Unify let-introduction in a single path

Remove branches on determining how let-bindings are introduced to the
scope. This is maybe a little more inefficient, but I think it is a huge
simplification.

One additional change this required was changing how fx suffixes are
checked. The current implementation would add additional constraints for
patterns in let bindings conditionally. However, this is unnecessary. I
believe it is sufficient to check the fx suffix by running the checks on
all introduced symbols after the type is well known (i.e. the body is
checked).

crates/compiler/can/src/constraint.rs

@@ -619,23 +619,6 @@ impl Constraints {
         Constraint::FxCall(constraint_index)
     }
 
-    pub fn fx_pattern_suffix(
-        &mut self,
-        symbol: Symbol,
-        type_index: TypeOrVar,
-        region: Region,
-    ) -> Constraint {
-        let constraint = FxSuffixConstraint {
-            kind: FxSuffixKind::Pattern(symbol),
-            type_index,
-            region,
-        };
-
-        let constraint_index = index_push_new(&mut self.fx_suffix_constraints, constraint);
-
-        Constraint::FxSuffix(constraint_index)
-    }
-
     pub fn fx_record_field_unsuffixed(&mut self, variable: Variable, region: Region) -> Constraint {
         let type_index = Self::push_type_variable(variable);
         let constraint = FxSuffixConstraint {

crates/compiler/constrain/src/pattern.rs

@@ -6,7 +6,7 @@ use roc_can::pattern::Pattern::{self, *};
 use roc_can::pattern::{DestructType, ListPatterns, RecordDestruct, TupleDestruct};
 use roc_collections::all::{HumanIndex, SendMap};
 use roc_collections::VecMap;
-use roc_module::ident::{IdentSuffix, Lowercase};
+use roc_module::ident::Lowercase;
 use roc_module::symbol::Symbol;
 use roc_region::all::{Loc, Region};
 use roc_types::subs::Variable;
@@ -249,16 +249,7 @@ pub fn constrain_pattern(
     expected: PExpectedTypeIndex,
     state: &mut PatternState,
 ) {
-    constrain_pattern_help(
+    constrain_pattern_help(types, constraints, env, pattern, region, expected, state);
-        types,
-        constraints,
-        env,
-        pattern,
-        region,
-        expected,
-        state,
-        true,
-    );
 }
 
 #[allow(clippy::too_many_arguments)]
@@ -270,7 +261,6 @@ pub fn constrain_pattern_help(
     region: Region,
     expected: PExpectedTypeIndex,
     state: &mut PatternState,
-    is_shallow: bool,
 ) {
     match pattern {
         Underscore => {
@@ -300,27 +290,6 @@ pub fn constrain_pattern_help(
                     .push(constraints.is_open_type(type_index));
             }
 
-            // Identifiers introduced in nested patterns get let constraints
-            // and therefore don't need fx_pattern_suffix constraints.
-            if is_shallow {
-                match symbol.suffix() {
-                    IdentSuffix::None => {
-                        // Unsuffixed identifiers should be constrained after we know if they're functions
-                        state
-                            .delayed_fx_suffix_constraints
-                            .push(constraints.fx_pattern_suffix(*symbol, type_index, region));
-                    }
-                    IdentSuffix::Bang => {
-                        // Bang suffixed identifiers are always required to be functions
-                        // We constrain this before the function's body,
-                        // so that we don't think it's pure and complain about leftover statements
-                        state
-                            .constraints
-                            .push(constraints.fx_pattern_suffix(*symbol, type_index, region));
-                    }
-                }
-            }
-
             state.headers.insert(
                 *symbol,
                 Loc {
@@ -350,7 +319,6 @@ pub fn constrain_pattern_help(
                 subpattern.region,
                 expected,
                 state,
-                false,
             )
         }
 
@@ -584,7 +552,6 @@ pub fn constrain_pattern_help(
                         loc_pattern.region,
                         expected,
                         state,
-                        false,
                     );
 
                     pat_type
@@ -683,7 +650,6 @@ pub fn constrain_pattern_help(
                             loc_guard.region,
                             expected,
                             state,
-                            false,
                         );
 
                         RecordField::Demanded(pat_type)
@@ -807,7 +773,6 @@ pub fn constrain_pattern_help(
                     loc_pat.region,
                     expected,
                     state,
-                    false,
                 );
             }
 
