Use anyhow more idiomatically (#31052)

https://github.com/zed-industries/zed/issues/30972 brought up another
case where our context is not enough to track the actual source of the
issue: we get a general top-level error without inner error.

The reason for this was `.ok_or_else(|| anyhow!("failed to read HEAD
SHA"))?; ` on the top level.

The PR finally reworks the way we use anyhow to reduce such issues (or
at least make it simpler to bubble them up later in a fix).
On top of that, uses a few more anyhow methods for better readability.

* `.ok_or_else(|| anyhow!("..."))`, `map_err` and other similar error
conversion/option reporting cases are replaced with `context` and
`with_context` calls
* in addition to that, various `anyhow!("failed to do ...")` are
stripped with `.context("Doing ...")` messages instead to remove the
parasitic `failed to` text
* `anyhow::ensure!` is used instead of `if ... { return Err(...); }`
calls
* `anyhow::bail!` is used instead of `return Err(anyhow!(...));`

Release Notes:

- N/A

crates/assistant_slash_commands/src/context_server_command.rs

@@ -1,4 +1,4 @@
-use anyhow::{Result, anyhow};
+use anyhow::{Context as _, Result, anyhow};
 use assistant_slash_command::{
     AfterCompletion, ArgumentCompletion, SlashCommand, SlashCommandOutput,
     SlashCommandOutputSection, SlashCommandResult,
@@ -84,9 +84,7 @@ impl SlashCommand for ContextServerSlashCommand {
 
         if let Some(server) = self.store.read(cx).get_running_server(&server_id) {
             cx.foreground_executor().spawn(async move {
-                let Some(protocol) = server.client() else {
-                    return Err(anyhow!("Context server not initialized"));
-                };
+                let protocol = server.client().context("Context server not initialized")?;
 
                 let completion_result = protocol
                     .completion(
@@ -139,21 +137,16 @@ impl SlashCommand for ContextServerSlashCommand {
         let store = self.store.read(cx);
         if let Some(server) = store.get_running_server(&server_id) {
             cx.foreground_executor().spawn(async move {
-                let Some(protocol) = server.client() else {
-                    return Err(anyhow!("Context server not initialized"));
-                };
+                let protocol = server.client().context("Context server not initialized")?;
                 let result = protocol.run_prompt(&prompt_name, prompt_args).await?;
 
-                // Check that there are only user roles
-                if result
-                    .messages
-                    .iter()
-                    .any(|msg| !matches!(msg.role, context_server::types::Role::User))
-                {
-                    return Err(anyhow!(
-                        "Prompt contains non-user roles, which is not supported"
-                    ));
-                }
+                anyhow::ensure!(
+                    result
+                        .messages
+                        .iter()
+                        .all(|msg| matches!(msg.role, context_server::types::Role::User)),
+                    "Prompt contains non-user roles, which is not supported"
+                );
 
                 // Extract text from user messages into a single prompt string
                 let mut prompt = result
@@ -192,9 +185,7 @@ impl SlashCommand for ContextServerSlashCommand {
 }
 
 fn completion_argument(prompt: &Prompt, arguments: &[String]) -> Result<(String, String)> {
-    if arguments.is_empty() {
-        return Err(anyhow!("No arguments given"));
-    }
+    anyhow::ensure!(!arguments.is_empty(), "No arguments given");
 
     match &prompt.arguments {
         Some(args) if args.len() == 1 => {
@@ -202,16 +193,16 @@ fn completion_argument(prompt: &Prompt, arguments: &[String]) -> Result<(String,
             let arg_value = arguments.join(" ");
             Ok((arg_name, arg_value))
         }
-        Some(_) => Err(anyhow!("Prompt must have exactly one argument")),
-        None => Err(anyhow!("Prompt has no arguments")),
+        Some(_) => anyhow::bail!("Prompt must have exactly one argument"),
+        None => anyhow::bail!("Prompt has no arguments"),
     }
 }
 
 fn prompt_arguments(prompt: &Prompt, arguments: &[String]) -> Result<HashMap<String, String>> {
     match &prompt.arguments {
-        Some(args) if args.len() > 1 => Err(anyhow!(
-            "Prompt has more than one argument, which is not supported"
-        )),
+        Some(args) if args.len() > 1 => {
+            anyhow::bail!("Prompt has more than one argument, which is not supported");
+        }
         Some(args) if args.len() == 1 => {
             if !arguments.is_empty() {
                 let mut map = HashMap::default();
@@ -220,15 +211,15 @@ fn prompt_arguments(prompt: &Prompt, arguments: &[String]) -> Result<HashMap<Str
             } else if arguments.is_empty() && args[0].required == Some(false) {
                 Ok(HashMap::default())
             } else {
-                Err(anyhow!("Prompt expects argument but none given"))
+                anyhow::bail!("Prompt expects argument but none given");
             }
         }
         Some(_) | None => {
-            if arguments.is_empty() {
-                Ok(HashMap::default())
-            } else {
-                Err(anyhow!("Prompt expects no arguments but some were given"))
-            }
+            anyhow::ensure!(
+                arguments.is_empty(),
+                "Prompt expects no arguments but some were given"
+            );
+            Ok(HashMap::default())
         }
     }
 }