> u64 { 99 }
+///
+/// parameter_types! {
+///   pub const Argument: u64 = non_const_expression();
+/// }
+/// ```
+
 #[macro_export]
 macro_rules! parameter_types {
 	(
 		$( #[ $attr:meta ] )*
 		$vis:vis const $name:ident: $type:ty = $value:expr;
 		$( $rest:tt )*
+	) => (
+		$( #[ $attr ] )*
+		$vis struct $name;
+		$crate::parameter_types!{IMPL_CONST $name , $type , $value}
+		$crate::parameter_types!{ $( $rest )* }
+	);
+	(
+		$( #[ $attr:meta ] )*
+		$vis:vis $name:ident: $type:ty = $value:expr;
+		$( $rest:tt )*
 	) => (
 		$( #[ $attr ] )*
 		$vis struct $name;
@@ -112,6 +147,18 @@ macro_rules! parameter_types {
 		$crate::parameter_types!{ $( $rest )* }
 	);
 	() => ();
+	(IMPL_CONST $name:ident , $type:ty , $value:expr) => {
+		impl $name {
+			pub const fn get() -> $type {
+				$value
+			}
+		}
+		impl<I: From<$type>> $crate::traits::Get<I> for $name {
+			fn get() -> I {
+				I::from($value)
+			}
+		}
+	};
 	(IMPL $name:ident , $type:ty , $value:expr) => {
 		impl $name {
 			pub fn get() -> $type {

substrate/frame/system/src/lib.rs

@@ -1877,7 +1877,7 @@ pub(crate) mod tests {
 		pub const MaximumExtrinsicWeight: Weight = 768;
 		pub const AvailableBlockRatio: Perbill = Perbill::from_percent(75);
 		pub const MaximumBlockLength: u32 = 1024;
-		pub const Version: RuntimeVersion = RuntimeVersion {
+		pub Version: RuntimeVersion = RuntimeVersion {
 			spec_name: sp_version::create_runtime_str!("test"),
 			impl_name: sp_version::create_runtime_str!("system-test"),
 			authoring_version: 1,