Use a shared env for desugaring and the rest of canonicalization

This refactor simplifies the desugar pass by reducing the number of
arguments threaded through each recursive function call.

- Add the module src string to `Env`.
- Add `line_info` to `Env` as a lazy-evaled function.
- Refactor desugar functions to take the `can::Env` struct in place of a
  number of params. This is mostly a find-and-replace, but in a few
  places `Vec::from_iter_in` was changed to `Vec::with_capacity_in`
  followed by a `for` loop in order to avoid lifetime issues.
- Remove unnecessary linter annotations for `clippy::too_many_arguments`

crates/compiler/can/src/env.rs

@@ -7,7 +7,7 @@ use roc_collections::{MutMap, VecSet};
 use roc_module::ident::{Ident, ModuleName};
 use roc_module::symbol::{IdentIdsByModule, ModuleId, PQModuleName, PackageModuleIds, Symbol};
 use roc_problem::can::{Problem, RuntimeError};
-use roc_region::all::{Loc, Region};
+use roc_region::all::{LineInfo, Loc, Region};
 use roc_types::subs::Variable;
 
 /// The canonicalization environment for a particular module.
@@ -44,11 +44,16 @@ pub struct Env<'a> {
     pub arena: &'a Bump,
 
     pub opt_shorthand: Option<&'a str>,
+
+    pub src: &'a str,
+
+    line_info: &'a mut Option<LineInfo>,
 }
 
 impl<'a> Env<'a> {
     pub fn new(
         arena: &'a Bump,
+        src: &'a str,
         home: ModuleId,
         module_path: &'a Path,
         dep_idents: &'a IdentIdsByModule,
@@ -57,6 +62,7 @@ impl<'a> Env<'a> {
     ) -> Env<'a> {
         Env {
             arena,
+            src,
             home,
             module_path,
             dep_idents,
@@ -69,6 +75,7 @@ impl<'a> Env<'a> {
             top_level_symbols: VecSet::default(),
             home_params_record: None,
             opt_shorthand,
+            line_info: arena.alloc(None),
         }
     }
 
@@ -219,4 +226,14 @@ impl<'a> Env<'a> {
     pub fn problem(&mut self, problem: Problem) {
         self.problems.push(problem)
     }
+
+    /// Lazily calculate line_info only if required. This way it there are no
+    /// `dbg` statements, we never pay the cast of scanning the source an extra
+    /// time.
+    pub fn line_info(&mut self) -> &LineInfo {
+        if self.line_info.is_none() {
+            *self.line_info = Some(LineInfo::new(self.src));
+        }
+        self.line_info.as_ref().unwrap()
+    }
 }