Improve hints for outlier warnings

Improve hints for outlier warnings if `--warmup` or `--prepare` are in
use already.

Closes #570

src/output/warnings.rs

@@ -4,12 +4,17 @@ use crate::benchmark::MIN_EXECUTION_TIME;
 use crate::output::format::format_duration;
 use crate::util::units::Second;
 
+pub struct OutlierWarningOptions {
+    pub warmup_in_use: bool,
+    pub prepare_in_use: bool,
+}
+
 /// A list of all possible warnings
 pub enum Warnings {
     FastExecutionTime,
     NonZeroExitCode,
-    SlowInitialRun(Second),
-    OutliersDetected,
+    SlowInitialRun(Second, OutlierWarningOptions),
+    OutliersDetected(OutlierWarningOptions),
 }
 
 impl fmt::Display for Warnings {
@@ -24,20 +29,40 @@ impl fmt::Display for Warnings {
                 MIN_EXECUTION_TIME * 1e3
             ),
             Warnings::NonZeroExitCode => write!(f, "Ignoring non-zero exit code."),
-            Warnings::SlowInitialRun(t_first) => write!(
+            Warnings::SlowInitialRun(time_first_run, ref options) => write!(
                 f,
                 "The first benchmarking run for this command was significantly slower than the \
-                 rest ({}). This could be caused by (filesystem) caches that were not filled until \
-                 after the first run. You should consider using the '--warmup' option to fill \
-                 those caches before the actual benchmark. Alternatively, use the '--prepare' \
-                 option to clear the caches before each timing run.",
-                format_duration(t_first, None)
+                 rest ({time}). This could be caused by (filesystem) caches that were not filled until \
+                 after the first run. {hints}",
+                time=format_duration(time_first_run, None),
+                hints=match (options.warmup_in_use, options.prepare_in_use) {
+                    (true, true) => "You are already using both the '--warmup' option as well \
+                    as the '--prepare' option. Consider re-running the benchmark on a quiet system. \
+                    Maybe it was a random outlier. Alternatively, consider increasing the warmup \
+                    count.",
+                    (true, false) => "You are already using the '--warmup' option which helps \
+                    to fill these caches before the actual benchmark. You can either try to \
+                    increase the warmup count further or re-run this benchmark on a quiet system \
+                    in case it was a random outlier. Alternatively, consider using the '--prepare' \
+                    option to clear the caches before each timing run.",
+                    (false, true) => "You are already using the '--prepare' option which can \
+                    be used to clear caches. If you did not use a cache-clearing command with \
+                    '--prepare', you can either try that or consider using the '--warmup' option \
+                    to fill those caches before the actual benchmark.",
+                    (false, false) => "You should consider using the '--warmup' option to fill \
+                    those caches before the actual benchmark. Alternatively, use the '--prepare' \
+                    option to clear the caches before each timing run."
+                }
             ),
-            Warnings::OutliersDetected => write!(
+            Warnings::OutliersDetected(ref options) => write!(
                 f,
                 "Statistical outliers were detected. Consider re-running this benchmark on a quiet \
-                 PC without any interferences from other programs. It might help to use the \
-                 '--warmup' or '--prepare' options."
+                 system without any interferences from other programs.{hint}",
+                hint=if options.warmup_in_use && options.prepare_in_use {
+                    ""
+                } else {
+                    " It might help to use the '--warmup' or '--prepare' options."
+                }
             ),
         }
     }