@@ -864,7 +829,6 @@ pub fn constrain_pattern_help(
                     loc_pattern.region,
                     expected,
                     state,
-                    false,
                 );
             }
 
@@ -930,7 +894,6 @@ pub fn constrain_pattern_help(
                 loc_arg_pattern.region,
                 arg_pattern_expected,
                 state,
-                false,
             );
 
             // Next, link `whole_var` to the opaque type of "@Id who"

crates/compiler/load/tests/test_reporting.rs

@@ -15615,7 +15615,7 @@ All branches in an `if` must have the same type!
         @r###"
     ── MISSING EXCLAMATION in /code/proj/Main.roc ──────────────────────────────────
 
-    This is an effectful function, but its name does not indicate so:
+    This function is effectful, but its name does not indicate so:
 
     10│  forEach! = \l, f ->
                         ^
@@ -15652,7 +15652,7 @@ All branches in an `if` must have the same type!
         @r###"
     ── UNNECESSARY EXCLAMATION in /code/proj/Main.roc ──────────────────────────────
 
-    This is a pure function, but its name suggests otherwise:
+    This function is pure, but its name suggests otherwise:
 
     12│  mapOk = \result, fn! ->
                           ^^^

crates/compiler/solve/src/solve.rs

@@ -211,18 +211,7 @@ enum Work<'a> {
         constraint: &'a Constraint,
     },
     CheckForInfiniteTypes(LocalDefVarsVec<(Symbol, Loc<Variable>)>),
-    /// The ret_con part of a let constraint that does NOT introduces rigid and/or flex variables
+    CheckSuffixFx(LocalDefVarsVec<(Symbol, Loc<Variable>)>),
-    LetConNoVariables {
-        scope: &'a Scope,
-        rank: Rank,
-        let_con: &'a LetConstraint,
-
-        /// The variables used to store imported types in the Subs.
-        /// The `Contents` are copied from the source module, but to
-        /// mimic `type_to_var`, we must add these variables to `Pools`
-        /// at the correct rank
-        pool_variables: &'a [Variable],
-    },
     /// The ret_con part of a let constraint that introduces rigid and/or flex variables
     ///
     /// These introduced variables must be generalized, hence this variant
@@ -288,56 +277,6 @@ fn solve(
 
                 continue;
             }
-            Work::LetConNoVariables {
-                scope,
-                rank,
-                let_con,
-                pool_variables,
-            } => {
-                // NOTE be extremely careful with shadowing here
-                let offset = let_con.defs_and_ret_constraint.index();
-                let ret_constraint = &env.constraints.constraints[offset + 1];
-
-                // Add a variable for each def to new_vars_by_env.
-                let local_def_vars = LocalDefVarsVec::from_def_types(
-                    env,
-                    rank,
-                    problems,
-                    abilities_store,
-                    obligation_cache,
-                    &mut can_types,
-                    aliases,
-                    let_con.def_types,
-                );
-
-                env.pools.get_mut(rank).extend(pool_variables);
-
-                let mut new_scope = scope.clone();
-                for (symbol, loc_var) in local_def_vars.iter() {
-                    check_ability_specialization(
-                        env,
-                        rank,
-                        abilities_store,
-                        obligation_cache,
-                        awaiting_specializations,
-                        problems,
-                        *symbol,
-                        *loc_var,
-                    );
-
-                    new_scope.insert_symbol_var_if_vacant(*symbol, loc_var.value);
-                }
-
-                stack.push(Work::Constraint {
-                    scope: env.arena.alloc(new_scope),
-                    rank,
-                    constraint: ret_constraint,
-                });
-                // Check for infinite types first
-                stack.push(Work::CheckForInfiniteTypes(local_def_vars));
-
-                continue;
-            }
             Work::LetConIntroducesVariables {
                 scope,
                 rank,
@@ -456,14 +395,8 @@ fn solve(
 
                     new_scope.insert_symbol_var_if_vacant(*symbol, loc_var.value);
 
-                    solve_suffix_fx(
+                    // At the time of introduction, promote explicitly-effectful symbols.
-                        env,
+                    promote_effectful_symbol(env, FxSuffixKind::Let(*symbol), loc_var.value);
-                        problems,
-                        host_exposed_symbols,
-                        FxSuffixKind::Let(*symbol),
-                        loc_var.value,
-                        &loc_var.region,
-                    );
                 }
 
