Move constrain and its deps into their own crates

compiler/problem/src/can.rs

@@ -0,0 +1,61 @@
+use inlinable_string::InlinableString;
+use roc_module::ident::Ident;
+use roc_module::symbol::{ModuleId, Symbol};
+use roc_parse::operator::BinOp;
+use roc_parse::pattern::PatternType;
+use roc_region::all::{Located, Region};
+
+/// Problems that can occur in the course of canonicalization.
+#[derive(Clone, Debug, PartialEq)]
+pub enum Problem {
+    UnusedDef(Symbol, Region),
+    UnusedImport(ModuleId, Region),
+    UnusedArgument(Symbol, Region),
+    PrecedenceProblem(PrecedenceProblem),
+    // Example: (5 = 1 + 2) is an unsupported pattern in an assignment; Int patterns aren't allowed in assignments!
+    UnsupportedPattern(PatternType, Region),
+    ShadowingInAnnotation {
+        original_region: Region,
+        shadow: Located<Ident>,
+    },
+    RuntimeError(RuntimeError),
+}
+
+#[derive(Clone, Debug, PartialEq)]
+pub enum PrecedenceProblem {
+    BothNonAssociative(Located<BinOp>, Located<BinOp>),
+}
+
+#[derive(Clone, Debug, PartialEq)]
+pub enum RuntimeError {
+    Shadowing {
+        original_region: Region,
+        shadow: Located<Ident>,
+    },
+    UnrecognizedFunctionName(Located<InlinableString>),
+    LookupNotInScope(Located<InlinableString>),
+    ValueNotExposed {
+        module_name: InlinableString,
+        ident: InlinableString,
+        region: Region,
+    },
+    ModuleNotImported {
+        module_name: InlinableString,
+        ident: InlinableString,
+        region: Region,
+    },
+    InvalidPrecedence(PrecedenceProblem, Region),
+    FloatOutsideRange(Box<str>),
+    IntOutsideRange(Box<str>),
+    InvalidHex(std::num::ParseIntError, Box<str>),
+    InvalidOctal(std::num::ParseIntError, Box<str>),
+    InvalidBinary(std::num::ParseIntError, Box<str>),
+    QualifiedPatternIdent(InlinableString),
+    CircularDef(
+        Vec<Located<Ident>>,
+        Vec<(Region /* pattern */, Region /* expr */)>,
+    ),
+
+    /// When the author specifies a type annotation but no implementation
+    NoImplementation,
+}

compiler/problem/src/lib.rs

@@ -0,0 +1,13 @@
+#![warn(clippy::all, clippy::dbg_macro)]
+// I'm skeptical that clippy:large_enum_variant is a good lint to have globally enabled.
+//
+// It warns about a performance problem where the only quick remediation is
+// to allocate more on the heap, which has lots of tradeoffs - including making it
+// long-term unclear which allocations *need* to happen for compilation's sake
+// (e.g. recursive structures) versus those which were only added to appease clippy.
+//
+// Effectively optimizing data struture memory layout isn't a quick fix,
+// and encouraging shortcuts here creates bad incentives. I would rather temporarily
+// re-enable this when working on performance optimizations than have it block PRs.
+#![allow(clippy::large_enum_variant)]
+pub mod can;