miniscript: add CONSENSUS and SANE consts to all contexts

We are going to move all of the logic currently ad-hoc spread across the
various "global"/"local" "consensus"/"policy" checks and other related
checks into a single validate() method implemented on Miniscript, Policy,
Semantic and possibly other types.

This validate() method will have an understanding of which checks are
"local" (i.e. computable from a single node) and which are "global" (i.e.
require looking at the whole script). This distinction is not meaningful
to an end user and it's hard for even developers to keep track of. For
a user, all the checks are just checks, and their knob belongs in the
big ValidationParams structure.

Author: apoelstra

src/miniscript/context.rs

@@ -15,7 +15,7 @@ use crate::miniscript::limits::{
 };
 use crate::miniscript::types;
 use crate::prelude::*;
-use crate::{hash256, Error, ForEachKey, Miniscript, MiniscriptKey, Terminal};
+use crate::{hash256, Error, ForEachKey, Miniscript, MiniscriptKey, Terminal, ValidationParams};
 
 /// Error for Script Context
 #[derive(Clone, PartialEq, Eq, Debug)]
@@ -176,6 +176,18 @@ where
 {
     /// The consensus key associated with the type. Must be a parseable key
     type Key: ParseableKey;
+
+    /// The validation parameters enforcing consensus limits in this context, and
+    /// nothing further.
+    const CONSENSUS: ValidationParams;
+
+    /// Sensible validation parameters for this context. Unless you have a good reason
+    /// to choose otherwise, these are the validation parameters you want.
+    ///
+    /// They are also the validation parameters used throughout this library when no
+    /// explicit choice of parameters is made.
+    const SANE: ValidationParams;
+
     /// Depending on ScriptContext, fragments can be malleable. For Example,
     /// under Legacy context, PkH is malleable because it is possible to
     /// estimate the cost of satisfaction because of compressed keys
@@ -359,6 +371,20 @@ pub enum Legacy {}
 
 impl ScriptContext for Legacy {
     type Key = bitcoin::PublicKey;
+
+    const CONSENSUS: ValidationParams = ValidationParams {
+        allow_compressed_keys: true,
+        allow_dup_if: false,
+        allow_uncompressed_keys: true,
+        allow_multi_a: false,
+        allow_or_i: false,
+        allow_x_only_keys: false,
+        max_opcode_count: MAX_OPS_PER_SCRIPT,
+        max_script_size: MAX_SCRIPT_ELEMENT_SIZE,
+        ..ValidationParams::CONSENSUS
+    };
+    const SANE: ValidationParams = Self::CONSENSUS.intersect(&ValidationParams::SANE);
+
     fn check_terminal_non_malleable<Pk: MiniscriptKey>(
         frag: &Terminal<Pk, Self>,
     ) -> Result<(), ScriptContextError> {
@@ -467,6 +493,22 @@ pub enum Segwitv0 {}
 
 impl ScriptContext for Segwitv0 {
     type Key = bitcoin::PublicKey;
+
+    const CONSENSUS: ValidationParams = ValidationParams {
+        allow_compressed_keys: true,
+        allow_uncompressed_keys: false,
+        allow_multi_a: false,
+        allow_x_only_keys: false,
+        max_opcode_count: MAX_OPS_PER_SCRIPT,
+        max_exec_stack_size: MAX_STACK_SIZE,
+        ..ValidationParams::CONSENSUS
+    };
+    const SANE: ValidationParams = ValidationParams {
+        max_script_size: MAX_STANDARD_P2WSH_SCRIPT_SIZE,
+        max_witness_items: MAX_STANDARD_P2WSH_STACK_ITEMS,
+        ..Self::CONSENSUS.intersect(&ValidationParams::SANE)
+    };
+
     fn check_terminal_non_malleable<Pk: MiniscriptKey>(
         _frag: &Terminal<Pk, Self>,
     ) -> Result<(), ScriptContextError> {
@@ -580,6 +622,20 @@ pub enum Tap {}
 
 impl ScriptContext for Tap {
     type Key = bitcoin::secp256k1::XOnlyPublicKey;
+
+    const CONSENSUS: ValidationParams = ValidationParams {
+        allow_compressed_keys: false,
+        allow_uncompressed_keys: false,
+        allow_multi: false,
+        allow_x_only_keys: true,
+        ..ValidationParams::CONSENSUS
+    };
+    const SANE: ValidationParams = ValidationParams {
+        max_script_size: MAX_STANDARD_P2WSH_SCRIPT_SIZE,
+        max_witness_items: MAX_STANDARD_P2WSH_STACK_ITEMS,
+        ..Self::CONSENSUS.intersect(&ValidationParams::SANE)
+    };
+
     fn check_terminal_non_malleable<Pk: MiniscriptKey>(
         _frag: &Terminal<Pk, Self>,
     ) -> Result<(), ScriptContextError> {
@@ -691,6 +747,20 @@ pub enum BareCtx {}
 
 impl ScriptContext for BareCtx {
     type Key = bitcoin::PublicKey;
+
+    const CONSENSUS: ValidationParams = ValidationParams {
+        allow_compressed_keys: true,
+        allow_dup_if: false,
+        allow_uncompressed_keys: true,
+        allow_multi_a: false,
+        allow_or_i: false,
+        allow_x_only_keys: false,
+        max_opcode_count: MAX_OPS_PER_SCRIPT,
+        max_script_size: MAX_SCRIPT_SIZE,
+        ..ValidationParams::CONSENSUS
+    };
+    const SANE: ValidationParams = Self::CONSENSUS.intersect(&ValidationParams::SANE);
+
     fn check_terminal_non_malleable<Pk: MiniscriptKey>(
         _frag: &Terminal<Pk, Self>,
     ) -> Result<(), ScriptContextError> {
@@ -801,6 +871,10 @@ pub enum NoChecks {}
 impl ScriptContext for NoChecks {
     // todo: When adding support for interpreter, we need a enum with all supported keys here
     type Key = bitcoin::PublicKey;
+
+    const CONSENSUS: ValidationParams = ValidationParams::MAX;
+    const SANE: ValidationParams = ValidationParams::MAX;
+
     fn check_terminal_non_malleable<Pk: MiniscriptKey>(
         _frag: &Terminal<Pk, Self>,
     ) -> Result<(), ScriptContextError> {