                 // Note that this vars_by_symbol is the one returned by the
@@ -473,20 +406,42 @@ fn solve(
                     mark: final_mark,
                 };
 
+                let next_work = [
+                    // Check for infinite types first
+                    Work::CheckForInfiniteTypes(local_def_vars.clone()),
                     // Now solve the body, using the new vars_by_symbol which includes
                     // the assignments' name-to-variable mappings.
-                stack.push(Work::Constraint {
+                    Work::Constraint {
                         scope: env.arena.alloc(new_scope),
                         rank,
                         constraint: ret_constraint,
-                });
+                    },
-                // Check for infinite types first
+                    // Finally, check the suffix fx, after we have solved all types.
-                stack.push(Work::CheckForInfiniteTypes(local_def_vars));
+                    Work::CheckSuffixFx(local_def_vars),
+                ];
+
+                for work in next_work.into_iter().rev() {
+                    stack.push(work);
+                }
 
                 state = state_for_ret_con;
 
                 continue;
             }
+
+            Work::CheckSuffixFx(local_def_vars) => {
+                for (symbol, loc_var) in local_def_vars.iter() {
+                    solve_suffix_fx(
+                        env,
+                        problems,
+                        host_exposed_symbols,
+                        FxSuffixKind::Let(*symbol),
+                        loc_var.value,
+                        &loc_var.region,
+                    );
+                }
+                continue;
+            }
         };
 
         state = match constraint {
@@ -1004,47 +959,12 @@ fn solve(
 
                 let offset = let_con.defs_and_ret_constraint.index();
                 let defs_constraint = &env.constraints.constraints[offset];
-                let ret_constraint = &env.constraints.constraints[offset + 1];
 
                 let flex_vars = &env.constraints.variables[let_con.flex_vars.indices()];
                 let rigid_vars = &env.constraints[let_con.rigid_vars];
 
                 let pool_variables = &env.constraints.variables[pool_slice.indices()];
 
-                if matches!(&ret_constraint, True) && rigid_vars.is_empty() {
-                    debug_assert!(pool_variables.is_empty());
-
-                    env.introduce(rank, flex_vars);
-
-                    // If the return expression is guaranteed to solve,
-                    // solve the assignments themselves and move on.
-                    stack.push(Work::Constraint {
-                        scope,
-                        rank,
-                        constraint: defs_constraint,
-                    });
-
-                    state
-                } else if rigid_vars.is_empty() && let_con.flex_vars.is_empty() {
-                    // items are popped from the stack in reverse order. That means that we'll
-                    // first solve then defs_constraint, and then (eventually) the ret_constraint.
-                    //
-                    // Note that the LetConSimple gets the current env and rank,
-                    // and not the env/rank from after solving the defs_constraint
-                    stack.push(Work::LetConNoVariables {
-                        scope,
-                        rank,
-                        let_con,
-                        pool_variables,
-                    });
-                    stack.push(Work::Constraint {
-                        scope,
-                        rank,
-                        constraint: defs_constraint,
-                    });
-
-                    state
-                } else {
                 // If the let-binding is generalizable, work at the next rank (which will be
                 // the rank at which introduced variables will become generalized, if they end up
                 // staying there); otherwise, stay at the current level.
@@ -1098,7 +1018,6 @@ fn solve(
 
                 state
             }
-            }
             IsOpenType(type_index) => {
                 let actual = either_type_index_to_var(
                     env,
@@ -1773,6 +1692,20 @@ fn solve_suffix_fx(
     }
 }
 
+fn promote_effectful_symbol(env: &mut InferenceEnv<'_>, kind: FxSuffixKind, variable: Variable) {
+    if kind.suffix() != IdentSuffix::Bang {
+        return;
+    }
+    if !matches!(
+        env.subs.get_content_without_compacting(variable),
+        Content::FlexVar(_)
+    ) {
+        return;
+    }
+    env.subs
+        .set_content(variable, Content::Structure(FlatType::EffectfulFunc));
+}
+
 fn chase_alias_content(subs: &Subs, mut var: Variable) -> (Variable, &Content) {
     loop {
         match subs.get_content_without_compacting(var) {
@@ -2147,7 +2080,7 @@ fn check_ability_specialization(
     }
 }
 
-#[derive(Debug)]
+#[derive(Debug, Clone)]
 enum LocalDefVarsVec<T> {
     Stack(arrayvec::ArrayVec<T, 32>),
     Heap(Vec<T>),