Auto merge of #13789 - feniljain:fix_enum_completion, r=Veykril

feat: show only missing variant suggestion for enums in patterns completion and bump them in list too

Fixes #12438

### Points to help in review:

- This PR can be reviewed commit wise, first commit is about bumping enum variant completions up in the list of completions and second commit is about only showing enum variants which are not complete
- I am calculating missing variants in analysis.rs by firstly locating the enum and then comparing each of it's variant's name and checking if arm string already contains that name, this is kinda hacky but I didn't want to implement complete missing_arms assist here as that would have been too bulky to run on each completion cycle ( if we can improve this somehow would appreciate some inputs on it )

### Output:

https://user-images.githubusercontent.com/49019259/208245540-57d7321b-b275-477e-bef0-b3a1ff8b7040.mov

Relevant Zulip Discussion: https://rust-lang.zulipchat.com/#narrow/stream/185405-t-compiler.2Frust-analyzer/topic/Issue.20.2312438

crates/ide-completion/src/context/analysis.rs

@@ -1,7 +1,7 @@
 //! Module responsible for analyzing the code surrounding the cursor for completion.
 use std::iter;
 
-use hir::{Semantics, Type, TypeInfo};
+use hir::{Semantics, Type, TypeInfo, Variant};
 use ide_db::{active_parameter::ActiveParameter, RootDatabase};
 use syntax::{
     algo::{find_node_at_offset, non_trivia_sibling},
@@ -1132,6 +1132,9 @@ fn pattern_context_for(
     pat: ast::Pat,
 ) -> PatternContext {
     let mut param_ctx = None;
+
+    let mut missing_variants = vec![];
+
     let (refutability, has_type_ascription) =
     pat
         .syntax()
@@ -1161,7 +1164,52 @@ fn pattern_context_for(
                         })();
                         return (PatternRefutability::Irrefutable, has_type_ascription)
                     },
-                    ast::MatchArm(_) => PatternRefutability::Refutable,
+                    ast::MatchArm(match_arm) => {
+                       let missing_variants_opt = match_arm
+                            .syntax()
+                            .parent()
+                            .and_then(ast::MatchArmList::cast)
+                            .and_then(|match_arm_list| {
+                                match_arm_list
+                                .syntax()
+                                .parent()
+                                .and_then(ast::MatchExpr::cast)
+                                .and_then(|match_expr| {
+                                    let expr_opt = find_opt_node_in_file(&original_file, match_expr.expr());
+
+                                    expr_opt.and_then(|expr| {
+                                        sema.type_of_expr(&expr)?
+                                        .adjusted()
+                                        .autoderef(sema.db)
+                                        .find_map(|ty| match ty.as_adt() {
+                                            Some(hir::Adt::Enum(e)) => Some(e),
+                                            _ => None,
+                                        }).and_then(|enum_| {
+                                            Some(enum_.variants(sema.db))
+                                        })
+                                    })
+                                }).and_then(|variants| {
+                                   Some(variants.iter().filter_map(|variant| {
+                                        let variant_name = variant.name(sema.db).to_string();
+
+                                        let variant_already_present = match_arm_list.arms().any(|arm| {
+                                            arm.pat().and_then(|pat| {
+                                                let pat_already_present = pat.syntax().to_string().contains(&variant_name);
+                                                pat_already_present.then(|| pat_already_present)
+                                            }).is_some()
+                                        });
+
+                                        (!variant_already_present).then_some(variant.clone())
+                                    }).collect::<Vec<Variant>>())
+                                })
+                        });
+
+                        if let Some(missing_variants_) = missing_variants_opt {
+                            missing_variants = missing_variants_;
+                        };
+
+                        PatternRefutability::Refutable
+                    },
                     ast::LetExpr(_) => PatternRefutability::Refutable,
                     ast::ForExpr(_) => PatternRefutability::Irrefutable,
                     _ => PatternRefutability::Irrefutable,
@@ -1183,6 +1231,7 @@ fn pattern_context_for(
         ref_token,
         record_pat: None,
         impl_: fetch_immediate_impl(sema, original_file, pat.syntax()),
+        missing_variants,
     }
 }